Abstract

In this study, we monitored the formation of cyanobacterial hyperscum and crust formation in Lake Hartbeespoort using satellite images and ground monitoring. The hyperscum that formed near the reservoir wall was characterised by a distinctive white surface layer of crust. Hyperscum is the result of exposure of the cells to high radiation, inflicting irreversible damage to the genetic constitution of the upper layer of Microcystis aeruginosa cells. Under the 3 mm thick layer of crust, dark (<0.93 µmol of photons m^-2s^-1) anaerobic conditions (0.4 mg/l, 3% saturation) prevailed with high levels of microcystin (12,300 µg/l) in the absence of sunlight irradiation and photolysis by UV light. Real time polymerase chain reaction (PCR) analysis indicated low levels of transcription of the mcyA, mcyB and mcyD genes which are responsible for synthesis of cyanotoxins under these low light intensity conditions. At other sampling sites where cyanobacterial scum occurred and hyperscum crust was absent, only the mcyB and mcyD genes were transcribed. A plausible explanation for the transcription of the mcyA gene in the hyperscum and not at the other sampling sites, was the presence of environmental stress-inducing factors, e.g. low light intensity (0.93 µmol of photon m^-2 s^-1) and pH 6.1. At the sampling site where no cyanobacterial scum was visible on the satellite images, low cell abundance (2.4 x 10⁴ µg/l) and chlorophyll a (12.2 µg/l) was measured in comparison with sites where cyanobacterial scum was visible on the satellite images.

Key words: Hyperscum crust, reverse-transcription PCR, mcyA levels, microcystin, satellite imaging, cyanobacteria.

Abbreviation

Abbreviation: ELISA, Enzyme-linked immunosorbent assay; chl-a, chlorophyll a;PCR, polymerase chain reaction.