Hydrogen metabolic patterns driven by Clostridium-Streptococcus community shifts in a continuous stirred tank reactor

Abstract

The hydrogen (H) production efficiency in dark fermentation systems is strongly dependent on the occurrence of metabolic pathways derived from the selection of microbial species that either consume molecular H or outcompete hydrogenogenic bacteria for the organic substrate. In this study, the effect of organic loading rate (OLR) on the H production performance, the metabolic pathways, and the microbial community composition in a continuous system was evaluated. Two bacterial genera, Clostridium and Streptococcus, were dominant in the microbial community depending on the OLR applied. At low OLR (14.7-44.1 g/L-d), Clostridium sp. was dominant and directed the system towards the acetate-butyrate fermentation pathway, with a maximum H yield of 2.14 mol/mol obtained at 29.4 g/L-d. Under such conditions, the volumetric hydrogen production rate (VHPR) was between 3.2 and 11.6 L/L-d. In contrast, relatively high OLR (58.8 and 88.2 g/L-d) favored the dominance of Streptococcus sp. as co-dominant microorganism leading to lactate production. Under these conditions, the formate production was also stimulated serving as a strategy to dispose the surplus of reduced molecules (e.g., NADH), which theoretically consumed up to 5.72 L/L-d. In such scenario, the VHPR was enhanced (13.7-14.5 L/L-d) but the H yield dropped to a minimum of 0.74 mol/mol at OLR = 58.8 g/L-d. Overall, this research brings clear evidence of the intrinsic occurrence of metabolic pathways detrimental for biohydrogen production, i.e., lactic acid fermentation and formate production, suggesting the use of low OLR as a strategy to control them.

Dynamics of Dark Fermentation Microbial Communities in the Light of Lactate and Butyrate Production

“…Lactate and acetate were transformed to butyrate at pH ≈ 7 when the substrate did not contain carbohydrates and or 5-6 when the substrate contained molasses or pure sucrose. Other studies reported pH in the range of 5.5-6.5 as optimal for hydrogen production and butyrate formation from lactate and acetate [13,21,24,[29][30][31]50].…”

“…The results of our research are generally consistent with those of other groups. On one hand they con rm that lactic acid bacteria compete with dark fermentation bacteria and inhibit their growth [9][10][11][12][13].…”

“…Substrate competition includes replacement of hydrogen fermentation by lactic acid or ethanol fermentation. Decrease in hydrogen production is observed with simultaneous increase of lactic acid and ethanol concentrations among nongaseous fermentation products [9][10][11][12][13].…”

Dynamics and Complexity of Dark Fermentation Microbial Communities Producing Hydrogen From Sugar Beet Molasses in Continuously Operating Packed Bed Reactors

“…Our observations regarding the low efficiency of hydrogen production in PBR4 operation at the point of PBR4_5 sample collection and PBR5 operation at the point of PBR5_12 sample collection confirm the commonly recognized fact about the negative role of the LAB in DF bioreactors. They are regarded as microorganisms that compete for the substrate and shift the type of fermentation toward lactic acid fermentation (Noike et al, 2002;Ren et al, 2007;Jo et al, 2008;Sreela-Or et al, 2011;Etchebehere et al, 2016;Palomo-Briones et al, 2018). Analysis of the non-gaseous fermentation products in PBR4_5 and PBR5_12 samples revealed lactic acid as the dominant fermentation product and a low TABLE 3 | A set of different studies presenting contribution of and proportions between hydrogen producing bacteria and lactic acid bacteria in hydrogen-producing microbial communities.…”

“…In the homolactic fermentation, two molecules of pyruvate are converted to two molecules of lactate whereas in the heterolactic fermentation the products are lactate, ethanol and carbon dioxide. The LAB can also secrete bacteriocins that inhibit the growth of the hydrogenproducing bacteria (Noike et al, 2002;Ren et al, 2007;Jo et al, 2008;Sreela-Or et al, 2011;Etchebehere et al, 2016;Palomo-Briones et al, 2018). On the other hand, lactate and acetate can be converted to butyrate, carbon dioxide and hydrogen.…”