LOW IRON REQUIREMENT FOR GROWTH IN OCEANIC PHYTOPLANKTON

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 (less-than-or-equal-to 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 species 8. 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-mu-mol: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.