Release and bioavailability of dissolved organic carbon derived from Fucus serratus and Zostera marina

The release and bioavailability of dissolved organic carbon (DOC) from widely distributed marine macrophytes such as Fucus serratus and Zostera marina remains inadequately quantified despite their potential significant contribution to carbon cycling and longer-lived carbon storage. Long-term incubation of macrophyte fragments (85 d) in flow-through chambers, which maintained continuous water exchange and reduced artefacts associated with closed incubations, revealed significant temporal dynamics in both the quantity and quality of DOC released. Microbial degradation of DOC was evident throughout the experiment, indicating the sustained release of bioavailable compounds by both species. However, the proportion of bioavailable DOC and the bacterial growth efficiency associated with its consumption decreased with macrophyte senescence and changes in physiological state. Despite these temporal trends, the overall relationship between DOC concentration and its bioavailable fraction indicated that across all samplingtime points, the microbial community was able to consume ~40% of the DOC released, while a considerable proportion (~60%) remained recalcitrant throughout the experimental period. The recalcitrant DOC pool from F. serratus and Z. marina equated to 13 and 11% of their total carbon biomass, respectively, indicating considerable potential for longer-lived carbon storage in the ocean.