Compensatory respiration supports survival during extended heat stress in Microcystis aeruginosa.

Phototrophic organisms must balance energy between photosynthesis and respiration to survive. In cyanobacteria, this coordination is especially critical as both processes share the same membrane and electron transport complexes. Although electron exchange between the two pathways is known, photosynthesis is often presumed to dominate under many conditions. Here, under an extended heat-stress condition, our data indicate that respiration can compensate for reduced photosynthetic activity. We compared two Microcystis aeruginosa strains (PCC7806 and Lake Kinneret isolated strain C-1004). PCC7806 maintained viability while increasing dark respiration and sustaining a larger, active plastocyanin pool and turnover. C-1004 maintained higher photosystem II activity but showed reduced respiration and reduced biomass. Overall, the results suggest that respiratory compensation, rather than residual photochemical throughput, aligns with survival under this condition. Such differences could contribute to seasonal shifts in strain dominance and point to respiration as an important survival mechanism in cyanobacteria during heat stress.