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

Abstract

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.