Microbial cycling of organosulfur gases in marine and freshwater environments

Abstract

Substantial quantities of volatile organic sulfur gases (OSG) are emitted from natural aquatic ecosystems to the atmosphere. Compounds such as dimethyl sulfide (DMS), methanethioi (MeSH), dimethyl disulfide (DMDS), carbonyl sulfide (COS) and carbon disulfide (CS₂) are important sources of sulfur to the atmosphere and play a significant role in the global sulfur cycle. This paper reviews the microbial processes which are responsible for much of the production and consumption of OSG in aquatic habitats. The gross production of OSG is controlled by the availability of specific precursor molecules which are ultimately derived from plants. DMS, for example, is the major OSG produced in marine systems because marine algae produce dimethylsul- foniopropionate (DMSP), which is metabolized to DMS. The methylated sulfur gases (DMS, DMDS, MeSH) and some of their precursors are readily inter-converted during microbial metabolism, but are ultimately mineralized. While few estimates exist, it appears that microbial decomposition of methylated sulfur gases removes most of the gross production of these compounds, leaving only a small fraction for exchange with the atmosphere. Considerably less is known about the production and removal mechanisms of COS and CS₂. COS appears to be derived from the photochemical destruction of organosulfur compounds, whereas CS₂ is thought to originate from microbial decomposition of organosulfur compounds in anoxic environments. Both COS and CS₂ are known to be utilized by bacteria, but the significance of their metabolism in natural environments is presently unknown. Microbial processes play a critical role in determining the net flux of reduced sulfur gases to the atmosphere. Considerable work remains to be done in order to understand the biogeochemical cycling of OSG in aquatic systems.