## 3 Results
### Results from PCR Analysis for Microcystin-Producing Strains
Bootstrap values are shown at branch points (percentage of 1050 resamplings). Legend: Tol—Reservoir Izvornik 2 (=Tulumdere), Dur—Lake Durankulak, Por—Reservoir Poroyd, Man (E)—Reservoir Mandra (site East), Man (W)—Reservoir Mandra (site West), Vai—Lake Vaya, Uz—Lake Uzungeren, Blu—Reservoir Sinyata Reka (=Blue River). For the identical sequences (IS), obtained during this study, only one accession number received from NCBI is provided in each cluster in Bold. The IS from Mandra (W), Izvornik 2 and Vaya in Cluster I have accession number MW187795. The IS from Mandra (W) and Vaya in Cluster II have accession number MW187803. The IS from Poroyd (Poro 4, 5, 13, 14) in Cluster I have accession number MN417095.1 because these sequences are identical with a strain submitted earlier by us, Vai1-2_11, NCBI:toxid2703880 with accession number MN417095.1 [9]. Comparison of data from this study and our previous results [9] allowed us to suppose that cluster II on Figure 7 contains 19 strains identical (100%) to _Microcytsis_ sp. Kot12/08-3 (NCBItxtid1402958), which we tentatively related with _Microcytsis nowacekii_ (Komarek) Compere identified by LM and submitted to NCBI as _Microcytsis_ sp. Man1_6 (MN417082.1). These strains are spread in the reservoirs Mandra (West--5 strains, and East -9) and Sinyata Reka (1), and in the lakes Durankulak (1), Vaya (2), and Uzungeren (1). However, cluster II is quite complicated and contains more sequences, grouped in clear subclusters, the affiliation of which so far was not possible and requires further studies. Up-to-now it was also not possible to affiliate with certainty to any species the six sequences of 100% homology with NCBI, namely Vai1, Vai4, Vai5, Por1, ManW4 and Dur 6 (Figure 7). However, according to the presence of NCBI strain _Microcytsis_ sp. Brat12/07-7 (NCBItxtid1402954), similarity with which was found in 2018 [9], we provisionally suppose that one of the six strains mentioned above, namely Dur 6, is similar with the strain of _Microcytsis matans_ (observed by [PERSON] in the same waterbody--Durankulak in 2018 [9] and submitted by us to NCBI as Uncultured _Microcytsis_ sp. clone Dur2_6 (MN417106.1). The _mcyE_ phylogenetic tree (Figure 7) demonstrated the rich biodiversity of _Microcytsis_ strains in the studied sites. The highest diversity was detected in the small inland reservoir Izvornik 2; there, all ten sequences were distant from each other and were distributed in four different clusters (I-IV). Rich diversity was found also in the coastal lake Durankulak, the eight sequences of which were distributed in three different clusters (I-III). From the coastal lake Vaya and reservoir Poroy a total of 15 sequences were obtained and most of them (11) belonged to cluster I. Much less heterogeneity was found in the small inland reservoir Sinyata Reka, from which eight sequences (out of 14) were concentrated in the single cluster IV. The lowest biodiversity was found in the coastal lake Mandra, most sequences of which (16 out of 19) were distributed in cluster II. By applying direct sequencing of PCR products with expected length of 850 bp and _mcyB_ marker, eight sequences were obtained and were included in the constructed phylogenetic tree (Figure 8). They belonged to two main clusters and were affiliated mainly to _M. aeruginosa_ strains registered in NCBI [55]. Figure 8: Neighbor-joining phylogenetic tree constructed from nucleotides sequences from direct sequencing of PCR products and _mcyB_ marker retrieved after BLAST search in NCBI database with the accession number presented. Bootstrap values are shown at branch points (percentage of 1050 resampling). Legend: Tol—Reservoir Izvornik 2 (=Tulumdere), Dur—Lake Durankulak, Por—Reservoir Poroy, Man (E)—Reservoir Mandra (site East), Man (W)—Reservoir Mandra (site West), Vai—Lake Vaya, Uz—Lake Uzungeren, Blu—Reservoir Sinyata Reka (=Blue River). For the identical sequences (IS), obtained during this study, only one accession number received from NCBI is provided in each cluster or subcluster.