Wijesooriya, M.M.Masakorala, K.Widana Gamage, S.M.K.2025-11-192025-11-192022-10-28Proceedings of the Postgraduate Institute of Science Research Congress (RESCON) -2022, University of Peradeniya, P 33978-955-8787-09-0https://ir.lib.pdn.ac.lk/handle/20.500.14444/6797In antagonistic interactions, cyanolytic bacteria may alter the physiology of cyanobacteria leading to cell death. The characterization of physiological alterations in cyanobacteria by the action of cyanolytic bacteria is important in the determination of cyanolytic mechanisms. Therefore, this study aimed to determine the oxidative stress and antioxidant responses generated in Pseudanabaena species against the cyanolytic bacteria Pseudomonas fluorescens BG-E (NCBI Acc. No. MZ007859) during the cyanolytic process as biomarkers. Based on the results of the preliminary screening study, 15% (v/v) of the total volume of bacterial suspension (6×10⁷ cells ml⁻¹ ) and cell-free supernatant of bacteria were inoculated into axenic cultures of Pseudanabaena sp. and P. lonchoides grown in BG11 medium at a cell density of 0.020 (OD730) respectively, in triplicates. The H₂O₂ content, peroxidase (POD) activity, and catalase (CAT) activity were analyzed on the date of incubation and following 2, 5, 8, and 10 days of experimental time. Initially, the H2O2 contents increased with time and reached the maximum values of 0.1622 and 0.1448 μM g⁻¹ f wt in Pseudanabaena sp. and P. lonchoides, respectively, at the end of the fifth day and decreased thereafter. The CAT and POD activities of both species showed a similar trend as H₂O₂ contents. On the fifth day, the CAT activities of both species were recorded at a similar value, 0.0029 U g⁻¹ f wt. Thereafter, in P. lonchoides, the CAT activity started to decrease and was even lower than the control on the tenth day. In Pseudanabaena sp., it was equal to the control (0.0011 U g⁻¹ f wt). The POD activity was highest in Pseudanabaena species (0.0081 U g⁻¹ f wt) compared with P. lonchoides (0.0069 U g⁻¹ f wt) on the fifth day. The elevated levels of H2O2 contents and CAT and POD activities infer that the cell suspension and cell-free supernatant of P. fluorescens BG-E have induced oxidative stress in Pseudanabaena sp. and P. lonchoides, respectively, and the antioxidant defence mechanisms in the cyanobacterial cells might have been activated to prevent damages as a response. The significant differences (p < 0.05) in the antioxidant levels of the two Pseudanabaena species could be due to their structural and morphological differences. The overall results infer the applicability of oxidative stress and antioxidant responses in cyanobateria as physiological biomarkers in the determination of cyanolytic mechanisms.en-USCyanolytic bacteriaCatalase activityH2O2 contentPeroxidase activityArticle