Dark carbon fixation in tropical-subtropical lagoon and bay regional intertidal sediments under the effect of vegetation: controlling factors and driving microorganisms

Coastal blue carbon ecosystems dominated by mangroves, salt marshes and seagrass beds are conventionally recognized as major carbon sources. Dark carbon fixation (DCF) by sediment chemoautotrophs also substantially contributes to sediment organic carbon (SOC), particularly in tropical and subtropical zones. But how vegetation types and regional differences could affect DCF remains poorly understood. Here, we integrated sediment biogeochemical assays, 16S rRNA amplicon sequencing, PCR and qPCR analyses to investigate microbial communities and Calvin-Benson-Bassham (CBB) cycle key genes (cbbL and cbbM) in rhizosphere and non-rhizosphere sediments across two tropical lagoons (Li’an Port, Xincun Bay) and one subtropical semi-enclosed bay (Tieshan Port). Our results revealed that the sediment inorganic carbon (SIC) content reached 13.42 g kg-1 in seagrass and mangrove rhizosphere sediments, significantly higher than 1.48 g kg-1 in non-rhizosphere sediments (one way ANOVA, P < 0.05), while SOC contents showed no significant discrepancy (0.68 vs 0.73 g kg-1, one way ANOVA, P > 0.05), suggesting vegetation strongly regulates SIC distribution. The dark carbon fixation rate ranged from 0.31 to 0.72 mmol C m-2 d-1 in surface rhizosphere sediments and 0.46 to 0.69 mmol C m-2 d-1 in non-rhizosphere areas, with no statistical difference between the two habitats (one way ANOVA, P > 0.05). Sulfur-oxidizing Proteobacteria were the primary chemoautotrophs within our sampling results. γ-Proteobacteria carrying cbbL dominated lagoon environments, while α- and β-Proteobacteria associated with cbbM prevailed in Tieshan Port. In lagoon rhizosphere sediments, cbbL abundance showed stronger environmental correlations than cbbM, and such difference was absent in bays and unvegetated regions. Mixed seagrass beds increased SOC storage relative to single-species habitats but suppressed cbbL and cbbM abundance. Moreover, anthropogenic interference notably lowered sediment cbbM abundance and weakened the DCF potential of vegetated sediments. This study clarifies DCF patterns and driving factors in tropical-subtropical coastal regions, providing theoretical support for precise ecosystem conservation and blue carbon management.