Microbial methanethiol-dependent dimethylsulfide production in different marine sediments

Methanethiol (MeSH) and dimethylsulfide (DMS) are volatile organic sulfur compounds (VOSCs) with important roles in sulfur cycling and atmospheric chemistryclimate regulation. The discovery of a new and widespread MeSH-dependent DMS producing S-methyltransferase, MddH, revealed that methylation of MeSH can be a non-negligible source of DMS in marine sediment. Therefore, the functionality of MeSH-dependent DMS production (Mdd) and Mdd-related microbial groups in marine sediments need to be further investigated. In this study, we performed a comprehensive comparation of Mdd activity in nearshore, pelagic and cold seep sediment. DMS is thought to be mainly produced via microbial dimethylsulfoniopropionate (DMSP) cleavage pathways. In addition, the bacterial methylation of H2S and MeSH can produce DMS by the methyltransferase MddA and MddH. The mddA gene and mddH gene are respectively abundant in soil environments and marine environments. Inorganic sulfides and organic sulfides are combined through the H2S- and Methanethiol-dependent DMS production (Mdd) pathways. However, the research so far is focused on the DMSP synthesis and cleavage pathways, very few marine sediment samples were detected their DMS production capacity from MeSH and H2S. Detailed bacterial Mdd pathways in marine sediments remains largely unknown. In this study, we explored the Mdd pathways with four different marine sediment samples by cultivation-dependent and cultivation-independent methods when supplied sufficient MeSH, indicating that their Mdd pathways were reliable. 16S rRNA gene amplicon sequencing results indicated that the groups with Mdd pathwaysH were enriched, such as Halomonas and Marinobacter. Meanwhile, metagenomics data showed that the abundance of mddH was higher than mddA after enrichments. Moreover, some DMS-producing bacteria isolated from these four marine environments lacked known genes which can process the Mdd pathways and can be used as model organisms to potentially identify alternative genes in these pathways, such as Cytophagia, Bacilli and Bacteroidia. Through analyzing the activity of all isolates, it was found that the process from H2S to MeSH may be prevalent in bacteria. This study reflects that the ecological role and contributions of the Mdd pathways has been underestimated in the past and highlights the requirement for discovering unidentified Mdd genes/pathways in the oceans.