Physiological limits to life in anoxic subseafloor sediment
Physiological limits to life in anoxic subseafloor sediment
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DOI：
10.1093/femsre/fuaa004
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发表时间：
2020-02
影响因子：
11.3
通讯作者：
W. Orsi;B. Schink;W. Buckel;W. Martin
中科院分区：
文献类型：
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作者：
W. Orsi;B. Schink;W. Buckel;W. Martin
摘要在海底沉积物中，微生物细胞密度随着发酵区深度的增加呈指数下降。在这里，我们解决了一个经典的问题：为什么细胞死亡的速度快于它们的生长速度？从生理学的角度来看。发酵区发酵产生和消耗的ATP的化学计量取决于旧细胞生物量向新细胞生物量的转化。深海海底沉积物中大多数可发酵的有机物是来自死亡细胞的氨基酸，因为按重量计算，细胞大多是蛋白质。发酵死细胞蛋白的碳转化为产甲烷蛋白的比例分别为∼2 0 0：1和10 0：1，而发酵罐的转化率接近6：1。氨基酸发酵在较低的底物和产物浓度下热力学效率较高，但由于转化的能量成本较高，从死细胞蛋白转化为发酵剂蛋白的转化率较低。海底下厌氧饲养链中的低碳转化系数是缺氧沉积物发酵区细胞生物量指数下降的原因。我们的分析指出，生命-死亡过渡区的存在，在这个过渡区中，最后生物催化的生命过程被不再与生命耦合的纯粹的化学反应所取代。
ABSTRACT In subseafloor sediment, microbial cell densities exponentially decrease with depth into the fermentation zone. Here, we address the classical question of 'why are cells dying faster than they are growing?’ from the standpoint of physiology. The stoichiometries of fermentative ATP production and consumption in the fermentation zone place bounds on the conversion of old cell biomass into new. Most fermentable organic matter in deep subseafloor sediment is amino acids from dead cells because cells are mostly protein by weight. Conversion of carbon from fermented dead cell protein into methanogen protein via hydrogenotrophic and acetoclastic methanogenesis occurs at ratios of ∼200:1 and 100:1, respectively, while fermenters can reach conversion ratios approaching 6:1. Amino acid fermentations become thermodynamically more efficient at lower substrate and product concentrations, but the conversion of carbon from dead cell protein into fermenter protein is low because of the high energetic cost of translation. Low carbon conversion factors within subseafloor anaerobic feeding chains account for exponential declines in cellular biomass in the fermentation zone of anoxic sediments. Our analysis points to the existence of a life–death transition zone in which the last biologically catalyzed life processes are replaced with purely chemical reactions no longer coupled to life.