Enhanced carbon sequestration in marginal seas through bacterial transformation

Labile organic carbon is a highly dynamic component of the marine carbon pool, traditionally thought to be respired within hours to days into carbon dioxide (CO₂) by bacteria, although there is a paucity of direct observational evidence. Here, we report that a significant portion of labile particulate organic carbon (POC) in marginal seas is converted into bacterial material. By exploiting D/L-amino acids, we trace the origins of labile POC and its transformation into bacterial POC in the marginal seas off Eastern China. Our results indicate that labile POC primarily originates from autochthonous primary production, with bacterial POC fractions closely paralleling those of labile POC. It appears that rapid bacterial POC transformation is driven by enhanced bacterial growth efficiency from abundant nutrients in marginal seas. We estimate that around 0.08 ± 0.03 Pg of bacterial organic carbon is buried annually in global marginal seas, accounting for ∼40 % of total organic carbon burial, thus contributing to long-term carbon sequestration. These findings highlight the critical role of bacterial transformation in carbon sequestration within marginal seas and provide a potential mechanism for the observed increase in CO₂ uptake in coastal regions.