ABSTRACT: Seagrasses are widely recognized as contributing to net ecosystem primary production and to supporting heterotrophy in estuarine systems. We investigated the linkage between seagrass (Thalassia testudinum) rhizosphere carbon exudation and sediment bacteria. In microcosms, we simulated summer conditions and enriched the water column DIC (dissolved inorganic carbon) pool with ¹³C, then followed the tracer into the sediment porewater DOC (dissolved organic carbon) and the bacterial biomarkers (phospholipid fatty acids, PLFAs). Subsequently, we developed an inverse analysis of the seagrass microcosm system and calculated the flux of carbon between biological and geochemical compartments. After 18 d, the bacterial pool was enriched by about Δ +4‰, while the DOC pool was enriched by about Δ +50 to +60‰. We estimate that about 15 to 30% of gross primary production was exuded from the root/rhizome and that this accounts for about 41 to 61% of the carbon required by sediment bacteria. Mineralization of detrital seagrass leaf material or refractory seagrass DOC rhizodeposition may account for the other 38 to 59% of the bacterial carbon demand. We suggest that the sediment bacteria in the seagrass rhizosphere rapidly utilize labile DOC and, consequently, build up ¹³C label in the refractory DOC pool. Our results conclusively show a direct linkage between seagrass carbon exudation and sediment biogeochemical processes.