Physiological limits to life in anoxic subseafloor sediment

Area of Science:

• Geomicrobiology
• Biogeochemistry
• Microbial Physiology

Background:

• Microbial cell densities decrease exponentially with depth in subseafloor sediments.
• The fermentation zone is characterized by a decline in microbial life.
• Understanding the physiological reasons for this decline is crucial.

Purpose of the Study:

• To investigate the physiological basis for the question: 'Why are cells dying faster than they are growing?' in subseafloor sediments.
• To analyze the efficiency of carbon conversion in subseafloor anaerobic feeding chains.
• To explain the exponential decline in cellular biomass with increasing depth.

Main Methods:

• Stoichiometric analysis of fermentative adenosine triphosphate (ATP) production and consumption.
• Evaluation of carbon conversion ratios in different microbial metabolic pathways (fermentation, hydrogenotrophic methanogenesis, acetoclastic methanogenesis).
• Thermodynamic assessment of amino acid fermentation at varying substrate and product concentrations.

Main Results:

• Carbon conversion ratios from dead cell protein to new microbial biomass are low, especially for methanogens (200:1 to 100:1) compared to fermenters (up to 6:1).
• Amino acid fermentation becomes more thermodynamically efficient at lower concentrations, but translation costs limit carbon conversion.
• Low carbon conversion factors in subseafloor food webs are the primary driver of biomass decline.

Conclusions:

• Inefficient carbon conversion within subseafloor anaerobic feeding chains explains the exponential decrease in cellular biomass with depth.
• A 'life-death transition zone' exists where biological processes cease and chemical reactions dominate.
• Physiological limitations on energy conversion and biomass synthesis dictate microbial survival and abundance in deep subseafloor environments.