In order to enhance lutein accumulation and to explain the reasons for the difference in lutein accumulation under photoautotrophic and heterotrophic conditions, different culture modes and the associated transcriptome profiles were investigated in Auxenochlorella protothecoides. The heterotrophic-photoautotrophic transition culture mode was investigated for lutein accumulation, changing from organic carbon to increase biomass in dark fermentation to irradiation under nitrogen rich conditions. This strategy increased the lutein content 10 times along with chloroplast regeneration and little biomass loss in 48 h. The highest lutein productivity and production in the heterotrophic-photoautotrophic transition culture reached 12.36 mg/L/day and 34.13 mg/L respectively within seven days. Furthermore, compared to the photoautotrophic conditions, most genes involved in lutein biosynthesis and photosystem generation were down-regulated during heterotrophic growth. By contrast, two β-ring hydroxylases were transiently upregulated, while violaxanthin de-epoxidase and zeaxanthin epoxidase were mostly downregulated, which explained the extremely low lutein content of heterotrophic cells. Nevertheless, the lutein proportion in total carotenoids reached nearly 100%. This study is the first to our knowledge to report on a comparative transcriptome analysis of lutein biosynthesis, and it provides a promising strategy to boost lutein production in A. protothecoides.

Cupriavidus necator can produce single-cell proteins (SCPs) using electrons produced by hydrogen as energy, oxygen as electron acceptors, and CO₂ as carbon sources. Gas fermentation is a process of microbial fermentation that uses gas substrates (such as hydrogen, carbon dioxide, etc.) which faces several challenges, mainly including the low solubility of gas substrates, the danger of hydrogen and oxygen mixing, and the optimization of fermentation conditions. To overcome these challenges, this article explores a variety of strategies—including the design of a self-developed bioreactor—to reduce the risk of static electricity. Without the addition of filler material, the results showed that the maximum cell dry weight (CDW) of 30% secondary seed inoculation was 20.41% higher than that of 10% secondary seed inoculum, and 5.99% higher than that of 20% secondary seed inoculum. Combined with the filler material and with the use of high-efficiency continuous fermentation technology, the average yield of continuous fermentation was 23.31 g/day, while the average yield of batch fermentation was 14.33 g/day. The daily yield of continuous fermentation is 1.63 times that of batch fermentation. These efforts are aimed at improving the efficiency and safety of gas fermentation.