MINIMUM IRON REQUIREMENTS OF MARINE-PHYTOPLANKTON AND THE IMPLICATIONS FOR THE BIOGEOCHEMICAL CONTROL OF NEW PRODUCTION

The Fe:PO4 ratio at which nutrient limitation of final cell yield shifts from one nutrient to the other was determined for 22 species of marine phytoplankton. Among eucaryotic phytoplankton, coastal species have subsistence optimum Fe:P molar ratios of 10(-2) to 10(-3.1), but most oceanic species have ratios of < 10(-4), indicating that oceanic species have been able to adapt their biochemical composition to the low availability of Fe in the open ocean. In contrast, both coastal and oceanic species of cyanobacteria have relatively high Fe:P molar ratio requirements, ranging from 10(-1.4) to 10(-2.7). A simple comparison of these requirement ratios with the ratios of the Fe and PO4 fluxes to the photic zone from deep water and the atmosphere indicates that new production of cyanobacterial biomass is Fe limited, but new production of eucaryotic algal biomass is not. Because of the large differences among species in their Fe requirements, especially between procaryotes and eucaryotes, changes in the relative inputs of Fe and PO4 to the photic zone are expected to lead to changes in the species composition of phytoplankton communities. Indeed, the ratio of atmospheric to deep-water inputs of nutrients and the resulting Fe:P input ratios appear to influence the relative abundance of unicellular cyanobacteria and Trichodesmium and their vertical and biogeographic distributions. Because some phytoplankton species have adaptations that reduce their dependence on combined N and Fe but not on P, it is concluded that PO4 is the ultimate limiting nutrient of new production of organic C on a geochemical and evolutionary time scale, even though N and Fe are important growth rate-limiting nutrients on an ecological time scale.