Cadmium uptake and trophic transfer in coastal plankton under contrasting nitrogen regimes

Abstract

Biological processes (uptake, trophic transfer, regeneration/excretion, and decomposition) are critical in controlling the fate of bioactive metals in the ocean. Whether nutrient conditions can affect metal uptake in marine phyto- plankton and its subsequent trophic transfer along planktonic food chains is little known. We demonstrate that Cd accumu- lation in marine diatoms is dependent on ambient nitrogen conditions and thus the physiological status of phytoplankton cells. Diatom cells Thalassiosira pseudonana and T. weiss- flogii inoculated under nitrogen-limited conditions accumu- lated considerably less Cd than cells maintained under nitro- gen-enriched conditions. The intracellular partitioning of Cd in the diatoms was also positively related to the level of ambi- ent N. Phosphate and silicate starvation did not affect Cd uptake in diatoms. The trophic transfer, quantified by mea- surement of the Cd assimilation efficiency (AE) from ingested diatoms in copepods Calanus sinicus, increased with an increase in the N quota of the diatom cells maintained in a semi-continuous culture. AEs were however comparable in copepods feeding on N-starved and N-enriched diatoms. There was a linear relationship between the Cd AE and the Cd distribution in the cytoplasm of diatoms. The Cd regener- ation rate in copepods was independent of the N status of ingested diatoms. Our study suggests that the cycling of Cd in marine planktonic food chains may be depressed under nitrogen-limited conditions. Conversely, nitrogen enrichment leads to an increase in Cd uptake in phytoplankton cells and zooplankton, elevating the potential exposure of plankton to toxic metals. Thus, nitrogen-stimulated eutrophication may not only affect the cycling of carbon, nitrogen and phosphorus in aquatic ecosystems, but can also result in changes in the cycling of toxic metals in marine food webs.