Using metagenomic sequencing across a realistic temperature gradient, researchers show that carcass decay triggers a surge in carbon-degradation genes, while warming selectively favors pathways that rapidly consume easily degradable carbon.
Animal death and decomposition are natural but powerful drivers of nutrient release. Each year, large quantities of animal carcasses enter terrestrial and aquatic ecosystems, releasing carbon-rich fluids that alter water chemistry and microbial activity. Aquatic systems are especially important, accounting for more than half of global primary production and playing a central role in carbon fixation and degradation. Microorganisms regulate these processes through specialized "carbon cycling genes." While temperature is known to influence microbial metabolism, its combined effects with sudden carbon pulses—such as those from carcass decomposition—have remained poorly understood, particularly at the level of functional genes.
A study (DOI:10.48130/biocontam-0025-0012) published in Biocontaminant on 05 December 2025 by Huan Li's team, Lanzhou University, highlights how climate warming and sudden carbon inputs can interact to redirect microbial carbon cycling, with implications for greenhouse gas emissions and aquatic ecosystem health.
Using a controlled carcass–water microcosm experiment across five temperature gradients (23–35 °C), this study employed metagenomic sequencing to comprehensively characterize microbial communities and functional genes involved in aquatic carbon cycling, while integrating multivariate statistics, network analysis, and pathway reconstruction to identify key drivers and mechanisms. The analysis showed that microorganisms carrying carbon-cycling genes spanned bacteria, eukaryotes, viruses, and archaea, but bacteria overwhelmingly dominated the system (mean 99.81%), with Proteobacteria, Actinobacteria, and Bacteroidetes as the most abundant groups. Temperature and carcass decomposition jointly reshaped microbial community structure, enriching Acidobacteria, Actinobacteria, Chloroflexi, Spirochaetes, and Firmicutes under warming alone, while favoring Verrucomicrobia, Proteobacteria, and genera such as Novosphingobium, Acidovorax, and Nocardioides during carcass decay. Functionally, carcass treatments produced a unimodal alpha-diversity pattern of carbon-cycling KEGG orthologs (KOs), peaking near 30 °C, and significantly altered beta diversity, with enrichment of carbon-degradation pathways including reductive TCA-related routes, gluconeogenesis, and the ethylmalonyl pathway. Carbohydrate-active enzyme (CAZy) profiles were dominated by glycosyltransferases, with key genes (e.g., GT2, GT4, CBM50, GH23, GT51) and hundreds of differential CAZy genes enriched in carcass conditions. Rising temperature strongly reduced carbon-cycling gene diversity in uncontaminated water, whereas this effect was buffered in carcass-contaminated systems by high nutrient availability. Approximately half of all carbon-cycling genes and over one-third of CAZy genes were temperature-sensitive, but substrate specificity diverged: warming promoted degradation of complex carbohydrates in controls, while only simple carbohydrate ester degradation increased with temperature during carcass decay, indicating preferential use of readily available carbon. Total carbon increased by nearly 87% following carcass decomposition and emerged as a key driver linking physicochemical conditions to functional gene structure. Network and pathway analyses further revealed a carcass-driven shift toward carbon degradation and fermentation, characterized by enhanced acetate and ethanol production and suppressed methane oxidation and parts of carbon fixation, demonstrating that carbon degradation, rather than fixation, dominates aquatic carbon cycling during carcass decomposition under warming conditions.
These findings have important implications for predicting carbon fluxes under climate change. As temperatures rise, aquatic environments experiencing sudden carbon inputs—such as mass fish deaths, livestock carcass disposal, or wildlife mortality events—may shift toward faster carbon turnover and increased release of carbon dioxide or other greenhouse gases. Understanding which microbial genes respond to warming helps refine models of carbon cycling and informs ecosystem management, particularly for freshwater bodies vulnerable to eutrophication and pollution.
