[1] Air-water gas transfer influences CO 2 and other climatically important trace gas fluxes on regional and global scales, yet the magnitude of the transfer is not well known. Widely used models of gas exchange rates are based on empirical relationships linked to wind speed, even though physical processes other than wind are known to play important roles. Here the first field investigations are described supporting a new mechanistic model based on surface water turbulence that predicts gas exchange for a range of aquatic and marine processes. Findings indicate that the gas transfer rate varies linearly with the turbulent dissipation rate to the 1 = 4 power in a range of systems with different types of forcing -in the coastal ocean, in a macro-tidal river estuary, in a large tidal freshwater river, and in a model (i.e., artificial) ocean. These results have important implications for understanding carbon cycling.

Sulphur is a universally required cell nutrient found in two amino acids and other small organic molecules. All aerobic marine bacteria are known to use assimilatory sulphate reduction to supply sulphur for biosynthesis, although many can assimilate sulphur from organic compounds that contain reduced sulphur atoms. An analysis of three complete 'Candidatus Pelagibacter ubique' genomes, and public ocean metagenomic data sets, suggested that members of the ubiquitous and abundant SAR11 alphaproteobacterial clade are deficient in assimilatory sulphate reduction genes. Here we show that SAR11 requires exogenous sources of reduced sulphur, such as methionine or 3-dimethylsulphoniopropionate (DMSP) for growth. Titrations of the algal osmolyte DMSP in seawater medium containing all other macronutrients in excess showed that 1.5 x 10(8) SAR11 cells are produced per nanomole of DMSP. Although it has been shown that other marine alphaproteobacteria use sulphur from DMSP in preference to sulphate, our results indicate that 'Cand. P. ubique' relies exclusively on reduced sulphur compounds that originate from other plankton.

About half the biogenic sulfur flux to the earth's atmosphere each year arises from the oceans. Dimethylsulfide (DMS), which constitutes about 90% of this marine sulfur flux, is presumed to originate from the decomposition of dimethylsulfoniopropionate produced by marine organisms, particularly phytoplankton. The rate of DMS release by phytoplankton is greatly increased when the phytoplankton are subjected to grazing by zooplankton. DMS production associated with such grazing may be the major mechanism of DMS production in many marine settings.

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