This study presents new insights into autotrophic carbon fixation in shallow-water hydrothermal systems, emphasizing the dominance of reductive tricarboxylic acid (rTCA) cycle-utilizing Campylobacteria. Low isotopic fractionation values in vent fluids, possibly due to very acidic pH levels, extend the isotopic fractionation range of rTCA. The study also highlights the ecological significance of chemosynthetically fixed carbon, observed in higher trophic levels like the vent endemic crab Xenograpsus testudinatus, improving our understanding of carbon cycling in extreme environments and the interplay between microbial chemosynthesis and higher organisms.