Abstract
BACKGROUND: Lignocellulosic feedstocks have attracted much attention as a potential carbon source for lactic acid (LA) production because of their ready availability, sustainability, and renewability. However, there are at least two major technical challenges to producing LA from lignocellulose. Inhibitors derived from lignocellulose pretreatment have a negative impact on the growth of cells producing LA. Furthermore, pentose sugars produced from the pretreatment are difficultly utilized by most LA producers, which is known as the carbon catabolite repression (CCR) effect. This complex feedstock can be utilized by a robust microbial consortium with high bioconversion efficiency. RESULTS: In this study, a thermophilic consortium DUT50 producing LA was enriched and employed to improve corn stover (CS) utilization. Enterococcus was the dominant family in the consortium DUT50, accounting for 93.66% of the total abundance, with Lactobacillus, Bacillus, Lactococcus, and Trichococcus accounted for the remaining 2.68%. This consortium could be resistant to inhibitors concentration up to 9.74 g/L (2.88 g/L acetic acid, 2.46 g/L furfural, 2.20 g/L 5-HMF, and 2.20 g/L vanillin derived from pretreatment of CS), and simultaneously metabolizes hexose and pentose without CCR effect. Based on the promising consortium features, an efficient process of simultaneous saccharification and co-fermentation (SSCF) was developed to produce LA from acid pretreated corn stover, in which solid-liquid separation and detoxification were avoided. The key influencing factors were investigated and optimized, including dry biomass and cellulase loading, corn steep liquor powder concentration, and the pre-hydrolysis time. The highest LA titer of 71.04 g/L with a yield of 0.49 g/g-CS was achieved at a dry biomass loading of 20% (w/v), which is the highest LA production from non-detoxified acid pretreated corn stover via the SSCF process without wastewater generation reported to date. The simultaneous metabolism of hexose and pentose revealed collaboration between Enterococcus in the consortium, whereas xylose may be efficiently metabolized by Lactobacillus and Bacillus with low abundance via the pentose phosphate pathway. CONCLUSIONS: The experimental results demonstrated the potential advantage of symbiosis in microbial consortia used for LA production from lignocellulosic biomass.