Extracellular chemosymbiont populations in a shallow-water thyasirid clam potentially shaped by priority effect

Chemosynthetic animal symbioses are common in marine ecosystems but remain overlooked as contributors to global carbon fixation. We show that the shallow-water thyasirid clam Thyasira tokunagai, dominant in Yellow Sea sediments, harbors sulfur-oxidizing Sedimenticola symbionts with remarkably consistent genomic contents and functionality across the region, showing active Calvin cycle gene expression and close-knit host-symbiont metabolic integration. Field surveys demonstrated densities up to 2015 individuals{middle dot}m{square}2, while radiocarbon tracing revealed assimilation rate constants (0.002-0.005 day{square}{superscript 1}) peaking at 14.8{degrees}C. Spatial modelling combining abundance and temperature estimated a carbon fixation of 0.89 Tg C{middle dot}yr{square}{superscript 1} in Yellow Sea, equivalent to 43% of the annual sedimental C burial from the Chinese coast. The species complex that includes T. tokunagai is widely distributed and constitutes a globally significant, previously unaccounted blue carbon sink. Our findings underscore the crucial role of shallow-water chemosymbioses in carbon cycling, emphasising the importance of incorporating them into climate models and conservation strategies focused on carbon sequestration.