Particulate Organic Carbon Released during Macroalgal Growth Has Significant Carbon Sequestration Potential in the Ocean

Substantial amounts of particulate organic carbon (POC) are released during macroalgal growth; however, the fate of these POCs and their carbon sequestration effects remain unclear. Here, field investigations found that Ulva prolifera caused a significant increase of POC in seawater below the surface during a macroalgal bloom. However, laboratory simulations revealed that 77.6% of these POC was easily degraded by microorganisms in a short period of time, concurrently resulting in the production of dissolved organic carbon (DOC) from POC transformation. Over a period of 3 months, the bioavailable components of macroalgae-released POC and POC-transformed DOC were degraded, leaving 39.6% of the antibiodegradable substances composed of biorecalcitrant POC and biorecalcitrant DOC. However, although the biorecalcitrant POC was rich in humic-like components resisting biodegradation, the biorecalcitrant POC exhibited greater sensitivity to photodegradation than biorecalcitrant DOC. The photodegradation removal rate of biorecalcitrant POC (14.1%) was more than 10 times that of biorecalcitrant DOC (1.2%). Ultimately, a substantial portion (36.3%) of the POC released by growing macroalgae could potentially perform long-term carbon sequestration after conversion to recalcitrant POC and recalcitrant DOC, and these inert carbons derived from macroalgal POC have been previously ignored and should also be included in macroalgal carbon sequestration accounting.