Water Studies Centre, School of Chemistry, Monash University, Melbourne, Victoria, Australia.
School of Earth, Atmosphere & Environment, Monash University, Melbourne, Victoria, Australia.
Nat Microbiol. 2019 Jun;4(6):1014-1023. doi: 10.1038/s41564-019-0391-z. Epub 2019 Mar 11.
Permeable (sandy) sediments cover half of the continental margin and are major regulators of oceanic carbon cycling. The microbial communities within these highly dynamic sediments frequently shift between oxic and anoxic states, and hence are less stratified than those in cohesive (muddy) sediments. A major question is, therefore, how these communities maintain metabolism during oxic-anoxic transitions. Here, we show that molecular hydrogen (H) accumulates in silicate sand sediments due to decoupling of bacterial fermentation and respiration processes following anoxia. In situ measurements show that H is 250-fold supersaturated in the water column overlying these sediments and has an isotopic composition consistent with fermentative production. Genome-resolved shotgun metagenomic profiling suggests that the sands harbour diverse and specialized microbial communities with a high abundance of [NiFe]-hydrogenase genes. Hydrogenase profiles predict that H is primarily produced by facultatively fermentative bacteria, including the dominant gammaproteobacterial family Woeseiaceae, and can be consumed by aerobic respiratory bacteria. Flow-through reactor and slurry experiments consistently demonstrate that H is rapidly produced by fermentation following anoxia, immediately consumed by aerobic respiration following reaeration and consumed by sulfate reduction only during prolonged anoxia. Hydrogenotrophic sulfur, nitrate and nitrite reducers were also detected, although contrary to previous hypotheses there was limited capacity for microalgal fermentation. In combination, these experiments confirm that fermentation dominates anoxic carbon mineralization in these permeable sediments and, in contrast to the case in cohesive sediments, is largely uncoupled from anaerobic respiration. Frequent changes in oxygen availability in these sediments may have selected for metabolically flexible bacteria while excluding strict anaerobes.
可渗透（沙质）沉积物覆盖了大陆架的一半，是海洋碳循环的主要调节者。这些高度动态沉积物中的微生物群落经常在有氧和缺氧状态之间转换，因此与粘性（泥质）沉积物中的群落相比，分层较少。因此，一个主要问题是，这些群落如何在有氧-缺氧转变过程中维持代谢。在这里，我们表明，由于细菌发酵和呼吸过程脱耦，在缺氧后，分子氢（H）在硅酸盐砂沉积物中积累。原位测量表明，在这些沉积物上方的水柱中，H 的过饱和度是 250 倍，其同位素组成与发酵产物一致。基于基因组的鸟枪法宏基因组分析表明，这些沉积物中蕴藏着丰富多样且具有高度专业化的微生物群落，其中含有大量[NiFe]-氢化酶基因。氢化酶图谱预测，H 主要由兼性发酵细菌产生，包括优势的γ变形杆菌科 Woeseiaceae，并且可以被需氧呼吸细菌消耗。流动式反应器和泥浆实验一致表明，在缺氧后，H 会通过发酵迅速产生，在重新充气后立即被需氧呼吸消耗，只有在长时间缺氧时才被硫酸盐还原消耗。还检测到了氢营养型硫、硝酸盐和亚硝酸盐还原菌，尽管与之前的假设相反，这些沉积物中微藻发酵的能力有限。总的来说，这些实验证实了发酵在这些可渗透沉积物的缺氧碳矿化中占主导地位，与粘性沉积物的情况相反，发酵与厌氧呼吸在很大程度上是脱耦的。这些沉积物中氧气供应的频繁变化可能选择了代谢灵活的细菌，而排除了严格的厌氧菌。
