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Under certain conditions, cyanobacteria can switch from photosynthesis to hydrogen production, which is a good energy carrier. However, the biological diversity of hydrogen-releasing cyanobacteria has a great unexplored potential. This study is aimed to investigate the ability of new strains of cyanobacteria Cyanobacterium sp. IPPAS B-1200, Dolichospermum sp. IPPAS B-1213, and Sodalinema gerasimenkoae IPPAS B-353 to release H 2 and to evaluate the effects of photosystem II inhibitor 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) on H 2 production under light and dark conditions. The results showed that cultures treated with DCMU produced several times more H 2 than untreated cells. The highest rate of H 2 photoproduction of 4.24 μmol H 2 (mg Chl a h)−1 was found in a Dolichospermum sp. IPPAS B-1213 culture treated with 20 μM DCMU. [Display omitted] • The ability of three new cyanobacterial strains to release hydrogen was studied. • The highest H2 yield was in the Dolichospermum sp. IPPAS B-1213 strain under light anaerobic conditions. • Sodalinema gerasimenkoae IPPAS B-353 released hydrogen only in the dark. • H2 production rate increased in presence of photosynthesis inhibitor. [ABSTRACT FROM AUTHOR]

Recently, there has been a propensity to postpone dealing with the world's climate concerns until later, resulting in a 1.5 °C rise in temperature over the last century. Therefore, interest in biologically derived, inexhaustible energy sources based on solar energy is growing. Cyanobacteria have the potential to produce clean, renewable fuels in the form of hydrogen (H 2) gas, derived from solar energy and water. The current study reports the screening 11 cyanobacterial strains isolated from rice paddies and hotsprings for efficient H 2 producers. According to our findings, H 2 concentrations in the species ranged from 3.6 to 48.9 μmol mg−1 Chl a h−1. H 2 production by isolated species was shown to have a 2% positive influence on oxygen (O 2) and carbon dioxide (CO 2) concentrations and a 2% negative effect on all nitrogen gas (N 2) concentrations. It was discovered that at high CO 2 concentrations, photosynthesis is enhanced but H 2 production is suppressed. Anabaena variabilis BTA-1047 was found to be the most active H 2 -producing species, with an H 2 production activity of 21.3 μmol mg−1 Chl a h−1. Moreover, a 1% O 2 : 2% CO 2 gas mixture doubled the strain activity of H 2 production. The findings of the study called into the question the notion that only an anaerobic environment is required for H 2 production by N 2 -fixing cyanobacterial species and explored whether H 2 productivity can be increased by stimulating the micro-anaerobic environment with a carbon source. [Display omitted] • Eleven nitrogen fixing cyanobacteria species were isolated and identified from different ecosystems. • Four cyanobacterial strains were distinguished by nitrogenase activity and hydrogen production. • 2% CO 2 and O 2 gases influence was favorably for most strains to produce molecular hydrogen. • Supply with 1% O 2 :2% CO 2 gas ratio increased H 2 production activity in Anabaena variabilis BTA-1047 by 2-fold. [ABSTRACT FROM AUTHOR]