Regulation of desiccation-immersion cycle on the rate and fate of dissolved organic carbon release by intertidal macroalgae

Macroalgae widely distribute in intertidal zones, one of blue carbon organisms. However, the regulatory mechanisms of tide on the carbon sequestration of macroalgae are still unclear. This study explored the effects of desiccation-rewetting cycles induced by tide on dissolved organic carbon (DOC) release from Ulva pertusa, which is prevalent from high to low tidal zones. Results showed that the DOC release of U. pertusa varied with desiccation levels, releasing 0.082, 0.22, and 0.35 mg g^-1 at 0%, 40%, and 80% water loss, respectively. During desiccation stage, DOC accumulated on the surface of U. pertusa at a rate of about 0.52 mg g^-1h^-1. Following 4 h of rewetting, DOC released surges to 3.95, 10.05, and 8.41 mg g^-1. Using a stable isotope (¹³C) tracer method, we found that most DOC released by U. pertusa come from early fixed carbon. At 40% water loss, partial DOC stemmed from newly fixed carbon. DOC composition varied with desiccation level, affecting its bioavailability. After 16 days of degradation, DOC concentrations from U. pertusa at 0%, 40%, and 80% desiccation were 1.99, 3.22, and 2.54 mg g^-1, respectively. The 80% water loss showed the highest degradation rate, while the non-water-loss treatment group had the most potential to form refractory DOC. This study underlines the complex relationship between tide and the dynamics of DOC release in U. pertusa, highlighting their role in coastal carbon cycling.