Active carbon-fixing microbes and their role in carbon fixation in mangrove sediments

Mangroves are vital blue carbon ecosystems, yet the microbial drivers of carbon fixation in their soils remain poorly understood. Here, this study investigated the patterns of drivers carbon-fixing microbes and their functional genes across three representative mangrove bays in the Beibu Gulf of the South China Sea (Lianzhou Bay, Maowei Sea and Zhenzhu Bay) using an integrated geochemical and metagenomic approach. The findings showed that: (1) the distribution of total organic carbon (TOC) in mangrove soils was significantly influenced by tidal zonation and mangrove plants, with TOC content in the mid-tidal zone consistently exceeding that in adjacent mudflats by 1.5- to 2.3-fold (p < 0.01); (2) potential dominant carbon fixation pathways inferred from soil microbial communities may vary significantly across different areas, including chemolithoautotrophic taxa (e.g., Nitrospira, Thiobacillus), phototrophic cyanobacteria (e.g., Synechococcus, Cyanobium), and mixotrophic assemblages. Correspondingly, the relative abundances of key functional genes (e.g., narH, narG, fabB, oadB) exhibited significant differences among these bays; (3) environmental factors including salinity, nutrients, and heavy metals jointly influenced the accumulation of carbon fixation genes and their microbial hosts, collectively explaining 63.9% of community variation at the species level. This study provides a mechanistic understanding of microbial functional diversity that underpins carbon cycling in mangrove soils, offering quantitative insights for the conservation and management of blue carbon ecosystems under anthropogenic pressures.