Novel insights into salinity-induced lipogenesis and carotenogenesis in the oleaginous astaxanthin-producing alga Chromochloris zofingiensis: a multi-omics study

Abstract

Background: Chromochloris zofingiensis, a freshwater alga capable of synthesizing both triacylglycerol (TAG) and astaxanthin, has been receiving increasing attention as a leading candidate producer. While the mechanism of oleaginousness and/or carotenogenesis has been studied under such induction conditions as nitrogen deprivation, high light and glucose feeding, it remains to be elucidated in response to salt stress, a condition critical for reducing freshwater footprint during algal production processes. Results:Firstly, the effect of salt concentrations on growth, lipids and carotenoids was examined for C. zofingiensis, and 0.2 M NaCl demonstrated to be the optimal salt concentration for maximizing both TAG and astaxanthin production. Then, the time-resolved lipid and carotenoid profiles and comparative transcriptomes and metabolomes were generated in response to the optimized salt concentration for congruent analysis. A global response was triggered in C. zofingiensis allowing acclimation to salt stress, including photosynthesis impairment, ROS build-up, protein turnover, starch degradation, and TAG and astaxanthin accumulation. The lipid metabolism involved a set of stimulated biological pathways that contributed to carbon precursors, energy and reductant molecules, pushing and pulling power, and storage sink for TAG accumulation. On the other hand, salt stress suppressed lutein biosynthesis, stimulated astaxanthin biosynthesis (mainly via ketolation), yet had little effect on total carotenoid flux, leading to astaxanthin accumulation at the expense of lutein. Astaxanthin was predominantly esterified and accumulated in a well-coordinated manner with TAG, pointing to the presence of common regulators and potential communication for the two compounds. Furthermore, the comparison between salt stress and nitrogen deprivation conditions revealed distinctions in TAG and astaxanthin biosynthesis as well as critical genes with engineering potential. Conclusions:Our multi-omics data and integrated analysis shed light on the salt acclimation of C. zofingiensis and underlying mechanisms of TAG and astaxanthin biosynthesis, provide engineering implications into future trait improvements, and will benefit the development of this alga for production uses under saline environment, thus reducing the footprint of freshwater.

The oleaginous astaxanthin-producing alga Chromochloris zofingiensis: potential from production to an emerging model for studying lipid metabolism and carotenogenesis

“…It is worth noticing that the micronutrient sulfur has a greater effect than phosphorus on C. zofingiensis growth, as suggested by the more severely impaired growth under sulfur starvation compared to under phosphorus starvation [ 17 ]. As a freshwater alga, C. zofingiensis is able to tolerate moderate salt levels (~ 0.25 M NaCl), yet at the expense of growth [ 18 , 32 ].…”

“…Under favorable growth conditions, algae contain predominantly polar membrane lipids with only a basal level of TAG; upon stress conditions, algae tend to slow down growth and accumulate TAG in bulk as the carbon and energy reservoir [ 3 ]. These stress conditions include but are not restricted to limitation/starvation of nutrients (e.g., nitrogen, phosphorus, sulfur, iron and zinc), high light, salinity, and abnormal temperature [ 13 , 17 , 18 , 71 – 78 ].…”

“…The robust performance in growth and simultaneous accumulation of TAG and astaxanthin in lipid droplets (LDs) enable C. zofingiensis an appealing alga for production uses [ 13 , 19 , 29 , 31 , 32 ]. Recently, the chromosome-level genome sequence of C. zofingiensis has been released [ 33 ], which, together with the workable genetic tools and random mutagenesis for screening target mutants [ 34 – 36 ], provide unprecedented opportunities to better understand the molecular mechanisms for lipid metabolism and carotenogenesis and the crosstalk between TAG and astaxanthin biosynthetic pathways [ 14 , 18 , 37 – 41 ]. The review centers around C. zofingiensis with an aim to (1) summarize recent progress on TAG and astaxanthin production, (2) update molecular understanding of lipid metabolism, carotenogenesis and the communications between TAG and astaxanthin biosynthesis, and (3) discuss engineering strategies for improving the synthesis of either TAG, astaxanthin or both.…”

Biosynthesis of Fatty Alcohols in Engineered Microbial Cell Factories: Advances and Limitations

“…From a process engineering standpoint, cultivation in medium with limiting amounts of a key non-carbon nutrient such as phosphorous, sulfur or nitrogen can be used to limit biomass accumulation and induce a switch toward lipogenesis ( Ratledge and Wynn, 2002 ; Wu et al, 2011 ; Wang et al, 2018 ). Elevated salinity can induce lipogenesis in some algal species ( Mao et al, 2020 ). Two-stage cultivations where biomass is initially accumulated in the presence of all required nutrients followed by a second phase in high carbon, limited nitrogen medium has been employed to achieve high titres of lipids ( Karamerou and Webb, 2019 ).…”

Regulation and remodeling of intermediate metabolite and membrane lipid during NaCl‐induced stress in freshwater microalga Micractinium sp. XJ‐2 for biofuel production

“…The mechanism under NaCl stress is definitely different from that under N and P deficiency because of ion effect and osmotic stress (Mao, Zhang, Wang, & Liu, 2020; Wang et al, 2016). Under NaCl stress, the adaptability of some amino acids (such as proline and homoserine) and carbohydrates (such as sucrose and trehalose) increases, resulting in the reduction of pyruvate, which is a precursor for TAG synthesis (Arora et al, 2019; Chokshi, Pancha, Ghosh, & Mishra, 2017; Hagemann & Pade, 2015; Yang & Guo, 2018).…”