Influence of simulated future climate CO2 scenarios on carbon allocation in keystone Cyanobacterium Synechococcus

Phytoplankton play a key role in aquatic carbon cycling. The partitioning of algal-derived carbon between particulate organic carbon (POC) and dissolved organic carbon (DOC), and its further fractionation within the DOC pool, determines the fate of fixed carbon. However, the regulatory mechanisms of phytoplankton carbon allocation under climate change remain insufficiently explored. Here, we investigated the carbon-partitioning response of a mode algae, Synechococcus to three carbon dioxide (CO ₂ ) conditions in different future climate contexts. It was found that when CO ₂ concentration increased to 850 ppm, the carbon allocation patterns did not have significantly changed compared to that under current CO ₂ concentration. In contrast, till the end of incubation, the extreme CO ₂ concentration of 1370 ppm led to significant decrease of 47.05% in the total organic carbon (TOC) concentration, with an increase of the concentration of extracellular DOC (DOC _ex ) and a reduction of POC. Notably, the final proportion of DOC _ex increased to 44.32% from 22.66% of the DOC fraction. Our observed changes in carbon allocation of Synechococcus under extreme CO ₂ concentrations suggests a reduced contribution of phytoplankton to aquatic carbon sequestration via the biological carbon pump through POC sinking, with a larger proportion of microalgal carbon being directly allocated to the surface DOC pool in the form of potentially recalcitrant DOC. Our study has important implications for accurately evaluating the role of phytoplankton in aquatic carbon sequestration in the context of future climate change.