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.

中文翻译：

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缺氧海底沉积物生命的生理极限。

在海底沉积物中，微生物细胞密度随着进入发酵区的深度呈指数下降。在这里，我们解决经典的问题：“为什么细胞死亡快于细胞生长？” 从生理学的角度来看。发酵区中发酵ATP产生和消耗的化学计量关系限制了旧细胞生物质向新细胞的转化。海底深海沉积物中最可发酵的有机物是死细胞中的氨基酸，因为细胞中的蛋白质多数是重量。通过氢营养和乙酰碎裂的甲烷生成，碳从发酵的死细胞蛋白转化为产甲烷蛋白的比例分别为〜200：1和100：1，而发酵罐的转化率可达到6：1。在较低的底物和产物浓度下，氨基酸发酵在热力学上变得更有效，但是由于翻译的高能成本，碳从死细胞蛋白到发酵罐蛋白的转化率很低。海底厌氧进料链中的低碳转化因子解释了缺氧沉积物发酵区细胞生物量的指数下降。我们的分析指出，存在一个生命死亡过渡区，其中最后的生物催化的生命过程被不再与生命相关的纯化学反应所取代。海底厌氧进料链中的低碳转化因子解释了缺氧沉积物发酵区细胞生物量的指数下降。我们的分析指出，存在一个生命死亡过渡区，其中最后的生物催化生命过程被不再与生命联系在一起的纯化学反应所取代。海底厌氧进料链中的低碳转化因子解释了缺氧沉积物发酵区细胞生物量的指数下降。我们的分析指出，存在一个生命死亡过渡区，其中最后的生物催化的生命过程被不再与生命相关的纯化学反应所取代。