With global economic development and industrialization, environmental pollution and energy shortage are becoming increasingly significant. The production of fuels from renewable feedstocks is essential for a fossil fuel independent economy (James and Srikanth, 2023). It is estimated that there are ∼360 billion tons of lignin resources in nature currently (Becker and Wittmann, 2019). Given the vast amount of lignin, its valorization benefits sustainable development of the world (Azubuike et al., 2022). However, a very limited amount of lignin is applied for high-value utilization currently (Abu-Omar et al., 2021). For example, approximately 50 million tons of lignin is generated from the pulp and paper industry per year, of which only 2% is utilized for lignin derivative products while the major part is burned for energy production or abandoned in landfills, which poses a significant threat to the environment (Beaucamp et al., 2022). Meanwhile, the valorization of lignin in biorefinery waste streams for biofuel production represents a unique opportunity for improving energy efficiency in biorefineries (Cao et al., 2021). Many countries have focused on biomass energy research as a priority project, such as the Energy Farm in the United States, the Alcohol Energy Program in Brazil, and the Sunshine Project in Japan (Tabe-Ojong and Naphtal, 2017). For example, The USA BioEnergy LLC claims that a biorefinery plant will be built in 2025, which can convert 1 million tons of wood waste into 139 million liters of renewable biofuel annually (Cao et al., 2021; Tabe-Ojong and Naphtal, 2017). Therefore, the value-added valorization of lignin is significant for alleviating energy and environmental challenges.

Bioconversion is a promising process for fuel production due to its mild operation conditions and environmentally friendly processes (Borchert et al., 2022). Owing to its catabolic diversity, Rhodococcus has emerged as a promising microorganism for converting biomass into high-value products (Zhao et al., 2020). Compared with other bacteria, Rhodococcus has strong environmental adaptability and can grow well in a medium with pH 6-8 (Wang et al., 2020). It can not only utilize common carbon sources like glucose, fructose, and lactose, but also can utilize alkanes (e.g., hexadecane), lignin-derived aromatics (Shields-Menard et al., 2019; Bhatia et al., 2019b), lignin polymer (Li et al., 2019a; Zhao et al., 2022b), and other aromatic compounds because it has unique metabolic pathways to degrade alkanes and aromatic hydrocarbons (Chen and Wan, 2017a). The carbon content within lignin and sugars (e.g., glucose and xylose) is around 60.4% and 40.0%, respectively. The oxygen content within lignin and sugars is around 33.9% and 53.3%, respectively. The hydrogen content within lignin and sugars is around 5.7% and 6.7%, respectively. It is deduced that sugars (e.g., glucose and xylose) are more oxidized than lignin (Zhang et al., 2007). Currently, sugars (e.g., glucose) are the major carbon sources used in the fermentation industry. Lipid production through fermentation mainly uses sugars as the substrates without exception (Patel et al., 2021). Given the huge amount of lignin, producing lipids with lignin can significantly reduce the cost of feedstock (Liu et al., 2022a).

R. opacus PD630, which was discovered and separated as early as in 1995, can synthesize lipids up to 76% of cell dry weight (CDW) utilizing gluconic acid under nitrogen source limitation conditions (Jiang et al., 2022; Wei et al., 2015c). With the natural tolerance to aromatics, diversity of metabolic pathways, and ability to utilize various carbon sources for lipid production, R. opacus PD630 has gained significant interest from both researchers and industrialists. Up to now, R. opacus PD630, as a model bacterium, has been recognized as one of the most promising candidate strains for the development of bioenergy and industrial-scale production of biodiesel/aviation fuel (Liu et al., 2022a). Table 1 shows that a large number of industrial raw materials can be used as carbon sources by Rhodococcus for lipid production. Not only limited to microbial lipids, Rhodococcus can also utilize various kinds of agricultural and industrial wastes to produce other high value-added bioproducts like polyhydroxyalkanoates (PHA) and pyridine-dicarboxylic acid (Jiang et al., 2022). For example, Salvachúa and coworkers employed R. jostii RHA1 to utilize alkaline pretreatment liquor of corn stover as a carbon source to produce PHA. The maximum PHA concentration reached 288 mg/L (Salvachúa et al., 2015). PHA is a kind of outstanding sustainable material as it presents biodegradability and biocompatibility (Zhao et al., 2023). It can be utilized in various value-added fields, such as biomaterials and medical industry (Ghosh et al., 2022). Spence and colleagues applied R. jostii RHA1 to utilize soda lignin to produce pyridine-dicarboxylic acid, which is an alternative to terephthalic acid for the synthesis of polyester bioplastics (Spence et al., 2021). The techno-economic assessment highlights that bacterial conversion of lignin to value-added products (e.g., lipids and polyhydroxyalkanoates) is significant for developing a bioeconomy to help address growing energy and materials demands (Liu et al., 2019).

Although converting lignin into lipids by Rhodococcus is very promising, its large-scale production has not been achieved yet. Lignin is a complex aromatic heteropolymer, which is composed of phenylpropane units, including guaiacyl (G), syringyl (S), and p-hydroxyl phenol (H). These monomers are cross-linked via various ether and C-C bonds, such as β-O-4-aryl ether, β-5-phenylcoumaran, and β-β-resinol linkage (Liu et al., 2019). Besides, the lignin-carbohydrate complex (LCC) is formed by covalent linkages between lignin and carbohydrates (e.g., cellulose and hemicelluloses), leading to the difficulty in separating and depolymerizing lignin from the lignocellulosic biomass (He et al., 2022). Due to the diversity of lignin monomers and inter-unit linkages, lignin structure varies considerably as a function of biomass source and pretreatment/fractionation methods. At present, the metabolic network to convert lignin into lipids has not been revealed completely, which limits an overall view and systematic optimization of the conversion process (Zhao et al., 2020). Since bacterial lignin degradation capacity is generally weak, either depolymerizing lignin through advanced pretreatment methods or building engineered microbial strains are essential to enhance lignin bioconversion (Liu et al., 2022a). Although Rhodococcus holds natural tolerance to various aromatic compounds, the high concentration of lignin-derived aromatics (e.g., phenolic compounds) generated during the lignocellulosic biomass pretreatment/fractionation process would inhibit microbial growth and metabolism. In light of these challenges, this review systematically summarizes the microbial conversion pathways to construct the metabolic network to convert lignin into lipids. Systems biology strategies were applied and analyzed to figure out the key enzymes and intermediates for improving lipids titer. Advances in creating powerful microbial strains through genetic engineering are reviewed. Besides modifying microbial strains, strategies for enhancing lignin bioaccessibility in aqueous fermentation media to facilitate microbial utilization are presented. Moreover, lignin depolymerization methods are systematically analyzed and compared to highlight their importance for biological valorization. Fermentation strategies are also important. Hence, advanced fermentation processes for promoting lignin bioconversion are summarized. At last, the effects of lipid compositions on biodiesel quality and the potential for manufacturing aviation fuels are analyzed. Overall, this study aims to unleash the capacity of Rhodococcus to convert lignin into lipids through a systematical approach, which includes synergistically designing microbial strains, modifying lignin substrates, and developing advanced fermentation processes.