The ruminal digestion system efficiently degrades lignocellulose into soluble sugars, which are further converted into volatile fatty acids (VFAs) for energy metabolism (Bhujbal et al., 2022). Lignocellulose degradation and metabolism are performed by the ruminal microbiota, comprising bacteria, fungi, protozoa, and archaea (Mizrahi et al., 2021). Diverse and complex ruminal microbes work together to efficiently degrade lignocellulose and produce volatile fatty acids (VFAs), exhibiting a dynamic balance between synergy and constraints (Basak et al., 2022). Ruminal bacteria efficiently degrade lignocellulosic biomass, convert it to VFAs, and participate in urea and ammonia metabolism as well as microbial protein synthesis (Gharechahi et al., 2023; Henderson et al., 2015). Furthermore, ruminal fungi and protozoa degrade lignocellulosic biomass via physical and chemical processes, and their coexistence with methanogens enables lignocellulose degradation and H2 consumption (Leahy et al., 2022a, Leahy et al., 2022b). Synergistically, these microbes facilitate the carbon, nitrogen, and hydrogen metabolism in the rumen.

Ruminal microbes (bacteria, fungi, and protozoa) represent the main carbohydrate degraders, mainly owing to their efficient carbohydrate-active enzyme (CAZyme) secreting abilities, including glycoside hydrolase (GHs) and glycosyl transferase (GTs) (Hess et al., 2011; Liang et al., 2020). Polysaccharide utilization loci (PULs) and cellulosomes present the main enzymatic strategies for ruminal microbial polysaccharide degradation (Tomazetto et al., 2020). Bacteroidetes represent the main cellulose- and hemicellulose-degrading bacteria, and they possess abundant PULs, which collaborate with multiple proteins to achieve integrated plant polysaccharide recognition, capture, and degradation (Feng et al., 2022). Cellulosomes are multi-enzyme complexes that efficiently degrade lignocellulose and are synthesized and secreted by ruminal anaerobic bacteria and fungi (Dorival et al., 2022). Currently, most studies on efficient enzymes mainly focus on CAZymes (Hinsu et al., 2021; Liang et al., 2020). The modes of interaction and regulatory mechanisms of PULs and cellulosomes in the ruminal ecosystem are not well elucidated.

Ruminal microbes have been employed as inocula or bioaugmentation agents in anaerobic fermentation to ferment biomass waste owing to their lignocellulosic biomass degradation performance (Bhujbal et al., 2022; Ozbayram et al., 2020). The use of ruminal microbes as biological pretreatment or bioaugmentation agents for biomass waste significantly improves lignocellulosic degradation efficiency and CH4 yield (Fonoll et al., 2021; Liang et al., 2020). Furthermore, ruminal microbes efficiently hydrolyze various biomass wastes, such as lawn grass, corn straw, and food waste to obtain a considerable VFA yield (Liang et al., 2020), and can further convert them into medium-chain fatty acids (MCFAs) (Ma et al., 2022). Using biomass as a substrate, ruminal microbes perform hydrolysis and acidification during anaerobic fermentation (Agarwal et al., 2015). However, biomass waste conversion to biofuels and organic acids by ruminal microbes remains preliminary.

Ruminal microbial composition, enzymes, and their applications to biofuels and value-added chemicals have been reviewed. However, previous reviews lack an in-depth discussion of the enzymatic and metabolic mechanisms (Carrillo-Reyes et al., 2016; Yue et al., 2013). Therefore, exploration of the mechanisms through which ruminal microbes interact and the strategies used by them to secrete enzymes and metabolize carbon, nitrogen, and hydrogen is needed to develop ideas and methods for ruminal microbe and enzyme resource development, along with biomass waste resource use. This review summarizes the key ruminal microbe function and mechanisms involved in efficient lignocellulose degradation. Furthermore, we elaborate on the interactive mechanisms of CAZymes and enzymatic strategies for lignocellulose degradation. We also investigate carbon, nitrogen, and hydrogen metabolism in the rumen in detail along with the research progress on ruminal microbes in biomass conversion into biomethane, biohydrogen, and organic acids. Finally, we present the current knowledges, challenges, and future perspectives associated with research on ruminal microbes and lignocellulose degradation.