Fate and carbon sequestration potential of sunken macroalgae in coastal oceans from long-term microbial degradation perspective

ABSTRACT Although deep-sea macroalgae sinking as a carbon sequestration strategy remains controversial, natural sinking of massive macroalgae frequently occurs in coastal oceans. In the Yellow Sea, millions of tons of the macroalga Ulva prolifera sink to the seafloor annually following green tides, yet their ultimate fate and carbon sequestration potential remain poorly understood. Microbial communities play a crucial role in decomposing organic matter and determining the fate of sunken macroalgae. Our 2-year simulated microbial degradation of U. prolifera revealed that approximately 38% of the carbon in sunken macroalgal biomass was ultimately sequestered in various forms. Of this retained carbon, 10% was transformed into dissolved inorganic bicarbonate ions, enhancing seawater alkalinity and contributing to inorganic carbon storage. Meanwhile, 28% was transformed into recalcitrant dissolved/particulate organic carbon and algal detritus, consisting of degradation-resistant compounds rich in humic-like substances, polycyclic aromatic hydrocarbons and highly aromatic compounds. Metagenomic analysis showed that these transformations were driven by a coordinated microbial succession from r-strategists to K-strategists, mediated by a microbial carbon pump and a ‘microbially driven alkalinity pump’. Our findings suggest that large-scale sinking of U. prolifera holds substantial potential for long-term ocean carbon sequestration, contributing to stable carbon pools in both organic and inorganic forms.