Effects of selected electron transport chain inhibitors on 24-h hydrogen production by Synechocystis sp. PCC 6803

Chemical Triggering Cyanobacterial Glycogen Accumulation: Methyl Viologen Treatment Increases Synechocystis sp. PCC 6803 Glycogen Storage by Enhancing Levels of Gene Transcript and Substrates in Glycogen Synthesis

Two-stage cultivation is effective for glycogen production by cyanobacteria. Cells were first grown under adequate nitrate supply (BG11) to increase biomass and subsequently transferred to nitrogen deprivation (-N) to stimulate glycogen accumulation. However, the two-stage method is time-consuming and requires extensive energy. Thus, one-stage cultivation that enables both cell growth and glycogen accumulation is advantageous. Such one-stage method could be achieved using a chemical triggering glycogen storage. However, there is a limited study on such chemicals. Here, nine compounds previously reported to affect cyanobacterial cellular functions were examined in Synechocystis sp. PCC 6803. 2-Phenylethanol, phenoxyethanol, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and methyl viologen can stimulate glycogen accumulation. The oxidative stress agent, methyl viologen significantly increased glycogen levels up to 57% and 69% [w/w dry weight (DW)] under BG11 and -N cultivation, respectively. One-stage cultivation where methyl viologen was directly added to the pre-grown culture enhanced glycogen storage to 53% (w/w DW), compared to the 10% (w/w DW) glycogen level of the control cells without methyl viologen. Methyl viologen treatment reduced the contents of total proteins (including phycobiliproteins) but caused increased transcript levels of glycogen synthetic genes and elevated levels of metabolite substrates for glycogen synthesis. Metabolomic results suggested that upon methyl viologen treatment, proteins degraded to amino acids, some of which could be used as a carbon source for glycogen synthesis. Results of oxygen evolution and metabolomic analysis suggested that photosynthesis and carbon fixation were not completely inhibited upon methyl viologen treatment, and these two processes may partially generate upstream metabolites required for glycogen synthesis.

Chemicals Affecting Cyanobacterial Poly(3-hydroxybutyrate) Accumulation: 2-Phenylethanol Treatment Combined with Nitrogen Deprivation Synergistically Enhanced Poly(3-hydroxybutyrate) Storage in Synechocystis sp. PCC6803 and Anabaena sp. TISTR8076

Various photoautotrophic cyanobacteria increase the accumulation of bioplastic poly(3-hydroxybutyrate) (PHB) under nitrogen deprivation (-N) for energy storage. Several metabolic engineering enhanced cyanobacterial PHB accumulation, but these strategies are not applicable in non-gene-transformable strains. Alternatively, stimulating PHB levels by chemical exposure is desirable because it might be applied to various cyanobacterial strains. However, the study of such chemicals is still limited. Here, 19 compounds previously reported to affect bacterial cellular processes were evaluated for their effect on PHB accumulation in Synechocystis sp. PCC6803, where 3-(3,4-dichlorophenyl)-1,1-dimethylurea, methyl viologen, arsenite, phenoxyethanol and 2-phenylethanol were found to increase PHB accumulation. When cultivated with optimal nitrate supply, Synechocystis contained less than 0.5% [w/w dry weight (DW)] PHB, while cultivation under -N conditions increased the PHB content to 7% (w/w DW). Interestingly, the -N cultivation combined with 2-phenylethanol exposure reduced the Synechocystis protein content by 27% (w/w DW) but significantly increased PHB levels up to 33% (w/w DW), the highest ever reported photoautotrophic cyanobacterial PHB accumulation in a wild-type strain. Results from transcriptomic and metabolomic analysis suggested that under 2-phenylethanol treatment, Synechocystis proteins were degraded to amino acids, which might be subsequently utilized as the source of carbon and energy for PHB biosynthesis. 2-Phenylethanol treatment also increased the levels of metabolites required for Synechocystis PHB synthesis (acetyl-CoA, acetoacetyl-CoA, 3-hydroxybutyryl-CoA and NADPH). Additionally, under -N, the exposure to phenoxyethanol and 2-phenylethanol increased the PHB levels of Anabaena sp. from 0.4% to 4.1% and 6.6% (w/w DW), respectively. The chemicals identified in this study might be applicable for enhancing PHB accumulation in other cyanobacteria.

Simazine Enhances Dark Fermentative H2 Production by Unicellular Halotolerant Cyanobacterium Aphanothece halophytica

The halotolerant cyanobacterium Aphanothece halophytica is a potential H₂ producer that induces H₂ evolution under nitrogen deprivation. H₂ is mainly produced via the catabolism of stored glycogen under dark anaerobic condition. H₂ evolution is catalyzed by O₂-sensitive bidirectional hydrogenase. The aim of this study was to improve H₂ production by A. halophytica using various kinds of inhibitors. Among all types of inhibitors, simazine efficiently promoted the highest H₂ production under dark conditions. High simazine concentration and long-term incubation resulted in a decrease in cell and chlorophyll concentrations. The optimal simazine concentration for H₂ production by A. halophytica was 25 µM. Simazine inhibited photosynthetic O₂ evolution but promoted dark respiration, resulting in a decrease in O₂ level. Hence, the bidirectional hydrogenase activity and H₂ production was increased. A. halophytica showed the highest H₂ production rate at 58.88 ± 0.22 µmol H₂ g⁻¹ dry weight h⁻¹ and H₂ accumulation at 356.21 ± 6.04 μmol H₂ g⁻¹ dry weight after treatment with 25 µM simazine under dark anaerobic condition for 2 and 24 h, respectively. This study demonstrates the potential of simazine for the enhancement of dark fermentative H₂ production by A. halophytica.

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

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 H₂g⁻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 H₂g⁻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.

Advances in the function and regulation of hydrogenase in the cyanobacterium Synechocystis PCC6803

In order to use cyanobacteria for the biological production of hydrogen, it is important to thoroughly study the function and the regulation of the hydrogen-production machine in order to better understand its role in the global cell metabolism and identify bottlenecks limiting H2 production. Most of the recent advances in our understanding of the bidirectional [Ni-Fe] hydrogenase (Hox) came from investigations performed in the widely-used model cyanobacterium Synechocystis PCC6803 where Hox is the sole enzyme capable of combining electrons with protons to produce H2 under specific conditions. Recent findings suggested that the Hox enzyme can receive electrons from not only NAD(P)H as usually shown, but also, or even preferentially, from ferredoxin. Furthermore, plasmid-encoded functions and glutathionylation (the formation of a mixed-disulfide between the cysteines residues of a protein and the cysteine residue of glutathione) are proposed as possible new players in the function and regulation of hydrogen production.

Expression of Shewanella oneidensis MR-1 [FeFe]-hydrogenase genes in Anabaena sp. strain PCC 7120

H(2) generated from renewable resources holds promise as an environmentally innocuous fuel that releases only energy and water when consumed. In biotechnology, photoautotrophic oxygenic diazotrophs could produce H(2) from water and sunlight using the cells' endogenous nitrogenases. However, nitrogenases have low turnover numbers and require large amounts of ATP. [FeFe]-hydrogenases found in other organisms can have 1,000-fold higher turnover numbers and no specific requirement for ATP but are very O(2) sensitive. Certain filamentous cyanobacteria protect nitrogenase from O(2) by sequestering the enzyme within internally micro-oxic, differentiated cells called heterocysts. We heterologously expressed the [FeFe]-hydrogenase operon from Shewanella oneidensis MR-1 in Anabaena sp. strain PCC 7120 using the heterocyst-specific promoter P(hetN). Active [FeFe]-hydrogenase was detected in and could be purified from aerobically grown Anabaena sp. strain PCC 7120, but only when the organism was grown under nitrate-depleted conditions that elicited heterocyst formation. These results suggest that the heterocysts protected the [FeFe]-hydrogenase against inactivation by O(2).

Redirecting the electron flow towards the nitrogenase and bidirectional Hox-hydrogenase by using specific inhibitors results in enhanced H2 production in the cyanobacterium Anabaena siamensis TISTR 8012

The inhibition of competitive metabolic pathways by various inhibitors in order to redirect electron flow towards nitrogenase and bidirectional Hox-hydrogenase was investigated in Anabaena siamensis TISTR 8012. Cells grown in BG11(0) supplemented with KCN, rotenone, DCMU, and DL-glyceraldehyde under light condition for 24 h showed enhanced H(2) production. Cells grown in BG11 medium showed only marginal H(2) production and its production was hardly increased by the inhibitors tested. H(2) production with either 20mM KCN or 50 μM DCMU in BG11(0) medium was 22 μmol H(2) mg chl a(-1) h(-1), threefold higher than the control. The increased H(2) production caused by inhibitors was consistent with the increase in the respective Hox-hydrogenase activities and nifD transcript levels, as well as the decrease in hupL transcript levels. The results suggested that interruption of metabolic pathways essential for growth could redirect electrons flow towards nitrogenase and bidirectional Hox-hydrogenase resulting in increased H(2) production.

Transcriptomic responses of Synechocystis sp. PCC 6803 encapsulated in silica gel

Global gene expression of Synechocystis sp. PCC 6803 encapsulated in silica gel was examined by microarray analysis. Cultures were encapsulated in gels derived from aqueous precursors or from alkoxide precursors and incubated under constant light for 24 h prior to RNA extraction. Cultures suspended in liquid media were exposed to 500 mM salt stress and incubated under identical conditions for comparison purposes. The expression of 414 genes was significantly altered by encapsulation in aqueous-derived gels (fold change ≥1.5 and P value < 0.01), the expression of 1,143 genes was significantly altered by encapsulation in alkoxide-derived gels, and only 243 genes were common to both encapsulation chemistries. Additional qRT-PCR analyses of four selected genes, ggpS, cpcG2, slr5055, and sll5057, confirmed microarray results for those genes. These results illustrate that encapsulation stress is quite different than salt stress in terms of gene expression response. Furthermore, a number of hypothetical and unknown proteins associated with encapsulation and alcohol stress have been identified with implications for improving encapsulation protocols and rationally engineering microorganisms for direct biofuel production.

Statistical optimization of microalgae Pavlova lutheri cultivation conditions and its fermentation conditions by yeast, Candida rugopelliculosa

In this study, sequential strategy based design was applied to optimize the microalgae, Pavlova lutheri mass culture conditions and fermentation conditions of the cultured algae by proteolytic yeast Candidia rugopelliculosa to obtain small peptide chains. This optimization of culture and fermentation conditions by response surface methodology (RSM) finally leads to effective purification of a bioactive peptide MPGPLSPL (793.01 Da) with hydroxyl radical scavenging activity. Collectively, these results indicated that microalgae P. lutheri can enhance the hydroxyl radical inhibiting effect through protein hydrolysis process under RSM optimal condition.