Nutrient enrichment by high aquaculture effluent input exacerbates imbalances between methane production and oxidation in mangrove sediments

Frequent aquaculture activities introduce substantial nutrients into mangrove ecosystems; however, the impact of this nutrient enrichment on methane (CH₄) emissions and the associated microbial communities remains largely unexplored. In this study, we used the static chamber method, combined with 16S rRNA-based, metagenomic sequencing and binning techniques, to investigate the emission patterns of greenhouse gases (GHGs), with a particular focus on CH₄, in mangroves subjected to different levels of effluents. The results showed that the effluent input decreased the mineral protection of sediment carbon (C) pools and increased C loss by more than double. In particular, high effluent input increased CH₄ emissions by 243.3 %. Random forest analysis revealed that changes in methanogens were an important factor in explaining the variation of CH₄ emissions. Amplicon data showed that the proportion of methylotrophic methanogens increased after effluent input, and metagenomic binning further attributed this change to the adaptability of methylotrophic methanogens to the substances transporting by the effluent. The enhanced hypoxia in sediments resulting from effluent input promoted the transition of methanotrophic communities from aerobic to anaerobic types and made anaerobic oxidation of CH₄ more reliant on sulfur reduction rather than nitrate reduction. The PLS model further revealed that the nutrients brought by effluent input stimulated an increase in DOC content which induced an imbalance between CH₄ production and oxidation in sediments by facilitating methanogens but inhibiting methanotrophs, ultimately resulting in an increase in CH₄ fluxes. These findings underscore the significance of mangroves receiving effluent input as critical consequent reactors, highlighting the necessity to consider effects of high nutrient enrichment by aquaculture effluent input on GHG emissions and blue C potential in mangroves.