The concentration of carbon dioxide in seawater may affect phytoplankton physiology and ecology and their role in marine biogeochemical cycles. In order to assess the effects of CO₂ on the elemental ...composition of marine phytoplankon, carbon, nitrogen, and phosphorus quotas were measured in 4 species of marine phytoplankton acclimated to 150 to 1500 ppm CO₂ (5 to 50 μM) in semi-continuous cultures. Nitrogen quotas declined steeply with increasing CO₂ in the centric diatomsThalassiosira pseudonanaandT. weissflogiiacclimated to 150 to 380 ppm (5 to 13 μM), but more slowly as the CO₂ increased from 380 to 1500 ppm (13 to 50 μM). Nitrogen demand varied little with CO₂ in the pennate diatomPhaeodactylum tricornutum, but was positively correlated with CO₂ over the range of 150 to 770 ppm in the prymnesiophyteIsochrysis galbana. Based on the nitrogen–CO₂ trends in centric diatoms, relief from carbon–nitrogen co-limitation could lead to 2-fold larger cells as CO₂ increases from 150 to 380 ppm, but only 15% larger cells from 380 to 770 ppm CO₂. Phosphorus quotas in the 3 diatoms decreased as CO₂ increased from 150 to 380 ppm. As previously observed in these and other species, C:N, C:P, and N:P ratios increased with increasing CO₂, but the present results show that much of this variation was due to differences in nitrogen and phosphorus rather than carbon quotas. Marine phytoplankton could provide a negative feedback against increasing CO₂ over the pCO₂ range of 150 to 380 ppm by supporting larger cells or higher biomass, but would support a smaller carbon sink as atmospheric CO₂ rises above 380 ppm.
Phytoplankton is a nineteenth century ecological construct for a biologically diverse group of pelagic photoautotrophs that share common metabolic functions but not evolutionary histories. In ...contrast to terrestrial plants, a major schism occurred in the evolution of the eukaryotic phytoplankton that gave rise to two major plastid superfamilies. The green superfamily appropriated chlorophyll b, whereas the red superfamily uses chlorophyll c as an accessory photosynthetic pigment. Fossil evidence suggests that the green superfamily dominated Palaeozoic oceans. However, after the end-Permian extinction, members of the red superfamily rose to ecological prominence. The processes responsible for this shift are obscure. Here we present an analysis of major nutrients and trace elements in 15 species of marine phytoplankton from the two superfamilies. Our results indicate that there are systematic phylogenetic differences in the two plastid types where macronutrient (carbon:nitrogen:phosphorus) stoichiometries primarily reflect ancestral pre-symbiotic host cell phenotypes, but trace element composition reflects differences in the acquired plastids. The compositional differences between the two plastid superfamilies suggest that changes in ocean redox state strongly influenced the evolution and selection of eukaryotic phytoplankton since the Proterozoic era.
We analyzed the elemental composition (C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Mo, and Ni) of five marine phytoplankton species representing the four major marine phyla grown over a range of ...growth irradiances. We found substantial variability in the elemental composition between different species, which is consistent with previously reported differences associated with evolutionary history. We found that many elements (Fe, Mn, Zn, Cu, Co, and Mo) are enriched relative to phosphorus by about two to three orders of magnitude under irradiances that are limiting for growth, and net steady-state uptake of element : P is often elevated under lower irradiances. For most elements, the variability in element : P due to irradiance is comparable to the variability due to phylogenetic differences at any irradiance, but often the interaction between genetic differences and the phenotypic response to irradiance amplifies the differences in elemental composition between species. The fractionation of trace elements relative to phosphorus into phytoplankton biomass under low light is consistent with depleted levels of Cu2+and Mn2+in deep chlorophyll maxima, suggesting that the export of lowlight-acclimated phytoplankton is a major source of trace element flux to the deep ocean and an important factor in the biogeochemical cycles of many of the biologically limiting elements in the oceans.
The primary mechanisms controlling the accumulation of methylmercury and inorganic mercury in aquatic food chains are not sufficiently understood. Differences in lipid solubility alone cannot account ...for the predominance of methylmercury in fish because inorganic mercury complexes (e.g., HgCl2), which are not bioaccumulated in fish, are as lipid soluble as their methylmercury analogs (e.g., CH3HgCl). Mercury concentrations in fish are ultimately determined by methylmercury accumulation at the base of the food chain, which is governed by water chemistry, primarily pH and chloride concentration. Our studies of mercury speciation, toxicity, and phytoplankton uptake demonstrate that passive uptake of uncharged, lipophilic chloride complexes is the principal accumulation route of both methylmercury and inorganic mercury in phytoplankton. The predominance of methylmercury in fish, however, is a consequence of the greater trophic transfer efficiency of methyl- mercury than inorganic mercury. In particular, methylmercury in phytoplankton, which accumulates in the cell cytoplasm, is assimilated by zooplankton four times more efficiently than inorganic mercury, which is principally bound in phytoplankton membranes. On the basis of these results, we constructed a simple model of mercury accumulation in fish as a function of the overall octanol−water partition coefficient of methylmercury. Our model can explain the variability of mercury concentrations in fish within, but not among, different lake regions.
Mercury (Hg) bound to fine aerosols (PM2.5-Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg ...isotopic compositions for 56 PM2.5 samples collected between 15 September and 16 October 2015 from Beijing, China, among which 26 were collected during daytime (between 08:00 and 18:30 LT) and 30 during night (between 19:00 and 07:30 LT). The results show that diel variation was statistically significant (p<0.05) for Hg content, Δ199Hg andΔ200Hg, with Hg content during daytime (0.32±0.14 µg g-1) lower than at night (0.48±0.24 µg g-1) and Δ199Hg and Δ200Hg values during daytime (mean of 0.26‰±0.40‰ and0.09‰±0.06‰, respectively) higher than during nighttime (0.04‰±0.22‰ and 0.06‰±0.05‰, respectively), whereas PM2.5 concentrations andδ202Hg values showed insignificant (p>0.05) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM2.5 source related) suggest that diel variation in Δ199Hg values resulted primarily from the photochemical reduction of divalent PM2.5-Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ199Hg and (i) Δ201Hg (positive, slope = 1.1), (ii) δ202Hg (positive, slope = 1.15), (iii) content of Hg in PM2.5 (negative), (iv) sunshine durations (positive) and (v) ozone concentration (positive) observed for consecutive day–night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM2.5-Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM2.5-Hg.
The bioaccumulation of trace elements in aquatic organisms can be described with a kinetic model that includes linear expressions for uptake and elimination from dissolved and dietary sources. Within ...this model, trace element trophic transfer is described by four parameters: the weight-specific ingestion rate (IR); the assimilation efficiency (AE); the physiological loss rate constant (
k
e
); and the weight-specific growth rate (
g). These four parameters define the trace element trophic transfer potential (TTP=IR·AE/
k
e
+
g) which is equal to the ratio of the steady-state trace element concentration in a consumer due to trophic accumulation to that in its prey. Recent work devoted to the quantification of AE and
k
e
for a variety of trace elements in aquatic invertebrates has provided the data needed for comparative studies of trace element trophic transfer among different species and trophic levels and, in at least one group of aquatic consumers (marine bivalves), sensitivity analyses and field tests of kinetic bioaccumulation models. Analysis of the trophic transfer potentials of trace elements for which data are available in zooplankton, bivalves, and fish, suggests that slight variations in assimilation efficiency or elimination rate constant may determine whether or not some trace elements (Cd, Se, and Zn) are biomagnified. A linear, single-compartment model may not be appropriate for fish which, unlike many aquatic invertebrates, have a large mass of tissue in which the concentrations of most trace elements are subject to feedback regulation.
The curious phenomenon of similar levels of methylmercury (MeHg) accumulation in fish from contaminated and pristine environments may be explained by the observation that the proportion of total ...mercury (HgT) present as MeHg is inversely related to HgT in natural waters. We hypothesize that this “MeHg accumulation paradox” is explained by the quantitative induction of bacterial enzymes that are encoded by the mercury resistance (mer) operon, organomercury lyase (MerB), and mercuric reductase (MerA) by inorganic Hg (HgII). We tested this hypothesis in two ecosystems in New Jersey: Berry's Creek in the Meadowlands (ML) and Pine Barren (PB) lakes. Across all sites, an inverse correlation (r 2 = 0.80) between the concentration of HgT (ML, 113−4220 ng L-1; PB, 0.3−5.4 ng L-1) and the proportion of HgT as MeHg (MeHg in ML and PB ranged from 0.08 to 1.6 and from 0.03 to 0.34 ng L-1, respectively) was observed. The planktonic microbial community in Meadowlands surface waters exhibited adaptation to mercury, the presence of mer genes and mRNA transcripts, and high rates of reductive demethylation (k deg = 0.19 day-1). In contrast, the microbial community of PB was not adapted to mercury and demonstrated low rates of oxidative demethylation (k deg = 0.01 day-1). These results strongly support our hypothesis and show that the degradation of MeHg by mer-encoded enzymes by the water column microbiota of contaminated environments can significantly affect the amount of MeHg that is available for entry into the aquatic food web.
Atmospheric concentrations of polycyclic aromatic hydrocarbons (PAHs) were measured at urban/industrial, suburban, coastal, and rural areas in New Jersey as part of the New Jersey Atmospheric ...Deposition Network. Concentrations of 36 PAH compounds were measured in the gas and particle phases in air and in precipitation at nine sites at regular intervals from October 1997 through May 2001. Gas-phase and particle-phase Σ36PAH concentrations ranged from 0.45 to 118 ng m-3 and from 0.046 to 172 ng m-3, respectively, and precipitation concentrations ranged from 11 to 16200 ng L-1. PAH concentrations vary spatially across the region, with the highest concentrations occurring at the most heavily urban and industrial locations. Average gas absorption deposition ranged from 0.004 (naphthacene) to 5040 (methylphenanthrenes) ng m-2 d-1, and dry particle deposition PAH fluxes ranged from 0.11 (naphthacene) to 300 (benzob+kfluoranthene) ng m-2 d-1 at the nine sites. Average atmospheric wet deposition PAH fluxes at the seven sites ranged from 0.40 (cyclopentacdpyrene) to 140 (methylphenanthrenes) ng m-2 d-1. These represent the first comprehensive estimates of PAH deposition to New Jersey and the Mid-Atlantic East Coast.
It is shown that HCO3 uptake by the marine diatom Thalassiosira weissflogii is modulated by the partial pressure of carbon dioxide and by the concentration of inorganic zinc.
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