二甲基巯基丙酸盐（DMSP）是地球上最丰富的有机硫分子之一，也是微生物产生“冷室气体”二甲基硫（DMS）的主要前体物质，在硫循环和气候变化中发挥重要作用。以前普遍认为只有海洋浮游植物等真核生物才能合成DMSP，但我们最新发现海洋异养细菌也可合成DMSP，并首次鉴定出DMSP合成的关键基因。由于浮游植物和异养细菌的营养方式不同，我们推测浮游植物在真光层对DMSP合成的贡献较大，而异养细菌在深水区贡献相对较大。马里亚纳海沟是验证该科学假设的理想生境，本项目拟从不同水层的异养细菌中筛选新的DMSP/DMS产生菌株，鉴定新型DMSP合成基因或DMSP裂解基因，并揭示其代谢通路；测定海水中DMSP的合成速率及过程；评估DMSP/DMS产生基因的多样性、丰度、活性及环境调控因子，并估算异养细菌和浮游植物对水体DMSP的相对贡献。该研究对揭示海洋细菌驱动硫循环的机制及在全球硫循环中的贡献具有重要作用。

Dimethylsulphoniopropionate (DMSP) is one of the planet’s most abundant organosulfur molecules, is believed to be the major precursor of the microbe-generated “cooling gas”, dimethyl sulphide (DMS), and plays key roles in global sulphur cycling and climate change. It has long been believed that only marine eukaryotes such as phytoplankton can make DMSP. However, we have recently discovered that marine heterotrophic bacteria can also make DMSP, and have also identified the key gene involved in DMSP synthesis in any organisms. As the trophic type of heterotrophic bacteria is completely different from that of phytoplankton, we proposed that phytoplankton acting as the major contributors to DMSP production is mainly valid within euphotic depth, while in deeper waters where very limited or no sunlight can penetrate, the contribution of heterotrophic bacteria to DMSP production is greater than phytoplankton. The Mariana Trench is a perfect environment to verify our hypothesis. In this project, we will isolate novel DMSP/DMS producing heterotrophic bacteria from depth-profiled water samples of the Mariana Trench, identify unknown DMSP synthesizing genes or ddd genes (genes involved in DMSP-dependent DMS production), and figure out the DMSP production pathway. We will also assess the DMSP production rates and process of natural microbial communities. With information of known and newly identified DMSP synthesizing genes and ddd genes, we will assess the diversity, abundance, activity and regulatory environmental factors of these genes, and estimate the relative contribution of heterotrophic bacteria versus phytoplankton to the DMSP production at different water depths. This study will have important implication in uncovering the mechanisms of marine bacteria in driving the global sulphur cycle as well as determining their contribution in this significant biogeochemical cycle.