Search

Diatoms are known to fractionate silicon isotopes during the formation of their frustules causing the silicon isotopic composition of biogenic silica to track the degree of silicic acid consumption in surface waters. Despite a growing body of work that uses this proxy to reconstruct past changes in silicic acid utilization, the understanding of the benthic silicon cycle, particularly the identification and quantification of the processes that potentially alter the silicon isotopic composition of biogenic silica during early diagenesis is still lacking. We investigated these processes by comparing the silicon isotopic composition of pore water silicic acid, biogenic silica and, for the first time, lithogenic silica from five sediment cores collected in the deep basin of the Southern Ocean representing a diversity of sedimentation regimes. Silicic acid concentrations and the isotopic composition of Southern Ocean pore waters were the result of a dynamic balance between the dissolution of biogenic silica, reactive lithogenic silica phases and Si re-precipitation with the relative importance of each processes differing significantly between regions. The results are consistent with the formation of authigenic alumino-silicates derived from dissolved biogenic silica in the Sub-Antarctic Zone and in the Antarctic Zone (on average 12 ± 5% and 17 ± 13%, respectively). Since this latter process can fractionate silicon isotopes, this implies that, even if the silicon isotopic composition of diatoms preserved in the sediments is a reliable proxy for silicic acid utilization in the past ocean, care must be taken to extract a clean biogenic silica phase free of authigenic clays and lithogenic phases from sediments to eliminate this potential bias when interpreting isotopic records. [ABSTRACT FROM AUTHOR]

This study examined the capacity of the giant kelp, Macrocystis pyrifera, to exhibit surge uptake as a mechanism to enhance nitrogen assimilation during seasons when nitrate is depleted. Surge uptake of nitrate, ammonium, and urea was assessed in the spring and summer with whole-blade incubation experiments using 15 N tracers. The incubation experiments showed evidence of surge uptake lasting up to 5 min for ammonium and 1 min for nitrate during the summer only. Giant kelp showed little capacity for surge uptake of urea regardless of season. The ecological importance of the patterns of surge uptake observed for ammonium and nitrate, however, is questionable, given the small scale and ephemeral nature of ammonium pulses most likely experienced by giant kelp, and the longer duration pulses of nitrate associated with internal waves and upwelling. Rather it seems more likely that uptake of ammonium and urea at ambient concentrations, combined with normal uptake of nitrate during longer duration pulses of high concentrations, sustains giant kelp growth during seasons when ambient concentrations of nitrate are low. [ABSTRACT FROM AUTHOR]

Transitions in phytoplankton community composition are typically attributed to ecological succession even in physically dynamic upwelling systems like the California Current Ecosystem (CCE). An expected succession from a high‐chlorophyll (~ 10 μg L−1) diatom‐dominated assemblage to a low‐chlorophyll (< 1.0 μg L−1) non‐diatom dominated assemblage was observed during a 2013 summer upwelling event in the CCE. Using an interdisciplinary field‐based space‐for‐time approach leveraging both biogeochemical rate measurements and metatranscriptomics, we suggest that this successional pattern was driven primarily by physical processes. An annually recurring mesoscale eddy‐like feature transported significant quantities of high‐phytoplankton‐biomass coastal water offshore. Chlorophyll was diluted during transport, but diatom contributions to phytoplankton biomass and activity (49–62% observed) did not decline to the extent predicted by dilution (18–24% predicted). Under the space‐for‐time assumption, these trends infer diatom biomass and activity and were stimulated during transport. This is hypothesized to result from decreased contact rates with mortality agents (e.g., viruses) and release from nutrient limitation (confirmed by rate data nearshore), as predicted by the Disturbance‐Recovery hypothesis of phytoplankton bloom formation. Thus, the end point taxonomic composition and activity of the phytoplankton assemblage being transported by the eddy‐like feature were driven by physical processes (mixing) affecting physiological (release from nutrient limitation, increased growth) and ecological (reduced mortality) factors that favored the persistence of the nearshore diatoms during transit. The observed connection between high‐diatom‐biomass coastal waters and non‐diatom‐dominated offshore waters supports the proposed mechanisms for this recurring eddy‐like feature moving seed populations of coastal phytoplankton offshore and thereby sustaining their activity. [ABSTRACT FROM AUTHOR]

Summary: A mix of adaptive strategies enable diatoms to sustain rapid growth in dynamic ocean regions, making diatoms one of the most productive primary producers in the world. We illustrate one such strategy off coastal California that facilitates continued, high, cell division rates despite silicic acid stress. Using a fluorescent dye to measure single‐cell diatom silica production rates, silicification (silica per unit area) and growth rates we show diatoms decrease silicification and maintain growth rate when silicon concentration limits silica production rates. While this physiological response to silicon stress was similar across taxa, in situ silicic acid concentration limited silica production rates by varying degrees for taxa within the same community. Despite this variability among taxa, silicon stress did not alter the contribution of specific taxa to total community silica production or to community composition. Maintenance of division rate at the expense of frustule thickness decreases cell density which could affect regional biogeochemical cycles. The reduction in frustule silicification also creates an ecological tradeoff: thinner frustules increase susceptibility to predation but reducing Si quotas maximizes cell abundance for a given pulse of silicic acid, thereby favouring a larger eventual population size which facilitates diatom persistence in habitats with pulsed resource supplies. [ABSTRACT FROM AUTHOR]

Abstract: Nitrate concentrations routinely fall below levels required to sustain growth of giant kelp (Macrocystis pyrifera) during summer and autumn in the Santa Barbara Channel, yet growth continues. We found urea to be consistently present at concentrations of 0.48–1.82 μM, accounting for greater than 20% of the dissolved fixed nitrogen pool during summer (14% overall). Field experiments indicate direct uptake of urea by giant kelp at a rate of 0.19 μmol N g dw−1 h−1, comparable to rates for ammonium (0.18 μmol N g dw−1 h−1) but lower than for nitrate (0.39 μmol N g dw−1 h−1). Co‐occurring phytoplankton took up nitrate, urea, and ammonium, 2‐, 15‐, and 39‐fold faster than giant kelp; however, the nitrogen uptake advantage of phytoplankton varies by substrate and season. Together, our results suggest that urea is readily used by giant kelp and may help to sustain growth throughout the year. [ABSTRACT FROM AUTHOR]

Abstract: The metabolic activity and growth of phytoplankton taxa drives their ecological function and contribution to biogeochemical processes. We present the first quantitative, taxon‐resolved silica production rates, growth rates, and silica content estimates for co‐occurring diatoms along two cross‐shelf transects off the California coast using the fluorescent tracer 2‐(4‐pyridyl)‐5‐((4‐(2‐dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole, and confocal microscopy. Taxon contribution to total diatom community silica production was predominantly a function of the surface area (SA) of new frustule that each taxon created as opposed to cell abundance or frustule thickness. The influential role of SA made large diatoms disproportionately important to community silica production over short time scales (< 1 d). In some cases, large taxa that comprised only ∼ 15% of numerical cell abundance accounted for over 50% of total community silica production. Over longer time scales relevant to bloom dynamics, the importance of SA declines and growth rate becomes the dominant influence on contribution to production. The relative importance of SA and growth rate in relation to silica production was modeled as the time needed for a smaller, faster‐growing taxon to create more SA than a larger, slower‐growing taxon. Differences in growth rate between the taxa effected the model outcome more than differences in SA. Shifts in relative silica production among taxa are time restricted by finite resources that limit the duration of a bloom. These patterns offer clues as to how taxa respond to their environment and the consequences for both species succession and the potential diatom contribution to elemental cycling. [ABSTRACT FROM AUTHOR]

Six clones of the marine cyanobacterium Synechococcus, representing four major clades, were all found to contain significant amounts of silicon in culture. Growth rate was unaffected by silicic acid, Si( OH)4, concentration between 1 and 120 μM suggesting that Synechococcus lacks an obligate need for silicon (Si). Strains contained two major pools of Si: an aqueous soluble and an aqueous insoluble pool. Soluble pool sizes correspond to estimated intracellular dissolved Si concentrations of 2-24 mM, which would be thermodynamically unstable implying the binding of intracellular soluble Si to organic ligands. The Si content of all clones was inversely related to growth rate and increased with higher [Si( OH)4] in the growth medium. Accumulation rates showed a unique bilinear response to increasing [Si( OH)4] from 1 to 500 μM with the rate of Si acquisition increasing abruptly between 80 and 100 μM Si( OH)4. Although these linear responses imply some form of diffusion-mediated transport, Si uptake rates at low Si (~1 μM Si) were inhibited by orthophosphate, suggesting a role of phosphate transporters in Si acquisition. Theoretical calculations imply that observed Si acquisition rates are too rapid to be supported by lipid-solubility diffusion of Si through the plasmalemma; however, facilitated diffusion involving membrane protein channels may suffice. The data are used to construct a working model of the mechanisms governing the Si content and rate of Si acquisition in Synechococcus. [ABSTRACT FROM AUTHOR]

The first inter-calibration study of the stable silicon isotope composition of dissolved silicic acid in seawater, δ30Si(OH)4, is presented as a contribution to the international GEOTRACES program. Eleven laboratories from seven countries analyzed two seawater samples from the North Pacific subtropical gyre (Station ALOHA) collected at 300 m and at 1000 m water depth. Sampling depths were chosen to obtain samples with a relatively low (9 μmol L−1, 300 m) and a relatively high (113 μmol L−1, 1000 m) silicic acid concentration as sample preparation differs for low- and high-concentration samples. Data for the 1000 m water sample were not normally distributed so the median is used to represent the central tendency for the two samples. Median δ30Si(OH)4 values of +1.66‰ for the low-concentration sample and +1.25‰ for the high-concentration sample were obtained. Agreement among laboratories is overall considered very good; however, small but statistically significant differences among the mean isotope values obtained by different laboratories were detected, likely reflecting inter-laboratory differences in chemical preparation including pre-concentration and purification methods together with different volumes of seawater analyzed, and the use of different mass spectrometers including the Neptune MC-ICP-MS (Thermo Fisher™, Germany), the Nu Plasma MC-ICP-MS (Nu Instruments™, Wrexham, UK), and the Finnigan™ (now Thermo Fisher™, Germany) MAT 252 IRMS. Future studies analyzing δ30Si(OH)4 in seawater should also analyze and report values for these same two reference waters in order to facilitate comparison of data generated among and within laboratories over time. [ABSTRACT FROM AUTHOR]

The distribution of silicon isotopes within silicic acid, δ 30 Si(OH) 4 , was examined along a section in the North Atlantic from the Cape Verde Islands off Africa to Cape Cod, Massachusetts in North America. Surface water displayed elevated δ 30 Si(OH) 4 associated with biological fractionation of Si during silica production. Below 300 m variations in δ 30 Si(OH) 4 were closely tied to the distribution of water masses as diagnosed through optimum multiparameter analysis, confirming a tight relationship between δ 30 Si(OH) 4 and the meridional overturning circulation in the Atlantic. A linear relationship between δ 30 Si(OH) 4 and the inverse of silicic acid concentration supported control of Si isotope distribution by conservative mixing of end member water masses of different isotopic composition in the Atlantic. There was a suggestion of a weak local minimum in δ 30 Si(OH) 4 in deep waters above the Trans-Atlantic Geotraverse hydrothermal zone on the mid-Atlantic Ridge consistent with the light δ 30 Si(OH) 4 of hydrothermal waters. The lightest δ 30 Si(OH) 4 values were observed in the deep western and deep eastern basins where Antarctic Bottom Water (AABW) dominated. The heaviest values in subsurface waters occurred in North Atlantic Deep Water due to strong ventilation and the contribution of heavy northern source waters that are influenced by the Arctic Ocean. The concept of a silicon isotope bipole is introduced to explain how the isotopic differences between the northern and southern end-member water masses arise, and how they influence Si isotope distributions. Northern end-member water masses are heavy due to the influence of the Arctic Ocean. Bottom topography prevents light deep waters from entering the Arctic and the further removal of light isotopes through local biological productivity results in extremely heavy δ 30 Si(OH) 4 within the Arctic. Light AABW dominates the southern end member. The Southern Ocean silicic acid trap distills heavier isotopes of Si out of the Southern Ocean as preformed silicic acid within Antarctic mode waters, while retaining light isotopes that become incorporated into AABW. The influence of the silicon isotope bipole is predicted to be strongest in the Atlantic as the net flow of waters through the Arctic Ocean is from the Pacific to the Atlantic. [ABSTRACT FROM AUTHOR]

Abstract: The growth of siliceous phytoplankton, mainly diatoms, in the Southern Ocean influences the preformed nutrient inventory in the ocean on a global scale. Silicic acid use by diatoms and deep circulation combine to trap dissolved Si in the Southern Ocean resulting in high levels of silica production and expansive diatom oozes in Southern Ocean sediments. The analysis of the silicon isotope composition of biogenic silica, or opal, and dissolved silicic acid provide insight into the operation of the global marine silicon cycle and the role played by the Southern Ocean in nutrient supply and carbon drawdown, both in the modern and in the past. Silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters, while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column; together these – and other – proxies reveal variations in the northward flow of Southern Ocean intermediate and mode waters and how changes in Southern Ocean productivity altered their preformed nutrient content. We present a new hypothesis – the “Silicic Acid Ventilation Hypothesis” (SAVH) – to explain the geographical variation of opal-based proxy records, in particular the contrasting patterns of opal burial change found in the low and high latitudes. By understanding the silicon isotope systematics of opal and silicic acid in the modern, we will be able to use opal-based proxies to reconstruct past changes in the Southern Ocean and so investigate its role in global carbon cycling and climate. [Copyright &y& Elsevier]

Forests of the giant kelp Macrocystis pyrifera found on coastal rocky reefs lack the large reservoirs for nutrient storage found in many terrestrial environments. Supporting their high year-round growth rates requires a continuous supply of nitrogen. Complementary timing of nutrient supply associated with the physical processes that deliver nitrate to reefs largely achieves this goal, but modeling studies indicate that the magnitude of nitrate delivery is inadequate to support the measured nitrogen demand of kelp forests during summer. Ammonium, from sediment efflux and excretion by reef consumers, likely fills the deficit. Together, the varied sources of inorganic nitrogen supplied to kelp forests support their high growth rates throughout the year. Kelp compensates for diminished nitrogen supply during summer by decreasing tissue nitrogen content, resulting in a doubling of kelp C:N ratios. [ABSTRACT FROM AUTHOR]

Abstract: Biogenic silica (bSiO2) cycling, diatom abundance and floristics were examined within summer-period diatom blooms in the North Pacific Subtropical Gyre (NPSG) in 2008 and 2009. Hemiaulus hauckii was the most abundant diatom observed in an expansive (100,000km2) bloom near the subtropical front in the northeastern NPSG in 2008 and the small pennate diatom Mastogloia woodiana dominated a smaller (30,000km2) bloom sampled in 2009 in the gyre interior. In both blooms, the bSiO2 stock and production rates were up to an order of magnitude higher relative to non-bloom areas. Remnants of a bSiO2 export event was sampled in the H. hauckii bloom area where the export rate at 300m exceeded that at 150m, and was among the highest values recorded in the NPSG. The M. woodiana bloom was very active with specific bSiO2 production rates of 0.50–0.75d−1 and net bSiO2 production rates were among the highest observed in any subtropical-gyre diatom bloom to date. Net silica production rates in the euphotic zone were strongly positive within blooms and near zero outside of blooms, consistent with an important role for blooms in bSiO2 export. The difference in the areal extent of the H. hauckii and M. woodiana blooms was consistent with remote-sensing observations that blooms in the northeastern portion of the NPSG, near the subtropical front, are typically more extensive than those in the gyre interior near Hawaii Ocean Time-series station ALOHA. Initial estimates suggest that blooms in the northeast region produced 3–25 times more bSiO2 in 2008 and 2009, respectively, than did blooms in the gyre interior; and due to the large areal extent these blooms, their area-integrated production of bSiO2 is similar to intense diatom blooms coastal upwelling systems (e.g. Monterey Bay, Santa Barbara Channel) despite significantly lower production rates and standing stock. [Copyright &y& Elsevier]

Lowman, H.E.; Hirsch, M.E.; Brzezinski, M.A., and Melack, J.M., 2023. Examining the potential of sandy marine sediments surrounding giant kelp forests to provide recycled nutrients for growth. Journal of Coastal Research, 39(3), 442–454. Charlotte (North Carolina), ISSN 0749-0208. Permeable marine sediments are biogeochemically active and may contribute dissolved nutrients to support primary production in coastal regions. This study examined the potential of permeable marine sediments near giant kelp forests in the Santa Barbara Channel, California as a source of ammonium (NH4+) to the overlying water column to support the observed growth of kelp during summer months when nitrate availability is low. Several nearshore sites located in coastal California in <20 m water depth were sampled for porewater nutrient concentrations, flushing rates, and nutrient fluxes in addition to diel fluctuations in nutrient concentrations of the overlying water column. Time-series analyses of porewater temperatures indicate that porewater flushed to a depth of 15 cm approximately every two hours, and mean NH4+ concentrations of porewater at these depths was 40 µM. The results of flow-through bioreactor incubations indicate that the top 2 cm of sediment are a net source of dissolved nitrogen to the overlying water column and are capable of supplying from 0.05 to 0.90 mmol NH4+ m–2 day–1. Diel water sampling demonstrates that kelp forests may be exposed to NH4+ concentrations greater than 1 µM for multiple hours (four–eight) over a day. These measured reservoirs and exchange rates of NH4+ suggest sandy marine sediments provide a significant source of nitrogen to the water column and may help meet the nitrogen demand by giant kelp during summer in the Santa Barbara Channel. [ABSTRACT FROM AUTHOR]

Abstract: Phytoplankton are a major food resource for filter-feeding organisms occupying intertidal and subtidal habitats of the inner continental shelf. Phytoplankton in these nearshore waters experience different anthropogenic and natural forcing compared to those offshore such that cross-shelf exchange would allow phytoplankton that were produced offshore to serve as a subsidy to inner shelf consumers when phytoplankton production on the shelf is negatively impacted. In the Santa Barbara Channel (SBC) the continental shelf is only a few kilometers wide facilitating exchange with offshore waters. Physical, chemical and biological gradients were examined monthly along a 3-km cross-shelf transect in the SBC from January, 2008 through April, 2009. Chemical and biological distributions followed temporal changes in physical forcing with higher nutrient concentrations and a more intense period of biological production associated with spring upwelling. Chlorophyll was relatively evenly distributed across the shelf during upwelling, but was present at higher concentrations on the inner shelf under stratified conditions. Similarly, cross-shelf gradients in the distribution of dominant phytoplankton genera were weakest during upwelling when blooms of the prymnesiophyte, Phaeocystis, and the diatoms, Eucampia spp. and Thalassiosira spp. occurred across most of the shelf. Upon stratification, blooms were largely confined to the inner shelf within 0.75km of the shoreline with an initial bloom of the diatom Leptocylindrus spp. followed by Pseudo-nitzschia spp. and series of dinoflagellate blooms with Prorocentrum spp. and Lingulodinium spp. attaining the highest abundances. Phytoplankton taxonomic similarity decreased with increasing distance separating stations along the transect and was inversely related to stratification intensity. The observed distribution patterns and the trends in taxonomic similarity imply that for most of the year consumers within rocky intertidal and subtidal ecosystems on the inner shelf are exposed to phytoplankton produced on the continental shelf, and often on the inner shelf. The contribution of phytoplankton produced beyond the shelf break occurs mainly during spring upwelling. [Copyright &y& Elsevier]

We examined Si limitation of silica production as a factor contributing to summer-bloom dynamics in the North Pacific subtropical gyre (NPSG). Substrate limitation of Si uptake was pervasive, but in most samples the degree of Si limitation was not severe enough to induce growth limitation as observed in the North Atlantic subtropical gyre. These results revise previous interpretations, because the difference in the degree of kinetic limitation between gyres appears to be driven by differences in both substrate and kinetic efficiency. In the NPSG, diatom bloom assemblages tended to show more efficient Si-uptake kinetics and higher Si-uptake rates than did nonbloom assemblages. A bloom dominated by Mastogloia woodiana exhibited the most efficient Si-uptake kinetics observed in the ocean to date. Enhanced kinetic efficiency for Si uptake was previously hypothesized to facilitate bloom development, but we suggest the efficient kinetics indirectly affects blooms because [Si(OH)4] does not appear to limit growth or division rates. Rather, efficient kinetics allows further depletion of the prevailing low [Si(OH)4] without inducing secondary growth limitation by Si, thereby maximizing biomass yield. Si-uptake limitation was not detected in the lower euphotic zone despite similar [Si(OH)4] as in well-lit waters, which suggests transition to light limitation of Si uptake with depth. These results suggest that enhanced silica production in the M. woodiana bloom was driven by a synergy between diatom species composition, where M. woodiana was highly efficient at Si uptake, and high irradiance, which lead to enhanced diatom division rates in the upper euphotic zone. [ABSTRACT FROM AUTHOR]

Abstract: Silicon is an important element for certain plankton groups in aquatic systems. In marine systems there is a strong coupling between the biogeochemical cycles of carbon and nitrogen with that of silicon due to the high proportion of primary productivity carried out by diatoms. Currently 32Si is the most used radiotracer for the measurement of biogenic silica production rates in aquatic systems with isotope activity typically quantified by Cherenkov or liquid scintillation counting. Here we describe a new method for quantifying 32Si activity using a gas-flow proportional counter optimized for low-level beta radiation analysis. The background activity is two-orders of magnitude lower and more stable than that of liquid scintillation counters, leading to consumption of 67–75% less isotope per sample. Sample preparation is non-destructive enabling reanalysis of samples indefinitely. Methods for counting samples under conditions of secular equilibrium and non-equilibrium between 32Si and its daughter isotope 32P are described. Analysis of equilibrated samples yields the highest sensitivity and accuracy, but requires that sample analysis be delayed for 120days for secular equilibrium. Non-equilibrium methods are less accurate, but provide results within 2days enabling experimental designs and protocols to be evaluated and optimized in near real time. [Copyright &y& Elsevier]

Abstract: Silica cycling in the upper 175m of the North Pacific Subtropical Gyre was examined over a two year period (January 2008–December 2009) at the Hawaii Ocean Time-series (HOT) station ALOHA. Silicic acid concentrations in surface waters ranged from 0.6 to 1.6μM, exhibiting no clear seasonal trends. Biogenic silica concentrations and silica production rates increased by an order of magnitude each summer following stratification of the upper 50m reaching values of 157nmolSiL−1 and 81nmolSiL−1 d−1, in 2008 and 2009, respectively. Sea surface height anomalies together with analyses of variability in isothermal surfaces at 150–175m indicated that the summer periods of elevated biogenic silica were associated with anticyclonic mesoscale features during both years. Lithogenic silica concentrations increased in the spring during the known period of maximum atmospheric dust concentrations with maximum values of 36nmolSiL−1 in the upper 10m. Dust deposition would enhance levels of dissolved iron in surface waters, but there was no response of diatom biomass or silica production to increases in near-surface ocean lithogenic silica concentrations suggesting iron sufficiency of diatom silica production rates. Low ambient silicic acid concentrations restricted silica production rates to an average of 43% of maximum potential rates. Si sufficiency only occurred during the summer period when diatom biomass was elevated suggesting that bloom diatoms are adapted to exploit low silicic acid concentrations. Annual silica production at HOT is estimated to be 63mmolSim−2 a−1 with summer blooms contributing 29% of the annual total. Diatoms are estimated to account for 3–7% of total phytoplankton primary productivity, but 9–20% of organic carbon export confirming past suggestions that diatoms are relatively minor contributors to primary productivity and autotrophic biomass, but important contributors to new and export production in oligotrophic open-ocean ecosystems. Annual silica production at HOT is nearly 4-fold lower than estimates at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea from the 1990s, but annual silica export at the base of the euphotic zone is similar between the two gyres indicating very different balances between silica production and its loss in surface waters. On a relative basis, BATS is a more productive system with respect to silica, where biogenic silica is recycled with high efficiency in surface waters; in contrast the NPSG is a lower productivity system with respect to silica, but where lower recycling efficiency leads to a much higher fraction of new silica production. The two gyres show contrasting long-term trends in diatom biomass as biogenic silica concentrations at HOT have been increasing since 1997, but they have been decreasing at BATS suggesting very different forcing of decadal trends in the contribution of diatoms in carbon cycling between these gyres. Combining the data from both gyres indicates that globally subtropical gyres produce 13TmolSia−1, which is only 51% of previous estimates reducing the contribution of subtropical gyres to 5–7% of global annual marine silica production. [Copyright &y& Elsevier]

Abstract: The distribution of silicon isotopes along a meridional transect at 140°W longitude in the Eastern Equatorial Pacific was used to test the hypothesis that δ30Si of silicic acid in surface waters should correlate with net silica production rates (gross silica production minus silica dissolution) rather than rates of gross silica production due to the opposing Si isotope fractionations associated with silica production and silica dissolution. Variations in δ30Si appeared significantly correlated with net silica production rates in equatorial surface waters and not with gross production rates. Around the Equator, values of δ30Si as low as deep water values occurred in the upper mesopelagic in a zone of net silica dissolution and high detrital biogenic silica content, where the release of low δ30Si silicic acid from opal dissolution would be expected to decrease δ30Si. The δ30Si of the deep water at 140°W appears constant for depths >2000m and is similar to the deep water at 110°W. This study brings to light the importance of considering Si fractionation during diatom silica dissolution, the biological fractionation during silica production and physical factors such as currents and mixing with adjacent water masses when interpreting silicon isotope distributions. [Copyright &y& Elsevier]

Abstract: The high-nitrate, low-silicic acid character of the eastern equatorial Pacific (EEP) has been attributed to the preferential export of diatom biogenic silica (bSiO2) over particulate organic nitrogen due to less efficient recycling of Si in surface waters. To gain insight into the strength of this silica pump, we examined [Si(OH)4] and [bSiO2] distributions and net bSiO2 production rates in two regions of the EEP, one spanning the equator from 4°N to 3.25°S at 140°W and the other along a tropical instability wave (TIW) at 0.5°N between 132.5 and 123.4°W, ending within a cold vortex at 1.75°N by 125°W. Large uncertainty in the net bSiO2 production rate measurement precluded a detailed examination of trends at high spatial resolution, but averaged data revealed clear differences in Si cycling between these two sampling areas. Surface [Si(OH)4] generally remained at levels <4μM across both, but [bSiO2] nearly doubled to values as high as 226nmolSiL−1 along the TIW. The mean integrated net rate of bSiO2 production along the meridional transect was no more than 0.29mmolSim−2 d−1 to the 0.1% light level depth and −0.31mmolSim−2 d−1 to a depth of 300m, implying net loss of bSiO2 to dissolution in the upper 300m in this area. In contrast, integrated net bSiO2 production rates were five times higher on average in the zonal sampling area, exhibiting a mean of 1.45mmolSim−2 d−1 within the euphotic zone that declined by only ∼15% to a depth of 300m, suggesting a significant potential for silica export to deeper waters along the TIW. In total, the fraction of bSiO2 produced in the euphotic zone that was supported by new inputs of Si(OH)4 was at least 3.7 times greater on average than the fraction of inorganic nitrogen taken up as , consistent with expectations for a silica pump. However, the mean integrated rate of uptake exceeded that of new Si(OH)4 uptake by at least five times, implying preferential nitrate depletion in contradiction to nutrient distributions that indicated preferential Si(OH)4 drawdown. This discrepancy can be resolved if, on average, ∼70% of the nitrate taken up was regenerated in the euphotic zone rather than being supplied by ‘new’ upwelled sources. These results suggest that the silica pump in the EEP arises only in part from the less efficient recycling of biogenic silica compared to organic matter, but also requires efficient recycling of nitrate in surface waters to reduce the rate of new nitrogen production well below the rate of gross nitrate uptake. Additional work will be necessary to confirm this apparent dependency and its relationship to variations in the physical and biological processes driving new production in this system. A relationship between bacterial protease activity and rates of silica recycling was also identified, but, unlike in coastal systems, the contribution of particle-associated bacteria was relatively unimportant in effecting bSiO2 dissolution in the EEP. [Copyright &y& Elsevier]

Abstract: The relative roles of silicon (Si) and iron (Fe) as limiting nutrients in the eastern equatorial Pacific (EEP) were examined in a series of nine microcosm experiments conducted over two years between 110°W and 140°W longitude. Si and Fe additions had consistently different but synergistic effects on macronutrient use, phytoplankton biomass and phytoplankton community structure. Silicon addition increased silicic acid use and biogenic silica production, but had no significant effect on the use of inorganic nitrogen or orthophosphate, chlorophyll accumulation, particulate inorganic (PIC) carbon accumulation, or plankton community composition relative to controls. That result, together with observations that Si addition increased the cellular Si content of the numerically dominant diatom by ∼50%, indicates that the main effect of Si was to regulate diatom silicification. Like the effect of Si, Fe addition increased the rate of silicic acid use and biogenic silica production and had no effect on PIC production. Unlike the effect of Si, Fe addition also enhanced rates of organic matter production, had no effect on cellular Si content of diatoms, and resulted in the growth of initially rare, large (>40μm) diatoms relative to controls, indicating that Fe limitation acts mainly through its effects on growth rate and phytoplankton community composition. A pennate diatom of the genus Pseudo-nitzschia dominated the diatom assemblage in situ, grew readily in the controls and did not show a strong growth response to either Fe or Si addition suggesting that its growth was regulated by other factors such as grazing or light. Addition of germanium, an inhibitor of diatom cell division, eliminated the effects of Fe on macronutrient use, biogenic silica production and chlorophyll accumulation and phytoplankton community composition, consistent with a predominantly diatom response to Fe addition. The lack of a response of PIC production to Fe suggests that coccolithophores were not Fe limited. Addition of Fe and Si together resulted in the greatest levels of nutrient drawdown and biomass accumulation through the effect of Fe in promoting the growth of large diatoms. The results suggest a form of co-limitation with Si regulating diatom silicification and the rate of biogenic silica production while Fe regulates the production of organic matter through limitation of phytoplankton growth rates, in particular those of large diatoms. The results argue against Si regulation of new production in the EEP under average upwelling conditions. Iron addition was necessary and sufficient to stimulate complete removal of nitrate within the equatorial upwelling zone suggesting that new production was restricted by low ambient dissolved Fe consistent with results from in situ Fe fertilization experiments conducted to the south of the equator outside of the equatorial upwelling zone. [Copyright &y& Elsevier]

Abstract: Silicon isotope ratios (δ30Si) of bulk mineral materials in soil integrate effects from both silicon sources and processing. Here we report δ30Si values from a climate gradient of Hawaiian soils developed on 170ka basalt and relate them to patterns of soil chemistry and mineralogy. The results demonstrate informative relationships between the mass fraction of soil Si depletion and δ30Si. In upper (<1m deep) soil horizons along the climate gradient, Si depletion correlates with decreases of residual δ30Si values in low rainfall soils and increases in high rainfall soils. Strong positive correlation between soil δ30Si and dust-derived quartz and mica content show that both trends are largely controlled by the abundance of these weathering-resistant minerals. The data also lend support to the idea that fractionation of Si isotopes in secondary phases is controlled by partitioning of silicon between dissolved and precipitated products during the initial weathering of primary basalt. Secondary mineral δ30Si values from lower (>1m deep) soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved Si in basalt-watershed rivers and driven by preferential 28Si removal from the dissolved phase during precipitation. In contrast, after correcting for the influence of dust, secondary mineral Si depletion and δ30Si values in shallow (<1m deep) soil horizons showed evidence of biocycling induced Si redistribution and substantially lower δ30Si values than predicted. Low δ30Si values in shallow soil horizons compared to predictions can be attributed to repeated fractionation as secondary minerals undergo additional cycles of dissolution and precipitation. Primary mineral weathering, secondary mineral weathering, dust accumulation, and biocycling are major processes in terrestrial Si cycling and these results demonstrate that each can be traced by δ30Si values interpreted in conjunction with mineralogy and measures of Si depletion. [Copyright &y& Elsevier]

Abstract: Silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization, primarily by diatoms, in surface waters of the ocean. Interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean, as driven by biological fractionation during the production of biogenic silica (bSiO2, or opal) by diatoms. However, other factors which may influence the silicon isotope composition of diatom opal, particularly post-formational aging and maturation processes, remain largely uninvestigated. Here, we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bSiO2 suspended in seawater under closed conditions. This fractionation acts counter to that occurring during bSiO2 production and at about half its absolute magnitude, with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor εDSi–BSi of −0.55‰, corresponding to a fractionation factor α30/28 of 0.99945. The enrichment factor did not vary with source material, indicating the lack of a significant species effect, or with temperature from 3 to 20°C. Thus, the dissolution of bSiO2 produces dissolved silicon with a δ30Si value that is 0.55‰ more negative than its parent bSiO2, an effect that must be accounted for when interpreting oceanic δ30Si distributions. The δ30Si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed, implying a measurable (±0.1‰) change in the relative δ30Si of opal samples whenever the difference in preservation efficiency between them is >20%. This effect could account for ∼10–30% of the difference in diatom δ30Si values observed between glacial and interglacial conditions. It is unlikely, however, that the inferred maximum possible change in δb30SiO2 of +0.55‰ would be manifested in situ, as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage. [Copyright &y& Elsevier]

Abstract: Silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4°N and 3°S latitude at 110°W longitude in the eastern Equatorial Pacific. Silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of ε estimated by applying a steady-state isotope fractionation model to data from all stations between 4°N and 3°S was −0.77±0.12‰ (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of ε was −1.08±0.27‰ (std. err.) similar to the value established in culture studies (−1.1‰). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of ε was significantly more positive, −0.61±0.16‰ (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of −1.1‰ for the fractionation factor for isotopes of Si in the sea. Without the assumption of steady state the value of ε can only be constrained to be between −0.6 and −1.1‰. Silicic acid in Equatorial Pacific Deep Water below 2000m had a near constant δ30Si of +1.32±0.05‰. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9–1.0‰) and it is slightly less positive than new measures of the δ30Si of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH)4 >180 μM, δ30Si=+1.46±0.12‰ (SD, n =4). We show that the data from the equator and Cascadia basin fit a general trend of increasing δ30Si(OH)4 with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water δ30Si(OH)4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea. [Copyright &y& Elsevier]

A procedure for the automated determination of isotopic abundances of silicon from biogenic and lithogenic particulate matter and from dissolved silicon in fresh or saltwaters is reported. Samples are purified using proven procedures through the reaction of Si with acidified ammonium molybdate, followed by precipitation with triethylamine and combustion of the precipitate to yield silicon dioxide. The silicon dioxide is converted to cesium hexafluosilicate by dissolution in hydrogen fluoride and the addition of cesium chloride. Isotopic analysis is accomplished by decomposing the cesium hexafluosilicate with concentrated sulfuric acid to generate silicon tetrafluoride gas. Silicon tetrafluoride is purified cryogenically and analyzed on a gas source isotope ratio mass spectrometer. Yields of silicon tetrafluoride are >99.5%. The procedure can be automated by modifying commercial inlet systems designed for carbonate analysis. The procedure is free of memory effects and isotopic biases. Reproducibility is ±0.03–0.10‰ for a variety of natural and synthetic materials. [ABSTRACT FROM AUTHOR]

We examined the role of Si limitation in mediating the response of siliceous biomass and silica production to mesoscale Fe fertilization in the high-silicic acid high-nitrate waters of the Antarctic and in the low-silicic acid high-nitrate waters of the Subantarctic during austral summer. Iron fertilization stimulated biogenic silica production and silicic acid depletion in both regions. Si limitation significantly curtailed the response of silica production to Fe in the Subantarctic, but not in the Antarctic. Additions of silicic acid to Fe-enriched waters of the Subantarctic more than doubled specific silica production rates beyond the increase caused by Fe alone. This result, combined with the presence of Si limitation both inside and outside of the fertilized patch in the Subantarctic, indicates that silica production in the Subantarctic is regulated by both Fe and Si during austral summer. Fe increased the ability of Subantarctic diatom assemblages to take up low concentrations of Si over an order of magnitude by increasing maximum uptake rates and lowering half-saturation constants for silicic acid uptake. Our observations indicate that the dramatic gradient in Si availability across the Antarctic Circumpolar Current exerts a strong control on the contribution of diatoms to new production following Fe enrichment in the Southern Ocean during austral summer. With sufficient Fe to allow nitrate depletion, the abundant silicic acid in the Antarctic would allow diatoms to dominate nitrate use; however, low silicic acid concentrations in the Subantarctic would restrict the fraction of available nitrate that would be consumed by diatoms to ca. 5%,. [ABSTRACT FROM AUTHOR]

Previous laboratory findings indicated that marine bacteria accelerate biogenic silica (bSiO[sub2]) dissolution rates in the sea by degrading the organic coating surrounding diatom frustules and exposing the underlying silica to chemical attack by undersaturated seawater. We examined the effectiveness of bacterial activity in facilitating in situ bSiO[sub2] dissolution during a diatom bloom in Monterey Bay, California, following moderate upwelling. Inhibition of bacterial activity with antibiotics and protease inhibitors reduced specific bSiO[sub2] dissolution rates (V[sub dis]) at five of six stations, with a reduction of 44 ± 27% (mean ± SD, n = 6, range 22-91%) over 24 h. Reduced V[sub dis], in inhibitor treatments corresponded with reductions in abundance, production, and proteolytic activity of attached bacteria. Dissolution rates were highly correlated with protease activity integrated from the surface down to the depth where each dissolution was measured, suggesting that increased V[sub dis] with depth in the upper 20-80 m of the ocean is caused by the progressive removal of organic matter from frustules during sinking. Facilitation of bSiO[sub2] dissolution by in situ bacterial assemblages varied between stations and was likely influenced by the physiological condition of resident diatom assemblages. Denaturing gradient gel electrophoresis and 16S rRNA gene sequencing of bacteria colonizing in situ diatom assemblages confirmed previous findings that specific bacterial phylotypes (Cytophaga/ Flavobacteria/Bacteriodes; &alpha and γ subclasses of Proteobacteria) mediate bSiO[sub2] dissolution. [ABSTRACT FROM AUTHOR]

Silicon isotope tracers were used to examine the relative magnitude of silica dissolution and silica production in the Monterey Bay, California, upwelling system. A diatom bloom dominated by Skeletonema costatum and Chaetoceros spp. was encountered under conditions of moderate upwelling. Profiles of silica production and dissolution rates were obtained at seven stations that sampled both inside and outside the bloom. Integrated silica production rates ranged from 5.4 to 108 mmol Si m[sup-2] d[sup-1], averaging 42.8 mmol Si m[sup-2] d[sup-1]. Integrated silica dissolution rates were considerably lower than production rates with values between 0.63 and 6.5 mmol Si m[sup-2] d[sup-1] (mean = 2.90 mmol Si m[sup-2] d[sup-1]). The mean ratio of integrated silica dissolution to integrated silica production (∫ D: ∫ P) between the surface and the 0.1% light depth was 0.075, omitting one station with an unusually high ∫ D: ∫ P of 0.61, indicating that, on average, 93% of silica production was supported by new silicic acid. The f-ratio for diatom nitrogen use estimated from silicic acid and nitrate depletion curves and the mean ∫ D: ∫ P ratio was found to be 0.83, indicating that silica was being regenerated at a rate that was only slightly slower than that for particulate organic nitrogen. These data provide direct evidence confirming earlier hypotheses that the silica pump is weak in Monterey Bay. Analysis of the global data set on ∫ D: ∫ P in the surface ocean leads to the hypothesis that low ∫ D: ∫ P (∼ 0.10 or less) are typical of diatom bloom events, with ∫ D: ∫ P rising to values in excess of... [ABSTRACT FROM AUTHOR]

The potential for carbon export and the role of siliceous plankton in the cycling of C and N was assessed in natural plankton assemblages in the Santa Barbara Basin, California, by examining uptake rates of inorganic carbon, nitrate and silicic acid. In April–August 1997, the concentrations of chlorophyll a, particulate organic carbon, particulate organic nitrogen and biogenic silica were measured twice monthly, and results revealed the occurrence of at least three blooms, the largest in June. Particulate elemental ratios of C, N and Si were similar to ratios of nutrient-replete diatoms, suggesting that they dominated this bloom. Mean integrated rates of carbon, nitrate and silicon uptake during the 4-month study period are similar to other productive coastal and upwelling regions (103, 8.3 and 13 mmol m−2 day−1, respectively). New production rates were twice as high as previously reported in this region and indicate that high rates of new production along eastern boundary currents are not confined to the major coastal upwelling regions. C/NO3−, Si/NO3− and Si/C uptake ratios varied widely, and mean integrated ratios were 14±5.4, 1.6±1.0 and 0.12±0.07 (S.D.), respectively. That mean C/NO3− uptake ratio corresponds to an f-ratio of about 0.5 indicating a large potential for particulate export. Based on the average Si/NO3− and Si/C uptake ratios, diatoms could perform all of the primary production and nitrate uptake that occurred during the study; these rates also suggest that export is controlled by diatoms in this system. The mean Si/C biomass ratio was lower than the mean Si/C uptake ratio, consistent with the preferential export of Si relative to C observed in sediment traps in the basin. The study took place during a period of surface-water warming, with nitrate and silicic acid concentrations decreasing throughout the onset of the 1997–1998 El Nin˜o conditions. Although diatoms contributed less to particulate biomass during the low nutrient conditions, high f-ratios (0.33–0.66) were maintained. [Copyright &y& Elsevier]

Nutrient uptake ratios and the Si:C:N composition ratios of particulate matter were measured within a diatom bloom located between the Antarctic Polar Front and the edge of the receding pack ice in the Pacific sector of the Southern Ocean along 170°W from 2 December 1997 to 3 January 1998. Rates of silicic acid, nitrate, ammonium and urea uptake were measured along with simultaneous estimates of net and gross carbon uptake derived from net oxygen production and 18O gross production rates. Concurrent measures of silica dissolution rates allowed net silicic acid uptake to be estimated. Comparison of integrated rates of gross silicic acid uptake, gross carbon uptake, and total nitrogen (nitrate+ammonium+urea) uptake revealed that gross uptake ratios within the bloom closely matched Redfield proportions. The mole ratio of gross Si uptake to total N uptake averaged 0.96±0.08, indistinguishable from the mean value of 0.90 characteristic of nutrient-replete diatoms in culture. The mean gross Si uptake: gross C mole uptake ratio, 0.20±0.02, was 50% higher than the typical diatom Si:C ratio of 0.13. Gross C:total N uptake mole ratios averaged 5.2±0.4, reasonably close to the Redfield value of 6.6. In contrast, average net uptake ratios were significantly higher than Redfield (C:NO3−=9.1±1.5, Si:C=0.52±0.12, Si:NO3−=3.7±0.4) and exceeded their corresponding gross uptake ratios by 180–380%. On average, net uptake was somewhat enriched in C, and greatly enriched in Si, over gross uptake; ratios of net uptake exceeded corresponding Redfield (C:N) or typical diatom (Si:C, Si:N) ratios by 38% for C:N, by a factor of 4.0 for Si:C and by a factor of 4.1 for Si:N. Thus, there was a transition from the plankton taking up nutrients in nearly Redfield proportions, to particles becoming slightly enriched in C over N and highly enriched in silica relative to organic matter through the combined effects of metabolic losses and food web processes. These results point to the operation of an efficient silicate pump that enriches particles in silica relative to organic matter prior to their export. We hypothesize that low [Fe] accentuated the decoupling between the cycling of silica and organic matter in the bloom. The average [Fe] in the mixed layer (0.13–0.29 nM) falls in a range that has been shown experimentally to dramatically increase Si(OH)4:NO3− uptake ratios by plankton assemblages in our study area. That result, together with the observation that gross uptake ratios of Si(OH)4 to total N were nearly in Redfield proportions, suggests that the main effect of low [Fe] was to increase reliance on regenerated nitrogen sources by inhibiting nitrate uptake. More efficient recycling of particulate nitrogen in surface waters compared to biogenic silica led to the selective depletion of silicic acid over nitrate, creating a positive feedback between [Fe] and the silicate pump, where low [Fe] accelerates the onset of Si limitation and diminishes the export of organic matter relative to silica. [Copyright &y& Elsevier]

Examines silica production and dissolution rates for better understanding of processes controlling silicic acid depletion and opal export. Occurrence of high diatom mass and high silica production rates in the marginal seas around Antarctica; Seasonal change in silica dissolution as observed from silica production rate ratios; Comparison of surface silica production rates with estimates of opal accumulation rates in the abyssal sediments.

Examines silicon limitation in the Pacific sector of the Southern Ocean from December 1997 through March 1998 using silicon radioisotope. Seasonal progression of silicic acid in surface waters; Details of the kinetics of silicic acid uptake; Seasonal silicic acid depletion and development of silicon limitation.

Deals with a study which investigated the relative roles of iron and silicic acid availabilities in regulating silicic uptake rates across the Polar Frontal Zone in the Pacific Sector of the Southern Ocean during the United States Joint Global Ocean Flux Study Southern Ocean program. Methods; Results; Discussion.

Studies the chemical form of dissolved silicon taken up by marine diatoms. Kinetics of silicon use; Use of silicon tracer methodology to determine uptake rates; Variation in half-saturation constant.

Reports on the development of a [sup 32]Si autoradiographic technique using a liquid photographic emulsion for the study of diatom silica deposition in culture or in natural water samples. Use of the method in the Central North Pacific to study silica deposition by diatoms of the genus Rhizosolenia; Composition of the Rhizosolenia mats examined.

The relatively non-toxic dye, rhodamine 123 (R123), was incorporated into the frustule of Thalassiosira weissflogii Grun. clone ACTIN in direct proportion to biogenic silica (BSi). R123 was used together with the DNA stain propidium iodide to track and quantify Si deposition during the cell cycle of T. weissflogii using flow cytometry, Silicon deposition was not continuous through the cell cycle. Deposition of the valves occurred during M phase. The hypocingulum was largely deposited during G 1 with some suggestion of minor or girdle band deposition during G2. Silicon deposition did not occur during S phase. Assuming that a complete pustule consists of an epivalve, epicingulum, hypocingulum, and hypavalve, then 40% of cellular BSi was contained within the cingulum of T. weissflogii with 60% present in the valves. These percentages correspond to 0.38 pmol Si in the two cingula and 0.57 pmol Si in the valves. Temporal differences in the timing of silicic acid uptake and deposition during the cell cycle of T. weissflogii suggested that deposition of both the new valves and the cingulum is supported by an internal pool of dissolved Si acquired during G2. [ABSTRACT FROM AUTHOR]

Neutral lipids, consisting primarily of triacylglycerols, were found to be a major form of carbon reserve in zoospores of the giant kelp Macrocystis pyrifera (L.) C. Ag. The fluoreseent stain Nile Red revealed large lipid droplets in the posterior end of the cell, which comprised 20-41% of cellular carbon in newly released spores. Flow cytometric analysts of newly released spores stained with ,Vile Red revealed considerable variation in the neutral lipid content among spores that was independent of spore ha,. Lipid droplets were consumed during germination in spores maintained either under constant light or in continual darkness. The availability of light appeared to delay, but did not preclude, lipid use. The rate of lipid use during germination varied considerably among germlings with some cells consuming all of their lipid reserves within 5 h after release. In addition to zoospores, lipid droplets were observed in both male and female gametes. Numerous droplets were observed in eggs, while single lipid droplets were observed in sperm. Neutral lipid droplets were not observed in gametophytes or sporophytes except in developing gametes and spores. Large lipid reserves thus seem to be confined to the microscopic life history stages that presumably have relatively high energy demands. By serving as a supplemental fuel reserve, neutral lipids may be important in extending the effective range of zoospore dispersal. [ABSTRACT FROM AUTHOR]

Presents a method for the purification, recovery and determination of isotopic abundances of silicon from biogenic and lithogenic particulate matter and dissolved silicic acid. Reagent cleaning and quantitative recovery of silicon; Fluorination yields and isotopic results; Isotopic analysis of seawater Si(OH)4.

The variability of marine diatom Si:C and Si:N composition ratio was examined to assess their utility as ecological under and 18:6 h LD cycle and sampled at the end of the light period gave mean ratios, by atoms, of 0.13 ± 0.04 and 1.12 ± 0.33 for Si:C and Si:N ratio, respectively (95% C.I. reported). The mean ratios, respectively under continuos illumination were 0.12 ± 0.03 for Si:C and 0.95 ± 0.23 for Si:N. the mean ratios of the clones grown under constant light were not statistically different from under constant light were not statistically different from those calculated for the same species grown under and 18.6 h LD photoperiod. The overall mean Si:C under an 18:6 h LD and continuous light experiments taken together, weighted by the number of species in each experiment, are 0.13 and 1.05, respectively. The average ratios for the nine nanoplandton species (<20 μm) examined were 0.09 ± 0.03 for Si:C and 0.80 ± 0.35 for Si:N. The eighteen netplankton species (>20 μm)had higher mean ratios, Si:C = 0.15 ± 0.04 and Si:N = 1.20 ± 0.37. Time course sampling throughout a 24 h period revealed twofold variations in both ratios for individual species grown on a 14:10 h LD cycle. Changes in irradiance can also produce factor of two variations, both ratios being higher under low light. Comparison of these data with those from the literature regarding the effects of temperature and nutrient limitation on diatom elemental composition suggest that use of these ratios to convert field estimates of biogenic silica production from [SUP14C] data, should yield results accurate to within a factor of three under most circumstances. [ABSTRACT FROM AUTHOR]

The California Current System displays a strong seasonal cycle in water properties, circulation, and biological production. Interactions of the alongshore current with coastal and topographic features lead to high spatial variability forced by seasonal winds that displace surface coastal water offshore. This process also supplies nutrients to the euphotic zone by Ekman transport and eventually supports phytoplankton blooms typically dominated by diatoms. Here, we investigate the relationship between biogenic silica production and mesoscale upwelling dynamics along the central region of the California Current System between 2013 and 2015, a period affected by a warm anomaly known as "the Blob." Changes in the upwelling phenology along California caused by this marine heatwave are investigated using an innovative index and related to patterns of diatom production during upwelling events to evaluate diatom resilience. Based on this new index, we estimated that the nutrient supply to the euphotic zone declined by 50% during the Blob, but the Blob had little impact on local production during individual upwelling events. A statistical analysis evaluating the relationship between production and environmental conditions reveals persistent biological hotspots characterized by high biomass, depleted nutrients, and high specific production rates (up to 0.7 d−1) throughout the study period. Lower observed biogenic silica to Chlorophyll a ratios during the Blob suggested a taxonomic shift from siliceous to nonsiliceous phytoplankton and/or lightly silicified diatoms signaling a change at the base of the food chain that could have ramifications for productivity in this eastern boundary coastal upwelling system. [ABSTRACT FROM AUTHOR]

Urea is an available and readily used source of nitrogen for giant kelp, Macrocystis pyrifera, but little is known about its potential importance for sustaining growth. Results of kinetic experiments indicate urea uptake saturates at an average maximum rate (Vmax) of 0.73–0.92 μmol N g dw−1 h−1 with a half saturation constant (Ks) of 1.02–1.08 μM. The affinity of giant kelp for urea was high relative to that reported for other seaweeds. However, results of similar kinetics experiments with natural, co‐occurring phytoplankton communities indicate that the rate of urea uptake by phytoplankton was > 10‐fold higher than that of giant kelp. Urea uptake by giant kelp decreased 3–12% in darkness (relative to in light) compared to a 66–85% decline for phytoplankton. Similar differences were observed for ammonium and nitrate, suggesting that light intensity and photocycles influence the outcome of competition for N between giant kelp and phytoplankton. Monthly measures of urease in kelp tissues revealed persistent activity at levels that were 100‐fold higher than rates of urea uptake (0.13–0.35 μmol N g fw−1 min−1). This finding, coupled with unsuccessful efforts to induce additional urease activity through substrate additions, suggests that urease plays a role in giant kelp physiology beyond that of processing urea taken up from the environment. Collectively, our results suggest giant kelp uses multiple forms of N including urea to sustain year‐round growth. Its consistent capacity to acquire N during both day and night may help offset its low uptake rates relative to phytoplankton and increase its ability to compete for N during periods of low N availability. [ABSTRACT FROM AUTHOR]

The article focuses on the significance of diatoms to manipulate silicon to produce biochemical underpinnings of biosilicification using nanotechnology. The role of the diatom in the ecology and biogeochemistry includes photosynthesis and global carbon cycling. It is stated that the diatoms are mostly dominated biogeochemical cycling of silicon through the sea.

Abstract The widely distributed marine picocyanobacterium Synechococcus is found throughout the upper oceans. Recent observations that Synechococcus accumulates silicon indicate that it may influence the global cycle of this element. While diatoms, the organisms that dominate the marine Si cycle, have absolute Si requirements due to their precipitation of exoskeleton frustules composed of hydrated opal-A (i.e., biogenic silica), no requirements or biological roles are known for the Si associated with Synechococcus. Even less is known about the biochemical form(s) of Si in picocyanobacteria or whether it differs from diatomaceous silica. Using bulk X-ray absorption near-edge spectroscopy (XANES) across the Si K-edge, we investigated the chemical speciation of Si in dried Synechococcus laboratory isolates from several clades, representing both coastal and oligotrophic isolates grown at a range of Si(OH) 4 concentrations. Silicon associated with Synechococcus is bound to oxygen and is spectroscopically distinct from opal-A precipitated by diatoms. The closest XANES spectral analogues found in the literature are dried Mg-Si gels precipitated under basic conditions, suggesting that some Synechococcus Si may be present in vivo as a hydrated siliceous network with Mg and/or Ca cations. Slight spectral variations across isolates and growth conditions suggest strain-specificity in Si chemistry and the potential for siliceous phases to bind organic matter. Highlights • Si-speciation in three cultured Synechococcus isolates is investigated using Si K-edge XANES. • Synechococcus Si is present as an oxyhydroxide or hydrous oxide, spectroscopically distinct from diatom opal. • No exact spectral analogues found; closest matches are cation-rich, hydrated Si-gel networks. [ABSTRACT FROM AUTHOR]

The article discusses the impact of ocean alkalinity enhancement (OAE) on coccolithophores and diatoms, two important phytoplankton groups. The study found that moderate and high alkalinity additions had a neutral effect on their growth and elemental ratios. However, abiotic mineral precipitation was observed, removing nutrients and alkalinity from the solution. Further research is needed to understand the effects of OAE on marine ecosystems.

Factors that affect the removal of organic carbon by heterotrophic bacterioplankton can impact the rate and magnitude of organic carbon loss in the ocean through the conversion of a portion of consumed organic carbon to CO2. Through enhanced rates of consumption, surface bacterioplankton communities can also reduce the amount of dissolved organic carbon (DOC) available for export from the surface ocean. The present study investigated the direct effects of elevated pCO2 on bacterioplankton removal of several forms of DOC ranging from glucose to complex phytoplankton exudate and lysate, and naturally occurring DOC. Elevated pCO2 (1000–1500 ppm) enhanced both the rate and magnitude of organic carbon removal by bacterioplankton communities compared to low (pre-industrial and ambient) pCO2 (250 –~400 ppm). The increased removal was largely due to enhanced respiration, rather than enhanced production of bacterioplankton biomass. The results suggest that elevated pCO2 can increase DOC consumption and decrease bacterioplankton growth efficiency, ultimately decreasing the amount of DOC available for vertical export and increasing the production of CO2 in the surface ocean. [ABSTRACT FROM AUTHOR]

Two novel and independent couplings of field data sets suggest that only ~10-20% of the biogenic silica (bSi) in the surface waters of the eastern equatorial Pacific upwelling zone is associated with living diatoms. Accounting for the ~80-90% contribution of detrital bSi reconciles discrepancies between estimates of diatom growth rates obtained by different methods. Approximately 50% of the bSi associated with living diatoms was found to be in cells whose greatest linear dimension was > 40 µm. The contribution of that > 40-µm fraction to bSi production was less than its contribution to biomass, as smaller diatoms, between 10 and 40 µm, contributed 59-100% of bSi production. A steady-state model, parameterized using field data, appears to explain the origin of the large detrital bSi pool, relative to the living fraction. Because specific rates of diatom growth are several times higher than those of detrital bSi dissolution, bSi must be predominantly detrital to balance gross rates of silica production and dissolution. Zooplankton grazing is the dominant process creating the detrital bSi pool, and the balance between diatom growth and zooplankton grazing rates maintains a living bSi pool that is a small fraction of the total bSi. Our results show near equivalence between the gross rates of detrital silica production by mesozooplankton grazing and bSi export from the euphotic zone, and also between the rates of detrital silica production by microzooplankton grazing and bSi dissolution, suggesting distinct roles for the two zooplankton size classes in regional Si biogeochemistry. [ABSTRACT FROM AUTHOR]

Coccolithophores are an important group of calcifying marine phytoplankton. Although coccolithophores are not silicified, some species exhibit a requirement for Si in the calcification process. These species also possess a novel protein (SITL) that resembles the SIT family of Si transporters found in diatoms. However, the nature of Si transport in coccolithophores is not yet known, making it difficult to determine the wider role of Si in coccolithophore biology. Here, we show that coccolithophore SITLs act as Na+‐coupled Si transporters when expressed in heterologous systems and exhibit similar characteristics to diatom SITs. We find that CbSITL from Coccolithus braarudii is transcriptionally regulated by Si availability and is expressed in environmental coccolithophore populations. However, the Si requirement of C. braarudii and other coccolithophores is very low, with transport rates of exogenous Si below the level of detection in sensitive assays of Si transport. As coccoliths contain only low levels of Si, we propose that Si acts to support the calcification process, rather than forming a structural component of the coccolith itself. Si is therefore acting as a micronutrient in coccolithophores and natural populations are only likely to experience Si limitation in circumstances where dissolved silicon (DSi) is depleted to extreme levels. [ABSTRACT FROM AUTHOR]

Ocean acidification is a threat to many marine organisms, especially those that use calcium carbonate to form their shells and skeletons. The ability to accurately measure the carbonate system is the first step in characterizing the drivers behind this threat. Due to logistical realities, regular carbonate system sampling is not possible in many nearshore ocean habitats, particularly in remote, difficult-to-access locations. The ability to autonomously measure the carbonate system in situ relieves many of the logistical challenges; however, it is not always possible to measure the two required carbonate parameters autonomously. Observed relationships between sea surface salinity and total alkalinity can frequently provide a second carbonate parameter thus allowing for the calculation of the entire carbonate system. Here, we assessed the rigor of estimating total alkalinity from salinity at a depth <15 m by routinely sampling water from a pier in southern California for several carbonate system parameters. Carbonate system parameters based on measured values were compared with those based on estimated TA values. Total alkalinity was not predictable from salinity or from a combination of salinity and temperature at this site. However, dissolved inorganic carbon and the calcium carbonate saturation state of these nearshore surface waters could both be estimated within on average 5% of measured values using measured pH and salinity-derived or regionally averaged total alkalinity. Thus we find that the autonomous measurement of pH and salinity can be used to monitor trends in coastal changes in DIC and saturation state and be a useful method for high-frequency, long-term monitoring of ocean acidification. [ABSTRACT FROM AUTHOR]

Abstract: Fractionation of silicon (Si) isotopes was measured in seven species (nine strains) of polar and sub-polar marine diatoms grown in semi-continuous unialgal cultures under optimal irradiance and temperature for each diatom strain. Results from this work provide the first evidence that Si isotope fractionation by diatoms is species-dependent. The greatest difference in the Si isotope fractionation factor (ε) was observed between two Southern Ocean diatoms, Fragilariopsis kerguelensis (−0.54‰, average for two strains) and Chaetoceros brevis (−2.09‰). The ε for the other species, both polar and sub-polar, ranged from −0.72‰ to −1.21‰. The two remaining polar diatoms had ε values of −0.74±0.05‰ for Thalassiosira antarctica, and −1.21±0.04‰ for Thalassiosira nordenskioeldii, while the sub-polar species had ε values of −0.72±0.04‰ for Thalassiosira weissflogii, −0.88±0.06‰ for Thalassiosira pseudonana (CCCM58), −0.97±0.14‰ for Thalassiosira pseudonana (CCMP1014), and −1.15±0.03‰ for Porosira glacialis. The range in ε for the diatoms evaluated in this study may be large enough to significantly impact the Si isotope composition measured in diatom opal (δ30Si-bSiO2) from marine sediments and its subsequent interpretation. To test the influence of diatom taxonomic composition on δ30Si-bSiO2, we developed a model that considered the relative abundance of diatom species and the ε values (from this study) for each species present within the sediment core (i.e. weighted-average ε). The model was applied to records from a Southern Ocean sediment core (TN057-13) where both diatom abundance and δ30Si-bSiO2 data were available. The analysis indicated that 67% of the variation in δ30Si-bSiO2 could be explained by species-dependent Si isotope fractionation. We suggest that future work should assess phytoplankton taxonomic composition when using δ30Si-bSiO2 as a proxy for Si utilization. [Copyright &y& Elsevier]

The summertime North Pacific subtropical gyre has widespread phytoplankton blooms between Hawaii and the subtropical front (∼30°N) that appear as chlorophyll (chl) increases in satellite ocean color data. Nitrogen-fixing diatom symbioses (diatom-diazotroph associations: DDAs) often increase 10²-10³ fold in these blooms and contribute to elevated export flux. In 2008 and 2009, two cruises targeted satellite chlorophyll blooms to examine DDA species abundance, chlorophyll concentration, biogenic silica concentration, and hydrography. Generalized observations that DDA blooms occur when the mixed layer depth is < 70 m are supported, but there is no consistent relationship between mixed layer depth, bloom intensity, or composition; regional blooms between 22-34°N occur within a broader temperature range (21-26°C) than previously reported. In both years, the Hemiaulus-Richelia and Rhizosolenia-Richelia DDAs increased 10²-10³ over background concentrations within satellite-defined bloom features. The two years share a common trend of Hemiaulus dominance of the DDAs and substantial increases in the >10 μm chl a fraction (∼40-90+% of total chl a). Integrated diatom abundance varied 10-fold over <10 km. Biogenic silica concentration tracked diatom abundance, was dominated by the >10 μm size fraction, and increased up to 5-fold in the blooms. The two years differed in the magnitude of the surface chl a increase (2009>2008), the abundance of pennate diatoms within the bloom (2009>2008), and the substantially greater mixed layer depth in 2009. Only the 2009 bloom had sufficient chl a in the >10 μm fraction to produce the observed ocean color chl increase. Blooms had high spatial variability; ocean color images likely average over numerous small events over time and space scales that exceed the individual event scale. Summertime DDA export flux noted at the Hawaii time-series Sta. ALOHA is probably a generalized feature of the eastern N. Pacific north to the subtropical front. [ABSTRACT FROM AUTHOR]