Metabolic pathways for photobiological hydrogen production by nitrogenase- and hydrogenase-containing unicellular cyanobacteria Cyanothece

A directed genome evolution method to enhance hydrogen production in Rhodobacter capsulatus

Nitrogenase-dependent H2 production by photosynthetic bacteria, such as Rhodobacter capsulatus, has been extensively investigated. An important limitation to increase H2 production using genetic manipulation is the scarcity of high-throughput screening methods to detect possible overproducing mutants. Previously, we engineered R. capsulatus strains that emitted fluorescence in response to H2 and used them to identify mutations in the nitrogenase Fe protein leading to H2 overproduction. Here, we used ultraviolet light to induce random mutations in the genome of the engineered H2-sensing strain, and fluorescent-activated cell sorting to detect and isolate the H2-overproducing cells from libraries containing 5 × 105 mutants. Three rounds of mutagenesis and strain selection gradually increased H2 production up to 3-fold. The whole genomes of five H2 overproducing strains were sequenced and compared to that of the parental sensor strain to determine the basis for H2 overproduction. No mutations were present in well-characterized functions related to nitrogen fixation, except for the transcriptional activator nifA2. However, several mutations mapped to energy-generating systems and to carbon metabolism-related functions, which could feed reducing power or ATP to nitrogenase. Time-course experiments of nitrogenase depression in batch cultures exposed mismatches between nitrogenase protein levels and their H2 and ethylene production activities that suggested energy limitation. Consistently, cultivating in a chemostat produced up to 19-fold more H2 than the corresponding batch cultures, revealing the potential of selected H2 overproducing strains.

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.

Hydrogen Dynamics in Trichodesmium Colonies and Their Potential Role in Mineral Iron Acquisition

N2-fixing cyanobacteria mediate H2 fluxes through the opposing processes of H2 evolution, which is a by-product of the N2 fixation reaction, and H2 uptake, which is driven by uptake hydrogenases. Here, we used microelectrodes to characterize H2 and O2 dynamics in single natural colonies of the globally important N2 fixer Trichodesmium collected from the Gulf of Eilat. We observed gradually changing H2 dynamics over the course of the day, including both net H2 evolution and net H2 uptake, as well as large differences in H2 fluxes between individual colonies. Net H2 uptake was observed in colonies amended with H2 in both light and dark. Net H2 evolution was recorded in the light only, reflecting light-dependent N2 fixation coupled to H2 evolution. Both net H2 evolution and H2 uptake rates were higher before 2 pm than later in the day. These pronounced H2 dynamics in the morning coincided with strong net O2 uptake and the previously reported diel peak in N2 fixation. Later in the afternoon, when photosynthesis rates determined by O2 measurements were highest, and N2 fixation rates decrease according to previous studies, the H2 dynamics were also less pronounced. Thus, the observed diel variations in H2 dynamics reflect diel changes in the rates of O2 consumption and N2 fixation. Remarkably, the presence of H2 strongly stimulated the uptake of mineral iron by natural colonies. The magnitude of this effect was dependent on the time of day, with the strongest response in incubations that started before 2 pm, i.e., the period that covered the time of highest uptake hydrogenase activity. Based on these findings, we propose that by providing an electron source for mineral iron reduction in N2-fixing cells, H2 may contribute to iron uptake in Trichodesmium colonies.

Rewiring of Cyanobacterial Metabolism for Hydrogen Production: Synthetic Biology Approaches and Challenges

With the demand for renewable energy growing, hydrogen (H₂) is becoming an attractive energy carrier. Developing H₂ production technologies with near-net zero carbon emissions is a major challenge for the "H₂ economy." Certain cyanobacteria inherently possess enzymes, nitrogenases, and bidirectional hydrogenases that are capable of H₂ evolution using sunlight, making them ideal cell factories for photocatalytic conversion of water to H₂. With the advances in synthetic biology, cyanobacteria are currently being developed as a "plug and play" chassis to produce H₂. This chapter describes the metabolic pathways involved and the theoretical limits to cyanobacterial H₂ production and summarizes the metabolic engineering technologies pursued.

Cyanobacterial nitrogenases: phylogenetic diversity, regulation and functional predictions

Cyanobacteria is a remarkable group of prokaryotic photosynthetic microorganisms, with several genera capable of fixing atmospheric nitrogen (N2) and presenting a wide range of morphologies. Although the nitrogenase complex is not present in all cyanobacterial taxa, it is spread across several cyanobacterial strains. The nitrogenase complex has also a high theoretical potential for biofuel production, since H2 is a by-product produced during N2 fixation. In this review we discuss the significance of a relatively wide variety of cell morphologies and metabolic strategies that allow spatial and temporal separation of N2 fixation from photosynthesis in cyanobacteria. Phylogenetic reconstructions based on 16S rRNA and nifD gene sequences shed light on the evolutionary history of the two genes. Our results demonstrated that (i) sequences of genes involved in nitrogen fixation (nifD) from several morphologically distinct strains of cyanobacteria are grouped in similarity with their morphology classification and phylogeny, and (ii) nifD genes from heterocytous strains share a common ancestor. By using this data we also discuss the evolutionary importance of processes such as horizontal gene transfer and genetic duplication for nitrogenase evolution and diversification. Finally, we discuss the importance of H2 synthesis in cyanobacteria, as well as strategies and challenges to improve cyanobacterial H2 production.

Strategy and design of Innovation Policy Road Mapping for a waste biorefinery

Looming energy crisis, climate change concerns coupled with decreasing fossil fuel resources has garnered significant global attention toward development of alternative, renewable, carbon-neutral and eco-friendly fuels to fulfil burgeoning energy demands. Waste utilization and its management are being pursued with renewed interest due to the gamut of biobased products it can offer apart from providing enough energy to meet a major fraction of the world's energy demand. Biorefining is the sustainable processing of biomass into a spectrum of marketable products and energy. Integrating all components of waste treatment culminating into biobased products and energy recovery in a single integrated waste biorefinery is self sufficient, highly sustainable and is very beneficial. Designing systematic innovation policies are essential for development and commercialization of new technologies in this important futuristic research area. This communication explores Innovation Policy Road Mapping (IPRM) methodology available in the literature and applies it to design integrated waste biorefinery.

Multi-Omic Dynamics Associate Oxygenic Photosynthesis with Nitrogenase-Mediated H2 Production in Cyanothece sp. ATCC 51142

To date, the proposed mechanisms of nitrogenase-driven photosynthetic H₂ production by the diazotrophic unicellular cyanobacterium Cyanothece sp. ATCC 51142 have assumed that reductant and ATP requirements are derived solely from glycogen oxidation and cyclic-electron flow around photosystem I. Through genome-scale transcript and protein profiling, this study presents and tests a new hypothesis on the metabolic relationship between oxygenic photosynthesis and nitrogenase-mediated H₂ production in Cyanothece 51142. Our results show that net-positive rates of oxygenic photosynthesis and increased expression of photosystem II reaction centers correspond and are synchronized with nitrogenase expression and H₂ production. These findings provide a new and more complete view on the metabolic processes contributing to the energy budget of photosynthetic H₂ production and highlight the role of concurrent photocatalytic H₂O oxidation as a participating process.

Novel properties of photofermentative biohydrogen production by purple bacteria Rhodobacter sphaeroides: effects of protonophores and inhibitors of responsible enzymes

Background

Biohydrogen (H₂) production by purple bacteria during photofermentation is a very promising way among biological H₂ production methods. The effects of protonophores, carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2,4-dinitrophenol (DNP), and inhibitors of enzymes, involved in H₂ metabolism, metronidazole (Met), diphenyleneiodonium (DPI), and dimethylsulphoxide (DMSO) on H₂ production by Rhodobacter sphaeroides MDC6522 isolated from Jermuk mineral springs in Armenia have been investigated in both nitrogen-limited and nitrogen-excess conditions.

Results

With the increase of inhibitors concentrations H₂ yield gradually decreased. The complete inhibition of H₂ production was observed in the presence of DPI and CCCP. DPI’s solvent—DMSO in low concentration did not significantly affect H₂ yield. N,N′-dicyclohexylcarbodiimide (DCCD)-inhibited the F_OF₁-ATPase activity of bacterial membrane vesicles was analyzed in the presence of inhibitors. Low concentrations of DPI and DMSO did not affect ATPase activity, whereas Met and CCCP stimulated enzyme activity. The effect of DNP was similar to CCCP.

Conclusions and significance

The results have shown the low concentration or concentration dependent effects of protonophores and nitrogenase and hydrogenase inhibitors on photofermentative H₂ production by Rh. sphaeroides in nitrogen-limited and nitrogen-excess conditions. They would be significant to understand novel properties in relationship between nitrogenase, hydrogenase and the F_OF₁-ATPase in Rh. sphaeroides, and regulatory pathways of photofermentation. The inhibitors of nitrogenase and hydrogenase can be used in biotechnology for regulation of H₂ production in different technology conditions and development of scale-up applications, for biomass and energy production using purple bacterial cells.

Cyanobacterial hydrogenases and hydrogen metabolism revisited: recent progress and future prospects

Cyanobacteria have garnered interest as potential cell factories for hydrogen production. In conjunction with photosynthesis, these organisms can utilize inexpensive inorganic substrates and solar energy for simultaneous biosynthesis and hydrogen evolution. However, the hydrogen yield associated with these organisms remains far too low to compete with the existing chemical processes. Our limited understanding of the cellular hydrogen production pathway is a primary setback in the potential scale-up of this process. In this regard, the present review discusses the recent insight around ferredoxin/flavodoxin as the likely electron donor to the bidirectional Hox hydrogenase instead of the generally accepted NAD(P)H. This may have far reaching implications in powering solar driven hydrogen production. However, it is evident that a successful hydrogen-producing candidate would likely integrate enzymatic traits from different species. Engineering the [NiFe] hydrogenases for optimal catalytic efficiency or expression of a high turnover [FeFe] hydrogenase in these photo-autotrophs may facilitate the development of strains to reach target levels of biohydrogen production in cyanobacteria. The fundamental advancements achieved in these fields are also summarized in this review.

Sustained H(2) production driven by photosynthetic water splitting in a unicellular cyanobacterium

The relationship between dinitrogenase-driven H(2) production and oxygenic photosynthesis was investigated in a unicellular cyanobacterium, Cyanothece sp. ATCC 51142, using a novel custom-built photobioreactor equipped with advanced process control. Continuously illuminated nitrogen-deprived cells evolved H(2) at rates up to 400 µmol ⋅ mg Chl(-1) ⋅ h(-1) in parallel with uninterrupted photosynthetic O(2) production. Notably, sustained coproduction of H(2) and O(2) occurred over 100 h in the presence of CO(2), with both gases displaying inverse oscillations which eventually dampened toward stable rates of 125 and 90 µmol ⋅ mg Chl(-1) ⋅ h(-1), respectively. Oscillations were not observed when CO(2) was omitted, and instead H(2) and O(2) evolution rates were positively correlated. The sustainability of the process was further supported by stable chlorophyll content, maintenance of baseline protein and carbohydrate levels, and an enhanced capacity for linear electron transport as measured by chlorophyll fluorescence throughout the experiment. In situ light saturation analyses of H(2) production displayed a strong dose dependence and lack of O(2) inhibition. Inactivation of photosystem II had substantial long-term effects but did not affect short-term H(2) production, indicating that the process is also supported by photosystem I activity and oxidation of endogenous glycogen. However, mass balance calculations suggest that carbohydrate consumption in the light may, at best, account for no more than 50% of the reductant required for the corresponding H(2) production over that period. Collectively, our results demonstrate that uninterrupted H(2) production in unicellular cyanobacteria can be fueled by water photolysis without the detrimental effects of O(2) and have important implications for sustainable production of biofuels.

IMPORTANCE

The study provides an important insight into the photophysiology of light-driven H(2) production by the nitrogen-fixing cyanobacterium Cyanothece sp. strain ATCC 51142. This work is also of significance for biotechnology, supporting the feasibility of "direct biophotolysis." The sustainability of the process, highlighted by prolonged gas evolution with no clear sign of significant decay or apparent photodamage, provides a foundation for the future development of an effective, renewable, and economically efficient bio-H(2) production process.