Metabolic labour division trade-offs in denitrifying microbiomes.

Division of metabolic labour is a defining trait of natural and engineered microbiomes. Denitrification-the stepwise reduction of nitrate and nitrite to nitrogenous gases-is inherently modular, catalysed either by a single microorganism (termed complete denitrifier) or by consortia of partial denitrifiers. Despite the pivotal role of denitrification in biogeochemical cycles and environmental biotechnologies, the ecological factors selecting for complete versus partial denitrifiers remain poorly understood. In this perspective, we critically review over 1500 published metagenome-assembled genomes of denitrifiers from diverse and globally relevant ecosystems. Our findings highlight the widespread occurrence of labour division and the dominance of partial denitrifiers in complex ecosystems, contrasting with the prevalence of complete denitrifiers only in simple laboratory cultures. We challenge current labour division theories centred around catabolic pathways, and discuss their limits in explaining the observed niche partitioning. Instead, we propose that labour division benefits partial denitrifiers by minimising resource allocation to denitrification, enabling broader metabolic adaptability to oligotrophic and dynamic environments. Conversely, stable, nutrient-rich laboratory cultures seem to favour complete denitrifiers, which maximise energy generation through denitrification. To resolve the ecological significance of metabolic trade-offs in denitrifying microbiomes, we advocate for mechanistic studies that integrate mixed-culture enrichments mimicking natural environments, multi-meta-omics, and targeted physiological characterisations. These undertakings will greatly advance our understanding of global nitrogen turnover and nitrogenous greenhouse gases emissions.