Responses of Zn assimilation by coastal plankton to macronutrients

We examined the Zn uptake in marine diatoms and its transfer to marine copepods under different nutrient-replete and -deplete conditions. Zn uptake, quantified by measurements of both total cellular and intracellular Zn accumulation in two coastal diatoms (Thalassiosira pseudonana and Thalassiosira weissflogii), was greatly dependent on the ambient nitrogen conditions. Semicontinuous culture experiments demonstrated that diatom cells accumulated much less Zn with decreasing ambient nitrate concentration. The calculated Zn uptake rate decreased by 1.8 times in T. pseudonana and 1.5 times in T. weissflogii with a decrease in ambient N concentration from 176 mM to 17.6 mM. N-starved cells also accumulated much less Zn compared with N-enriched cells. The uptake rate decreased by 2.5–2.6 times when both diatoms were starved of N for 2 d. The intracellular partitioning of Zn in the diatom T. pseudonana was positively related to the ambient N level within the short-term exposure period (5 h). P starvation, however, resulted in an increase in Zn accumulation in the diatoms, whereas Si starvation did not significantly affect the rate of Zn uptake in diatoms. The trophic transfer was quantified by measurement of Zn assimilation efficiency in two copepods (Calanus sinicus and Acartia spinicauda) feeding on diatoms with a different N quota. Assimilation generally increased with an increase in the N quota of the diatoms. In one experiment (C. sinicus feeding on diatom T. weissflogii), the influence of different N status in diatoms on Zn assimilation by copepods was statistically significant. In contrast, the elimination rate (physiological turnover rate) was independent of the N quota of the cells. Nitrogen enrichment may lead to an increase in Zn uptake and transfer in marine plankton. Our study therefore suggests that there is considerable interaction between macronutrients and Zn uptake by plankton in aquatic systems. Many trace metals are biologically required by aquatic organisms but can be toxic at elevated concentrations. Trace metal uptake in marine phytoplankton has been extensively investigated over the past decades because of the significance of phytoplankton in ecosystem dynamics (Sunda 1994; Sunda and Huntsman 1998). Various geochemical and biological processes are known to affect metal uptake in these organisms (Campbell 1995; Sunda and Huntsman 1998, 2000). It is now well established that metal speciation critically affects trace metal accumulation in marine phytoplankton. In contrast to the extensive studies on the influences of metal geochemical factors on metal uptake, relatively few studies have considered the influence of environmental factors on metal accumulation. Recent studies have shown that the ambient pCO2 level can greatly affect Zn and Cd uptake in marine phytoplankton (Morel et al. 1994; Cullen et al. 1999). The influence of the pCO2 level on Zn uptake is primarily due to the involvement of Zn as a cofactor in carbonic anhydrase that converts inorganic biocarbonate into CO2 for photosynthesis. Similarly, Cd is involved in carbonic anhydrase, and its uptake is influenced by the ambient pCO2 level (Cullen et al. 1999). Recently, we have shown that the nutritional conditions of the algal cells can influence metal uptake in marine phytoplankton (Wang and Dei 2001a). Among the different species of phy1 Corresponding author ([email protected]).