The diel light cycle modulates microcystin content in continuous cultures and natural communities of Microcystis: Implications for bloom monitoring

Microcystis aeruginosa PCC 7806 microcystin concentrations were significantly higher at the end of the dark period (64 ± 8 g MC L-1; 630 ± 40 ag MC m-3) than at the end of the light period (45 ± 7 g MC L-1; 390 ± 60 ag MC m-3).

• Why it matters: Monitoring programs often compare microcystin across sampling times, but how diel light variation affects these estimates is poorly resolved, risking misinterpretation of bloom toxin levels.
• What they did: Axenic Microcystis aeruginosa PCC 7806 was grown in cyclostats under a 16 h light:8 h dark cycle, and microcystin was measured at end-of-light versus end-of-dark; a Lake Erie bloom community was exposed to 60 min sunlight versus paired dark controls.
• The result: Microcystin showed a reproducible diel pattern without changes in cell abundance or photosynthetic efficiency, and sunlight exposure reduced microcystin by ~19% after 60 min (p = 0.048), implying time-of-day standardization is needed for monitoring.