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The unicellular halotolerant cyanobacterium  Aphanothece halophytica  is a potential dark fermentative producer of molecular hydrogen (H 2 ) that produces very little H 2  under illumination. One factor limiting the H 2  photoproduction of this cyanobacterium is an inhibition of bidirectional hydrogenase activity by oxygen (O 2 ) 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 2  production of  A .  halophytica  under light and dark conditions and on photosynthetic and respiratory activities. The results showed that  A .  halophytica  treated with CCCP and DCMU produced H 2  at three to five times the rate of untreated cells, when exposed to light. The highest H 2  photoproduction rates, 2.26 ± 0.24 and 3.63 ± 0.26   μ mol H 2  g −1  dry weight h −1 , were found in cells treated with 0.5  μ M CCCP and 50  μ M DCMU, respectively. Without inhibitor treatment,  A .  halophytica  incubated in the dark showed a significant increase in H 2  production compared with cells that were incubated in the light. Only CCCP treatment increased H 2  production of  A .  halophytica  during dark incubation, because CCCP functions as an uncoupling agent of oxidative phosphorylation. The highest dark fermentative H 2  production rate of 39.50 ± 2.13   μ mol H 2  g −1  dry weight h −1  was found in cells treated with 0.5  μ M CCCP after 2 h of dark incubation. Under illumination, CCCP and DCMU inhibited chlorophyll fluorescence, resulting in a low level of O 2 , which promoted bidirectional hydrogenase activity in  A .  halophytica  cells. In addition, only CCCP enhanced the respiration rate, further reducing the O 2  level. In contrast, DCMU reduced the respiration rate in  A .  halophytica.

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Effects of the Photosystem II Inhibitors CCCP and DCMU on Hydrogen Production by the Unicellular Halotolerant Cyanobacterium<i>Aphanothece halophytica</i>

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