Energy is a crucial entity for the development and it has various alternative forms of energy sources. Recently, the synthesis of nanoparticles using benign biocatalyst has attracted increased attention. In this study, silver nanoparticles were synthesized and characterized using Azadirachta indica plant-derived phytochemical as the reducing agent. Biomass of the microalga Chlorella sp. cultivated in BG11 medium increased after exposure to low concentrations of up to 0.48 mg L-1 AgNPs. In addition, algal cells treated with 0.24 mg L-1 AgNPs and cultivated in BG110 medium which contained no nitrogen source showed the highest hydrogen yield of 10.8 mmol L-1, whereas the untreated cells under the same conditions showed very low hydrogen yield of 0.003 mmol L-1. The enhanced hydrogen production observed in the treated cells was consistent with an increase in hydrogenase activity. Treatment of BG110 grown cells with low concentration of green synthesized AgNPs at 0.24 mg L-1 enhanced hydrogenase activity with a 5-fold increase of enzyme activity compared to untreated BG110 grown cells. In addition, to improve photolytic water splitting efficiency for hydrogen production, cells treated with AgNPs at 0.24 mg L-1 showed highest oxygen evolution signifying improvement in photosynthesis. The silver nanoparticles synthesized using phytochemicals derived from plant enhanced both microalgal biomass and hydrogen production with an added advantage of CO2 reduction which could be achieved due to an increase in biomass. Hence, treating microalgae with nanoparticles provided a promising strategy to reduce the atmospheric carbon dioxide as well as increasing production of hydrogen as clean energy.

In the present study, a dark fermentation system inoculated with mixed culture bacteria (MCB) was developed using prepared alkali-based magnetic nanosheets (AMNSs) to facilitate biohydrogen (BioH2) production. The highest BioH2 yields of 232.8 ± 8.5 and 150.3 ± 4.8 mL/g glucose were observed at 100 (mesophilic condition) and 400 (thermophilic condition) mg/L AMNSs groups, which were 65.4% and 43.3%, respectively, above the 0 mg/L AMNSs group. The fermentation pathway revealed that AMNSs enhanced the butyrate-type metabolic pathway and the corresponding nicotinamide adenine dinucleotides (NADHand NAD+) ratio, and hydrogenase activity was enhanced in mesophilic fermentation. The interaction of AMNSs and MCB suggested that AMNSs could assist in electron transfer and that the released metal elements might be responsible for elevated hydrogenase activity. AMNSs also promoted the evolution of the dominant microbial community and altered the content of extracellular polymers, leading to increased production of BioH2.

BACKGROUND: The chemolithoautotrophic β-proteobacterium Ralstonia eutropha H16 (Cupriavidus necator) is one of the most studied model organisms for growth on H2 and CO2. R. eutropha H16 is also a biologically significant bacterium capable of synthesizing O2-tolerant [NiFe]-hydrogenases (Hyds), which can be used as anode biocatalysts in enzyme fuel cells. For heterotrophic growth of R. eutropha, various sources of organic carbon and energy can be used.
RESULTS: Growth, bioenergetic properties, and oxidation-reduction potential (ORP) kinetics were investigated during cultivation of R. eutropha H16 on fructose and glycerol or lignocellulose-containing brewery spent grain hydrolysate (BSGH). BSGH was used as carbon and energy source by R. eutropha H16, and the activities of the membrane-bound hydrogenase (MBH) and cytoplasmic, soluble hydrogenase (SH) were measured in different growth phases. Growth of R. eutropha H16 on optimized BSGH medium yielded ~ 0.7 g cell dry weight L-1 with 3.50 ± 0.02 (SH) and 2.3 ± 0.03 (MBH) U (mg protein)-1 activities. Upon growth on fructose and glycerol, a pH drop from 7.0 to 6.7 and a concomitant decrease of ORP was observed. During growth on BSGH, in contrast, the pH and ORP stayed constant. The growth rate was slightly stimulated through addition of 1 mM K3[Fe(CN)6], whereas temporarily reduced growth was observed upon addition of 3 mM dithiothreitol. The overall and N,N\'-dicyclohexylcarbodiimide-sensitive ATPase activities of membrane vesicles were ~ 4- and ~ 2.5-fold lower, respectively, upon growth on fructose and glycerol (FGN) compared with only fructose utilization (FN). Compared to FN, ORP was lower upon bacterial growth on FGN, GFN, and BSGH.
CONCLUSIONS: Our results suggest that reductive conditions and low ATPase activity might be signals for energy depletion, which, in turn, leads to increased hydrogenase biosynthesis to overcome this unfavorable situation. Addition of fructose or microelements have no, or a negative, influence on hydrogenase activity. Organic wastes (glycerol, BSGH) are promising carbon and energy sources for the formation of biomass harboring significant amounts of the biotechnologically relevant hydrogenases MBH and SH. The results are valuable for using microbial cells as producers of hydrogenase enzymes as catalysts in enzymatic fuel cells.

Conversion of organic matter to biohydrogen possesses promising application potential. In this study, low-cost ferrihydrite nanorods were used to enhance hydrogen production by Clostridium pasteurianum. The maximum cumulative hydrogen production and the hydrogen yield were 1.03 mmol and 3.55 mol H2/mol glucose, respectively, which were 68.9% and 15.6% higher than those of the batch groups without ferrihydrite addition. Moreover, in comparison with magnetite and hematite nanoparticles, ferrihydrite presented the best stimulation for hydrogen evolution. The enhancement mechanisms were explored based on metabolic distribution, gene expression, enzymatic activity, and metabolite determination, such as Fe(II) concentration and pH value. The potential stimulation mechanisms are summarized as follows: ferrihydrite improves glucose conversion efficiency and promotes cell growth; ferrihydrite elevates the transcripts and activity of hydrogenase; and ferrihydrite reduction via its buffer function could ease acidification. This study demonstrates that ferrihydrite addition is an effective and green strategy to enhance fermentative hydrogen production.