**5. Occurrence**

Genome mining has shown that cyanobacteria have the potential to generate much more microviridin than is typically found under normal growth conditions. A study of this type has contributed to the expansion of knowledge on the chemical and genetic diversity of microviridins. They have been detected in various cyanobacterial genera and species, and these microorganisms are notorious producers of different groups of peptides and can be found in many environments, whether in fresh or salt water (Table 2). Due to this great variety, these bacteria have been evaluated for their significant biotechnological potential. The genus *Microcystis* and the species *M. aeruginosa* are the largest producers of microviridins—currently, of the 25 isolated microviridins, 11 belong to the genus *Microcystis*, and eight of these belong to the species *M. aeruginosa* [21,23,27,29–31]. Microviridin gene clusters have also been found in genomes of a number of bacteria, such as bacteroidetes and proteobacteria phyla [20].


**Table 2.** Occurrence of microviridins.


**Table 2.** *Cont.*

N.I.: not informed.

The genus *Microcystis* was the first to be described in the literature as a cyanobacteria producer of microviridins. This peptide was isolated from the bloom-forming *M. viridis* (NIES-102) on Kasumigaura Lake by Ishitsuka et al. (1990) [21]. This new oligopeptide demonstrated a noncanonical structure and was named microviridin by the name of the viridis species. In addition, other microviridins from the cyanobacteria of the genus *Microcystis* have been identified as microviridin B, C, L, SD1684, SD1634, SD1652, LH1667, 1777, O and M. Each of these microviridins has a considerable inhibition for at least one serine protease, such as elastase or trypsin [21,23,27,29–31].

An in-situ diversity investigation of the *Microcystis* communities present in lakes located around and in the city of Berlin, Germany demonstrated that 20% of 165 colonies analyzed were capable of producing microviridin. These cyanobacteria were present in almost all investigated areas. The majority of the microviridins producers also synthesized microcystins and cyanopeptolins. The coproduction of microviridin-aeruginosins and -microginins was rarely reported among the strains, being present in only 4% and 2%, respectively. The metabolomic profile of the peptides can be utilized to distinguish *Microcystis* strains with elevated morphological similarity whose visualization in the light microscope is not sufficient to differentiate them [42].

Martins and collaborators [43] isolated strains of cyanobacteria *M. aeruginosa* from a large range of lakes, rivers and reservoirs in Portugal. These strains were examined for the presence of secondary metabolites, such as aeruginosins, microviridins and microcystins. In this analysis, 47 strains from different sites were isolated among the identified peptides; microcystin was the most recurrent, appearing in 26 strains, and microviridins were contained in only three. The results of the analysis of the coproduction showed that the strains that produced microviridins did not produce microcystins. In another study, Walker et al [44] isolated the microviridin-producing strains of the *Planktothrix* genus from Maxsee in Germany incapable of producing microcystins.

In a study accomplished by Andreote [45], the purpose of which was to obtain information on the cyanobacterial community present in the phyllosphere of native plants from the Atlantic Forest, identified 40 cyanobacterial strains belonging to the genera *Nostocaceae*, *Desmontosc* and *Chroococcidiopsis* as microviridin producers obtained from *Merostachys neesii* (bamboo), *Euterpe edulis* (palmeira jacura), *Guapira opposita* and *Garcinia gardneriana*.

Andreote [45] was the pioneer in the identification of these peptides in the *Desmontosc* and *Chroococcidiopsis* genera. To identify the presence of this peptide in the strains, PCR amplifications of the *mdnA*, *mdnB* and *mdnC* genes were performed, which were related to the biosynthetic pathways of the microviridins. The strains *Nostocaceae* sp. CENA358 and CENA376, *Desmonostoc* sp. CENA365 and *Chroococcidiopsis* sp. CENA353 demonstrated the presence of these genes. Other strains lacked at least one of these genes, which did not rule out the synthesis of this peptide by these microorganisms due to the primers utilized that were constructed for strains of *Microcystis*, causing low-amplification performances, which implies that they might have more strains producing microviridins or possessing a biosynthetic cluster [45].

Eleven cyanopeptides from four different groups were reported from samples of cyanobacterial bloom in the Salto Grande reservoir, located in the State of São Paulo, Brazil, including the microviridin variant 1706. Cyanopeptides such as aeroguniosins, microcystins and cyanopeptolin were also detected. The morphological research showed that the bulk of the population of cyanobacteria belonged to the genus *Microcystis* [46].

Variants of microviridins were characterized in two cyanobacteria isolated from Brazilian reservoirs. *R. fernandoii* strain 28 was obtained from the Furnas Reservoir, which is situated in the southeastern region of Brazil and is described as an oligo-to-mesotrophic aquatic environment that receives organic matter contributions from domestic, farming and agriculture wastewaters. The *R. fernandoii* 86 strain was identified in an urban eutrophic reservoir located in the city of Belo Horizonte, Brazil, which suffers a great impact from domestic pollution, industrial sewage. A total of twelve peptides were found in the two strains. In the *R. fernadoii* 28 strain, a microviridin MV-1709 was found, and, in the strain *R. fernadoii* 86, two microviridins were reported, MV-1707 and MV-1739. Along with microviridins, peptides such as microcystins, cyanopeptolin and an unidentified peptide were also detected [47].
