Effect of marine anoxia on the conversion of macroalgal biomass to refractory dissolved organic carbon.

The input of macroalgal biomass into the deep sea is a crucial process for macroalgal carbon sequestration, but this process may be affected by anoxia. We compared the breakdown of kelp biomass in both normoxic (>4 mg/L O₂) and anoxic (<2 mg/L O₂) environments. Following 240 days of decomposition experiment, complete degradation of the kelp biomass occurred in normoxic conditions, whereas under anoxic conditions, relatively 13.58% residual biomass remained. Our results suggest that microorganisms facilitated the conversion of dissolved organic carbon (DOC) derived from kelp degradation into refractory dissolved organic carbon (RDOC), a process observed under both normoxic and anoxic conditions. However, different dissolved oxygen levels lead to different bacterial community successions, which affected the conversion process from labile dissolved organic carbon (LDOC) to RDOC differently. Bacteroidia, which possess sulfur metabolic capabilities, play a significant role in RDOC generation under both normoxic and anoxic conditions. In normoxic conditions, the relative abundance of CHO molecules was 2.57% less than that under anoxic conditions, whereas the proportions of CHON was 3.83% higher. Additionally, DBE_wa and Almod_wa values were 11.04% and 15.63% higher than those observed under anoxic conditions. At the end of the experiment, the relative content of RDOC under normoxic and anoxic conditions was 9.18% and 5.45%, respectively. Despite the reduced production of RDOC, anoxic conditions promote the preservation of a larger amount of macroalgae biomass. However, uncertainty exists regarding the extent to which stored POC reaches deep-sea sequestration. Consequently, it is challenging to assert that anoxia positively influences carbon sequestration in macroalgae.