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.

Anaerobic digestion of bean straw applying a fungal pre-treatment and using cow manure as co-substrate.

The significant amounts of agriculture residues such as bean straw (BS) in rural areas, advises its valorisation for energy recovery. The feasibility of using BS for biogas production through anaerobic digestion was assessed. Prior to this, a fungal pre-treatment to hydrolyse BS with Pleutorus ostreatus was studied at 30°C and 100 rpm in orbital incubators with 1, 10 and 30 mg fungus/g straw for 14, 21 and 28 days. Then, anaerobic digestion experiments were performed in batch with cow manure (CM) as co-substrate and pre-treated BS at ratios (g/g total solids) of 1/2, 1/3, 1/5 and 0/1. Maximum lignin (18%) and hemicellulose (44%) degradation occurred at 30 mg fungus/g straw and 28 days, along with the highest total methane yield (38 mL CH4/g VS loaded). The total amount of methane decreased when increasing CM in the experiments (701.4-191.5 mL CH4), suggesting inhibition owed to a component of CM. Self-sustained biogas production of BS occurred due to the presence of bacteria (i.e. Bacilli and Bacteroidia) and archea (i.e. Methanobacteria and Methanomicrobia). However, the usage of a full-active inoculum should be studied for higher biogas production rates.

Improvement of continuous hydrogen production using individual and binary enzymatic hydrolysates of agave bagasse in suspended-culture and biofilm reactors.

Continuous hydrogen (H2) production from individual (Stonezyme, IH) and binary (Celluclast-Viscozyme, BH) enzymatic hydrolysates of agave bagasse was evaluated in continuous stirred-tank reactors (CSTR) and trickling bed reactors (TBR). The volumetric H2 production rates (VHPR) in CSTR were 13 and 2.25 L H2/L-d with BH and IH, respectively. Meanwhile, VHPR of 5.76 and 2.0 L H2/L-d were obtained in the TBR configuration using BH and IH, respectively. Differences on VHPR between reactors could be explained by substrate availability, which is intrinsic to the growth mode of each reactor configuration; while differences of VHPR between hydrolysates were possibly related to the composition of enzymatic hydrolysates. Furthermore, homoacetogenesis was strongly influenced by H2 and substrate transfer conditions. Considering VHPR, H2 yields, and costs of hydrolysis, hydrogen production from binary hydrolysates of agave bagasse was identified as the most promising alternative evaluated with scale-up potential for the production of energy biofuels.