“…On the other hand, environmental factors can adjust and control the expression of microcystin genes. Several field studies have shown that certain environmental factors are associated with the amount of toxins found in cyanobacterial blooms (Wicks and Thiel, 1990;Zheng et al, 2004). A wide range of laboratory studies also have examined the effects of various environmental factors on microcystin production, including trace metal supply (Lukac and Aegerter, 1993;Utkilen and Gjølme, 1995), nitrogen and phosphorus (Sivonen, 1990), light and temperature (van der Westhuizen and Eloff, 1985), pH (de Maagd et al, 1999), culture medium (Song et al, 1998), water column stability (Huisman et al, 1999) and grazing pressure by zooplankton (Paerl, 1996;Hyenstrand et al, 1998).…”