Supplementation of green-synthesized nickel oxide nanoparticles enhances biohydrogen production of Klebsilla sp. using lignocellulosic hydrolysate and elucidation of the regulatory mechanism

Abstract

Background In recent years, adding nanoparticles to fermentative hydrogen production system has become an effective way to increase the biohydrogen yield, however, the application of green synthesized nanoparticles to hydrogen production system is rarely studied, even to the interpretation of the regulatory mechanism, there are few reports on the regulation of hydrogen production pathway and related gene expression by addition of nanoparticles. Thus, we herein reported the green synthesis of nickel oxide nanoparticles (NiO-NPs) from Eichhornia crassipes (Ec) extract for the first time, and evaluated the regulatory effect of these NPs on fermentative hydrogen production. Results Characterization of the Ec-NiO-NPs revealed their spherical shape, small diameter (9.1 ± 2.6 nm) and high purity. The maximum cumulative hydrogen production and hydrogen yield Y(H2/S) reached 4842.19 ± 23.43 mL/L and 101.45 ± 3.32 mL/gsubstrate, respectively, in the presence of 20 mg/L Ec-NiO-NP, which were 47.29% and 37.78% higher than the control without NPs addition. Evaluation of glucose and xylose utilization efficiency as well as key node metabolites further established the potential of Ec-NiO-NP to improve the reducing sugar utilization and metabolic flux distribution in hydrogen synthesis pathway. Furthermore, addition of 20 mg/L Ec-NiO-NP resulted in enhanced hydrogenase activity with a maximum increase of 623% comparing to the control, and led to changes in the gene expression of both hydrogenase and formate-hydrogen lyase, which play important roles in promoting hydrogen production at different stages of fermentation. Conclusions The results prove that supplementation with green-synthesized Ec-NiO-NP effectively improves fermentative hydrogen production and regulates key node metabolites alteration and functional gene expression. This study provides a cheap and eco-friendly method to enhance fermentative hydrogen production and new insights to reveal the regulatory mechanism underlying NP mediated increases in biohydrogen synthesis.