Microbial mechanisms of carbon sequestration in mangrove sediments

Mangrove ecosystems are efficient natural carbon (C) sinks and play key roles in mitigating global change. Microorganisms are crucial for maintaining ecosystem functions, thus understanding microbial mechanisms in C sequestration is essential for regulating the C sink function in mangrove ecosystems. To address this issue, we compared the microbially-driven C, nitrogen (N) and sulfur (S) cycling genes/pathways and their associated taxa, as well as mangrove-microbe interactions between native and introduced mangrove species with different C sequestration capacities. The native mangrove sediment showed a higher carbon sequestration capacity, characterized by significantly (P<0.05) higher total carbon and microbial necromass carbon content, along with lower methane emissions compared to the introduced mangrove sediment. The native mangrove sediment had lower alpha-diversity of methanogenic communities and higher abundances of methanotrophs, leading to reduced methane production and greater methane oxidation than the introduced mangrove sediment. Also, the native mangrove sediments exhibited higher functional potentials of chemoautotrophic C fixation, C degradation, N₂ fixation, S oxidation and sulfate reduction compared with introduced mangroves. The higher microbial necromass carbon in the native mangrove sediment was related with sulfur oxidation, and chemoautotrophic sulfur-oxidizing Burkholderiales could play a key role in the transformation of plant-derived to microbially-derived carbon. In addition, flavonoid catechin and sulfur-oxidizing Rhodobacteraceae were identified as key root exudate and microbial group dominating mangrove-microbe interactions in the mangrove rhizosphere, which could subsequently influence mangrove growth and total carbon content. Our findings highlighted the importance of microorganisms in C sequestration, and provides new strategies for microbiome engineering to enhance carbon sink functions in mangrove wetlands.