Effects of the Photosystem II Inhibitors CCCP and DCMU on Hydrogen Production by the Unicellular Halotolerant Cyanobacterium .

The unicellular halotolerant cyanobacterium is a potential dark fermentative producer of molecular hydrogen (H) that produces very little H under illumination. One factor limiting the H photoproduction of this cyanobacterium is an inhibition of bidirectional hydrogenase activity by oxygen (O) obtained from splitting water molecules via photosystem II activity. The present study aimed to investigate the effects of the photosystem II inhibitors carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on H production of . under light and dark conditions and on photosynthetic and respiratory activities. The results showed that . treated with CCCP and DCMU produced H at three to five times the rate of untreated cells, when exposed to light. The highest H photoproduction rates, 2.26 ± 0.24 and 3.63 ± 0.26  mol Hg dry weight h, were found in cells treated with 0.5 M CCCP and 50 M DCMU, respectively. Without inhibitor treatment, . incubated in the dark showed a significant increase in H production compared with cells that were incubated in the light. Only CCCP treatment increased H production of . during dark incubation, because CCCP functions as an uncoupling agent of oxidative phosphorylation. The highest dark fermentative H production rate of 39.50 ± 2.13  mol Hg dry weight h was found in cells treated with 0.5 M CCCP after 2 h of dark incubation. Under illumination, CCCP and DCMU inhibited chlorophyll fluorescence, resulting in a low level of O, which promoted bidirectional hydrogenase activity in . cells. In addition, only CCCP enhanced the respiration rate, further reducing the O level. In contrast, DCMU reduced the respiration rate in .