The algal osmolyte dimethylsulphoniopropionate (DMSP) and its enzymatic cleavage product dimethylsulphide (DMS) contribute significantly to the global sulphur cycle, yet their physiological functions are uncertain. Here we report results that, together with those in the literature, show that DMSP and its breakdown products (DMS, acrylate, dimethylsulphoxide, and methane sulphinic acid) readily scavenge hydroxyl radicals and other reactive oxygen species, and thus may serve as an antioxidant system, regulated in part by enzymatic cleavage of DMSP. In support of this hypothesis, we found that oxidative stressors, solar ultraviolet radiation, CO(2) limitation, Fe limitation, high Cu(2+) (ref. 9) and H(2)O(2) substantially increased cellular DMSP and/or its lysis to DMS in marine algal cultures. Our results indicate direct links between such stressors and the dynamics of DMSP and DMS in marine phytoplankton, which probably influence the production of DMS and its release to the atmosphere. As oxidation of DMS to sulphuric acid in the atmosphere provides a major source of sulphate aerosols and cloud condensation nuclei, oxidative stressors--including solar radiation and Fe limitation--may be involved in complex ocean atmosphere feedback loops that influence global climate and hydrological cycles.

Iron concentrations in open ocean are orders of magnitude lower than levels in coastal waters. Experiments with coastal and oceanic phytoplankton clones representing different algal groups and cell sizes indicate that cellular iron uptake rates are similar among the species when rates are normalized to cell surface area. This similarity in rates apparently is explained by evolutionary pressures that have pushed iron uptake in all species toward the maximum limits imposed by diffusion and ligand exchange kinetics. Because of these physical/chemical limits on uptake, oceanic species have been forced to decrease their cell size and/or to reduce their growth requirements for cellular iron by up to 8-fold. The biochemical mechanisms responsible for this reduction in metabolic requirements are unknown.

Surface waters of the subtropical Sargasso Sea contain dissolved inorganic phosphate (DIP) concentrations of 0.2 to 1.0 nanomolar, which are sufficiently low to result in phosphorus control of primary production. The DIP concentrations in this area (which receives high inputs of iron-rich dust from arid regions of North Africa) are one to two orders of magnitude lower than surface levels in the North Pacific (where eolian iron inputs are much lower and water column denitrification is much more substantial). These data indicate a severe relative phosphorus depletion in the Atlantic. We hypothesize that nitrogen versus phosphorus limitation of primary production in the present-day ocean may be closely linked to iron supply through control of dinitrogen (N2) fixation, an iron-intensive metabolic process. Although the oceanic phosphorus inventory may set the upper limit for the total amount of organic matter produced in the ocean over geological time scales, at any instant in geological time, oceanic primary production may fall below this limit because of a persistent insufficient iron supply. By controlling N2 fixation, iron may control not only nitrogen versus phosphorus limitation but also carbon fixation and export stoichiometry and hence biological sequestration of atmospheric carbon dioxide.

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Zinc is used extensively in the metabolism of higher organisms; cobalt's usage is minimal. We found an unusual pattern of requirement for these metals in marine phyto,plankton in which the cyanobacterium Synechococcus bacillaris needed Co but not Zn for growth, the coccolithophore Emiliania huxleyi had a Co requirement that could be partly met by Zn, and the diatoms Thalassiosira pseudonana and Thalassiosira oceanica had Zn requirements that could be largely met by Co. These results indicate that Co and Zn can replace one another metabolically in the eucaryotic species. Associated with this replacement, there was up to a 700-fold increase in cellular Co uptake rates with decreasing 2.n. ion concentration, indicating that Zn should have a major influence on biological scavenging of Co. This hypothesis is consistent with Zn and Co distributions within the oceanic nutricline which show Co depletion only after Zn has become depleted. Zn ion concentrations and Co : Zn ratios vary widely in the ocean, and these variations could influence the relative growth of diatoms and coccolithophores, with potential effects on global carbon cycles.