Fermentative Bacteria Influence the Competition between Denitrifiers and DNRA Bacteria.

Area of Science:

• Biogeochemistry
• Microbial Ecology
• Environmental Microbiology

Background:

• Denitrification and dissimilatory reduction to ammonium (DNRA) are key microbial processes governing nitrogen cycling.
• The carbon source and electron donor significantly influence the competition between these two nitrate reduction pathways.

Purpose of the Study:

• To investigate the impact of varying lactate to nitrate (Lac/N) ratios on the competition between denitrification and DNRA.
• To elucidate the mechanistic basis of these microbial conversions under anoxic conditions.

Main Methods:

• Utilized an open chemostat culture enriched from activated sludge.
• Integrated results from batch tests, molecular biomass analysis, and a computational model.
• Investigated microbial communities under strict anoxia with varying Lac/N ratios.

Main Results:

• At high Lac/N ratios, both fermentative and respiratory DNRA occurred alongside lactate fermentation and oxidation.
• At lower Lac/N ratios, denitrification became coupled to lactate oxidation, reducing the proportion of DNRA.
• Identified specific bacterial groups, including *Geobacter* and *Clostridiales* members, likely responsible for DNRA and fermentation, and *Propionivibrio militaris* for denitrification.

Conclusions:

• The substrate ratio and electron donor type critically affect the competition between DNRA and denitrification.
• Fermentative bacteria play a key role by consuming electron donors, thereby influencing substrate availability for nitrate reducers.