Allelopathic effects of cyanotoxins on the physiological responses of Chlorella vulgaris.

Contributing to the assessment of potential physiological changes in microalgae subjected to different concentrations and types of cyanotoxins, this study investigated the inhibitory effects of cyanotoxins on the growth, density, biomass, and ecotoxicity of Chlorella vulgaris. Chlorella vulgaris was exposed to crude extracts of cyanobacteria producing microcystin-LR (MC-LR), saxitoxin (SXT), anatoxin-a (ATX-A), and cylindrospermopsin (CYN) with initial concentrations of 5.0, 2.05, 0.61, and 1.42 μg.L, respectively. The experiments were conducted under controlled conditions, and monitoring of growth and cell inhibition occurred at 24h, 48h, 72h, and 96h. Chlorophyll-a content and ecotoxicity assessment were conducted with samples collected after 96h of exposure to cyanotoxins. The growth assays of Chlorella vulgaris, with results expressed in terms of average growth rates (doublings/day), indicated the following order for cyanotoxins: SXT (2.03) > CYN (1.66) > MC-LR (1.56) > ATX-A (0.18). This assay revealed the prominent inhibitory potential of ATX-A on Chlorella vulgaris growth compared to the other toxins evaluated. Regarding the inhibition of the photosynthetic process, expressed in terms of the percentage inhibition of Chlorophyll-a, the following order for cyanotoxins was obtained: ATX-A (82%) > MC-LR (76%) > STX (46%) > CYN (16%). These results also indicated that among the cyanotoxins, ATX-A was the most detrimental to the photosynthetic process. However, contrary to the observations in the growth study, SXT proved to be more harmful than CYN in terms of Chlorophyll-a inhibition. Finally, the results of the toxicity assay revealed that only ATX-A and MC-LR exerted a chronic influence on Chlorella vulgaris under the investigated conditions.