Elucidating the Competition between Heterotrophic Denitrification and DNRA Using the Resource-Ratio Theory

Heterotrophic denitrification and
dissimilatory nitrate reduction
to ammonium (DNRA) are two microbial processes competing for two shared
resources, namely, nitrate and organic carbon (COD). Their competition
has great implications for nitrogen loss, conservation, and greenhouse
gas emissions. Nevertheless, a comprehensive and mechanistic understanding
of the governing factors for this competition is still lacking. We
applied the resource-ratio theory to study this competition and validated
the theory with experimental data from continuous cultures reported
in the literature. Based on this theory, we revealed that influent
COD/N ratio alone was not sufficient to predict the competition outcome
as the boundary values for different competition outcomes changed
substantially with influent resource concentrations. The stoichiometry
of the two processes was determinative for the boundaries, whereas
the affinity for the shared resources (KS), maximum specific growth rate (μmax) of the two
species, and the dilution rate had significant impacts as well but
mainly at low influent resource concentrations (e.g., <100 μM
nitrate). The presented approach allows for a more comprehensive understanding
of the parameters controlling microbial competition. The computational
comparison between continuous and batch cultures could explain seemingly
conflicting experimental results as to the impact of the COD/N ratio.
The results also include testable hypotheses and tools for understanding
and managing the fate of nitrate in ecosystems, which could also be
applied more widely to other species competing for two shared resources.