Effects of light intensity on cylindrospermopsin production in the cyanobacterial HAB species Cylindrospermopsis raciborskii

Abstract

The role of light intensity on growth and the production of the hepatotoxin cylindrospermopsin (CYN) in the cyanobacterial harmful algal bloom species Cylindrospermopsis raciborskii was investigated using cultured isolates grown in N-free media under a series of neutral density screens. Maximum growth as indicated by chlorophyll a concentrations was measured at 75μmol photons m⁻² s⁻¹, whereas maximum intracellular and extracellular CYN concentrations occurred in cultures grown under the highest light intensity (140μmol photons m⁻² s⁻¹). During exponential growth phase, N-limited C. raciborskii cultures grown under light intensities of 18–75μmol photons m⁻² s⁻¹ exhibited a strong linear relationship between light intensity and both intracellular and extracellular CYN concentrations. Extracellular CYN concentrations increased significantly as the culture moved from log to stationary growth phase. These results indicate that the highest intracellular toxin concentrations in the field are likely to occur in C. raciborskii populations that have been actively growing at light intensities of 75–150μmol photons m⁻² s⁻¹ for more than two weeks, and that peak soluble toxin levels will be found at the end of a bloom.