Vitamin K1 Can Effectively Promote the Photosynthesis and Lipid Production of Chlorella pyrenoidosa

Whether exogenous vitamin K1 (VK1) promotes microalgal growth is unknown. This study reports for the first time that Bacillus megaterium can promote the growth of Chlorella pyrenoidosa, an effect possibly mediated by VK1. To confirm this finding and clarify the corresponding mechanism, the effects of exogenous VK1 on the growth and lipid production of C. pyrenoidosa were studied. The results showed that the microalgal cell density, chlorophyll a content, lipid content, and lipid productivity increased by 117%, 90%, 291%, and 247%, respectively, following the addition of 0.3 g/L exogenous VK1. Additionally, the microalgal Fv/Fo, Fv/Fm, and Fm/Fo were also improved by the VK1, indicating that VK1 promoted microalgae growth by increasing microalgal photosynthesis activities. The microalgal genes of lipid synthesis, including the acetyl-CoA carboxylase gene, malonyl-transferase gene, 3-oxoacylsynthase I gene, and 3-oxoacyl-[acyl-carrier-protein] synthase II gene, were all up-regulated by VK1 at the transcript level, revealing the promotion mechanism of VK1 on microalgal lipid production. Moreover, the total phosphorus utilization of the C. pyrenoidosa reached nearly 100%, indicating its excellent phosphorus utilization ability. The results are beneficial for exploring a more effective technology of utilizing C. pyrenoidosa for biofuel production and provide a new perspective on understanding the interaction mechanism between bacteria and microalgae.