DOE Joint Genome Institute, Berkeley, California, USA.
Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia, Canada.
mSystems. 2024 Jan 23;9(1):e0093623. doi: 10.1128/msystems.00936-23. Epub 2024 Jan 3.
Estuarine wetlands harbor considerable carbon stocks, but rising sea levels could affect their ability to sequester soil carbon as well as their potential to emit methane (CH). While sulfate loading from seawater intrusion may reduce CH production due to the higher energy yield of microbial sulfate reduction, existing studies suggest other factors are likely at play. Our study of 11 wetland complexes spanning a natural salinity and productivity gradient across the San Francisco Bay and Delta found that while CH fluxes generally declined with salinity, they were highest in oligohaline wetlands (ca. 3-ppt salinity). Methanogens and methanogenesis genes were weakly correlated with CH fluxes but alone did not explain the highest rates observed. Taxonomic and functional gene data suggested that other microbial guilds that influence carbon and nitrogen cycling need to be accounted for to better predict CH fluxes at landscape scales. Higher methane production occurring near the freshwater boundary with slight salinization (and sulfate incursion) might result from increased sulfate-reducing fermenter and syntrophic populations, which can produce substrates used by methanogens. Moreover, higher salinities can solubilize ionically bound ammonium abundant in the lower salinity wetland soils examined here, which could inhibit methanotrophs and potentially contribute to greater CH fluxes observed in oligohaline sediments.IMPORTANCELow-level salinity intrusion could increase CH4 flux in tidal freshwater wetlands, while higher levels of salinization might instead decrease CH4 fluxes. High CH4 emissions in oligohaline sites are concerning because seawater intrusion will cause tidal freshwater wetlands to become oligohaline. Methanogenesis genes alone did not account for landscape patterns of CH4 fluxes, suggesting mechanisms altering methanogenesis, methanotrophy, nitrogen cycling, and ammonium release, and increasing decomposition and syntrophic bacterial populations could contribute to increases in net CH4 flux at oligohaline salinities. Improved understanding of these influences on net CH4 emissions could improve restoration efforts and accounting of carbon sequestration in estuarine wetlands. More pristine reference sites may have older and more abundant organic matter with higher carbon:nitrogen compared to wetlands impacted by agricultural activity and may present different interactions between salinity and CH4. This distinction might be critical for modeling efforts to scale up biogeochemical process interactions in estuarine wetlands.
滨海湿地蕴藏着大量的碳储量，但海平面上升可能会影响其土壤碳封存能力以及潜在的甲烷（CH）排放能力。虽然海水入侵带来的硫酸盐负荷可能会由于微生物硫酸盐还原的更高能量产量而减少 CH 生产，但现有研究表明，可能还有其他因素在起作用。我们对跨越旧金山湾和三角洲自然盐度和生产力梯度的 11 个湿地复合体进行了研究，发现尽管 CH 通量通常随盐度下降而下降，但在寡盐湿地（约 3-ppt 盐度）中最高。产甲烷菌和产甲烷基因与 CH 通量弱相关，但单独解释不了观察到的最高速率。分类和功能基因数据表明，需要考虑影响碳氮循环的其他微生物类群，以便更好地预测景观尺度上的 CH 通量。在淡水边界附近发生的甲烷产量较高，伴有轻微盐化（和硫酸盐入侵），可能是由于硫酸盐还原发酵菌和共生种群的增加所致，这些菌群可以产生产甲烷菌使用的基质。此外，较高的盐度可以溶解这里所研究的低盐湿地土壤中丰富的离子结合态铵，这可能会抑制甲烷营养菌，并可能导致寡盐沉积物中观察到的更大 CH 通量。重要的是，低水平的海水入侵可能会增加潮汐淡水湿地的 CH4 通量，而较高水平的盐化可能会降低 CH4 通量。寡盐地区高 CH4 排放令人担忧，因为海水入侵会导致潮汐淡水湿地变成寡盐湿地。产甲烷基因本身并不能解释 CH4 通量的景观格局，这表明改变产甲烷作用、甲烷营养作用、氮循环和铵释放的机制，以及增加分解和共生细菌种群，可能会导致寡盐度下净 CH4 通量的增加。更好地了解这些因素对净 CH4 排放的影响，可以改善恢复工作，并更好地估算河口湿地的碳封存。与受农业活动影响的湿地相比，更原始的参考湿地可能具有更古老、更丰富的有机物质，其碳氮比更高，而且可能会出现盐度和 CH4 之间的不同相互作用。这一区别对于将生物地球化学过程相互作用扩展到河口湿地的建模工作可能至关重要。
