Abstract
Microalgal heterotrophic cultivation is an emerging technology that can enable producing high cell-density algal cell cultures, which can be coupled with photoautotrophic cultivation for valuable chemicals such as lipids manufacturing. However, how the heterotrophically grown algal cells respond to the lipid-inducing conditions has not been fully elucidated so far. In this study, when the heterotrophically grown Scenedesmus acuminatus cells were subjected to the high light (HL) and nitrogen-limited (NL) conditions, both the biomass and lipid productivity were enhanced as compared to that of the photoautotrophically grown counterparts. The chlorophyll a fluorometry analysis showed that the Fv/Fm and Y(II) of the heterotrophically grown cells subjected to the HL and NL conditions was recovered to the maximum value of 0.75 and 0.43, respectively, much higher than those of the photoautotrophically grown cells under the same stress conditions. Transcriptomic analysis revealed that heterotrophically grown cells fully expressed the genes coding for the photosystems proteins, including the key photoprotective proteins D1, PsbS, light-harvesting-complex (LHC) I and LHC II. Meanwhile, downregulation of the carotenoid biosynthesis and upregulation of the glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathways were observed when the heterotrophically grown cells were subjected to the HL and N-limited conditions for lipid production. It was deduced that regulation of these pathways not only enhanced the light utilization but also provided the reducing power and ATP by which the biomass accumulation was significantly elevated. Besides, upregulation of the acetyl-CoA carboxylase/biotin carboxylase, digalactosyl diacylglycerol synthase and diacylglycerol acyltransferase 2 encoding genes may be attributable to the enhanced lipid production. Understanding the cellular responses during the trophic transition process could guide improvement of the strength of trophic transition enhancing microalgal biomass and lipid production.
Highlights
Microalgae are promising feedstocks for liquid biofuels production because of their high photosynthetic efficiencies, fast growth rates, high lipid contents and no competition with food production for arable land during cultivation (Yin et al, 2020)
Growth and fatty acid methyl esters (FAMEs) Contents of the heterotrophically grown cells (HC) and photoautotrophically grown ones (PC) of S. acuminatus Subjected to the high light (HL) and NL Conditions
When the HC and PC of S. acuminatus were subjected to the HL and NL conditions at the same initial chlorophyll concentration, the volumetric chlorophyll concentration of the HC culture increased more drastically than that of the PC culture within 24 h (Figure 1A)
Summary
Microalgae are promising feedstocks for liquid biofuels production because of their high photosynthetic efficiencies, fast growth rates, high lipid contents and no competition with food production for arable land during cultivation (Yin et al, 2020). Various cultivation strategies besides photoautotrophic cultivation have been emerging to enhance microalgal lipid production so as to balance the costs, which include heterotrophic cultivation, photoautotrophy-to-heterotrophy cultivation and heterotrophyto-photoautotrophy cultivation (Xiong et al, 2010; Han et al, 2012; Zheng et al, 2013) Among these cultivation modes, the heterotrophic cultivation mode offers many advantages that can enable fast growth and high biomass productivity for microalgae (Chen et al, 2020), while photoautotrophic cultivation is essential for producing many compounds of which the synthesis are highly dependent on photosynthesis, such as lipids requiring large amounts of photosynthetically produced reducing equivalent and fatty acids (Li-Beisson et al, 2015). It is noteworthy that when the coupled cultivation mode was utilized for Chlorella spp. and S. acuminatus, the biomass yield and lipid productivity were significantly enhanced as compared with traditional photoautotrophic cultivation (Han et al, 2012; Jin et al, 2020)
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