Microbial oils for biotechnology: strain diversity, advanced fermentation and industrial scale-up

Oleaginous microorganisms are generally defined as microorganisms capable of accumulating more than 20% of their dry biomass as lipids, with lipid accumulation reaching substantially higher levels in some strains under optimized conditions. Various yeasts, filamentous fungi, bacteria and microalgae are of industrial interest, producing lipid products ranging from triacylglycerols (TAGs) to polyunsaturated fatty acids (PUFAs). Commercial microbial lipid production dates to the 1980s. However, large-scale industrial adoption remains limited, primarily because production costs remain higher than conventional oils and fats. Extensive efforts in strain discovery, selection, and metabolic improvement have led to substantial advances in lipid titers and productivity. Although yeasts have traditionally received the greatest attention, interest in filamentous fungi and microalgae has increased, particularly to produce PUFAs. While bioprocess development is highly strain-specific, the carbon-to-nitrogen (C/N) ratio is a key factor governing lipid accumulation across microorganisms. Lipid accumulation is commonly promoted under nitrogen-limited (relatively high C/N ratio) conditions, causing high initial carbon concentrations, substrate inhibition, osmotic stress and reduced overall productivity. Fed-batch and two-stage cultivation strategies can help overcome these limitations. Fed-batch processes enable controlled substrate feeding and nutrient supplementation, preventing excessive substrate concentrations while extending the productive cultivation period. Maximal lipid productivity can be achieved through a two-stage strategy consisting of an initial nitrogen-sufficient phase to maximize biomass formation, followed by a nitrogen-limited phase to induce lipid accumulation. Stepwise control of parameters such as temperature and dissolved oxygen, when combined across the phases, can further enhance lipid production. However, although two-stage strategies have shown considerable promise, the optimal timing and conditions for transition between growth and lipid-accumulation phases remain poorly characterized across strains and feedstocks.