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Biodegradable and biobased surface active agents are renewable and environmentally friendly alternatives to petroleum derived or oleochemical surfactants. However, they are accompanied by relatively high production costs. In this study, the aim was to reduce the production costs for an innovative type of microbial biosurfactant: bolaform sophorolipids, produced by the yeast Starmerella bombicola ΔsbleΔat. A novel continuous retentostat set-up was performed whereby continuous broth microfiltration retained the biomass in the bioreactor while performing an in situ product separation of bolaform sophorolipids. Although a mean volumetric productivity of 0.56 g L^(−1) h^(−1) was achieved, it was not possible to maintain this productivity, which collapsed to almost 0 g L^(−1) h^(−1). Therefore, two process adaptations were evaluated, a sequential batch strategy and a phosphate limitation alleviation strategy. The sequential batch set-up restored the mean volumetric productivity to 0.66 g L^(−1) h^(−1) for an additional 132 h but was again followed by a productivity decline. A similar result was obtained with the phosphate limitation alleviation strategy where a mean volumetric productivity of 0.54 g L^(−1) h^(−1) was reached, but a productivity decline was also observed. Whole genome variant analysis uncovered no evidence for genomic variations for up to 1306 h of retentostat cultivation. Untargeted metabolomics analysis identified 8-hydroxyguanosine, a biomarker for oxidative RNA damage, as a key metabolite correlating with high bolaform sophorolipid productivity. This study showcases the application of a retentostat to increase bolaform sophorolipid productivity and lays the basis of a multi-omics platform for in depth investigation of microbial biosurfactant production with S. bombicola.

Agro and food industries generate various side- and waste streams that, in addition to being an economic loss, also contribute to the emission of greenhouse gases and to a substantial waste of resources. Apart from offering solutions for the issues above, microbial conversion of food waste to value-added products like microbial biosurfactants will reduce the carbon footprint and price of these products, as it replaces first-generation substrates that are the main contributors to cost and impact. In order to achieve this goal, strain- and process engineering has been applied to enable the use of different waste streams as feedstock for the yeast Starmerella bombicola. S. bombicola is a natural producer of sophorolipids, a class of glycolipid biosurfactants consisting of a hydrophilic sophorose group attached to a hydrophobic fatty acid tail. Within this research, in the framework of the European BBI-JU funded WASTE2FUNC project and the FWO SB scholarship of ir. T. Van de Craen, the focus lays on various new-to-nature biosurfactants produced by modified S. bombicola strains. These novel biosurfactants are similar to sophorolipids but contain other functionalities that help to further broaden the application range of glycolipid biosurfactants . Physical and enzymatic pretreatment methods for the waste streams as well as genetic engineering towards achieving higher productivities of these new-to-nature biosurfactants are investigated. The final goal is to develop scalable, economic and sustainable production processes for novel glycolipid biosurfactants.

The marine environment is an excellent resource for natural products with therapeutic potential. Its microbial inhabitants, often associated with other marine organisms, are specialized in the synthesis of bioactive secondary metabolites. Similar to their terrestrial counterparts, marine Actinobacteria are a prevalent source of these natural products. Here, we discuss 77 newly discovered alkaloids produced by such marine Actinobacteria between 2017 and mid-2021, as well as the strategies employed in their elucidation. While 12 different classes of alkaloids were unraveled, indoles, diketopiperazines, glutarimides, indolizidines, and pyrroles were most dominant. Discoveries were mainly based on experimental approaches where microbial extracts were analyzed in relation to novel compounds. Although such experimental procedures have proven useful in the past, the methodologies need adaptations to limit the chance of compound rediscovery. On the other hand, genome mining provides a different angle for natural product discovery. While the technology is still relatively young compared to experimental screening, significant improvement has been made in recent years. Together with synthetic biology tools, both genome mining and extract screening provide excellent opportunities for continued drug discovery from marine Actinobacteria.