Denitrification Modularity and Its Environmental Controls in Transiently Versus Permanently Anoxic Marine Systems

Denitrification is a modular process that is mediated by an assemblage of microbes with varying denitrification gene combinations. The controls on these gene combinations, known as modularity, are poorly understood and marine observations are mostly limited to permanently anoxic systems. In this global metagenomic analysis representing 69 water column metagenome samples we report different modularity patterns associated with environmental parameters based on the permanence of anoxia. Thermodynamic favourability alone is not enough to explain the distribution of modularity patterns. Instead, variables such as the permanence (or transience) of anoxia, oxygen availability, biogeography and ratios of organic matter to nitrogen supply all help shape the denitrifier community gene assemblage. Environmental correlates in transiently anoxic compared to permanently anoxic systems suggest that the pressures of a more complex environment may favour shorter pathways due to resource allocation trade-off regardless of organic matter availability. Nitrate reduction is the dominant step compared to the rest of the denitrification pathway irrespective of anoxia type. As increases in global temperature result in more seasonally anoxic and hypoxic waters, these results highlight the importance of understanding the controls on denitrification modularity under varying states of anoxia.