Effects of the Photosystem II Inhibitors CCCP and DCMU on Hydrogen Production by the Unicellular Halotolerant CyanobacteriumAphanothece halophytica

The unicellular halotolerant cyanobacteriumAphanothece halophyticais a potential dark fermentative producer of molecular hydrogen (H2) that produces very little H2under illumination. One factor limiting the H2photoproduction of this cyanobacterium is an inhibition of bidirectional hydrogenase activity by oxygen (O2) 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 H2production ofA.halophyticaunder light and dark conditions and on photosynthetic and respiratory activities. The results showed thatA.halophyticatreated with CCCP and DCMU produced H2at three to five times the rate of untreated cells, when exposed to light. The highest H2photoproduction rates,2.26 ± 0.24and3.63 ± 0.26 μmol H2 g−1dry weight h−1, were found in cells treated with 0.5μM CCCP and 50μM DCMU, respectively. Without inhibitor treatment,A.halophyticaincubated in the dark showed a significant increase in H2production compared with cells that were incubated in the light. Only CCCP treatment increased H2production ofA.halophyticaduring dark incubation, because CCCP functions as an uncoupling agent of oxidative phosphorylation. The highest dark fermentative H2production rate of39.50 ± 2.13 μmol H2 g−1dry weight h−1was found in cells treated with 0.5μM CCCP after 2 h of dark incubation. Under illumination, CCCP and DCMU inhibited chlorophyll fluorescence, resulting in a low level of O2, which promoted bidirectional hydrogenase activity inA.halophyticacells. In addition, only CCCP enhanced the respiration rate, further reducing the O2level. In contrast, DCMU reduced the respiration rate inA.halophytica.