High altitude may limit production of secondary metabolites by cyanobacteria.

• High altitude connected with high UV radiation may limit the production of secondary metabolites by Cyanobacteria. • Strains isolated from two microbial mats in which no secondary metabolites were detected produced them in laboratory conditions. Ever-new cyanotoxins are being discovered, with planktic Cyanobacteria being the most studied communities, although records of cyanotoxins from benthic communities are becoming increasingly common. ​​Thus, the latter also started threatening users of water for recreational or drinking purposes. However, vast areas of the globe, i.a. Central Asia, are still understudied in this respect. Our recent study of benthic Cyanobacteria in the mountains of Eastern Pamir (Tajikistan) suggested that cyanotoxin production in high mountain environments is very limited. Here we present further study of Cyanobacteria, cyanotoxins, and other biologically active compounds such as geosmin and 2-Methylisoborneol (MIB) in microbial mats and water above them. Samples were collected in Eastern Pamirs, in UV (7–11) and altitude gradient (1000–4000 m a.s.l.). In the study, we used microscopic and genetic identification of Cyanobacteria based on NGS of V3-V4 16S rRNA amplicon and toxin assays using LC-QTOF-MS. The analyses demonstrated that the studied microbial mats contained potentially toxic Cyanobacteria (Anabaena, Lyngbya, Nostoc, Oscillatoria, and Phormidium). The production of cyanotoxins and taste and odor compounds (T&O) in natural environments was restricted to altitudes up to 3000 m a.s.l. Three water samples at 1000, 2000 and 3000 m a.s.l. contained MIB and/or geosmin, while debromoaplysiatoxin was noted at 2000 m a.s.l. Additionally, two strains (Hillbrichtia pamiria gen. nov. sp. nov. and Nostoc paludosum) isolated from sites at about 4000 m, in which no cyanotoxins or T&O were identified, produced debromoaplysiatoxin and microginin in laboratory conditions. The results suggest that in a stressful environment Cyanobacteria do not produce toxins. [ABSTRACT FROM AUTHOR]