Description

Lignocellulosic biomass

Lignocellulosic biomass is the most plentiful renewable resource with an annual yield of about 10 billion tons on the earth (Nguyen et al., 2019). However, most of lignocellulosic biomass are discarded or burned in the field, leading to resource waste and environmental pollution (Liang et al., 2021a). Reasonable and effective utilization of lignocellulosic biomass is of great significance for relieving energy crisis and environmental pollution.

Anaerobic fermentation is an effective way to convert lignocellulose into volatile fatty acids (VFAs) (Fang et al., 2020). VFAs, as important products of anaerobic fermentation, can be used in many industries like chemical, food, and pharmaceutical, as industrial raw materials to generate a variety of products, such as bioplastics, biodiesel, biogas, etc., and as carbon source to remove nutrients from wastewater (Liu et al., 2023a; Liu et al., 2023b).

However, hydrolysis of lignocellulosic biomass is the rate-limiting step in anaerobic fermentation owing to its complex structure (Liang et al., 2023). Some physical, chemical, biological, and combined pretreatments have been used to improve the hydrolysis efficiency of lignocellulosic biomass, but these pretreatments have disadvantages such as operational complexity and secondary contamination (Hoang et al., 2023; Singh et al., 2022).

Research Focus Areas

The rumen digestive system of ruminants is an efficient ecosystem for hydrolyzing lignocellulosic biomass (Moraïs and Mizrahi; 2019a). rumen microorganisms produce VFAs that can supply 70% of the nutritional requirements of ruminants (Gharechahi et al., 2023). The efficient hydrolysis and acidogenesis of lignocellulosic biomass in rumen is mainly due to the complex and stable microbial ecosystem, which is mainly composed of bacteria, fungi, protozoa and archaea (Liang et al., 2020). Recently, rumen microorganisms were used as efficient anaerobic inoculants for VFA production owing to their high capacity to hydrolyze lignocellulosic biomass (Mizrahi et al., 2021; Ozbayram et al., 2020).

Rumen microorganisms as inoculum efficiently degraded a wide range of straw biomass, and the VFA yield was 0.3-0.4 g/g volatile solid (VS) after only 3 days of fermentation (Liang et al., 2021; Liu et al., 2016). Anaerobic fermentation of biomass by rumen microorganisms produced four times more VFAs than the activated sludge (Nguyen et al., 2019). Moreover, the VFA production efficiency of rumen microorganisms from untreated lignocellulose was comparable to or even higher than that of activated sludge from pretreated lignocellulose (Liang et al., 2023). Therefore, rumen microorganisms have great potential as inoculum in biomass conversion to VFAs. To our knowledge, the changes of fungi composition, key functional genes, and CAZymes during lignocellulose hydrolysis and acidogenesis with rumen microorganisms are still unclear. The effect of high substrate loads on VFA production during long-term rumen fermentation has not been explored.

In rumen fermentation, does the greater the amount of nutrients, the greater the production of volatile fatty acids?

Therefore, the aims of this study are to reveal the changes of rumen bacteria and fungi community composition, functional genes for VFA production in metabolic pathway, and CAZymes composition in rumen anaerobic fermentation. Meanwhile, a rumen semi-continuous reactor was designed with corn straw, a classic biomass waste, as the fermentation substrate. The effect of hydrolysis and acidogenesis in a rumen semi-continuous reactor at different corn straw loads were investigated. To be specific, the VFA and soluble chemical oxygen demand (SCOD) concentration, fermentation broth pH and straw physicochemical properties were measured during anaerobic fermentation. Rumen bacterial and fungal community compositions, key functional genes associated with VFA metabolic pathways, and CAZymes involving in carbohydrate metabolism were explored through the metagenomic sequencing.

The VS removal, pH, VFA concentration and composition at different corn straw loads were measured. The difference in diversity, community, and gene expression of bacteria at different corn straw loads were analyzed. The correlation differences between bacteria at different corn straw loads was elucidated; The correlation of bacteria with VS removal and VFA concentration at different corn straw loads were explored. This study provided some new insights into rumen microorganisms at genetic levels, providing a theoretical reference for the application of rumen microorganisms in practice.