6. Three hundred microliters of 50 mM DMSO was placed in the bulb of the side arm and was then used to initiate the reaction. The oxidation of MV was monitored by the decrease in A600 nm and the rate of oxidation was determined using the millimolar extinction coefficient of the reduced form, being 1.13 mM−1 cm−1 (Kelly & Wood, 1994). Cell-free extracts learn more prepared from H. sulfonivorans S1T grown heterotrophically
on dimethylsulfone were used as the positive control. ATP production experiments were performed essentially as described previously (Boden et al., 2010) using 1 mM DMS as an energy source in place of thiosulfate. The kinetic parameters derived from the growth of S. stellata in chemostat culture on fructose (12 mM) or succinate (2 mM) are given in Table 1. The maximum yield coefficient (Ymax) increased in the presence of DMS, which was oxidized stoichiometrically to DMSO SB431542 without assimilation into biomass. No DMS was detected in the cultures in a steady state. Upon the addition of DMS to a succinate or a fructose-limited chemostat, there was no immediate perturbation of the steady state and the dissolved oxygen concentration did not begin to decrease for approximately 6 h in the case of fructose or 3 h in the case of succinate, independent of the dilution rate.
The delay in oxygen consumption in the presence of DMS would indicate that the enzyme system for DMS oxidation was not constitutively expressed and the culture essentially underwent a lag phase while expression was induced. While the Ymax increased, it should be noted that the maintenance coefficient (mS) remained constant in the case of both carbon sources used. This was also the case when thiosulfate was used to support
the chemolithoheterotrophic growth of Methylophaga thiooxydans (Boden et al., 2010) and mixotrophic growth of Acidithiobacillus thiooxidans (Mason & Kelly, 1988). As stated previously, it is not possible to compare these data with those of Green et al. (2011) second owing to insufficient data being available from their paper to calculate Y– i.e. without quantifying substrate disappearance, Y cannot be calculated. The theoretical Ymax for growth on succinate is 37.1 g dry biomass mol−1 succinate (9.23 g dry biomass mol−1 substrate carbon), calculated using the assumption that 32% of succinate carbon is assimilated to biomass, as per the determinations performed by Anthony (1982) in a range of organisms. The experimental Ymax for succinate was found to be 33.6 g dry biomass mol−1 succinate (8.4 g dry biomass mol−1 substrate carbon), which increased in the presence of DMS to 38.9 g dry biomass mol−1 succinate (9.7 g dry biomass mol−1 substrate carbon) – this is higher than the theoretical Ymax and a 16% increase on the Ymax in the absence of DMS. The theoretical Ymax for growth on fructose dissimilated to 3-phosphoglycerate via the Entner–Doudoroff pathway is 73.7 g dry biomass mol−1 fructose (12.