High Diversity of Microcystin Chemotypes within a Summer Bloom of the Cyanobacterium Microcystis botrys

Abstract

The fresh-water cyanobacterium Microcystis is known to form blooms world-wide, and is often responsible for the production of microcystins found in lake water. Microcystins are non-ribosomal peptides with toxic effects, e.g. on vertebrates, but their function remains largely unresolved. Moreover, not all strains produce microcystins, and many different microcystin variants have been described. Here we explored the diversity of microcystin variants within Microcystis botrys, a common bloom-former in Sweden. We isolated a total of 130 strains through the duration of a bloom in eutrophic Lake Vomb, and analyzed their microcystin profiles with tandem mass spectrometry (LC-MS/MS). We found that microcystin producing (28.5%) and non-producing (71.5%) M. botrys strains, co-existed throughout the bloom. However, microcystin producing strains were more prevalent towards the end of the sampling period. Overall, 26 unique M. botrys chemotypes were identified, and while some chemotypes re-occurred, others were found only once. The M. botrys chemotypes showed considerable variation both in terms of number of microcystin variants, as well as in what combinations the variants occurred. To our knowledge, this is the first report on microcystin chemotype variation and dynamics in M. botrys. In addition, our study verifies the co-existence of microcystin and non-microcystin producing strains, and we propose that environmental conditions may be implicated in determining their composition. Key Contribution:This study demonstrates that a single bloom of Microcystis botrys consists of strains with different microcystin chemotypes, each producing from zero up to 12 different microcystin variants. We established that microcystin and non-microcystin producing strains co-occur, and that the chemotype composition varies during the bloom.Toxins 2019, 11, 698 2 of 16 bloom-forming cyanobacteria, and that harmful blooms will increase both in frequency and duration, due to direct and indirect effects of changes in hydrological cycling, increased water temperatures, and nutrient loading [4][5][6][7][8][9].In addition to forming dense blooms which can deteriorate water quality per se, many cyanobacterial genera produce secondary metabolites with significant bioactivity and toxicity to aquatic organisms and humans [10][11][12][13][14]. In freshwaters, microcystins are among the most common cyanotoxins [7,15]. These inhibit the eukaryotic protein phosphatases 1 and 2a, and have hepatotoxic effects in vertebrates [10,16,17]. Drinking water from surface water supplies must therefore be checked for microcystin content, and may not be higher than 1 µg microcystin L −1 according to WHO regulation. Microcystins are cyclic heptapeptides (Figure 1) and are produced by several cyanobacteria including Microcystis, Planktothrix, and Anabaena (Dolichospermum) [14,18,19]. The common structure and biosynthetic pathway of microcystin synthesis in these genera have been described [20][21][22][23][24][25]. In Microcystis, the microcystin synthetase gene cluster co...

In Vitro Toxicological Screening of Stable and Senescing Cultures of Aphanizomenon, Planktothrix, and Raphidiopsis

“…These so-called ecotypes may vary in their adaptation across both large geographic regions and adjacent water bodies [54]. In addition, a high variation in toxicity has been shown to exist within strains forming blooms [55,56]. In light of these reports, the high strain variability observed in our study is unsurprising, and it clearly supports the importance of strain variation in understanding the influence of potentially toxic cyanobacteria on other organisms.…”

Characterization and Diversity of Microcystins Produced by Cyanobacteria from the Curonian Lagoon (SE Baltic Sea)

“…The significant differences in MC profiles recorded in these samples indicate changes at the sub-population level of the cyanobacteria community. In water bodies, several cyanobacteria chemotypes characterized by different MC patterns usually coexist [56][57][58]. In the Curonian Lagoon, the environmental conditions on the two days when the highest MC structural diversity was recorded (23 July and 16 August) apparently favored the proliferation of MC-rich chemotypes, and thus, indirectly influenced the presence of numerous, including the more rare, MC variants (e.g., MC-HtyR, [Ser 1 ]MC-RR, MC-LW, or MC-LY).…”

Harmful Cyanobacterial Blooms: Going beyond the “Green” to Monitor and Predict HCBs

“…As discussed above, the taxonomic confusion and controversy has also had an impact on the characterization of which cyanotoxins can be produced by each cyanobacterial species [65]. Additionally, each species can have toxic and non-toxic genotypes [69,[136][137][138], which brings even more entropy into toxicological studies. The synthesis of different cyanotoxins relies on the presence of specific gene clusters [139][140][141], and some cyanobacteria can also have biosynthesis gene clusters for more than one cyanotoxin [11], meaning that cyanobacterial blooms can be also co-dominated by multiple-toxin-producing strains, as previously reported [142,143].…”