Small Suspended Particulate Organic Carbon from Kelp Farming: A Neglected Pathway for Microbially Persistent Carbon Retention

Seaweed farming is increasingly recognized for its potential role in ocean carbon dioxide removal, yet the fate of small suspended particulate organic carbon (sPOC, 0.7-20 μm) released during macroalgal growth remains poorly constrained. Here, we investigated sPOC production, transformation, and microbial persistence in Sanggou Bay, China, one of the world's most intensive Saccharina japonica cultivation systems. During the farming season, sPOC concentrations increased by 103.4% and 117.0% in surface and bottom waters, respectively, relative to the nonfarming period, accompanied by shifts in sPOC molecular composition toward kelp-derived signatures. In situ mesocosm experiments showed increasing sPOC release with kelp development, reaching 13.4-63.5 μmol L-1 over 64 h across growth stages. The 180-day microbial incubations revealed that 18.1-55.6% of kelp-derived sPOC was retained in three microbially persistent carbon pools: recalcitrant sPOC (R-sPOC; 7.6-27.3%), recalcitrant dissolved organic carbon (4.6-9.7%), and bicarbonate-associated inorganic carbon (5.9-18.6%). Residual R-sPOC exhibited molecular characteristics associated with enhanced microbial resistance, with formula-level evidence of direct kelp release and microbial transformation (48.2% and 51.8%, respectively). These findings identify sPOC as a neglected, mechanistically distinct carbon-retention pathway in seaweed farming and provide a basis for assessing its potential contribution to longer-term blue carbon sequestration.