Search

The foundation of methylmercury (MeHg) bioaccumulation in aquatic systems is uptake and trophic transfer into phytoplankton and zooplankton. Phytoplankton and zooplankton samples were collected seasonally from Long Island Sound (LIS) and its adjacent shelf waters, located along the northeast coast of the United States. Phytoplankton samples were size fractioned into 0.2–5 μm, 5–20 μm, and seston of > 20 μm. Average phytoplankton MeHg concentrations were 0.01–1.5 pmol g−1 (wet weight), and MeHg bioconcentration factors (logBCF) ranged from 2.5 to 5.5. There was higher MeHg concentration and logBCF values with increasing phytoplankton size. Zooplankton samples were size fractioned into 0.2–0.5 mm, 0.5–1.0 mm, 1.0–2.0 mm, and > 2.0 mm classes and analyzed for Hg and MeHg as well as carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) isotopes. The %MeHg in organisms was highest in the > 2.0 mm size class for coastal LIS and shelf species, indicating higher MeHg bioaccumulation with increasing zooplankton sizes. Eastern LIS zooplankton populations yielded higher overall Hg accumulation than western LIS populations, in alignment with phytoplankton fractions. Seasonal differences were evident for plankton %MeHg, although only ELIS displayed an increase in summertime %MeHg throughout the fractions. The results suggest the importance of biodilution in determining plankton MeHg concentrations. The δ13C values infer that zooplankton were opportunistically feeding on food containing variable levels of Hg and MeHg. The δ15N isotopes indicated enrichment characteristic of anthropogenic input in populations at the western end of LIS. The δ34S isotopes signified that organisms were primarily feeding on pelagic plankton.

Phytoplankton may serve as a key entry for methylmercury (MeHg) into aquatic food webs however very few studies have quantified the bioconcentration of MeHg in marine phytoplankton from seawater, particularly for non-diatoms. Experiments using 203Hg to measure MeHg uptake rates and concentration factors in six marine phytoplankton species belonging to different algal classes were conducted and the influence of light, temperature, and nutrient conditions on MeHg bioaccumulation were determined. All algal species greatly concentrated MeHg out of seawater, with volume concentration factors (VCFs) ranging from 0.2 × 105 to 6.4 × 106. VCFs were directly related to cellular surface area-to-volume ratios. Most of the cellular MeHg was found in the cytoplasm. Temperature, light, and nutrient additions did not directly affect MeHg uptake in most species, with the exception that the dinoflagellate Prorocentrum minimum displayed significantly greater uptake per cell at 18°C than at 4°C, suggesting an active uptake for this species. Passive transport seemed to be the major pathway for most phytoplankton to acquire MeHg and was related to the surface area-to-volume ratio of algal cells. Environmental conditions that promoted cell growth resulted in more total MeHg associated with cells, but with lower concentrations per unit biomass due to biodilution. The very high bioconcentration of MeHg in marine phytoplankton is by far the largest bioconcentration step in marine food chains and variations in algal uptake may account for differences in the amount of MeHg that ultimately builds up in different marine ecosystems.

Eutrophication can have opposite effects on mercury (Hg) bioavailability in aquatic systems, by increasing methylmercury (MeHg) production but reducing Hg biomagnification. Globally, the effect of eutrophication on Hg dynamics remains largely untested at lower latitudes such as eastern China, a productive subtropical ecoregion with Hg emission and deposition rates that are among the highest worldwide. Here, we quantify Hg (both MeHg and total Hg, THg) concentrations, Hg bioaccumulation, and Hg biomagnification rates in reservoir food webs across a gradient of eutrophication indicated by chlorophyll a (Chl a), zooplankton density, and total phosphorus (TP). We also assess the effect of hydrogeomorphic (HGM) features on Hg concentrations in water and biota. Water THg concentrations were strongly correlated with TP and were greater in reservoirs at higher elevations with short water retention times (WRT). Zooplankton and top predator THg concentrations were negatively correlated with Chl a, suggesting algal biodilution; evidence for zooplankton density dilution was also found when subtropical reservoirs were compared at a global scale with temperate lakes. Mercury bioaccumulation and biomagnification factors, respectively, did not correlate with increasing Chl a or zooplankton density suggesting no effect of plankton density on Hg trophic transfer. In subtropical reservoirs, eutrophication is associated with lower Hg concentrations in biota but does not explain Hg biomagnification; HGM features (i.e., elevation, WRT) and land use (i.e., % crop) appear to also influence Hg concentrations and bioaccumulation in reservoir food webs.

We describe interactive effects of total phosphorus (total P 5 0.1–4.0 mmol L21; added as H2NaPO4), irradiance (40 and 150 mmol quanta m22 s21), and the partial pressure of carbon dioxide (PCO2 ; 19 and 81 Pa, i.e., 190 and 800 ppm) on growth and CO2- and dinitrogen (N2)-fixation rates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii (WH0003) isolated from the Pacific Ocean near Hawaii. In semicontinuous cultures of C. watsonii, elevated PCO2 positively affected growth and CO2- and N2-fixation rates under high light. Under low light, elevated PCO2 positively affected growth rates at all concentrations of P, but CO2- and N2-fixation rates were affected by elevated PCO2 only when P was low. In both high-light and low-light cultures, the total P requirements for growth and CO2 -a nd N 2-fixation declined as PCO2 increased. The minimum concentration (Cmin) of total P and half-saturation constant (KK) for growth and CO2- and N2-fixation rates with respect to total P were reduced by 0.05 mmol L21 as a function of elevated PCO2 . We speculate that low P requirements under high PCO2 resulted from a lower energy demand associated with carbon-concentrating mechanisms in comparison with lowPCO2 cultures. There was also a 0.10 mmol L21 increase in Cmin and KK for growth and N2 fixation with respect to total P as a function of increasing light regardless of PCO2 concentration. We speculate that cellular P concentrations are responsible for this shift through biodilution of cellular P and possibly cellular P uptake systems as a function of increasing light. Changing concentrations of P, CO2, and light have both positive and negative interactive effects on growth and CO2-, and N2-fixation rates of unicellular oxygenic diazotrophs like C. watsonii.

In laboratory experiments we examined the interplay of light and iron availability on the intracellular iron concentrations, specific growth rates, and photosynthetic physiology of Southern (S.) Ocean diatoms (Eucampia antarctica and Proboscia inermis) and the haptophyte Phaeocystis antarctica. Intracellular iron concentrations and iron : carbon (Fe : C) molar ratios increased with decreasing irradiance in temperate coastal (Thalassiosira weissflogii) and oceanic (Thalassiosira oceanica) diatoms, in support of the well-established antagonistic iron–light relationship. In contrast, S. Ocean species required lower cellular iron concentrations and Fe : C ratios than temperate diatoms to grow at comparable rates, and their iron requirements decreased or remained relatively constant with decreasing light. These results suggest that the current paradigm that low light increases algal cellular iron requirements (supplied through ‘‘biodilution’’) is not applicable to S. Ocean phytoplankton. Although iron use efficiencies decreased at sub-saturating light in all species, these reductions were due primarily to lower growth rates, but not higher intracellular Fe : C ratios, in S. Ocean species. We propose that S. Ocean species have overcome the antagonistic iron–light relationship by increasing the size, rather than the number, of photosynthetic units under low irradiances, resulting in an acclimation strategy that does not increase their cellular iron requirements.

We conducted culture experiments with Thalassiosira pseudonanato determine the effect of CO2, photoperiod, and light intensity on cellular zinc concentrations and zinc requirements for growth. Cellular zinc requirements were dependent on the supply of CO2 relative to its photosynthetic demand. Decreasing the CO2 concentration (via an increase in pH from 8.2 to 9.0) increased the cellular zinc required to achieve a given growth rate or that needed for maximum growth. This increase is apparently linked to a greater demand for the zinc enzyme carbonic anhydrase (CA), which is needed for cellular CO2 acquisition. A decrease in photoperiod had a similar effect. Based on the present and previous results, a decrease in photoperiod from 24 h d 21 (continuous light) t o7hd 21 was accompanied by an estimated 2.2-fold increase in the net specific rate of photosynthetic C fixation, which increased the cellular demand for CA. The higher cellular requirement for zinc under decreased CO2 or photoperiod was accentuated at high growth rates because of a disproportionate increase in the cellular demand for CA with increasing specific rate of C fixation. The increased demand for cellular zinc was largely met by a decrease in the daily specific growth rate, which increased cellular zinc concentrations by decreasing biodilution rates. In addition, there was an approximately twofold increase in cellular zinc uptake rates at high zinc concentrations (and high growth rates) for cells grown at low CO2 concentration. In contrast to the effects of decreased CO 2 or photoperiod, a tenfold decrease in light intensity reduced the cellular zinc requirement, apparently linked to a 2.8-fold decrease in the maximum specific growth rate, and resultant decreased demand for CA and other biosynthetic zinc enzymes. Other factors (e.g., iron limitation) that decrease specific growth rate should have a similar effect.

We conducted culture experiments with the diatom Thalassiosira pseudonanato determine the interactive effects of day length and biologically available concentrations of iron and zinc on cellular iron (Fe), zinc (Zn), chlorophyll a (Chl a), and fixed carbon (C) throughout the light period. Specific rates of C-fixation and growth were also measured. Specific C-fixation rates showed a single linear relation with the cellular Fe : C ratio regardless of the photoperiod. Decreasing the photoperiod from 14 to 7 h increased the mean daytime cellular Fe : C ratio by 40%, the specific C-fixation rate by 34%, and the Chla : C ratio by 91% in mildly iron-limited cultures. These changes reflect a cellular acclimation to the shortened light period. The higher cellular iron level apparently allowed for synthesis of additional iron-rich proteins (e.g., those utilized in photosynthetic electron transport) needed to support the increased rate of C-fixation. Mean cellular Chla concentration decreased linearly with decreasing specific growth rate under iron and zinc limitation, thereby allowing the cells to maintain a balance between light harvesting and biosynthesis. Cellular concentrations of carbon, Chl a, zinc, and iron typically varied during the light period because of the day‐night differences in rates of C-fixation, Chl a synthesis, growth, and metal uptake. Cell carbon concentrations increased by 36‐96% during the light period, reflecting daytime storage of fixed carbon to support nighttime respiration and growth. Cellular zinc concentrations decreased by 25% during the light period owing to higher daytime specific growth rates and resulting higher rates of biodilution. By contrast, the direction of change in cellular iron concentrations was dependent on the extent of photochemical redox cycling of iron chelates, which increased iron uptake rates during the day. The direction and magnitude of daytime changes in cellular zinc and iron were also dependent on the parameter (cell volume, cell numbers, or carbon) to which the cellular metal was normalized, as each of these parameters exhibited its own unique diurnal pattern.

Recent studies have emphasized the need for understanding the accumulation and fate of metal contaminants at different trophic levels and across a broad spectrum of lake types. To address both issues, metal concentrations (Hg, Zn, Cd, As, and Pb) were measured in the water, two size fractions of zooplankton, and fish from 20 lakes in contaminated to pristine watersheds in the northeastern United States. Our goals were to examine links between watershed characteristics and aqueous metal levels in lakes and relationships between aqueous concentrations, metal burdens in different plankton groups and in fish. Two pairs of metals, (1) Hg and Zn and (2) As and Pb, exhibited strong similarities both in the factors that predict their concentrations in water and in the patterns of accumulation in particular trophic levels. Aqueous concentrations of Hg and Zn were highest in cool water lakes, whereas As and Pb were highest in more eutrophic lakes in agricultural areas. Aqueous Cd concentrations were closely correlated with the land-use variables, percentage of agricultural land, and road densities. Similarly, Hg and Zn both biomagnified from small plankton (45‐202mm) to macrozooplankton (.202 mm) and from macrozooplankton to fish. In contrast, bioaccumulation of both As and Pb diminished with increasing trophic level. Although aqueous metal and zooplankton metal levels were not significant predictors of As and Pb levels in fish, metal levels in zooplankton were predictive of Hg and Zn in fish, suggesting that sources of bioaccumulation differ for different metals. Our findings demonstrate the importance of investigating upper and lower trophic levels separately, to fully understand metal transfer pathways in aquatic food webs.