Environmentally relevant decrease in temperature increases mcy gene transcription and cellular microcystin content in continuous cultures of Microcystis aeruginosa PCC 7806

Microcystins produced during harmful cyanobacterial blooms are a growing public health concern. Although trends have emerged, the environmental cues that stimulate production of microcystin remain confusing, hindering our ability to predict fluctuations in bloom toxicity. Growth at environmentally relevant cool temperatures (18 degrees C vs. 26 degrees C) was shown in earlier work to increase microcystin quota in Microcystis aeruginosa NIES-843. Here, we tested this response in M. aeruginosa PCC 7806. We used continuous cultures to examine the temporal dynamics of and RNA sequencing to investigate the physiological nature of the response. Growth at 19 degrees C vs. 26 degrees C increased microcystin quota ~2-fold, from an average of 464 ag micrometer-3 cell volume to 891 ag micrometer-3. This increase occurred over a 7-9 d period. Returning the temperature to 26 degrees C reduced quota to an average of 489 ag micrometer-3 over a period of 8 d. Long periods (31-42 d) at 19 degrees C did not increase or decrease microcystin quota from that observed at 7-9 d. Nitrogen concentration had little effect on the cold response. Decrease in temperature to 19 degrees C induced a classic cold stress response in Microcystis that seemed to operate at a different timescale than the increase in microcystin production. Microcystin quota showed a strong 48- to 72-h time-lag correlation to mcy gene expression, but no correlation to concurrent mcy expression. This work validates the effect of cool temperature on microcystin quota and improves our understanding of the physiological nature of the response.