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.
