Deciphering the hidden defensive behavior and binding mechanism of anammox process to microcystin from the perspective of granular properties, intracellular metabolism and molecular docking

Microcystin-LR (MC-LR), released during algal blooms, poses severe threats to human beings and ecosystem because of its high persistence and toxicity. Due to anammox bacteria's vulnerability to adverse environmental factors, and the impact of MC-LR on anammox should not be overlooked. In this work, the response mechanism of anammox to MC-LR was dissected from macroscopic to microscopic level. Spectroscopy and rheological tools demonstrated that hydrogen bond force provided by extracellular polymeric substances (EPS) and relatively stable yield stress of sludge endowed anammox granules with ability to resist in deformation for maintaining nitrogen metabolism. Anammox process can adaptively against MC-LR at biology level. At 1 mg/L of MC-LR, the relative abundance of functional microorganism (Candidatus Brocadia and Candidatus Kuenenia) and core gene (hzs and hdh) were increased for self-maintenance, while the abundance of denitrifying bacteria was increased by 7.72 % at 3 mg/L of MC-LR to prevent further collapse of nitrogen metabolism. Notably, high concentration of MC-LR with degradation-resistant property would penetrate EPS barrier to disturb intracellular anti-oxidation balance and attack respiratory chain, thus directly disrupting nitrogen transformation and electron transfer, which led to a 25.6 % decrease in the nitrogen removal efficiency. Computational chemistry further demonstrated that MC-LR conjugated intracellular biomolecules to affect the biological functions. This work provides the insights into the fate and potential risk of MC-LR to anammox process.