Effects of two toxic cyanobacterial crude extracts containing microcystin-LR and cylindrospermopsin on the growth and photosynthetic capacity of the microalga Parachlorella kessleri

Secondary metabolites produced by cyanobacteria like microcystins (MC) and cylindrospermopsin (CYN) have been frequently studied because of their toxicity in humans and other animals. However, the function(s) of these metabolites remain largely unknown. The objective of this work was to deepen the knowledge in this research area and characterize the allelopathic effects of the cyanotoxins MC and CYN on the green alga Parachlorella kessleri. Several assays were carried out under controlled laboratory conditions with cyanobacterial cell extracts from Microcystis aeruginosa and Chrysosporum ovalisporum, producers of MC-LR and CYN respectively, at concentrations of 55 and 150 μg/L. The effects of CYN at 150 μg/L was also studied in P. kessleri growing in Z8 medium with altered nutrient composition. The data obtained indicate that growth rate and photosynthetic pigments were not affected by the exposure to cyanobacterial toxic extracts compared to control. However, the growth rate and photosynthetic pigments content changed during the 14 days of P. kessleri cultivation, in both groups (control and treatment), which may reflect the culture conditions and P. kessleri growth curve. It was also possible to verify that nutrients' concentration of the microalga culture medium does not influence the growth rate of the green microalga or the biological activity of the cyanobacterial extracts. In conclusion, under these specific laboratory conditions, there were no toxic or allelopathic effects of M. aeruginosa and C. ovalisporum crude extracts, which demonstrates the high tolerance of this phytoplanktonic species for the cyanobacterial toxins MC and CYN and other metabolites.