Phosphorus limitation of nitrogen fixation by Trichodesmium in the centralAtlantic Ocean

Marine fixation of atmospheric nitrogen is believed to be an important sour ce of biologically useful nitrogen to ocean surface waters(1), stimulating productivity of phytoplankton and so influencing the global carbon cycle(2) . The majority of nitrogen fixation in tropical waters is carried out by th e marine cyanobacterium Trichodesmium(3), which supplies more than half of the new nitrogen used for primary production(4). Although the factors contr olling marine nitrogen fixation remain poorly understood, it has been thoug ht that nitrogen fixation is limited by iron availability in the ocean(2,5) . This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms(6) and the higher apparent densities of Trichodes mium where aeolian iron inputs are plentiful(7). Here we report that nitrog en fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium coloni es. Nitrogen fixation was, instead, highly correlated to the phosphorus con tent of Trichodesmium and was enhanced at higher irradiance. Furthermore, o ur calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated(6). Although iron deficiency could still potentially limit growth of nitrogen-f ixing organisms in regions of low iron availability-for example, in the sub tropical North Pacific Ocean-our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.