Anaplerotic processes are key contributors to dark carbon fixation in the ocean

Anaplerotic carbon fixation is ubiquitous in heterotrophic organisms including those inhabiting the ocean1. Despite its prevalence, the drivers of this process and its significance in ocean carbon cycling remain poorly understood2,3. Here we combined global ocean metatranscriptomic analysis, laboratory experiments on a bacterial model strain, and microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH) on marine microbial communities, to uncover the global prevalence of anaplerotic processes in oceanic dark dissolved inorganic carbon (DIC) fixation. Metatranscriptomic analysis revealed high expression levels of key anaplerotic genes, especially in mesopelagic waters, comparable to those of photo- and chemolithoautotrophic DIC fixation genes. Alteromonas emerged as the main contributor to anaplerotic DIC fixation gene expression, highlighting its role in DIC assimilation in the global ocean. Laboratory incubations with a marine Alteromonas representative confirmed their capability to fix DIC, which varied with organic matter availability and temperature. MICRO-CARD-FISH on oceanic samples revealed that Alteromonas contributed 0–40% (14 ± 16%, mean ± s.d.) to the dark DIC fixation in the pelagic ocean. Considering that Alteromonas is an obligate heterotroph lacking chemoautotrophic DIC fixation genes, its contribution to DIC fixation should be attributed to anaplerotic processes. Based on these results, we estimated a contribution of anaplerotic processes to dark DIC fixation of 0–0.5 C Pg y-1 in the global dark ocean. Yet, since Alteromonas is not the only taxon performing anaplerotic DIC fixation, our results represent a baseline conservative estimate. Collectively, our findings place anaplerotic DIC fixation as a relevant processes in the oceanic carbon cycling.