Filamentous green algae Spirogyra regulates methane emissions from eutrophic rivers.

Excessive growth of filamentous green algae in rivers has attracted much attention due to their functional importance to primary production and carbon cycling. However, comprehensive knowledge of how filamentous green algae affect carbon cycling, especially the CH emissions from river ecosystems, remains limited. In this study, incubation experiments were conducted to examine the factors regulating CH emissions from a eutrophic river with dense growth of filamentous green algae Spirogyra through combinations of biogeochemical, molecular biological, and stable carbon isotope analyses. Results showed that although water dissolved oxygen (DO) in the algae+sediment (A+S) incubation groups increased up to 19 mg L, average CH flux of the groups was 13.09 μmol m day, nearly up to two times higher than that from sediments without algae (S groups). The significant increase of sediment CH oxidation potential and methanotroph abundances identified the enhancing sediment CH oxidation during Spirogyra bloom. However, the increased water CH concentration was consistent with depleted water [Formula: see text] and decreased apparent fractionation factor (α), suggesting the important contribution of Spirogyra to the oxic water CH production. It can thus be concluded that high DO concentration during the algal bloom promoted the CH consumption by enhancing sediment CH oxidation, while algal-linked oxic water CH production as a major component of water CH promoted the CH emissions from the river. Our study highlights the regulation of Spirogyra in aquatic CH fluxes and will help to estimate accurately CH emissions from eutrophic rivers with dense blooms of filamentous green algae. Graphical abstract.