Transcriptional responses to oxygen gradients in cyanobacterial aggregates.

Cyanobacterial aggregates (CAs) are the main cause of harmful cyanobacterial blooms in freshwater lakes, posing serious risks to water quality and ecosystem health. The ecological success of CAs is closely linked to their abilities to adapt to fluctuating dissolved oxygen (DO) levels. In this study, we investigated the transcriptional responses of CA-associated microbial communities across a gradient of DO concentrations (0-6 mg/L) using incubation experiments combined with 16S rRNA transcript and metatranscriptomic sequencing. Distinct transcriptional patterns of clusters of genes were revealed for both cyanobacterial and phycospheric communities. Notably, Microcystis, the dominating cyanobacteria in the CAs, demonstrated markedly elevated transcriptional activities under oxygen-deficient conditions. Under low DO, cyanobacteria actively cope with reactive oxygen species (ROS) stress, and utilized fermentation and O-independent alternative electron sink to maintain anaerobic metabolism. Upregulation of gas vesicle protein genes also suggests a role in buoyancy regulation to escape low-oxygen zones. These transcriptomic findings were further supported by physiological assays of Microcystis, which exhibited increased ROS level, extracellular polysaccharides (EPS) content and alcohol production under oxygen-deficient conditions. Moreover, intensified competition for nutrients between cyanobacteria and phycospheic bacteria under low DO were revealed, although the latter may also support cyanobacterial growth through cobalamin (vitamin B) provisioning. Collectively, our findings uncover key adaptive responses of Microcystis under oxygen-deficient conditions and underscore the importance of redox regulation in shaping the metabolic dynamics of CA-associated microbial communities.