Metabolism of dimethylsulfide in the bacteria

Abstract

Dimethylsulfide (DMS) is a volatile organosulfur compound which has been implicated as playing key roles in climate control and in the biogeochemical cycling of sulfur. Metabolism of DMS by Bacteria has been previously identified as an important sink of DMS in soils and in the marine environment; however, relatively little is known about the physiology or biochemistry of Bacteria that metabolism DMS. The key enzyme of DMS oxidation in Hyphomicrobium spp. – DMS monooxygenase - has been purified and characterised from H. sulfonivorans. It has been shown to be a two-componant monooxygenase, related to bacterial luciferase, comprising two subunits – an FMNH2-dependent DMS monooxygenase (DmoA) and an NADHdependent FMN oxidoreductase (DmoB). For DMS, DMS monooxygenase from H. sulfonivorans has a Vmax of 1250 nmol DMS oxidised min-1 (mg protein)-1 and a kM of 16.5μM, corresponding to a kCAT of 5.2s-1. DMS oxidation in terms of acting as a sole-carbon source and as a supplementary energy source has been demonstrated in methylotrophic and heterotrophic bacteria. Chemolithoheterotrophic growth in which DMS carbon is assimilated to biomass whilst DMS sulfur is oxidised to tetrathionate with a net energy gain has been demonstrated in “M. thiooxidans”. Both “internal” and “external” chemolithoheterotrophy has been observed in “M. thiooxidans”, with endogenous and exogenous thiosulfate being oxidised to tetrathionate with a net energy gain. As far as can be found from the literature, this is the first recorded production of a polythionate from an organosulfur compound, as such, representing a potential new step in the biogeochemical sulfur cycle. Stable-isotope probing with [13C2]-DMS has been performed for the first time and has confirmed Methylophaga spp. as dominant DMS-oxidising Bacteria in the marine environment. The oxidation of marine thiosulfate to tetrathionate has been demonstrating during a phytoplankton bloom, indicating that chemolithoheterotrophic Bacteria are active during the bloom. Preliminary analyses have been carried out on the genome sequence of “Methylophaga thiooxidans” and the genes encoding the major enzymes of formaldehyde assimilation via the KDPG aldolase variant RuMP pathway have been identified. Genes encoding key enzymes involved in the dissimilation of methanol and methylated amines have been indentified, in addition to those involved in nitrogen uptake from ammonia, nitrate, nitrite and urea. Chemoorganoheterotrophic growth, coupling the oxidation of DMS to DMSO with ATP formation, has been demonstrated in Sagittula stellata E-37T, though the enzyme(s) responsible for this oxidation remain unclear