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Nostocalean cyanobacteria differentiate heterocysts to fix dissolved nitrogen (N2) and dormant cells (akinetes) to survive harsh environmental conditions. The distribution and proliferation of Nostocales and their hibernation strategies in deep stratified lakes of different trophic states and climate zones were investigated. Studies were carried out in the meso-eutrophic Lake Kinneret (Israel) and in the mesotrophic Lake Scharmützelsee and the oligo-mesotrophic Lake Stechlinsee (Germany).
The spatio-temporal distribution of akinetes in sediments was analysed. The abundance in the sediment surface increased with water depth and differed due to basin morphometry. Light and temperature distribution revealed shallow areas of sedimentary akinete pool as a potential inoculum to contribute to the formation of a pelagic population. Hence, a small viable akinete pool was deposited in shallow sediments.
A CARD-FISH protocol was implemented to identify and quantify the akinetes in sediments. Tested Nostocales strains and akinetes in sediment samples were successfully labelled with 16S rRNA targeted probes and microscopically detected by their fluorescence signal. Akinete enumeration in field samples confirmed the suitability of the CARD-FISH approach.
Species- or genus-specific differences in the overwintering strategies of Nostocales were observed. Akinetes were the only overwintering form of Anabaena spp. On the other hand, Aphanizomenon flos-aquae overwintered as vegetative filaments in the pelagial of Lake Stechlinsee. In the other lakes studied, Aphanizomenon spp. performed an intermediate life cycle, with akinetes and a small pelagic population in winter. This suggested that Aphanizomenon spp. had another overwintering strategy contrary to the exclusively dormant strategy of Anabaena spp.
Putative strains of cylindrospermopsin (CYN)-producing Aphanizomenon ovalisporum were isolated from Lake Kinneret. Their morphologic and phylogenetic diversity, as well as the presence of CYN-encoding gene cluster and CYN production, were analysed. Four of the six strains were assigned to Anabaena bergii. Although all the isolated strains possess at least fragments of CYN gene cluster, no CYN production was detected. Non-CYN-producing Aphanizomenon ovalisporum strains and the presence of non-toxic Anabaena bergii in Lake Kinneret have not been reported before. This suggested that toxic and non-toxic Nostocales strains can coexist in lakes.