Long-Term Euxinia Restricts Microbial Methane Removal in Eutrophic Coastal Basins

In eutrophic coastal waters, aerobic methane-oxidizing bacteria (MOB) mitigate methane emissions by oxidizing benthic methane even in the stratified, anoxic water column. However, ongoing warming and eutrophication lead to extended stratification periods, enhancing anoxic and sulfidic conditions (euxinia), potentially affecting methane removal capacity. Here we compared overall water column methane removal between sites with irregular, seasonal and longer-term euxinia in the Stockholm Archipelago during summer 2022. The highest water–air methane emissions, bottom water–methane and sulfide accumulation, and the lowest methane oxidation potential were observed under longer-term euxinic bottom water conditions. While MOB relative abundance and potential activity indicated high functioning of the methane biofilter in the seasonally euxinic bottom water layer, the methane-filtering potential was much lower in the longer-term euxinic bottom water. Under persistent euxinic conditions, overall bacterial diversity and microbial network connectivity were lower, likely following a simultaneous shift in redox conditions and a shift toward anaerobic sulfur-cycling. This shift may force MOB to retreat from the euxinic bottom water into the narrow oxycline, reducing the capacity of the methane biofilter and resulting in higher methane emissions. These findings highlight the positive feedback loop that can further amplify oceanic methane emissions, particularly from eutrophic and shallow coastal waters prone to prolonged stratification under global warming.在富营养化沿海水域中，好氧甲烷氧化菌（aerobic methane-oxidizing bacteria, MOB）即便在分层缺氧水柱中，也可通过氧化底栖甲烷来缓解甲烷排放。然而，持续变暖与富营养化会延长水体分层周期，加剧缺氧与硫化物环境（euxinia），进而可能影响甲烷去除能力。本研究于2022年夏季在斯德哥尔摩群岛开展，对比了存在不规则、季节性及长期硫化缺氧环境的位点间全水柱甲烷去除效率。研究发现，在长期硫化缺氧底水环境下，水-气甲烷排放、底水甲烷与硫化物积累量均达到最高，而甲烷氧化潜势最低。尽管季节性硫化缺氧底水层中，MOB的相对丰度与潜在活性显示其甲烷生物滤池功能较强，但长期硫化缺氧底水层的甲烷过滤能力显著更低。在持续硫化缺氧环境中，整体细菌多样性与微生物网络连通性均较低，这可能与氧化还原条件的同步改变以及群落向厌氧硫循环的转变有关。这种转变可能迫使MOB从硫化缺氧底水撤退至狭窄的氧化跃层（oxycline），削弱甲烷生物滤池的功能，最终导致更高的甲烷排放。本研究结果揭示了一种可进一步放大海洋甲烷排放的正反馈循环，尤其在全球变暖下易发生长期分层的富营养化浅海沿岸水域中更为显著。