Effects of inorganic nitrogen addition to okara-utilized medium on the oleaginous yeast Lipomyces starkeyi and assessment of metabolism involved in increased oil production

Background

Oleaginous microorganisms contain oil and fat at amounts greater than 20% of their biomass weight, with fatty acid and chemical compositions often similar to those of vegetable oil and animal fat. Oleaginous yeasts, including Lipomyces starkeyi, are particularly promising because of their high oil accumulation capacity, broad sugar utilization, and ability to use non-edible biomass, making them suitable for large-scale, cost-effective oil production. However, reducing production costs remains a major challenge, as media costs account for the majority of total microbial oil production costs. Okara, a byproduct of tofu and soy milk production, is a potential low-cost nitrogen source. Although previous study have reported the use of solid okara for oil production and its lower oil yield than that of yeast extract medium, the potential benefits of adding inorganic nitrogen to improve yield have not been fully explored.

Results

We tested the effect of inorganic nitrogen addition on oil production in okara medium using culture experiments and found that the addition of ammonium sulfate significantly increased not only the cell concentration but also the oil yield by 1.61-fold (19.7 ± 0.44 [g/L]). In addition, the presence of both sulfate and ammonium ions was important for increasing the oil production. Metabolome analysis of the culture supernatant showed that sulfate ions contribute to glutathione synthesis, whereas ammonium ions provide nitrogen and affect the glutathione synthesis pathway through the ammonia assimilation pathway, which may result in increased oil productivity.

Conclusions

The use of okara medium supplemented with ammonium sulfate can reduce the cost of nitrogen source materials to a level of several percentages of that of conventional yeast extract medium, presenting the possibility of inexpensive oil production by L. starkeyi. In addition, the dual roles of ammonium sulfate in enhancing oil production were proposed. Furthermore, this is the first study to confirm the relationship between an enhanced glutathione synthesis pathway and increased oil production by L. starkeyi. These findings provide a foundation for the further development of sustainable and economically viable microbial oil production bioprocesses.