Enhanced carbon storage in dissolved organic matter in a future oligotrophic ocean

Marine dissolved organic carbon (DOC) is one of the oceans largest carbon reser-voirs, which persists for millennia and exceeds the carbon stored in terrestrial and marine biomass combined (Hansell et al, 2012; Dittmar et al, 2021; Moran et al, 2022). Yet its response to climate change remains uncertain, largely because the ecological mechanisms behind its microbial degradation are poorly understood (Wagner et al, 2020; Legendre et al, 2015; Lonborg et al, 2020). Here we show how observed global-scale distribution of DOC emerges from bottom-up ecological controls acting on microbial DOC consumers. We combined large-scale metagenomic data (Larkin et al, 2021) with incubation experiments (Hale et al, 2017) to map global patterns of nutrient limitation in heterotrophic microbial communities responsible for DOC uptake. We then integrated a new microbial-DOC component into an Earth system model, which successfully reproduced observed global distributions of nutrient stress and DOC concentrations. Linking carbon and nutrient cycles reveals quantitatively significant consequences for the future ocean. Under a highemission scenario (SSP5-8.5), the DOC pool is projected to increase by 18-44 gigatons by 2200. This increase, driven by intensified nutrient limitation in surface waters, contributes substantially to the biological carbon pump, accounting for about[~] 30% of the increase in deep-ocean carbon storage associated with particulate organic carbon export. Overall, our results indicate that the marine DOC reservoir is more dynamic than previously thought. Reduced DOC remineralisation in an increasingly oligotrophic ocean constitutes a quantitatively significant negative feedback on centennial timescales.