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Trichodesmium, a major diazotroph in tropical and subtropical oceans, provides a significant amount of bioavailable nitrogen in oligotrophic oceans. Remarkably, the cyanobacteria carry out nitrogen fixation and photosynthesis simultaneously under high light (HL) conditions. Here, we hypothesize that nickel superoxide dismutase (NiSOD) protects the nitrogen fixation process from oxidative stress generated in photosynthesis under high solar radiation conditions. We found that the variations in nitrogen fixation rates closely corresponded to Ni availability under HL conditions. The expression of NiSOD and two representative photosynthetic proteins also corresponded to variations in both nitrogen fixation rates and Ni availability under HL conditions. Trichodesmium collected in the field had a high Ni quota and elevated SOD activity. The evidence supports the conclusion that NiSOD protects nitrogen fixation in Trichodesmium from oxidative stress. The ability to utilize NiSOD may be a major advantage for Trichodesmium under high solar radiation conditions. [ABSTRACT FROM AUTHOR]

We investigated the effects of Ni concentration on hydrogen (H2) net production and nitrogen (N2) fixation rates in a model unicellular diazotrophic cyanobacterium, Cyanothece. We quantified NiFe uptake hydrogenase of the diazotroph to examine how Ni deficiency affects the expression of hydrogenase and nitrogenase and N2 fixation rates. We found that Ni availability controls its H2 net production and the expression of HupL, the major subunit of the uptake hydrogenase. Low Ni availability induced low HupL expression levels while yielding high H2 net production, and vice versa. We also identified the threshold range of Ni concentrations that may differentiate low and high H2 net production patterns in the diazotroph. However, the substantial variations in Ni availability and H2 net production did not affect nitrogenase expression and N2 fixation rates in Cyanothece. From two independent studies in the Atlantic tropical and subtropical oceans, we found that dissolved Ni concentrations and H2 oversaturation percentage exhibited a statistically significant negative correlation, which is consistent with the findings of this culture study. Overall, these results indicate that Ni availability may affect H2 net production and concentrations in the euphotic zone of the oceanic regions. H2 concentrations may be used as a promising proxy to reflect N2 fixation activities in tropical and subtropical oceans only under Ni‐deficient conditions. [ABSTRACT FROM AUTHOR]

Author(s): Antonietta Quigg (corresponding author) [1]; Zoe V. Finkel [1]; Andrew J. Irwin [1]; Yair Rosenthal [1, 2]; Tung-Yuan Ho [5]; John R. Reinfelder [3]; Oscar Schofield [1, 4]; Francois [...]

The North Pacific Ocean is located at the end of the thermohaline circulation of deep water. This study reports on basin‐scale full‐depth sectional distributions of total dissolvable (td), dissolved (d), and labile particulate (lp) Cd, Ni, Zn, and Cu along three transects: the GEOTRACES transects GP18 (165°E) and GP02 (47°N), and along 160°W. We find that scavenging is an important factor that significantly affects the distributions of dZn, dNi, and dCu, of which the magnitude of influence increases in the order of Cd < Ni, Zn < Cu. The relationships between the four dissolved metals with Si(OH)4 and PO4 differed considerably from those in other oceans. The spot concentration ratio of dCd/PO4 was 0.34 ± 0.02 nmol/μmol (n = 296) in waters >800 m deep, which is in the range of the phytoplankton Cd/P ratio. This is indicative of the dominant effect of water circulation and biological processes on dCd distribution. The dissolved metals (dMs) to PO4 ratios of other examined metals were either partially or completely outside the range of typical biomass ratios. They generally increased with depth in waters >800 m deep; the magnitude of increase was the highest for Cu and moderate for Ni and Zn. Below 800 m, an increase in the apparent oxygen utilization from 150 to 300 μmol/kg was concurrent with a decrease in the dMs/PO4 ratios: 4 ± 3% for Cd, 21 ± 4% for Zn, 21 ± 3% for Ni, and 69 ± 7% for Cu. Key Points: Basin‐scale distributions of total dissolvable, dissolved, and labile particulate Cd, Ni, Zn, and Cu in the North Pacific are reportedCd is controlled by internal cycling and biological processes, and its stoichiometry with major nutrients is modified via ocean circulationThe distributions of Ni, Zn, and Cu are affected by scavenging and redissolution from sinking particles and sediments [ABSTRACT FROM AUTHOR]

Warming and nutrient limitation are stressors known to weaken the health of microalgae. In situations of stress, access to energy reserves can minimize physiological damage. Because of its widespread requirements in biochemical processes, iron is an important trace metal, especially for photosynthetic organisms. Lowered iron availability in oceans experiencing rising temperatures may contribute to the thermal sensitivity of reef‐building corals, which rely on mutualisms with dinoflagellates to survive. To test the influence of iron concentration on thermal sensitivity, the physiological responses of cultured symbiotic dinoflagellates (genus Breviolum; family Symbiodiniaceae) were evaluated when exposed to increasing temperatures (26 to 30°C) and iron concentrations ranging from replete (500 pM Fe') to limiting (50 pM Fe') under a diurnal light cycle with saturating radiance. Declines in photosynthetic efficiency at elevated temperatures indicated sensitivity to heat stress. Furthermore, five times the amount of iron was needed to reach exponential growth during heat stress (50 pM Fe′ at 26–28°C vs. 250 pM Fe′ at 30°C). In treatments where exponential growth was reached, Breviolum psygmophilum grew faster than B.minutum, possibly due to greater cellular contents of iron and other trace metals. The metal composition of B.psygmophilum shifted only at the highest temperature (30°C), whereas changes in B.minutum were observed at lower temperatures (28°C). The influence of iron availability in modulating each alga's response to thermal stress suggests the importance of trace metals to the health of coral‐algal mutualisms. Ultimately, a greater ability to acquire scarce metals may improve the tolerance of corals to physiological stressors and contribute to the differences in performance associated with hosting one symbiont species over another. [ABSTRACT FROM AUTHOR]

Ni is an essential cofactor in NiFe‐uptake hydrogenase, an enzyme regulating H2 metabolism in diazotrophic cyanobacteria, the major H2 producers in the surface ocean globally. Here, we investigated the effect of Ni supply on H2 production and N2 fixation by using a model marine cyanobacterial diazotroph, Cyanothece. By mediating total dissolved Ni concentrations from 100 to 0.03 nmol L−1 in a trace metal‐defined culture medium, we demonstrated that Ni deficiency results in H2 accumulation, coupled with decreasing Ni quotas, growth rates, and occasionally relatively low N2 fixation rates. These results indicate that Ni deficiency limits the growth of the Cyanothece to some extent, considerably decreases H2 uptake by hydrogenase and leads to H2 accumulation and N2 fixation variation in the diazotroph. The findings show that Ni availability is a critical factor on controlling H2 production and N2 fixation in marine diazotrophic cyanobacteria. The information of Ni bioavailability for diazotrophic cyanobacteria is thus essential to evaluate the importance of Ni for H2 cycling and N2 fixation in oceanic surface waters. [ABSTRACT FROM AUTHOR]

The effects of the diel (involving a 24 hour period) variations in the surface concentrations of chlorophyll a (C) on the use of once-daily remotely sensed C as the diel average were assessed from the diel records in the derived depth-weighted C (Cd) that should be detected by remote sensing and the in situ surface C at two time-series stations in the North Pacific: the SEATS (SouthEast Asian Time-series Study) station in the northern South China Sea and the ALOHA (A Long-Term Oligotrophic Habitat Assessment) station in the North Pacific subtropical gyre. In situ surface C varied by a factor of about 2.0 and 1.3 over a diel cycle, and by ±20% and ±9% over the diel average at the SEATS and ALOHA stations, respectively. As the overpass-times of the different satellites were not identical, Cd was satellite-dependent. While the Cd corresponding to MODerate resolution Imaging Spectroradiometer on Aqua (MODIS-Aqua) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) overpass-times agreed to ±10%, the Cd corresponding to MEdium Resolution Imaging Spectrometer (MERIS) overpass-time could differ from the other two by −22% to +28% at the SEATS station and −1% to +12% at the ALOHA station. In addition, Cd corresponding to the overpass-times of the three satellites deviated from the observed diel average in situ surface C by −19% to +32% at the SEATS station and by −6% to +13% at the ALOHA station. These results indicate that, as a result of diel variations, neither a one-time remotely-sensed nor a one-time observed in situ surface C can represent the diel average in situ surface C accurately. Furthermore, diel variations are an inherent source of uncertainty when data from multiple satellites are pooled for use. The magnitudes of these discrepancies can be comparable to the commonly claimed uncertainties in remotely sensed C and thus should be taken into consideration in its interpretation and use. [ABSTRACT FROM AUTHOR]

The Kuroshio, flowing through the eastern end of the East China Sea, transports terrestrial material to the Northwestern Pacific Ocean. Along the Kuroshio path, we collected size‐fractionated suspended particles in the upper 200 m to investigate the composition and sources of particulate trace metals. Demonstrated by Al‐ and P‐normalized elemental ratios, we found that anthropogenic aerosol deposition was the major source of most particulate trace metals in the Kuroshio region although the impact of bottom resuspension and riverine input may be significant at some specific regions. As seen in our previous studies in the South China Sea and the Western Philippine Sea, this study concluded that anthropogenic aerosol deposition is the dominant particulate trace metal source in the surface water of the Northwestern Pacific Ocean and its marginal seas. Compared to particulate trace metal composition obtained in other open oceans, we found that the distribution patterns of particulate trace metal composition in the surface waters are closely associated with aerosol deposition fluxes. In regions with low deposition, the particulate trace metal concentrations were extremely low and metal to P ratios were all comparable to their intracellular quota previously proposed. In regions with high deposition, metal to P ratios were highly elevated in comparison to their intracellular quota and metal to Al ratios were deviated from lithogenic ratios to various extents. The globally consistent distribution patterns validate trace metal stoichiometry concept in plankton intracellularly and the primary role of aerosol deposition on deciding particulate trace metal composition in oceanic surface water globally. Plain Language Summary: The Kuroshio adjacent to the East China Sea is an ideal region to investigate the material exchange and transport from the marginal sea to the Northwestern Pacific Ocean. Particulate trace metals may serve as useful proxies or tracers for the sources of particulate materials in the Kuroshio and material exchange with the marginal seas and islands. In addition to aerosol transport, both anthropogenic and lithogenic materials carried by riverine or sediment transport may be additional sources of particulate trace metals in the studied region. In this study, we found that anthropogenic aerosol deposition is the dominant particulate trace metal source in the surface water. Compared to the data obtained in other open oceans, we further revealed that the distribution patterns of particulate trace metal composition in the surface waters are closely associated with aerosol deposition globally. The globally consistent distribution patterns also validate that plankton possess relatively constrained trace metal composition intracellularly. Key Points: Anthropogenic aerosol deposition is the dominant source of most particulate trace metals collected in the surface water of the KuroshioThe distribution patterns of particulate metal composition in the global surface water are associated with aerosol deposition fluxesThe distribution patterns of particulate metal composition validate intracellular trace metal stoichiometry concept in plankton [ABSTRACT FROM AUTHOR]

Abstract: Desert dust storms are frequent in the Northern Red Sea region, providing nutrients (i.e., PO4) and trace‐metals (i.e., Fe) that may stimulate dinitrogen (N2) fixation. Dust also carries a high diversity of airborne microbes (bacteria and archaea), including diazotrophs, that may remain viable during transport and upon deposition. Here we evaluate the impact of atmospheric deposition and its associated airborne diazotrophs on N2 fixation in the surface water of the low‐nutrient Northern Red Sea, using mesocosm bioassay experiments. We compared the chemical (nutritional) and sole airborne microbial impact of aerosol additions on N2 fixation using “live‐dust” (release nutrients/trace metals and viable airborne microorganisms) and “UV‐killed dust” (release only chemicals). Airborne diazotrophy accounted for about one third of the measured N2 fixation (0.35 ± 0.06 nmol N · L−1 · day−1 and 0.29 ± 0.06 nmol N · L−1 · day−1, for “February 2017” and “May 2017,” “live‐dust” additions, respectively). Two nifH sequences related to cluster III diazotrophs were amplified from the dust samples, consistent with the N2 fixation measurement results. We postulate that the deposition of viable airborne diazotrophs may enhance N2 fixation, especially in marine provinces subjected to high aerosol loads. We speculate that the relative contribution of airborne N2 fixation may increase in the future with the predicted increase in dust deposition. [ABSTRACT FROM AUTHOR]

Light and trace metals are critical growth factors for algae but how the interdependence of light quality and metal availability affects algal growth remains largely unknown. Our previous studies have demonstrated the importance of Ni and Fe on the growth of Trichodesmium and Symbiodinium, respectively, two important marine primary producers inhabiting environments with high light intensities. Here, we investigated the effects of light quality and intensity with availability of either Ni or Fe on their growth. For Trichodesmium, we found that specific growth rates for high Ni treatments were all significantly higher than in corresponding low Ni treatments with varying light quality and intensity. The inhibitory effect of low intensity red light was also countered by sufficient Ni supply. For Symbiodinium, we found that growth rates and biomass were reduced by 75% under low intensity red light and the stress can only be partially relieved by sufficient Fe supply. The results show that trace metal availability plays an important role in relieving the stress induced by low red light condition for both Trichodesmium and Symbiodinium although the cyanobacterium performs better in this growth condition. The difference may be attributed to the presence of phycocyanin, a unique pigment attuned to absorption of red light, in Trichodesmium. Our study shows that the concerted effects of light intensity and quality compounded with trace metal availability may influence the growth of photosynthetic organisms in the ocean. [ABSTRACT FROM AUTHOR]

The seasonal and spatial variations of phytoplankton pigment distribution and their interaction with environmental controlling parameters were investigated in the Northern South China Sea (NSCS) at 37 stations covering the coastal region, the continental shelf, the slope, and the deep-water basin during two summer and two winter cruises from 2010 to 2012. Strong spatial, interseasonal, and intraseasonal variations of pigment distribution were observed in the diverse biogeochemical regions, exhibiting extremely dynamic phytoplankton community structure in the NSCS. In addition to chlorophyll a , the major pigments observed included divinyl chlorophyll a (DV Chl a ), total chlorophyll b (Chl b ), zeaxanthin (Zea), 19′-hexanoyloxyfucoxanthin (Hex), 19′-but-fucoxanthin (But), fucoxanthin (Fuco), and prasinoxanthin (Pras). Overall, Fuco, Chl b , Zea, Hex, But, and Pras were the dominant pigments in the coastal and shelf region; DV Chl a , Chl b , Hex, But, Zea, and Fuco were the major pigments in the offshore water. After analyzing marker pigment correlations and cellular pigment concentrations, we conclude that Zea, Hex (with But), and Fuco can be used as specific marker pigments for Synechococcus , coccolithophores, and diatoms in the NSCS, respectively. We have also derived that prasinophytes accounted for most of the elevated Chl b observed in the coastal water and appeared to be an important pico-eukaryotic group (Pico) in the coastal region. With varying cellular pigment concentrations observed vertically and seasonally, it is essential to separate sampling stations to different biogeochemical domains to apply pigment to Chl a ratios to estimate phytoplankton community structure in the NSCS. We observed close association between soluble reactive phosphate and Chl a abundance in coastal, shelf, and deep-water regions during summer cruises. Major nutrient supply appears to be the main controlling factor on the temporal and spatial variations of major pigment distribution. The supply is mainly driven by terrestrial input in summer and water mixing strength in winter. During the summer periods, major nutrients are primarily supplied from riverine and groundwater discharge; during winter periods, strong winter monsoon intensifies sub-surface water mixing and results in nutrient and pigment elevation in the euphotic zone. Also driven by the monsoon, the intrusion of the low-temperature coastal current from Taiwan Strait to the coastal region of the NSCS may replace or alter the phytoplankton community in the coastal region in winter. Overall, the temporal and spatial variations of phytoplankton pigment distribution in the NSCS are regulated by the fluctuations of these two major environmental forcings. [ABSTRACT FROM AUTHOR]

This study applied a regional air quality model, incorporated with an emission module, to quantitatively differentiate the atmospheric iron sources originating from lithogenic dusts or coal-burning fly ashes deposited in the Northwestern Pacific Ocean and its marginal seas. Particular attention was paid to the high iron content of fly ashes emitted from steel and iron plants burning coals. Using the year 2007 as an example, the modeling results exhibit large seasonal variations in iron deposition, with highest deposition fluxes occurred during spring and autumn, which are comparable to the seasonal fluctuation of chlorophyll a concentrations estimated by satellite images in the oceanic regions. Fly ash from coal burning accounted for 7.2% of the total iron deposited over the northwestern Pacific Ocean and 15% of that over the northern South China Sea. After considering the difference of iron solubility in the aerosols, anthropogenic aerosol associated with coal burning would be the major bioavailable iron source in the surface water of the oceanic regions. [ABSTRACT FROM AUTHOR]

We determined Zn isotope composition (δ66Zn) in the water column of the Northwestern Pacific Ocean (NWPO) and its marginal seas to investigate the processes driving vertical and spatial variations. Comparable to previous studies, dissolved δ66Zn was relatively low in the top 200 m, ranging from −0.91 to +0.24‰ and increased with depth toward an averaged value, +0.38±0.10‰, in deep water. We found that δ66Zn observed in the deep water of the NWPO was much lower than the Northeastern Pacific Ocean. Box model approaches suggest that spatial variations in the deep water may be attributed to isotopically light Zn originating from sedimentary input or anthropogenic aerosols. In the surface water, the fractionation factors derived from both closed or open system fractionation models are all less than one in the NWPO. However, both closed and open system models show a relatively poor fit to the measured data. We thus propose that external input is a major factor causing the variations of [Zn] and δ66Zn values in surface water, in addition to the effects of scavenging, physical mixing, and biological uptake. As elevated [Zn] and relatively light δ66Zn in the surface water were observed in high aerosol input regions, aerosol deposition may play a dominant role in regulating Zn elemental and isotopic composition in the surface ocean. Key Points: We observed spatial variations of δ66Zn in the deep water of the NWPO and found considerably lighter δ66Zn than the NEPOThe sedimentary input may be a major cause of relatively light dissolved δ66Zn in the deep water of the NWPOAerosol Zn input is a major cause of the variations of [Zn] and δ66Zn in the surface water of the NWPO [ABSTRACT FROM AUTHOR]

Abstract: We determined the Cd isotopic composition of seawater and sinking particles collected in the deep basin of the northern South China Sea (SCS) to investigate the controlling mechanisms on the Cd isotopic composition in the water column. The isotopic composition in the water column decreased with depth, with ε114/110Cd values (ε114/110Cd=[(114Cd/110Cd)sample/(114Cd/110Cd)JMC Cd Münster −1]×104) ranging from +8.7 to +9.9 in the top 80m, from +4.6 to +5.5 between 100 and 150m, decreasing from +5.5 to +3.6 at depths from 150 to 1000m, and remaining at +3.4±0.5 from 1000 to 3500m. The isotopic composition and concentrations of Cd observed in the deep waters of the SCS are similar to the values that were previously reported in the North Pacific Ocean. In the thermocline, the variations in the Cd isotopic composition and concentrations were consistent with the relative volumetric percentages of the subsurface water, the intermediate water, and the deep water in the water column, indicating that water mixing is the dominant process determining the isotopic composition in the thermocline. Comparable to the isotopic composition value in the seawater of the mixed layer, the ε114/110Cd in the sinking particles collected at 30m was +9.3±0.9. Because our previous studies demonstrated that the particulate Cd was predominantly biogenic organic matter, the comparable isotopic composition between the surface seawater and the sinking particles indicates that net biological isotopic fractionation on Cd in the surface water was insignificant. The result indicates that phytoplankton do not necessarily take up relatively light Cd in the oceanic surface waters. It is necessary to directly and systematically investigate how marine phytoplankton fractionate Cd isotopes. [Copyright &y& Elsevier]