###### Abstract  
The rising interest in harmful cyanoprokaryote blooms promotes an increase of phycological and ecological research on potentially toxic species and their hazardous substances. The present study aimed to identify the main microcystin (MC) producers and their contribution to the phytoplankton of shallow waterbodies in Bulgaria, applying different methods. The sampling was performed in August 2019 in nine lakes and reservoirs, two of which (reservoirs Kriva Reka and Izvornik 2) were studied for the first time. The high contribution of cyanoprokaryotes to the total species composition and phytoplankton abundance was proved by light microscopic (LM) observations and HPLC analysis of marker pigments. The LM identification of potential MC-producers was supported by PCR amplification of _mcyE_ and _mcyB_ genes. The MCs amounts, detected by HPLC-DAD, varied by sites with a range from undetectable concentrations to 0.46 \(\mathrm{\SIUnitSymbolMicro m}\)[1] with only one recorded variant, namely MC-LR. It was found only in the reservoirs Mandra and Durankulak, while toxicenic MC-strains were obtained by PCR from five more waterbodies. Both LM and PCR demonstrated that the MC-producers were _Microystis aeruginosa_ and _M. wesenbergii_, despite their occurrence in low amounts (<0.5-5% of the total biomass) when filamentous cyanoprokaryotes dominated. c ontological waterbodies; cyanobacteria; cyanoprokaryotes; cyanotoxins; harmful blooms; lakes; microcystins; shallow waterbodies; reservoirs; toxins +
Footnote †: journal: _Sci._  
_Microystis aeruginosa_ and _M. wesenbergii_ Were the Primary Planktonic Microcystin Producers in Several Bulgarian  
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Simultaneously with the increased awareness of CyanoHABs at a global scale [2] there is a rising interest in physiological and ecological studies of potentially toxic species and their hazardous substances [4; 6]. However, many questions related to the taxonomy, ecology, distribution and toxicity of some important bloom-causative cyanoprokaryotes are still awaiting their answers. _Microcystis_, despite being one of the first and best-known toxin producers and successful phytoplankton competitors due to numerous adaptive features (e.g., [7]), is among these genera with unresolved taxonomy and disputable species in regard to their toxicity [8; 9]. It is the most often reported toxic genus worldwide [2] and besides the ability to produce renowned potent hepatotoxic cyclic peptides microcystitis (MCs), its species have biosynthesis genes for other peptides (aeruginosins, anabaenopeptins, micrognins, microviridins, and cyanopeptolins), some of which can deleteriously affect aquatic ecosystems [6]. Additional concern comes from studies indicating that future elevated temperatures can positively influence MC gene expression and favor toxic _Microcystis_ genotypes over their nontoxic counterparts [10; 11]. There is also evidence that higher concentrations of nutrients may promote toxic genotypes and MC production [5; 12]. The discussions of all accumulated data on _Microcystis_ beyond different levels of its investigation at a global scale, are complicated because: (1) the main morphological features are variable, can overlap and transitional colonies between different morphospecies have been observed (for details see [9]); (2) this prokaryotic genus contains both toxicenic and non-toxicing strains; (3) its typical toxins, MCs, are known to be produced by other algae which have MC synthase (_mcy_) genes. The most frequently reported planktonic genera with _mcy_ genes are _Anabaena_ s.l. (recently including _Dolichospermum_, _Spharosperomopsis_, _etc._), _Planktothrix_ (former _Oscillatoria_ p.p.) and, occasionally, _Nostoc_[2; 3; 6; 13; 14; 15]. Additionally, there is evidence that some species of _Aphanizomenon_ s.l. (including _A.