Mangroves form the basis of coastal wetlands, where land, sea, and sky meet. Beneath their tangled roots, tiny microbes in the mud are hard at work capturing and storing large amounts of carbon from the atmosphere. Many do this through dark carbon fixation (DCF)—a process like photosynthesis but taking place in the dark. And now, scientists are shedding new light on the microbes and functions involved at different depths in the sediment. The team examined sediment samples from 3 mangrove wetlands in Fujian Province, China, finding high variation in DCF rates according to depth and environmental factors like dissolved inorganic carbon, total sulfur, ammonium, nitrate, and nitrite. Distinct bacterial groups were found to be key players in the different sediment layers: At the surface, Gammaproteobacteria drove DCF, mainly through sulfur and hydrogen oxidation with oxygen reduction, while Campylobacteria and Desulfobacteria were more important in the middle layers, and methane-producing Methanosarcinia archaea played the greatest role at the greatest sampled depth. These findings provide crucial new insight into the patterns and processes driving DCF in mangrove wetlands, improving our understanding of the microbial impacts on carbon cycling in the world’s oceans.