CYANOBACTERIAL TOXINS - Cyanobacteria - page 556

River can be seen in Fig. 23. Toxic blooms of Anabaena , Aphanizomenon , Microcystis , Oscillatoria and
Nodularia have been reported from Canada (Carmichael, 1992). Detectable limits of MC-LR greater
than 1 µg g -1 dry weight were found in 70% of the samples collected from 19 lakes of Alberta
(Hrudey et al ., 1994). However, MC-LR concentrations of phytoplankton samples from lakes and
dugouts ranged from 4 to 605 µg g -1 dry weight (Kotak et al ., 1993). Maximum values of MC-LR
exceeding 1500 µg g -1 dry weight were also reported (Hrudey et al ., 1994; Zurawell et al ., 1999). The
occurrence of blooms of M . aeruginosa in drinking water lakes of Canada with toxin levels much
higher than permissible limits in both natural and treated waters has been reported (Gupta et al .,
2001). Cyanobacterial abundance and toxicity were predicted on the basis of studies conducted
on 22 lakes of southern Qubec, Canada. Three important fi ndings emerged out of these studies.
Firstly, the biomass of the phytoplankton increased linearly with increase in total phosphorus
concentration. Secondly, the biomass of the toxic genera, i.e. Microcystis and Anabaena correlated with
the MC concentration. Thirdly, the most important factor appeared to be epilimnetic nitrogen that
is responsible for the biomass of toxic species. Moreover, the level of MC per unit biomass did not
vary signifi cantly among the lakes (Giani et al ., 2005). Further studies on four eutrophic lakes in the
eastern parts of Qubec (Canada) were chosen for a study of seasonal changes in the composition of
cyanobacterial community. MC content was determined using PP-inhibition assay by using extracts
of lyophilized plankton. Three lakes showed maximum toxicity in summer while the fourth showed
maximum toxicity during spring due to the abundance of genera Microcystis and Anabaena (Rolland
et al ., 2005). In a study on Lake Erie, Rinta-Kanto (2005) observed the persistence of toxic blooms of
Microcystis responsible for MC-LR equivalents exceeding 1 µg L -1 , the safety limit set by WHO. By
applying a combination of molecular probes and techniques of real-time PCR assay utilizing specifi c
Primer-Taq Man probe, Microcystis -specifi c 16S rDNA fragment and a microcystin toxin synthetase
Figure 23: Bloom of Microcystis aeruginosa and Anabaena circinalis on the St. Johns River, Florida, USA. Picture courtesy John
Burns (Cyanolab), Mark Schneegurt (Wichita State University) and Cyanosite (www.cyanosite.bio.purdue.edu).
Color image of this figure appears in the color plate section at the end of the topic.