Dark carbon fixation in intertidal sediments: Controlling factors and driving microorganisms

Dark carbon fixation (DCF) contributes approximately 0.77 Pg C y^-1 to oceanic primary production and the global carbon budget. It is estimated that nearly half of the DCF in marine sediments occurs in estuarine and coastal regions, but the environmental factors controlling DCF and the microorganisms responsible for its production remain under exploration. In this study, we investigated DCF rates and the active chemoautotrophic microorganisms in intertidal sediments of the Yangtze Estuary, using ¹⁴C-labeling and DNA-stable isotope probing (DNA-SIP) techniques. The measured DCF rates ranged from 0.27 to 3.37 mmol C m^-2 day^-1 in intertidal surface sediments. The rates of DCF were closely related to sediment sulfide content, demonstrating that the availability of reductive substrates may be the dominant factor controlling DCF in the intertidal sediments. A significant positive correlation was also observed between the DCF rates and abundance of the cbbM gene. DNA-stable isotope probing (DNA-SIP) results further confirmed that cbbM-harboring bacteria, rather than cbbL-harboring bacteria, played a dominant role in DCF in intertidal sediments. Phylogenetic analysis showed that the predominant cbbM-harboring bacteria were affiliated with Burkholderia, including Sulfuricella denitrificans, Sulfuriferula, Acidihalobacter, Thiobacillus, and Sulfurivermis fontis. Moreover, metagenome analyses indicated that most of the potential dark-carbon-fixing bacteria detected in intertidal sediments also harbor genes for sulfur oxidation, denitrification, or dissimilatory nitrate reduction to ammonium (DNRA), indicating that these chemoautotrophic microorganisms may play important roles in coupled carbon, nitrogen, and sulfur cycles. These results shed light on the ecological importance and the underlying mechanisms of the DCF process driven by chemoautotrophic microorganisms in intertidal wetlands.