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

Sunisa Pansook,Saranya Phunpruch,Aran Incharoensakdi

doi:10.1155/2019/1030236

Abstract

The unicellular halotolerant cyanobacterium Aphanothece halophytica is a potential dark fermentative producer of molecular hydrogen (H2) that produces very little H2 under illumination. One factor limiting the H2 photoproduction 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 H2 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 H2 at three to five times the rate of untreated cells, when exposed to light. The highest H2 photoproduction rates, 2.26 ± 0.24 and 3.63 ± 0.26 μmol H2 g−1 dry weight h−1, were found in cells treated with 0.5 μM CCCP and 50 μM DCMU, respectively. Without inhibitor treatment, A. halophytica incubated in the dark showed a significant increase in H2 production compared with cells that were incubated in the light. Only CCCP treatment increased H2 production of A. halophytica during dark incubation, because CCCP functions as an uncoupling agent of oxidative phosphorylation. The highest dark fermentative H2 production rate of 39.50 ± 2.13 μmol H2 g−1 dry weight h−1 was found in cells treated with 0.5 μM 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 in A. halophytica cells. In addition, only CCCP enhanced the respiration rate, further reducing the O2 level. In contrast, DCMU reduced the respiration rate in A. halophytica.

Highlights

Molecular hydrogen (H2) has attracted a great deal of interest from researchers because H2 combustion liberates a high heating value with 141.6 MJ kg−1 [1] and does not emit polluting gases to the environment
After A. halophytica cells were incubated in a nitrogen-deprived medium containing various concentrations of carbonyl cyanide m-chlorophenyl hydrazone (CCCP) (0–5 μM) and DCMU (0–250 μM) under light for 2, 24, 48, 72, and 96 h, cell and chlorophyll-a concentrations were measured
The results showed that the chlorophyll fluorescence emission spectra of A. halophytica cells treated with higher concentrations of CCCP or DCMU were significantly lower than those that were not treated (Figure 4), suggesting that these inhibitors could inhibit photosystem II (PSII) efficiency, leading to a decrease in the O2 level in vials, an increase in bidirectional hydrogenase activity (Figure 3), and an increase in H2 production rate (Figure 2)

Summary

Introduction

Molecular hydrogen (H2) has attracted a great deal of interest from researchers because H2 combustion liberates a high heating value with 141.6 MJ kg−1 [1] and does not emit polluting gases to the environment. Biological H2 production can be established in many kinds of microorganisms such as photosynthetic bacteria, fermentative bacteria, green algae, and cyanobacteria [2]. Among these microorganisms, cyanobacteria show high capability because they can generate H2 using electrons obtained from a light reaction of the photosynthetic pathway and/or from the degradation of storage carbohydrates within cells in darkness [3, 4]. A. halophytica produces a large amount of dark fermentative H2 compared with other marine cyanobacteria [6, 7]. H2 production by A. halophytica is catalyzed by bidirectional hydrogenase and occurs under nitrogen-deprived and dark

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