Germination of Cylindrospermopsis raciborskii and Aphanizomenon species under natural and experimental conditions
Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju and Aphanizomenon spp. are both freshwater cyanobacteria of the order Nostocales. C. raciborskii was thought confined to tropical and sub-tropical environments but spread to temperate climatic regions on all continents except Antarctica (Padisák, 1997). It is known to be widely distributed in Northern Germany reaching the northernmost margin of its distribution at latitudes of 53 - 54°N (Krienitz & Hegewald, 1996; Stüken et al., 2006). Strains of this species are detected to produce the hepatotoxin cylindrospermopsin, from which human injury has been clearly identified (Falconer & Humpage, 2006). Nostocales are characterized by trichomal structures such as vegetative cells, heterocysts and akinetes. The heterocysts can fix atmospheric nitrogen (N2) when aquatic nitrogen is depleted (Kim et al., 2005). Akinetes are non-motile, resistant cells that accumulate proteinaceous reserves in the form of cyanophycin granules (Wetzel, 2001). These akinetes can survive low temperatures, desiccation and other adverse environmental conditions. When favourable conditions return, they germinate to produce trichomes. The ability of Nostocales to form akinetes confers a distinct advantage in environmental adaptation and subsequent bloom formation (Kim et al., 2005; Wetzel, 2001). C. raciborskii akinetes are more commonly observed in subtropical and temperate populations late in the population cycle as a component of seasonal population dynamics (Kravchuk et al., 2006). Particular set of physico-chemical conditions stimulate akinetes to germinate (Moore et al., 2005). Light has been implicated as well as temperature and nutrients as triggering factors (Huber, 1985). According to Moore (2004) germination of akinetes occurs at temperatures between 15 °C and 30 °C in tropical Australian strains of C. raciborskii. Mischke (2003) found filaments of this species at temperatures of about 17 oC in temperate lakes with those in the Scharmützelsee region in Germany. We therefore, tried to find out at what temperature the akinetes germinate in temperate region hypothesizing it should be below 15 oC. Since Aphanizomenon species like A. gracile and A. flosaquae are the most abundant natural Nostocales in that habitat, we compare it to the invaded C. raciborskii. To figure out the germination temperature for both species, field observations are complemented by experiments at different temperatures.