High Ca2+ concentrations enhance Microcystis colony formation through upregulating polysaccharide-, energy metabolism-, and transmembrane transport-related pathways

Abstract

Colony formation plays a critical role in Microcystis blooms. Previous studies have demonstrated that high Ca²⁺ concentrations can bring about the rapid aggregation of Microcystis aeruginosa, leading to the formation of colonies with morphologies resembling those observed in the wild over extended periods. However, the mechanisms through which high Ca²⁺ levels enhance colony formation remain inadequately understood. This study investigated the impact of Ca²⁺ concentrations on M. aeruginosa colony formation and elucidated the underlying mechanisms. The results indicated that high Ca²⁺ concentrations (≥50 mg/L) significantly enhanced colony formation, with an increase in colony size observed as Ca²⁺ concentrations rose within the range of 1–400 mg/L. In addition, the cell surface hydrophobicity and zeta potential increased with Ca²⁺ concentrations, primarily due to the augmented secretion of extracellular polymeric substances (EPS) and compression of the double electric layer. This decreased interaction energy among Microcystis cells and facilitated colony formation. The energy barrier decreased from 2684.35 KT to 123.64 KT as the Ca²⁺ concentration rose from 10 mg/L to 400 mg/L, indicating a significantly greater propensity for aggregation in the 400 mg/L Ca²⁺ group compared to the control. Additionally, this study found that high Ca²⁺ concentrations upregulated extracellular polysaccharide-, transmembrane transport-, and energy metabolism-related pathways while downregulating photosynthesis-related pathways. This enhanced polysaccharide content in EPS and ultimately promoted colony formation. These findings provide new insights into the role of elevated Ca²⁺ concentrations in Microcystis colony formation, contributing to the advancements in the knowledge of cyanobacterial bloom mechanisms.