Minimum iron requirements of marine phytoplankton and the implications for the biogeochemical control of new production

Abstract

The Fe : PO., 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 lo-* to 10-3.1, but most oceanic species have ratios of < 1 O--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 1O-1.4 to 1O-2.7. A simple comparison of these requirement ratios with the ratios of the Fe and PO, 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 PO, 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 phytoplanlcton species have adaptations that reduce their dependence on combined N and Fe but not on P, it is concluded that PO, 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-limiling nutrients on an ecological time scale. What controls the rate of new production of organic C in the ocean is of great interest because of its influence on atmospheric 0, and C02, sedimentation and organic burial, and fisheries. One of the first to consider the geochemical control of new production in the ocean, Redfield (195 8) conducted a gedanken experiment in which he compared the ratio of NO3 to PO, in deep water with the ratio of N to P in marine biota and asked which nutrient would be depleted first by the biota if the deep water was upwelled into the photic zone. At that time,, trace metals were not generally considered as important limiting nutrients because of the high concentrations observed in the ocean as a result of inadequate techniques and serious contamination problems. As a result of greatly improved techniques, we now know that many trace metals are present in the ocean