Lipid biomarkers indicate the dynamics of particulate organic carbon and its carbon sequestration effects during the degradation of Ulva prolifera.

Millions of tons of Ulva prolifera sink to the seafloor and gradually degrade after green tide occurred annually in the Yellow Sea, releasing substantial amounts of particulate organic carbon (POC) into marine environments. However, monitoring the dynamics of macroalgae-derived POC and its carbon sequestration effects is challenging due to severe environmental disturbances. Here, we conducted a long-term simulated degradation experiment with U. prolifera in the laboratory. During degradation, 86-90 % of U. prolifera-derived POC was readily degraded by microorganisms, while 10-14 % was stabilized in seawater as bio-recalcitrant POC. Microbial community structure underwent significant succession, driving the degradation of U. prolifera and the release and transformation of POC. 28-isofucosterol and POC concentrations changed concurrently and showed a significant positive correlation throughout the degradation. Hence, we propose that lipid biomarkers, i.e. 28-isofucosterol, can be used to track the release of U. prolifera-derived POC and to potentially reveal its carbon sequestration in marine environments.