Biological uptake and assimilation of iron by marine plankton: influences of macronutrients

We examined the biological uptake of iron by a coastal diatom (Thalassiosira pseudonana, clone 3H) and natural phytoplankton assembledge and its subsequent trophic transfer to marine copepods under various macronutrient regimes. Diatom cells maintained in semi-continuous cultures generally exhibited a higher intracellular Fe concentration with increasing nitrate concentration, whereas the ammonium concentration did not significantly affect the intracellular Fe uptake in the cells. At the similar concentrations, the calculated Fe uptake rate by the diatoms was 1.5–2.4× higher in cells maintained in nitrate substrate than in cells maintained in ammonium substrate. Intracellular Fe uptake was also much higher in diatom cells maintained under N-enriched conditions than in cells under N starvation. In contrast, the intracellular Fe uptake in diatoms was not dependent on the ambient P and Si conditions. The significant influence of macronutrients was also observed in the natural phytoplankton assembledge collected from a harbor. There was marked increase in Fe uptake with the addition of all major nutrients or P alone, whereas N addition alone did not greatly affect Fe uptake because of the high background N concentration. More Fe was also partitioned into the intracellular compartment with the addition of macronutrients. The Fe assimilation efficiency (AE) in two marine copepods (Calanus sinicus and Acartia spinicauda) increased with increasing nitrate concentration in diatoms, primarily due to a greater partitioning of Fe in the intracellular and cytoplasmic pools of the cells. Under N-limited conditions, a greater fraction of phytoplankton Fe may be packed into fecal pellets and transported to deeper waters. Conversely, the higher biological uptake of Fe by phytoplankton and the higher assimilation by marine copepods under nitrate-enriched conditions may increase the Fe residence time in the surface waters.