2 mg ATP mg−1 dry biomass being formed per mole of DMS oxidized to DMSO (which is in the same order of magnitude as that produced during thiosulfate oxidation by M. thiooxydans [0.13 mg, Boden et al. (2010)]. It is interesting to note that the production of ATP here apparently follows an exponential rather than a logarithmic pattern – as observed in M. thiooxydans and Halothiobacillus Erastin neapolitanus during thiosulfate oxidation (Kelly & Syrett, 1964; Boden et al., 2010).

There is also a slight lag as ATP formation begins, suggesting that the oxidation of DMS is not immediate and that DMS must first be transported into the cells – possibly by active transport. Alternatively, this lag could be due to a high ATP demand of the cells for example, to fuel motility. This is in contrast to the immediate ATP formation during thiosulfate oxidation in M. thiooxydans and H. neapolitanus, which is thought to occur in the periplasm. The oxidation of DMS to DMSO alone provides 2 mol of electrons per mole of DMS oxidized. This is not sufficient to provide the 14–16% increases in Ymax observed here. The same amount of electrons buy PD0325901 from thiosulfate oxidation in M. thiooxydans provides only a 9% increase in Ymax during growth on methanol (Boden et al., 2010). This could indicate that, in addition to providing electrons to the respiratory chain, the oxidation affects some other system within the cell that generates

an increased yield of reducing equivalents that are responsible for a larger conservation of carbon into biomass. More complex radiorespirometric or metabolomic studies are required to GNA12 fully investigate the pathway of DMS-dependent energy metabolism in S. stellata; however, we have demonstrated

that DMS acts as an energy source for the chemoorganoheterotrophic growth of this organism on different carbon sources and that the oxidation of DMS to DMSO is coupled to ATP synthesis. Few data are available on the kinetics and growth yields in mixotrophic bacteria – particularly those capable of chemoorganoheterotrophy – and the data we present here add to this understudied area of bacterial physiology. The regulation and environmental significance of mixotrophic Bacteria are unknown, although the substrates and products of their energy-yielding oxidations can be compounds of global biogeochemical significance – such as DMS and DMSO, which we report here. Further work is required to better the understanding of these mixed metabolic modes, their use by Bacteria in the environment and their contribution to the flux of compounds through biogeochemical cycles. We thank Don Kelly for many stimulating discussions on growth kinetics and Gez Chapman is thanked for technical support. We thank the Natural Environment Research Council (UK) for funding via a studentship to R.B. and fellowships to H.S. (NE/B501404/1 and NE/E013333/1). Ann P. Wood and Ben Berks are thanked for the kind donation of strains.