IRON LIMITATION AND THE CYANOBACTERIUM SYNECHOCOCCUS IN EQUATORIAL PACIFIC WATERS

Iron enrichments in bottle experiments in high nutrient, low chlorophy ll (HNLC) surface waters typically stimulate the net growth of nanophy toplankton (2.0-20 mum) but not picophytoplankton (< 2.0 mum), suggest ing either that picophytoplankton are not Fe limited or that any incre ased picophytoplankton growth is balanced by increased microzooplankto n grazing rates. Using a novel approach in which we diminish iron avai lability in seawater with the fungal siderophore deferriferrioxamine B , we demonstrate that growth of the abundant picoplanktonic cyanobacte rium Synechococcus is not strongly rate limited by Fe in the HNLC equa torial Pacific Ocean. However, experiments at the equator and 5-degree s-S show an unsuspected non-uniformity in iron nutrition within this H NLC region. Furthermore, our results indicate that a major portion of ''dissolved'' (<0.4 mum) iron in these waters is unavailable to Synech ococcus, providing the first evidence that a significant fraction of d issolved iron occurs in forms other than the simple hydroxy species pr edicted by thermodynamic models. Deferriferrioxamine B affords a power ful new tool for probing the iron nutrition of marine phytoplankton an d can provide unique insight into the role of iron in marine biogeoche mical cycles.