Do furanic and phenolic compounds of lignocellulosic and algae biomass hydrolyzate inhibit anaerobic mixed cultures? A comprehensive review.

Nowadays there is a growing interest on the use of both lignocellulosic and algae biomass to produce biofuels (i.e. biohydrogen, ethanol and methane), as future alternatives to fossil fuels. In this purpose, thermal and thermo-chemical pretreatments have been widely investigated to overcome the natural physico-chemical barriers of such biomass and to enhance biofuel production from lignocellulosic residues and, more recently, marine biomass (i.e. macro and microalgae). However, the pretreatment technologies lead not only to the conversion of carbohydrate polymers (ie cellulose, hemicelluloses, starch, agar) to soluble monomeric sugar (ie glucose, xylose, arabinose, galactose), but also the generation of various by-products (i.e. furfural and 5-HMF). In the case of lignocellulosic residues, part of the lignin can also be degraded in lignin derived by-products, mainly composed of phenolic compounds. Although the negative impact of such by-products on ethanol production has been widely described in literature, studies on their impact on biohydrogen and methane production operated with mixed cultures are still very limited. This review aims to summarise and discuss literature data on the impact of pre-treatment by-products on H2-producing dark fermentation and anaerobic digestion processes when using mixed cultures as inoculum. As a summary, furanic (5-HMF, furfural) and phenolic compounds were found to be stronger inhibitors of the microbial dark fermentation than the full anaerobic digestion process. Such observations can be explained by differences in process parameters: anaerobic digestion is performed with more complex mixed cultures, lower substrate/inoculum and by-products/inoculum ratios and longer batch incubation times than dark fermentation. Finally, it has been reported that, during dark fermentation process, the presence of by-products could lead to a metabolic shift from H2-producing pathways (i.e. acetate and butyrate) to non-H2-producing pathways (i.e. lactate, ethanol and propionate) and whatever the metabolic route, metabolites can be all further converted into methane, but at different rates.

Identification of different alkane hydroxylase systems in Rhodococcus ruber strain SP2B, an hexane-degrading actinomycete.

AIMS: To investigate the alkane-hydroxylating system of isolate SP2B, closely related to Rhodococcus ruber DSM 43338(T) and uncharacterized so far for its alkane degradation genes. METHODS AND RESULTS: Although isolate SP2B and reference strain can grow on by-products from hexane degradation, the type strain R. ruber was unable, unlike SP2B isolate, to use short-chain alkanes, as assessed by gas chromatography. Using PCR with specific or degenerated primers, inverse PCR and Southern blot, two alkane hydroxylase encoding genes (alkB) were detected in both bacteria, which is in agreement with their alkane range. The first AlkB was related to Rhodococcus AlkB7 enzymes and contains a nonbulky residue at a specific position, suggesting it might be involved in medium- and long-chain alkane oxidation. The second partial alkB gene potentially belongs to alkB5-type, which was found in bacteria unable to use hexane. Moreover, a partial P450 cytochrome alkane hydroxylase, thought to be responsible for the hexane degradation, was detected only in the isolated strain. CONCLUSIONS: Rhodococcus ruber SP2B should prove to be a promising candidate for bioremediation studies of contaminated sites because of its large degradation range of alkanes. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first thorough study on R.ruber alkane degradation systems.