Quantifying population-specific growth in benthic bacterial communities under low oxygen using H₂¹⁸O

Area of Science:

• Environmental Microbiology
• Geochemistry
• Benthic Ecology

Background:

• Estuarine benthos frequently experiences hypoxia and anoxia, impacting biogeochemical cycles.
• The effect of oxygen depletion on uncultured microbial populations in these environments is poorly understood.

Purpose of the Study:

• To quantify the growth of diverse, uncultured bacterial populations in response to low oxygen in estuarine sediments.
• To elucidate the redox conditions influencing microbial growth and biogeochemical cycling.

Main Methods:

• Application of H218O quantitative stable isotope probing (qSIP).
• Incubation of estuarine sediments under varying redox conditions (hypoxia to euxinia) for 7 and 28 days.
• Analysis of 18O labeling patterns in bacterial populations and dsrB genes.

Main Results:

• Distinct 18O labeling patterns indicated micro-aerophilic, anaerobic, facultative anaerobic, and aerotolerant anaerobic growth.
• Anaerobic growth was phylogenetically non-random, with significant representation from uncultured clades (Planctomycetes, Actinobacteria, Latescibacteria, Verrucomicrobia, Acidobacteria).
• 18O labeling of dissimilatory sulfate reductase (dsrB) genes occurred only under euxinic conditions, with Acidobacteria identified as dominant sulfate-reducing bacteria.

Conclusions:

• This study provides the first experimental data linking specific redox conditions to the growth of previously uncultured microbial groups.
• Acidobacteria play a crucial role in estuarine sulfur cycling as sulfate-reducing bacteria.
• Findings validate previous metagenomic hypotheses regarding the ecological roles of microbial dark matter.