Redox potential drives divergent microbial carbon fixation in mangrove wetland sediments, with ammonium exerting...

Mangrove wetlands represent dynamic coastal interfaces where redox conditions and nutrient cycling shape microbial communities and their biogeochemical functions. However, tidal-driven siltation continuously transports sediment from low- to high-tide zones, altering sediment redox potential and nutrient content. The microbial responses to these changes, particularly the response mechanisms of carbon-fixing microorganisms, remain unclear. We integrated metagenomic and metatranscriptomic sequencing with ¹³C and ¹⁵N isotope labeling to examine how oxidation-reduction potential (ORP) and ammonium (NH₄⁺) availability regulate microbial assembly and metabolism in mangrove sediments. ORP emerged as the primary determinant of microbial composition and diversity, while NH₄⁺ exerted variable effects on microbial traits. Under high ORP, CBBL-microorganisms predominantly utilized the cmmG CO₂-concentrating mechanism, and carbon fixation rates decreased with increasing NH₄⁺ concentration. Under low ORP, CBBM-harboring genera dominated, primarily utilizing another mechanism cmmE, while NH₄⁺ had little effect and total organic carbon (TOC) exerted stronger control. ORP thus acts as the dominant environmental filter, with NH₄⁺ selectively affecting nitrifiers and carbon-fixing taxa. This was the first study to simultaneously measure ammonia oxidation and carbon fixation rate in mangrove sediments, revealing their mechanistic coupling. This work provides new mechanistic insights into the regulation of microbial metabolic potential in mangrove ecosystems and contributes to a broader understanding of their resilience and function in coastal biogeochemical cycles under fluctuating climate and environmental conditions.