Efficient lignin biodegradation triggered by alkali-tolerant ligninolytic bacteria through improving lignin solubility in alkaline solution

Low lignin solubility in aqueous solution is one of the major bottlenecks for lignin biodegradation and bioconversion.Alkaline solution contributes to improving lignin solubility,whereas most microbes can not survive in alkaline conditions.Herein,lignin dissolution behaviors in different pH solutions were systematically investigated,which indicated that solution pH above 10.5 contributed to high solubility of alkali lignin.To match with alkaline lignin aqueous system,several alkali-tolerant ligninolytic bacteria were isolated,most of which are distinct to previously reported ones.Then,the ligninolytic capabilities of these isolates were assessed in different pH conditions by determining their assimilation on alkali lignin,lignin-derived monomers and dimers,their decolorization capabilities,and their lignin peroxidase activities.Thereafter,the underlying ligninolytic and alkali-tolerant mechanisms of Sutcliffiella sp.NC1,an alkalophilic bacterium,was analyzed on the basis of its genome information.The results not only provide valuable information for lignin biodegradation and lignin valorization,but also expand knowledge on alkali-tolerant bacteria.

Optimizing bio-physical conditions and pre-treatment options for breaking lignin barrier of maize stover feed using white rot fungi

The greatest limitation to utilization of maize stover by ruminants as a feed is the high concentration of lignin, which limits fibre digestibility. However, ruminants can effectively utilize maize stover if its nutritive value is improved using white rot fungal species. This study was designed to determine optimal biophysical conditions for mycelial growth and select the most ideal fungal species and pre-treatment options for improving nutritive value of maize stover. Four popular edible Pleurotus fungal species(viz.Pleurotus florida,Pleurotus ostreatus, Pleurotus sajor caju and Pleurotus pulmonarius) were subjected to varying temperatures, pH levels, hydrogen peroxide(H_2 O_2) concentration and illumination to establish the extent of mycelial growth rate.Inclusion of H_2 O_2 was used to determine optimal levels for preservation and prevention of contamination from other indigenous microbiota. Effects of pre-treatment options on chemical composition and nutritive value of maize stover were also examined. Mycelial growth rate of Pleurotus species on potato dextrose agar(PDA) varied(P < 0.05) with temperature, pH level and H_2 O_2 concentration following a quadratic trend. Optimal temperature, pH and H_2 O_2 concentration for mycelial growth on PDA were 25 ℃,5 and 0.01 mL/L, respectively. Under the different bio-physical conditions,P. sajor caju had the highest mycelia density and growth rate. Chemical composition of solid-state fermented maize stover differed(P < 0.05) among the Pleurotus species. Maize stover fermented with P. sajor caju had the highest crude protein(CP) of 86.6 g/kg DM, in-vitro dry matter digestibility(IVDMD) of 731 g/kg DM, in-vitro organic matter digestibility(IVOMD) of 670.4 g/kg DM and metabolizable energy(ME) of10.0 MJ/kg DM but with the lowest lignin(sa) of 50 g/kg DM. At 25℃, P. sajor caju had the highest mycelial growth rate on PDA and highest lignin(sa) breakdown in the maize stover substrate. It was, therefore,selected as the most ideal fungal species for improving nutritive value of maize stover. Pre-treatment of maize stover with Lactobacillus plantarum and molasses under anaerobic condition for 7 days before inoculation with P. sajor caju resulted into a substrate with the highest(P< 0.05) CP(96.6 g/kg DM), IVDMD(752.3 g/kg DM), IVOMD(687.2 g/kg DM) and ME(10.2 MJ/kg DM). However, neutral detergent fiber exclusive of residual ash(NDFom) and lignin(sa) fractions decreased(P < 0.05) as a result of subjecting maize stover to pre-treatment with L. plantarum and molasses prior to fermentation with P. sajor caju.Therefore, pre-treatment of maize stover with L. plantarum and molasses for 7 days prior to fermentation with P. sajorcaju for 14 days in darkness at 25℃ offered the greatest potential for breaking the lignin barrier.