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Applied and Environmental Microbiology, March 2003, p. 1475-1481, Vol. 69, No. 3
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.3.1475-1481.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Effects of Light on the Microcystin Content of Microcystis Strain PCC 7806

Claudia Wiedner,1* Petra M. Visser,1 Jutta Fastner,2 James S. Metcalf,3 Geoffrey A. Codd,3 and Luuc R. Mur1

Institute for Biodiversity and Ecosystem Dynamics, Aquatic Microbiology, University of Amsterdam, 1018 VW Amsterdam, The Netherlands,1 Federal Environmental Agency, 14195 Berlin, Germany,2 School of Life Sciences, University of Dundee, Dundee DD1 4HN, United Kingdom3

Received 12 July 2002/ Accepted 3 December 2002

Many cyanobacteria produce microcystins, hepatotoxic cyclic heptapeptides that can affect animals and humans. The effects of photosynthetically active radiation (PAR) on microcystin production by Microcystis strain PCC 7806 were studied in continuous cultures. Microcystis strain PCC 7806 was grown under PAR intensities between 10 and 403 µmol of photons m-2 s-1 on a light-dark rhythm of 12 h -12 h. The microcystin concentration per cell, per unit biovolume and protein, was estimated under steady-state and transient-state conditions and on a diurnal timescale. The cellular microcystin content varied between 34.5 and 81.4 fg cell-1 and was significantly positively correlated with growth rate under PAR-limited growth but not under PAR-saturated growth. Microcystin production and PAR showed a significant positive correlation under PAR-limited growth and a significant negative correlation under PAR-saturated growth. The microcystin concentration, as a ratio with respect to biovolume and protein, correlated neither with growth rate nor with PAR. Adaptation of microcystin production to a higher irradiance during transient states lasted for 5 days. During the period of illumination at a PAR of 10 and 40 µmol of photons m-2 s-1, the intracellular microcystin content increased to values 10 to 20% higher than those at the end of the dark period. Extracellular (dissolved) microcystin concentrations were 20 times higher at 40 µmol of photons m-2 s-1 than at 10 µmol of photons m-2 s-1 and did not change significantly during the light-dark cycles at both irradiances. In summary, our results showed a positive effect of PAR on microcystin production and content of Microcystis strain PCC 7806 up to the point where the maximum growth rate is reached, while at higher irradiances the microcystin production is inhibited.

* Corresponding author. Present address: Leibniz Institute of Freshwater Ecology and Inland Fisheries, Alte Fischerhütte 2, 16775 Neuglobsow, Germany. Phone: 49 3308269963. Fax: 49 3308269917. E-mail: c.wiedner{at}igb-berlin.de.

Applied and Environmental Microbiology, March 2003, p. 1475-1481, Vol. 69, No. 3
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.3.1475-1481.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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