Influence of algal iron content on the assimilation and fate of iron and carbon in a marine copepod
We conducted pulse-chase experiments to study the assimilation and excretion of iron and carbon by the copepod Acartia tonsa fed three algal species cultured under Fe-replete or Fe-depleted conditions. Mean assimilation efficiency of Fe (AEFe) for copepods ranged from 47% to 53% when fed the diatom Thalassiosira oceanica, 43% to 45% when fed the cryptophyte Rhodomonas salina, and 50% to 69% when fed the prymnesiophyte Isochrysis galbana. Carbon assimilation efficiency (AEC) ranged from 82% to 90% for all species. No consistent difference in AEFe or AEC was observed between Fe treatments. Instead, AEFe was linearly correlated with the fraction of Fe contained in algal cytosol (r2 = 0.75, p = 0.02). The excretion rate constants of Fe were generally higher when A. tonsa individuals were fed Fe-depleted cells, especially for R. salina and I. galbana, contrary to expectations if the copepods were maintaining elemental homeostasis. Mean respiratory rates were 30–60% higher for the copepods fed Fe-depleted food among the three algal diets, suggesting that those animals were “dumping” excess C. However, respiration was also closely related to cell size of algal food. Biokinetic modeling based on parameters measured here suggests that A. tonsa cannot maintain stoichiometric homeostasis of their tissue Fe : C ratios. The lack of regulation of tissue Fe contents is in marked contrast to the well-established regulation of the macronutrients nitrogen and phosphorus for some crustacean zooplankton.