Microcystin aids in photo-acclimation during prolonged cold stress treatment in Microcystis aeruginosa strain PCC7806

Microcystis aeruginosa PCC7806 has been shown to increase toxin production in response to prolonged cold stress. To test whether this increase in toxin production is a beneficial adaptation that aids in cold fitness, the non-toxic mutant Microcystis aeruginosa PCC 7806 ΔmcyB was grown in duplicate chemostat cultures alongside chemostats containing the wildtype isolate. Strains reached steady-state at 26℃, then were subjected to an 11-day shift to 19℃, followed by a reversion back to 26℃. Throughout the experimental period, samples were collected to measure cell abundance, excitation pressure, effective quantum yield, microcystin production, reactive oxygen species (ROS) formation, and for RNA-seq analyses. During the 19℃ period, the mutant experienced elevated excitation pressure and ROS formation relative to the wildtype strain. Microcystin quota doubled in the wildtype strain by day 7 of the 19℃ treatment, followed by decreased effective quantum yields 24 h later. This was not observed in the mutant strains. Both mutant and wildtype populations began to recover after 8 days at 19℃. Microcystin did not aid in growth recovery during cold stress, although it seemed to play a part in the photo-acclimatory process based on excitation pressure and quantum yield readings.