Sustained algal-bacterial interactions promote accumulation of recalcitrant particulate carbon with accelerated radiocarbon aging signatures.

Algal-bacterial interactions regulate the production and fate of marine particulate organic carbon (POC), yet their capacity to generate long-lived carbon sinks remains unclear. Using a two-year, nutrient-self-sustaining co-culture of Synechococcus and its mutualistic microbiota, we demonstrate that sustained algal-bacterial interactions promote the progressive accumulation of recalcitrant POC. Continuous release of algal-derived organic substrates, coupled with repeated microbial enzymatic reworking, produced inert organic particles, with ~26% resisting remineralization and forming a stable recalcitrant particulate carbon pool. Radiocarbon (Δ14C) analyses revealed an apparent age offset exceeding 400 years for POC formed within only two-year experiment. This "aged" signature likely results from algal assimilation of fossil-fuel-14C-depleted atmospheric CO2 followed by accumulation in generated recalcitrant POC. By demonstrating that apparent radiocarbon age can become decoupled from actual residence time, these results provide insights into the interpretation of "old" marine POC and highlight the important roles of algal-bacterial interactions in ocean particulate carbon sequestration.