Photosynthetic prokaryotes known as cyanobacteria produce an extensive range of primary and secondary metabolites that serve multiple biotechnological and biomedical purposes. The non-model filamentous Phormidium species capture researchers’ attention through their biotechnological potentials from diverse metabolites and their ability to thrive in tough environments while producing bioactive compounds. In this study, a thermotolerant strain of Phormidium ambiguum was isolated from the Chundzha thermal springs in southeastern Kazakhstan and characterized morphologically, physiologically, and biochemically. This cyanobacterium demonstrated fast growth in its culture media along with significant accumulation of proteins (44.8% DM), carbohydrates (45% DM), and photosynthetic pigments like chlorophyll a and valuable carotenoids, including b-carotene, myxoxantophyll and zeaxanthin. The LC-ESI-MS/MS analysis of cyanobacterial non-polar extract identified 150 potential metabolites which include fatty acid derivatives, terpenoids and carotenoid-related compounds known for their antioxidant and antimicrobial properties, as well as immune system stimulation. Biological assays confirmed a weak antioxidant capacity in the DPPH test, while in immunological assays, the extract of P. ambiguum stimulated T lymphocyte proliferation and activation, as well as NK cell proliferation in vitro. It also exhibited moderate antibacterial activity towards tested Gram-negative and Gram-positive bacterial strains. While additional studies are required to address environmental robustness, biosynthetic regulation, and practical scalability, the present findings indicate that P. ambiguum represents a valuable non-model cyanobacterium for exploratory bioprospecting. Its metabolite profile and observed bioactivities support further investigation of this thermotolerant strain as a potential source of immunomodulatory, antioxidant, and antimicrobial compounds under controlled conditions.

Extreme ecosystems represent natural models for studying the mechanisms of microbial adaptation to a complex of abiotic stress factors, including elevated mineralization, thermal stress, pH fluctuations, radiation exposure, and limited nutrient availability. Kazakhstan is characterized by a wide diversity of such unique natural environments; however, the biodiversity of phototrophic microorganisms inhabiting these ecosystems remains insufficiently studied to date. Contemporary climate challenges and the demand for sustainable bioenergy solutions have increased interest in extremophilic phototrophic microorganisms demonstrating adaptation to harsh environmental conditions. The goal of the present study was to assess the biodiversity and taxonomic structure of microalgae and cyanobacteria isolated from extreme aquatic and soil ecosystems in various regions of Kazakhstan. To achieve this objective, field expeditions were conducted at designated monitoring sites, and microbiological methods were employed to isolate axenic cultures. As a result of the study, 171 species and subspecies of phototrophic microorganisms were identified in soil samples, and 14 species in aquatic ecosystems, predominantly belonging to the classes Chlorophyceae, Cyanophyceae, and Bacillariophyceae. Axenic cultures representing six genera Chlorella, Anabaena, Synechocystis, Coelastrum, Leptolyngbya, and Phormidium were successfully obtained. The identified taxa exhibited morphological and ecological plasticity, reflecting their adaptation to thermal, saline, highly mineralized, and radiation-active environmental conditions. The obtained results indicate a high adaptive potential of phototrophic microorganisms inhabiting extreme ecosystems and highlight their prospects for the development of sustainable biotechnological processes, including biomass and lipid production for biofuel generation. Keywords: extreme ecosystems, biodiversity, phototrophic microorganisms