海洋异养细菌对DMSP/DMS产生的贡献及驱动硫循环的机制：以马里亚纳海沟为例

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.

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。

二甲基巯基丙酸内盐（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，但最近研究发现海洋异养细菌也可合成DMSP，并鉴定出DMSP合成的关键基因。然而，浮游植物和异养细菌对海洋环境中DMSP的相对贡献尚不清楚。本项目以马里亚纳海沟为重点研究区域，聚焦海洋异养细菌对DMSP/DMS产生的贡献及驱动硫循环的机制。我们首先研究了马里亚纳海沟全水深微生物的群落结构变化及功能活性，发现细菌在全水柱中占主导地位，烷烃降解菌在海沟底部勃发，认为烷烃可能是超深渊带微生物群落结构差异的重要驱动力；发现深渊海沟的氨氧化古菌具有特殊的渗透保护剂和能量代谢机制，从而适应高压环境。其次，基于新的DNA稳定同位素标记技术（SIP）鉴定了海水中活跃的DMSP裂解微生物及活性酶，发现DMSP的降解方式主要为裂解途径，海洋螺菌目的巴氏发菌属、海单胞菌属、海细菌属等是主要的DMSP降解类群，而DMSP裂解酶DddD是对外界环境中DMSP浓度变化响应最活跃的酶类。此外，从马里亚纳海沟水体和沉积物中鉴定出多种新型DMSP合成及降解细菌和DMS产生菌，并鉴定出新型DMS产生基因mddH，该基因所编码的同源蛋白广泛存在于多种细菌中，可能是海洋环境中DMS的重要来源。最后，本项目研究了马里亚纳海沟水体和沉积物的DMSP循环，发现马里亚纳海沟异养细菌的DMSP合成基因及转录活性随水深的增加而增加，而DMSP降解基因呈相反的变化趋势，并且发现DMSP在细菌压力保护方面的重要作用，指出异养细菌是深海海水和沉积物中DMSP的重要生产者。综上所述，我们的研究结果揭示了海洋异养细菌对DMSP/DMS产生的贡献、驱动硫循环的机制以及深海微生物的高压适应机制，并为重新估算全球DMSP的产量、通量及其气候效应提供了科学依据。