Ecological dynamics explain modular denitrification in the ocean

Area of Science:

• Marine microbial ecology
• Biogeochemical cycles
• Biogeochemistry

Background:

• Marine oxygen minimum zones (OMZs) host microorganisms crucial for global biogeochemical processes.
• Multistep denitrification (NO3-→NO2-→NO→N2O→N2) in OMZs significantly impacts nitrogen loss and nitrous oxide (N2O) production.
• Current models often simplify denitrification as a single step, overlooking the prevalence of partial pathways (modules) in OMZ denitrifiers.

Purpose of the Study:

• To identify ecological mechanisms sustaining diverse denitrifiers in OMZs.
• To explain the prevalence of specific denitrification modules within OMZ microbial communities.
• To examine the implications of these microbial strategies for nitrogen loss and N2O production.

Main Methods:

• Developed an idealized OMZ ecosystem model incorporating microbial functional types.
• Described denitrifier modules based on redox chemistry and thermodynamic constraints.
• Applied pathway length penalties to model microbial growth yields and community succession.

Main Results:

• Microbial biomass yields increase along the denitrification pathway under organic matter limitation, explaining the survival of intermediate-metabolizing populations.
• Predicted denitrifier community succession correlated with environmental gradients (organic matter vs. nitrogen limitation).
• The model successfully explained the observed dominance and oxygen tolerance of the NO3-→NO2- module and identified NO3- as the primary N2O source.

Conclusions:

• Microbial ecology and functional diversity are critical for understanding nitrogen cycling in OMZs.
• The study provides a mechanistic framework for the relationship between microbial community structure and biogeochemical process rates.
• The findings advance our understanding of nitrogen loss and N2O production in OMZs and can be applied to other environments.