Low iron requirement for growth in oceanic phytoplankton

Abstract

DESPITE the controversy on the importance of iron in limiting phytoplankton growth and affecting air–sea exchange of CO₂ in the ocean^1–4, there is very little information on cellular iron requirements for growth. The few data available^5,6 come from species isolated from coastal sea water where dissolved Fe levels are 10–1,000 times higher than those (≤0.1 nM) in the open ocean^1,7. Species from oceanic waters require much lower external Fe concentrations for growth than do comparable coastal species⁸. Here we report that an oceanic diatom was able to grow at a near maximum specific rate of about 1.0 per day at a cellular Fe:C ratio of 2 μmol: mol, about 25% of the amount needed for the same rate in a related estuarine species, and 2–20% of values previously used to estimate algal Fe requirements in sea water^1,2. These results have important implications concerning iron limitation of primary productivity in the ocean and cell biology of iron in oceanic algae.