ABSTRACT
Carbon, nitrogen, and phosphorus are required by all phytoplankton and silicon is required by diatoms and some other organisms as major constituents of cellular material. Nitrogen gas forms 78% of the modern atmosphere and must be fixed in a reduced state into organic material before it can used in cellular processes. Sulfur is an essential element for all organisms, constituting about 1% of cellular mass, mainly occurring in the amino acids methionine and cysteine and in various coenzymes and metalloproteins. Microbes need special mechanisms to acquire this iron and many bacteria achieve this by production of siderophores, which bring iron into the cell via binding of the organic complexes to surface receptors and specialized transport mechanisms to bring the iron into the cell. Biogeochemical cycling of phosphorus is closely linked with carbon flux and the availability of phosphorus has a major limiting or co-limiting effect on oceanic primary production rates and microbial community composition.