The secretion of organics by living Microcystis under the dark/anoxic condition and its enhancing effect on nitrate removal

Recent studies indicated that the algal decomposition produces particulate and dissolved organic carbon (DOC), and can enhance denitrification in eutrophic lakes. However, the effects of the living cyanobacteria on nitrogen cycling in eutrophic lakes were still an unknown question. This study explores a new underlying mechanism of nitrate removal which is driven by living Microcystis. The results suggested that living Microcystis significantly enhanced the nitrate removal at sediment-water interface, with a nitrate removal rate of 0.54 d⁻¹, which was 2.57 times higher than the nitrate removal rate in the treatment without the addition of Microcystis. Measurements of Chl a and Fv/Fm confirmed that Microcystis was tolerant to the dark/anoxic condition, and the recovery experiments suggested that Microcystis could survive under such stress conditions for at least seven days. Meanwhile, DOC secreted by living Microcystis reached to 4.55 mg C mg⁻¹ Chl a. These secretions were biodegradable hydrophilic and contained carbohydrates and proteins. Our study indicated that during blooms, sinking Microcystis cells could directly provide DOC as carbon source, then consequently enhanced the denitrification at sediment-water interface, and the interactive relationship between living cyanobacteria and permanent nitrate removal should be taken into account while studying nitrogen cycling in aquatic ecosystem.