Impact of algicidal fungus Aspergillus welwitschiae GF6 on harmful bloom-forming cyanobacterium Microcystis aeruginosa: Growth and physiological responses.

Harmful cyanobacterial blooms (HCB_s) have become a common issue in freshwater worldwide. Biological methods for controlling HCB_s are relatively cost effective and environmentally friendly. The strain of ascomycete GF6 was isolated from a water sample collected from the estuarine zone of the eastern part of the Gulf of Finland. Based on cultural and morphological features and data of phylogenetic analysis, the strain was identified as Aspergillus welwitschiae GF6. The isolated GF6 strain has algicidal activity against both cyanobacteria and green algae. The highest sensitivity to the algicidal action of strain GF6 was found in cyanobacteria (98.5-100%). The algicidal effect on green algae did not exceed 63-70%. It was shown that GF6 strain exhibited an indirect attack mode by releasing metabolites that inhibit and/or degrade algal cells. In this study, significantly increased malondialdehyde content in Microcystis aeruginosa cells indicated that GF6 strain caused oxidative damage to the algal cell membrane. Enhanced production of phytosynthetic pigments, increase in lifetime chlorophyll a fluorescence and in the levels of antioxidants were noted in Microcystis aeruginosa cells. Besides this, GF6 strain could reduce the microcystins content in the medium under inhibiting the growth of M. aeruginosa. Apart from the growth inhibition and cell degradation of M. aeruginosa, GF6 strain is able to remove microcystin-LR (MC-LR). The content of MC-LR at an initial concentration of 0.51 μg/mL decreased by 61% after 72 h of A.welwitschiae GF6 strain cultivation. In the process of MC-LR biodestruction, transformation products were identified - the conjugate of microcystin with glutathione and the linearized form of MC-LR. The isolated strain with algicidal activity and the ability to degrade microcystin is of interest for further research in order to be able to use it for convergent technology to prevent the mass development of cyanobacteria and detoxification of cyanotoxins in water bodies.