Microalgae Intracellular Plastoquinone And Ubiquinone Synthesis Regulation And Influence On Growth And Metabolism

Microalgae have efficient photosynthetic carbon fixation efficiency and high growth rate.Microalgae can use CO2 to produce a variety of commercial products.Plastoquinone(PQ)and ubiquinone(UQ)are important electron and proton carriers in the electron transport chain and involved in the synthesis of various metabolites in microalgae.The changes of their redox state and content can affect the physiological metabolism of microalgae.However,there are relatively few studies on the synthesis and related metabolic regulation mechanisms of PQ and UQ of microalgae.Therefore,it is urgent to explore the influence of PQ and UQ content changes on the growth and physiological metabolism of microalgae.In this study,we overexpressed geranyl diphosphate:4-hydroxybenzoate geranyltransferase(LePGT)from Lithospermum erythrorhizon in the model microalgae Synechocystis and Chlamydomonas reinhardtii to drain both 4-hydroxybenzoic acid(4-HB)and geranyl diphosphate(GPP).We could reduce the contents of PQ and UQ in microalgae by expressing LePGT.Furthermore,we could increase the contents of PQ and UQ in microalgae by the addition of the precursor substance 4-hydroxybenzoic acid(4-HB).We could affect the biosynthetic pathway and quantities of PQ and UQ in microalgae by genetic engineering and additing 4-HB.Combined with analysis of photosystem efficiency,respiration rate and cell composition,we explored the metabolic regulation mechanism of the changes of PQ and UQ content in microalgae.Our results showed that the expression of LePGT in Synechocystis had no effect on the growth of Synechocystis.PQ content was reduced by 22.18%compared to wild type.The expression of LePGT in Synechocystis promoted the electron transfer efficiency of the photosystem and respiration rate and significantly facilitated the synthesis of intracellular lipids.The content of long-chain fatty acids and neutral lipid was increased by 10.56%and 58.18%compared with wild type,respectively.Adding low concentrations of 4-HB had no obvious effect on the growth of Synechocystis.The intracellular PQ content of Synechocystis with 1 mM 4-HB was increased by 14.76%.The intracellular PQ content of Synechocystis with 2 mM 4-HB was increased by 70.86%.The addition of 1 mM 4-HB increased the photosystem efficiency and respiration rate of Synechocystis.The addition of 2 mM 4-HB significantly reduced the photosystem efficiency and respiration rate of Synechocystis.It increased the content of intracellular PQ and promoted the production of neutral lipid and glycogen.The contents of neutral lipid and glycogen of Synechocystis with 1 mM 4-HB was improved by 54.82%and 34.92%compared to the control group,respectively.The expression of LePGT in Chlamydomonas reinhardtii resulted in a decrease in intracellular PQ and UQ content by 49.14%and 98.30%,respectively.It inhibited the efficiency of photosystem and respiration.The expression of LePGT in Chlamydomonas reinhardtii significantly reduced the growth and cytochrome content and promoted the synthesis of intracellular starch and neutral lipid.The yield of neutral lipid and starch was increased by 63.68%and 19.42%,respectively.Adding low-concentration 4-HB stimulated growth of Chlamydomonas reinhardtii.It enhanced the biosynthesis of PQ and UQ and reduced the production of fatty acids.The larger carbon flux was controlled leading to the synthesis pathway of starch.The starch content of the Chlamydomonas reinhardtii with 1 mM 4-HB was increased by 14.91%compared with the control group.This article initially explored the metabolic regulation mechanism of the changes of intracellular PQ and UQ content in microalgae.We found differences in the physiological phenotypic changes caused by regulating the quantities of PQ and UQ in prokaryotic cyanobacteria and eukaryotic green algae.It provides a new idea for promoting the development of "green cell factory" based on microalgae.