| A combination of targeted
field sampling of surface water particulate material and controlled shipboard
and laboratory incubation studies were used to investigate the role of
phytoplankton uptake in the geochemical cycling of bioactive trace elements
in ocean surface waters. The study addressed two problems in marine
geochemistry: 1) What factors control the dissolved Cd/P ratio in ocean
surface waters?, and 2) How does the availability of dissolved Fe affect
the Fe content, growth and nitrogen fixation rates of marine cyanobacteria?
Novel techniques were developed to collect trace element clean, size-fractionated
particulate samples from marine surface waters for multi-element analysis
Phytoplankton Cd/P ratios were found to have systematic geographical variations spanning 2.5 orders of magnitude around the global average upwelled dissolved ratio. These variations were found to be governed largely by the availability of dissolved Cd, Zn and Mn to the phytoplankton and, for the first time, are shown to depend on the pCO2 of surface waters. The dissolved Cd/P ratio in surface water is postulated to be intimately tied to the trace metal and carbon acquisition physiology of phytoplankton that dominate the vertical export of particulate material. The supply to the photic zone of dissolved CO2, Cd, Zn and Mn in relation to the principal algal nutrients likely govern the degree of Cd removal and, therefore, the dissolved Cd/P ratio of ocean surface waters.
The proposed link between iron availability and nitrogen fixation was revisited in laboratory isolates and natural populations of the globally significant marine cyanobacteria Trichodesmium spp.. In Fe limited laboratory cultures there were clear relationships between iron availability, cellular iron quotas and rates of nitrogen fixation which displayed a critical dissolved inorganic iron threshold of ca. 300-400 pM. Field populations of Trichodesmium, collected from coastal blooms, showed high iron quotas consistent with high nitrogen fixation rates. Using seasonal maps of aeolian iron fluxes and model derived maps of surface water total dissolved Fe, the potential for nitrogen fixation by Trichodesmium in the global ocean was calculated. The results suggest that in 75% of the global ocean, iron availability limits nitrogen fixation by this organism.