Quantifying genome-specific carbon fixation in a 750-meter deep subsurface hydrothermal microbial community

Area of Science:

• Geomicrobiology
• Environmental Microbiology
• Deep Biosphere Research

Background:

• Microbial chemoautotrophy is crucial for deep subsurface carbon cycling.
• Dissolved inorganic carbon is a potential substrate for chemoautotrophic activity.
• Understanding carbon fixation pathways in deep environments is essential.

Purpose of the Study:

• To investigate the impact of dissolved inorganic carbon on microbial chemoautotrophy.
• To identify microbial populations and carbon fixation pathways stimulated by bicarbonate.
• To experimentally test the hypothesis of inorganic carbon acting as a biological sink.

Main Methods:

• Quantitative DNA stable isotope probing using 13C-labeled bicarbonate.
• Metagenome-assembled genomes (MAGs) analysis from deep subsurface hydrothermal fluids.
• Assessing microbial assimilation of 13C-labeled bicarbonate at varying concentrations (1-10 mM).

Main Results:

• Microbial community composition shifted with increasing bicarbonate concentrations.
• Four distinct carbon fixation pathways were identified in 13C-labeled MAGs.
• The Calvin-Benson-Bassham cycle dominated carbon fixation across all tested concentrations.
• Aquificae bacteria utilizing the reverse tricarboxylic acid (rTCA) pathway showed increased bicarbonate assimilation with higher concentrations.

Conclusions:

• Increased dissolved inorganic carbon concentrations can promote chemoautotrophy in deep subsurface ecosystems.
• The rTCA pathway, particularly in Aquificae, plays a significant role in utilizing elevated bicarbonate levels.
• This study provides experimental evidence for inorganic carbon acting as a biological sink in the deep subsurface.