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Sophorolipids are among the best-positioned microbial biosurfactants to reach large-scale industrial production and application. However, the structural variety of sophorolipids offered by wild-type strains is rather limited, requiring their efficient modification to expand the application areas of sophorolipids. A combination of genetic engineering and green chemical modification via ozonolysis was applied in this work to produce key precursors useful in the development of a library of sophorolipid derivatives. Uniform symmetrical α,ω-bola sophorosides, produced by a novel strain of Starmerella bombicola, were investigated as substrates for the first time to generate 100% ω-C9 sophorosides (key precursors in the development of a sophorolipid library) via ozonolysis in water. Ozonolysis yielded a mixture of C9:0 ω-sophoroside aldehydes and C9:0 ω-sophorolipid acids. The selectivity toward the C9:0 ω-sophoroside aldehyde was increased using catalase, limiting the overoxidation of the aldehyde by the in situ formed H2O2. The C9:0 ω-sophorolipid acid could be produced selectively by extending the ozonolysis time. Moreover, using water as the solvent during ozonolysis proved to be beneficial in suppressing the formation of ozonides, therefore eliminating the need to perform a reductive or oxidative workup. Consequently, an efficient, safe, and scalable route has been established for the production of key sophoroside precursors.

Poly(3-hydroxybutyrate) (PHB) is a microbially produced biopolymer that is emerging as a propitious alternative to petroleum-based plastics owing to its biodegradable and biocompatible properties. However, to date, the relatively high costs related to the PHB production process are hampering its widespread commercialization. Since feedstock costs add up to half of the total production costs, ample research has been focusing on the use of inexpensive industrial side streams as carbon sources. While various industrial side streams such as second-generation carbohydrates, lignocellulose, lipids, and glycerol have been extensively investigated in liquid fermentation processes, also gaseous sources, including carbon dioxide, carbon monoxide, and methane, are gaining attention as substrates for gas fermentation. In addition, recent studies have investigated two-stage processes to convert waste gases into PHB via organic acids or alcohols. In this review, a variety of different industrial side streams are discussed as more sustainable and economical carbon sources for microbial PHB production. In particular, a comprehensive overview of recent developments and remaining challenges in fermentation strategies using these feedstocks is provided, considering technical, environmental, and economic aspects to shed light on their industrial feasibility. As such, this review aims to contribute to the global shift towards a zero-waste bio-economy and more sustainable materials.