Salt and heat stress enhances hydrogen production in cyanobacteria.

Cyanobacteria are among the most suitable organisms for the capture of excessive amounts of CO2 and can be grown in extreme environments. In our research we use the single-celled freshwater cyanobacteria Synechococcus elongatus PCC7942 PAMCOD strain and Synechocystis sp. PCC6714 for the production of carbohydrates and hydrogen. PAMCOD strain and Synechocystis sp. PCC6714 synthesize sucrose when exposed to salinity stress, as their main compatible osmolyte. We examined the cell proliferation rate and the sucrose accumulation in those two different strains of cyanobacteria under salt (0.4 M NaCl) and heat stress (35 0C) conditions. The intracellular sucrose (mol sucrose content per Chl a) was found to increase by 50% and 108% in PAMCOD strain and Synechocystis sp. PCC6714 cells, respectively. As previously reported, PAMCOD strain has the ability to produce hydrogen through the process of dark anaerobic fermentation (Vayenos D, Romanos GE, Papageorgiou GC, Stamatakis K (2020) Photosynth Res 146, 235-245). In the present study, we demonstrate that Synechocystis sp. PCC6714 has also this ability. We further examined the optimal conditions during the dark fermentation of PAMCOD and Synechocystis sp. PCC6714 regarding H2 formation, increasing the PAMCOD H2 productivity from 2 nmol H2 h- 1 mol Chl a- 1 to 23 nmol H2 h- 1 mol Chl a- 1. Moreover, after the dark fermentation, the cells demonstrated proliferation in both double BG-11 and BG-11 medium enriched in NaNO3, thus showing the sustainability of the procedure.

Influences of bisphenol A on hydrogen production from food waste by thermophilic dark fermentation.

The extensive use of plastic products in food packaging and daily life makes them inevitably enter the treatment process of food waste (FW). Plasticizer as a new pollutant is threatening the dark fermentation of FW. Our study showed that bisphenol A (BPA) at > 250 mg/L had a significant inhibition on hydrogen production from FW by thermophilic dark fermentation. The endogenous ATP content and lactate dehydrogenase (LDH) release showed that high level of BPA not only inhibited the growth of hydrogen-producing consortium, but also led to cell death. In addition, BPA mainly affects the hydrogen-producing consortium by reducing cell membrane fluidity, damaging cell membrane integrity and reducing cell membrane potential, resulting in cell death. This study provides some new insights into the mechanism of the effect of BPA on hydrogen production from FW by thermophilic dark fermentation, and lays the foundation on the utilization of FW.

Enhanced thermophilic dark fermentation of hydrogen production from food waste by Fe-modified biochar.

The industrialization of hydrogen production through dark fermentation of food waste faces challenges, such as low yields and unpredictable fermentation processes. Biochar has emerged as a promising green additive to enhance hydrogen production in dark fermentation. Our study demonstrated that the introduction of Fe-modified biochar (Fe-L600) significantly boosted hydrogen production during thermophilic dark fermentation of food waste. The addition of Fe-L600 led to a remarkable 31.19% increase in hydrogen yield and shortened the time needed for achieving stabilization of hydrogen production from 18 h to 12 h. The metabolite analysis revealed an enhancement in the butyric acid pathway as the molar ratio of acetic acid to butyric acid decreased from 3.09 to 2.69 but hydrogen yield increased from 57.12 ± 1.48 to 76.78 ± 2.77 mL/g, indicating Fe-L600 improved hydrogen yield by regulating crucial metabolic pathways of hydrogen production. The addition of Fe-L600 also promoted the release of Fe2+ and Fe3+ and increased the concentrations of Fe2+ and Fe3+ in the fermentation system, which might promote the activity of hydrogenase and ferredoxin. Microbial community analysis indicated a substantial increase in the relative abundance of Thermoanaerobacterium after thermophilic dark fermentation. The relative abundances of microorganisms responsible for hydrolysis and acidogenesis were also observed to be improved in the system with Fe-L600 addition. This research provides a feasible strategy for improving hydrogen production of food waste and deepens the understanding of the mechanisms of biochar.

Combining pretreatments and co-fermentation as successful approach to improve biohydrogen production from dairy cow manure.

The present paper is focused on enhancing the production of biohydrogen (bioH2) from dairy cow manure (DCM) through dark fermentation (DF). Two enhancement production strategies have been tested: i) the combination of H2O2 with sonification as pretreatment and ii) the co-fermentation with cheese whey as co-substrate. Concerning the pretreatment, the best combination was investigated according to the response surface methodology (RSM) by varying H2O2 dosage between 0.0015 and 0.06 g/gTS and ultrasonic specific energy input (USEI) between 35.48 and 1419.36 J/gTS. The increase of carbohydrates concentration was used as target parameter. Results showed that the combination of 0.06 g/gTS of H2O2 with 1419.36 J/gTS of USEI maximized the concentration of carbohydrates. The optimized conditions were used to pretreat the substrate prior conducting DF tests. The use of pretreatment resulted in obtaining a cumulative bioH2 volume of 51.25 mL/L and enhanced the bioH2 production by 125% compared to the control test conducted using raw DCM. Moreover, the second strategy, i.e. co-fermentation with cheese whey (20% v/v) as co-substrate ended up to enhancing the DF performance as the bioH2 production reached a value of 334.90 mL/L with an increase of 1372% compared to the control DF test. To further improve the process, dark fermentation effluents (DFEs) were valorized via photo fermentation (PF), obtaining an additional hydrogen production aliquot.

Investigating the efficiency of a two-stage anaerobic-aerobic process for the treatment of confectionery industry wastewaters with simultaneous production of biohydrogen and polyhydroxyalkanoates.

The scope of the current study was to investigate the efficiency of a two-stage anaerobic-aerobic process for the simultaneous treatment and valorization of selective wastewater streams from a confectionary industry. The specific wastewater (confectionary industry wastewater, CIW) was a mixture of the rinsing eluting during washing of the cauldrons in which jellies and syrups were produced, and contained mainly readily fermentable sugars, being thus of high organic load. The first stage of the process was the dark fermentation (DF) of the CIW in continuous, attached-biomass systems, in which the effect on hydrogen yields and distribution of metabolites were studied for different packing materials (ceramic or plastic), hydraulic retention times, HRTs (12 h-30 h) and feed substrate concentration (20 g COD/L- 50 g COD/L). In the second stage, the effectiveness of the aerobic treatment of the DF effluents was evaluated in terms of the reduction of the organic load and the production of polyhydroxyalkanoates (PHAs) through an enriched mixed microbial culture (MMC). The MMC was developed in a continuous draw and fill system, in which the accumulation potential of PHAs was studied. It was shown that the hydrogen production rates decreased for increasing substrate concentration and HRTs, with a maximum of 12.70 ± 0.35 m3 H2/m3 initial CIW achieved for the lowest HRT and feed concentration and using ceramic beads as packing material. Butyrate, acetate and lactate were the main metabolites generated in all cases, in different ratios. The distribution of metabolites during DF was shown to highly affect the efficiency of the second process in terms of both the reduction of organic load and the PHAs yields. The highest removal of organic load achieved after 48 h of aerobic treatment was 84.0 ± 0.9 %, whereas the maximum PHAs yield was 21.46 ± 0.13 kg PHAs/m3 initial CIW.

Recent advancements in wastewater treatment via anaerobic fermentation process: A systematic review.

This manuscript delves into the realm of wastewater treatment, with a particular emphasis on anaerobic fermentation processes, especially dark, photo, and dark-photo fermentation processes, which have not been covered and overviewed previously in the literature regarding the treatment of wastewater. Moreover, the study conducts a bibliometric analysis for the first time to elucidate the research landscape of anaerobic fermentation utilization in wastewater purification. Furthermore, microorganisms, ranging from microalgae to bacteria and fungi, emphasizing the integration of these agents for enhanced efficiency, are all discussed and compared. Various bioreactors, such as dark and photo fermentation bioreactors, including tubular photo bioreactors, are scrutinized for their design and operational intricacies. The results illustrated that using clostridium pasteurianum CH4 and Rhodopseudomonas palustris WP3-5 in a combined dark-photo fermentation process can treat wastewater to a pH of nearly 7 with over 90% COD removal. Also, integrating Chlorella sp and Activated sludge can potentially treat synthetic wastewater to COD, P, and N percentage removal rates of 99%,86%, and 79%, respectively. Finally, the paper extends to discuss the limitations and future prospects of dark-photo fermentation processes, offering insights into the road ahead for researchers and scientists.

Sustainable biorefining and bioprocessing of green seaweed (Ulva spp.) for the production of edible (ulvan) and non-edible (polyhydroxyalkanoate) biopolymeric films.

A sustainable biorefining and bioprocessing strategy was developed to produce edible-ulvan films and non-edible polyhydroxybutyrate films. The preparation of edible-ulvan films by crosslinking and plasticisation of ulvan with citric acid and xylitol was investigated using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) analysis. The edible ulvan film was tested for its gut-friendliness using Lactobacillus and Bifidobacterium spp. (yoghurt) and was shown to improve these gut-friendly microbiome's growth and simultaneously retarding the activity of pathogens like Escherchia coli and Staphylococcus aureus. Green macroalgal biomass refused after the extraction of ulvan was biologically processed by dark fermentation to produce a maximum of 3.48 (± 0.14) g/L of volatile fatty acids (VFAs). Aerobic processing of these VFAs using Cupriavidus necator cells produced 1.59 (± 0.12) g/L of biomass with 18.2 wt% polyhydroxybutyrate. The present study demonstrated the possibility of producing edible and non-edible packaging films using green macroalgal biomass as the sustainable feedstock.

Application of a novel biological-nanoparticle pretreatment to Oscillatoria acuminata biomass and coculture dark fermentation for improving hydrogen production.

BACKGROUND: Energy is the basis and assurance for a world's stable development; however, as traditional non-renewable energy sources deplete, the development and study of renewable clean energy have emerged. Using microalgae as a carbon source for anaerobic bacteria to generate biohydrogen is a clean energy generation system that both local and global peers see as promising. RESULTS: Klebsiella pneumonia, Enterobacter cloacae, and their coculture were used to synthesize biohydrogen using Oscillatoria acuminata biomass via dark fermentation. The total carbohydrate content in O. acuminata was 237.39 mg/L. To enhance the content of fermentable reducing sugars, thermochemical, biological, and biological with magnesium zinc ferrite nanoparticles (Mg-Zn Fe2O4-NPs) pretreatments were applied. Crude hydrolytic enzymes extracted from Trichoderma harzianum of biological pretreatment were enhanced by Mg-Zn Fe2O4-NPs and significantly increased reducing sugars (230.48 mg/g) four times than thermochemical pretreatment (45.34 mg/g). K. pneumonia demonstrated a greater accumulated hydrogen level (1022 mLH2/L) than E. cloacae (813 mLH2/L), while their coculture showed superior results (1520 mLH2/L) and shortened the production time to 48 h instead of 72 h in single culture pretreatments. Biological pretreatment + Mg-Zn Fe2O4 NPs using coculture significantly stimulated hydrogen yield (3254 mLH2/L), hydrogen efficiency)216.9 mL H2/g reducing sugar( and hydrogen production rate (67.7 mL/L/h) to the maximum among all pretreatments. CONCLUSION: These results confirm the effectiveness of biological treatments + Mg-Zn Fe2O4-NPs and coculture dark fermentation in upregulating biohydrogen production.

Optimization of hydrogen production in Enterobacter aerogenes by Complex I peripheral fragments destruction and maeA overexpression.

As a concentrated energy source with high added value, hydrogen has great development prospects, with special emphasis on sustainable microbial production as a replacement for traditional fossil fuels. In this study, λ-Red recombination was used to alter the activity of Complex I by single and combined knockout of nuoE, nuoF and nuoG. In addition, the conversion of malic to pyruvic acid was promoted by overexpressing the maeA gene, which could increase the content of NADH and formic acid in the bacterial cells. Compared to the original strain, hydrogen production was 65% higher in the optimized strain IAM1183-EFG/M, in which the flux of the formic acid pathway was increased by 257%, the flux of the NADH pathway was increased by 13%, and the content of metabolites also changed significantly. In further bioreactor, the total hydrogen production of the scale-up IAM1183-EFG/M after 44 h of fermentation was 4.76 L, which increased by 18% compared with the starting strain. This study provides a new direction for future exploration of microbial hydrogen production by combinatorial modification of multiple genes.

Metatranscriptomic Analysis Reveals the Coexpression of Hydrogen-Producing and Homoacetogenesis Genes in Dark Fermentative Reactors Operated at High Substrate Loads.

Microbial communities in dark fermentation continuous systems are affected by substrate type, concentration, and product accumulation (e.g., H2 and CO2). Metatranscriptomics and quantitative PCR (qPCR) were used to assess how high organic loading rates (OLR) from 60 to 160 g total carbohydrates (TC)/L-d modify the microbial community diversity and expression of key dark fermentative genes. Overall, the microbial communities were composed of H2-producing bacteria (Clostridium butyricum), homoacetogens (Clostridium luticellarii), and lactic acid bacteria (Enteroccocus gallinarum and Leuconostoc mesenteroides). Quantification through qPCR showed that the abundance of genes encoding the formyltetrahydrofolate synthetase (fthfs, homoacetogens) and hydrogenase (hydA, H2-producing bacteria) was strongly associated with the OLR and H2 production performance. Similarly, increasing the OLR influenced the abundance of the gene transcripts responsible for H2 production and homoacetogenesis. To evaluate the effect of decreasing the H2 partial pressure, silicone oil was added to the reactor at an OLR of 138 and 160 g TC/L-d, increasing the production of H2, the copies of genes codifying for hydA and fthfs, and the genes transcripts related to H2 production and homoacetogenesis. Moreover, the metatranscriptomic analysis also showed that lactate-type fermentation and dark fermentation simultaneously occurred without compromising the reactor performance for H2 production.

Comparative effect of acid and heat inoculum pretreatment on dark fermentative biohydrogen production.

This study delves into the impact of various pretreatment methods on the inoculum in dark fermentation trials, specifically exploring thermal shock at different temperatures (60, 80, and 100 °C) and durations (15, 30, and 60 min), as well as acid shock at pH 5.5. Initial acidification of the substrate/inoculum mixture facilitates H2 generation, making acid shock an effective pretreatment option. However, it is also observed that combining thermal and acid pretreatments boosts H2 production synergistically. The synergy between thermal and acid pretreatments results in a significant improvement, increasing the overall hydrogen production efficiency by more than 9% compared to assays involving acidification alone. This highlights the considerable potential for optimizing pretreatment strategies. Furthermore, the study sheds light on the critical role of inoculum characteristics in the process, with diverse hydrogen-generating bacteria significantly influencing outcomes. The established equivalent performance of HCl and H2SO4 in inoculum pretreatment demonstrates the versatility of these acids in shaping the microbial community and influencing hydrogen production. The analysis of glucose conversion data highlights a prevalence of butyric acid in all trials, irrespective of the pretreatment method, emphasizing the dominance of the butyrate pathway in hydrogen generation. Additionally, an examination of the microbial community offers valuable insights into the intricate relationships between temperature, pH, and microbial diversity. Bacteroidota established its dominance among the bacterial populations, with a relative abundance exceeding 20-25% in the raw inoculum, and this dominance further increased following the treatment. Thermal and acid pretreatments result in significant shifts in dominant microbial communities, with some non-dominant phyla like Cloacimonadota and Spirochaetota becoming more prominent. These shifts in microbial diversity underscore the sensitivity of microbial communities to environmental conditions and pretreatment methods, further highlighting the importance of understanding their dynamics in dark fermentation processes.

Optimization of dark fermentative biohydrogen production from rice starch by Enterobacter aerogenes MTCC 2822 and Clostridium acetobutylicum MTCC 11274.

Dark fermentative biohydrogen production (DFBHP) has potential for utilization of rice starch wastewater (RSWW) as substrate. The hydrogen production of Enterobacter aerogenes MTCC 2822 and Clostridium acetobutylicum MTCC 11274, in pure culture and co-culture modes, was evaluated. The experiments were performed in a 2 L bioreactor, for a batch time of 120 h. The co-culture system resulted in highest cumulative hydrogen (1.13 L H2/L media) and highest yield (1.67 mol H2/mol glucose). Two parameters were optimized through response surface methodology (RSM)-substrate concentration (3.0-5.0 g/L) and initial pH (5.5-7.5), in a three-level factorial design. A total of 11 runs were performed in duplicate, which revealed that 4.0 g/L substrate concentration and 6.5 initial pH were optimal in producing hydrogen. The metabolites produced were acetic, butyric, propionic, lactic and isobutyric acids. The volumetric H2 productions, without and with pH adjustments, were 1.24 L H2/L media and 1.45 L H2/L media, respectively.

Strategies to control pH in the dark fermentation of sugarcane vinasse: Impacts on sulfate reduction, biohydrogen production and metabolite distribution.

pH is notably known as the main variable defining distinct metabolic pathways during sugarcane vinasse dark fermentation. However, different alkalinizing (e.g. sodium bicarbonate; NaHCO3) and/or neutralizing (e.g. sodium hydroxide; NaOH) approaches were never directly compared to understand the associated impacts on metabolite profiles. Three anaerobic structured-bed reactors (AnSTBR) were operated in parallel and subjected to equivalent operational parameters, except for the pH control: an acidogenic-sulfidogenic (R1; NaOH + NaHCO3) designed to remove sulfur compounds (sulfate and sulfide), a hydrogenogenic (R2; NaOH) aimed to optimize biohydrogen (bioH2) production, and a strictly fermentative system without pH adjustment (R3) to mainly evaluate lactic acid (HLa) production and other soluble metabolites. NaHCO3 dosing triggered advantages not only for sulfate reduction (up to 56%), but also to enhance the stripping of sulfide to the gas phase (75-96% of the theoretical sulfide produced) by the high and constant biogas flow resulting from the CO2 released during NaHCO3 dissociation. Meanwhile, molasses-based vinasse presented higher potential for bioH2 (up to 4545 mL-H2 L-1 d-1) and HLa (up to 4800 mg L-1) production by butyric-type and capnophilic lactic fermentation pathways. Finally, heterolactic fermentation was the main metabolic route established when no pH control was provided (R3), as indicated by the high production of both HLa (up to 4315 mg L-1) and ethanol (1987 mg L-1). Hence, one single substrate (from which one single source of inoculum was originated) offers a wide range of metabolic possibilities to be exploited, providing substantial versatility to the application of anaerobic digestion in sugarcane biorefineries.

Towards industrial biological hydrogen production: a review.

Increased production of renewable energy sources is becoming increasingly needed. Amidst other strategies, one promising technology that could help achieve this goal is biological hydrogen production. This technology uses micro-organisms to convert organic matter into hydrogen gas, a clean and versatile fuel that can be used in a wide range of applications. While biohydrogen production is in its early stages, several challenges must be addressed for biological hydrogen production to become a viable commercial solution. From an experimental perspective, the need to improve the efficiency of hydrogen production, the optimization strategy of the microbial consortia, and the reduction in costs associated with the process is still required. From a scale-up perspective, novel strategies (such as modelling and experimental validation) need to be discussed to facilitate this hydrogen production process. Hence, this review considers hydrogen production, not within the framework of a particular production method or technique, but rather outlines the work (bioreactor modes and configurations, modelling, and techno-economic and life cycle assessment) that has been done in the field as a whole. This type of analysis allows for the abstraction of the biohydrogen production technology industrially, giving insights into novel applications, cross-pollination of separate lines of inquiry, and giving a reference point for researchers and industrial developers in the field of biohydrogen production.

Debottlenecking the biological hydrogen production pathway of dark fermentation: insight into the impact of strain improvement.

Confronted with the exhaustion of the earth's fossil fuel reservoirs, bio-based process to produce renewable energy is receiving significant interest. Hydrogen is considered as an attractive energy carrier that can replace fossil fuels in the future mainly due to its high energy content, recyclability and environment-friendly nature. Biological hydrogen production from renewable biomass or waste materials by dark fermentation is a promising alternative to conventional routes since it is energy-saving and reduces environmental pollution. However, the current yield and evolution rate of fermentative hydrogen production are still low. Strain improvement of the microorganisms employed for hydrogen production is required to make the process competitive with traditional production methods. The present review summarizes recent progresses on the screening for highly efficient hydrogen-producing strains using various strategies. As the metabolic pathways for fermentative hydrogen production have been largely resolved, it is now possible to engineer the hydrogen-producing strains by rational design. The hydrogen yields and production rates by different genetically modified microorganisms are discussed. The key limitations and challenges faced in present studies are also proposed. We hope that this review can provide useful information for scientists in the field of fermentative hydrogen production.

Prokaryotic population dynamics and interactions in an AnSBBR using tequila vinasses as substrate in co-digestion with acid hydrolysates of Agave tequilana var. azul bagasse for hydrogen production.

AIMS: The purpose of this study was to characterize the prokaryotic community and putative microbial interactions between H2 -producing bacteria (HPB) and non-HPB using two anaerobic sequencing batch biofilm reactors (AnSBBRs) fed with tequila vinasses in co-digestion with acid hydrolysates of Agave tequilana var. azul bagasse (ATAB). METHODS AND RESULTS: Two AnSBBRs were operated for H2 production to correlate changes in physicochemical and biological variables by principal component analysis (PCA). Results indicated that H2 yield was supported by Ethanoligenens harbinense and Clostridium tyrobutyricum through the PFOR pathway. However, only E. harbinense was able to compete for sugars against non-HPB. Competitive exclusion associated with competition for sugars, depletion of essential trace elements, bacteriocin production and resistance to inhibitory compounds could be carried out by non-HPB, increasing their relative abundances during the dark fermentation (DF) process. CONCLUSIONS: The global scenario obtained by PCA correlated the decrease in H2 production with the lactate:acetate molar ratio in the influent. At the beginning of co-digestion, this ratio had the minimum value considered for a net gain of ATP. This fact could cause the reduction of the relative abundance of C. tyrobutyricum. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first study that demonstrated the feasibility of H2 production by Clostridiales from acid hydrolysates of ATAB in co-digestion with tequila vinasses.

Effect of pH on the performance of hydrogen production by dark fermentation coupled denitrification.

High concentration nitrogen-containing organic wastewater is a potential substrate for hydrogen production by dark fermentation. In this study, the effect of initial pH on the performance of hydrogen production by dark fermentation coupled denitrification was investigated. The hydrogen production, liquid metabolites, nitrate, nitrite and microbial community were monitored under the condition of pH varying from 4 to 11. Results showed that the highest hydrogen production (70.94 ± 4.750 mL/g VSS), chemical oxygen demand (COD) removal rate (37.13 ± 1.86%) and nitrate reduction rate (1.57 ± 0.27 mg/L/h) were obtained at pH of 5. Under this condition, the nitrate was mainly reduced to N2 with hydrogen as the electron donor. When the initial pH was 6-11, nitrate mainly reduced to N2 through co-action with acetate. Microbial community analysis revealed that as the initial pH increased from 4 to 11, the main hydrogen-producing microorganisms were gradually converted from Clostridium sensu stricto 12 sp. into Clostridium sensu stricto 1 sp, which leaded to changes in the hydrogen production process.

New insights into microbial interactions and putative competitive mechanisms during the hydrogen production from tequila vinasses.

This study aimed to characterize the prokaryotic community and putative microbial interactions involved in hydrogen (H2) production during the dark fermentation (DF) process, applying principal components analysis (PCA) to correlate changes in operational, physicochemical, and biological variables. For this purpose, a continuous stirred-tank reactor-type digester fed with tequila vinasses was operated at 24, 18, and 12 h of hydraulic retention times (HRTs) to apply organic loading rates of 20, 36, and 54 g-COD L-1 d-1, corresponding to stages I, II, and III, respectively. Results indicated high population dynamics for Archaea during the DF process toward a decrease in total sequences from 6299 to 99. Concerning the Bacteria community, lactic acid bacteria (LAB) were dominant reaching a relative abundance of 57.67%, while dominant H2-producing bacteria (HPB) decreased from 25.76% to 21.06% during stage III. Putative competitive exclusion mechanisms such as competition for substrates, bacteriocins production, and micronutrient depletion carried out by Archaea and non-H2-producing bacteria (non-HPB), especially LAB, could negatively impact the dominance of HPB such as Ethanoligenens harbinense and Clostridium tyrobutyricum. As a consequence, low maximal volumetric H2 production rate (672 mL-H2 L-1 d-1) and yield (3.88 mol-H2 assimilated sugars-1) were obtained. The global scenario obtained by PCA correlations suggested that C. tyrobutyricum positively impacted H2 molar yield through butyrate fermentation using the butyryl-CoA:acetate CoA transferase pathway, while the most abundant HPB E. harbinense decreased its relative abundance at the shortest HRT toward the dominance of non-HPB. This study provides new insights into the microbial interactions and helps to better understand the DF performance for H2 production using tequila vinasses as substrate. KEY POINTS: • E. harbinense and C. tyrobutyricum were responsible for H2 production. • Clostridiales used acetate and butyrate fermentations for H2 production. • LAB won the competition for sugars against Clostridiales during DF. • Putative bacteriocins production and micronutrients depletion could favor LAB.

Novel strategies towards efficient molecular biohydrogen production by dark fermentative mechanism: present progress and future perspective.

In the scenario of alarming increase in greenhouse and toxic gas emissions from the burning of conventional fuels, it is high time that the population drifts towards alternative fuel usage to obviate pollution. Hydrogen is an environment-friendly biofuel with high energy content. Several production methods exist to produce hydrogen, but the least energy intensive processes are the fermentative biohydrogen techniques. Dark fermentative biohydrogen production (DFBHP) is a value-added, less energy-consuming process to generate biohydrogen. In this process, biohydrogen can be produced from sugars as well as complex substrates that are generally considered as organic waste. Yet, the process is constrained by many factors such as low hydrogen yield, incomplete conversion of substrates, accumulation of volatile fatty acids which lead to the drop of the system pH resulting in hindered growth and hydrogen production by the bacteria. To circumvent these drawbacks, researchers have come up with several strategies that improve the yield of DFBHP process. These strategies can be classified as preliminary methodologies concerned with the process optimization and the latter that deals with pretreatment of substrate and seed sludge, bioaugmentation, co-culture of bacteria, supplementation of additives, bioreactor design considerations, metabolic engineering, nanotechnology, immobilization of bacteria, etc. This review sums up some of the improvement techniques that profoundly enhance the biohydrogen productivity in a DFBHP process.

Dark fermentative biohydrogen production from vinicultural biomass without exogenous inoculum in a semi-batch reactor: A kinetic study.

This work employed a unique kind of vinicultural biomass (grape residues) to generate fermentative hydrogen. This form of biomass serves two purposes (contains substrate and inoculum). Four mathematical model methods were established; these models were used to represent the fluctuation of hydrogen generation and other fermentation products (organic acids, alcohols), the consumption of substrates included in biomass, and bacterial growth. One of these models was verified using experimental data and used to represent all of the metabolic pathways of bacteria contained in the medium and the interaction between products and substrates. The optimal biomass load, 60 g COD (Chemical Oxygen Demand)/L with a concentration of 0.22 mol of hexose and 0.0444 mol of tartrate offers the best hydrogen yield.