Optimization of seawater-based triacylglycerol accumulation in a freshwater green alga, Chlorella kessleri , through simultaneous imposition of lowered-temperature and enhanced-light intensity

slr2103, a homolog of type-2 diacylglycerol acyltransferase genes, for plastoquinone-related neutral lipid synthesis and NaCl-stress acclimatization in a cyanobacterium, Synechocystis sp. PCC 6803

A cyanobacterium, Synechocystis sp. PCC 6803, contains a lipid with triacylglycerol-like TLC mobility but its identity and physiological roles remain unknown. Here, on ESI-positive LC-MS² analysis, it is shown that the triacylglycerol-like lipid (lipid X) is related to plastoquinone and can be grouped into two subclasses, X_a and X_b, the latter of which is esterified by 16:0 and 18:0. This study further shows that a Synechocystis homolog of type-2 diacylglycerol acyltransferase genes, slr2103, is essential for lipid X synthesis: lipid X disappears in a Synechocystis slr2103-disruptant whereas it appears in an slr2103-overexpressing transformant (OE) of Synechococcus elongatus PCC 7942 that intrinsically lacks lipid X. The slr2103 disruption causes Synechocystis cells to accumulate plastoquinone-C at an abnormally high level whereas slr2103 overexpression in Synechococcus causes the cells to almost completely lose it. It is thus deduced that slr2103 encodes a novel acyltransferase that esterifies 16:0 or 18:0 with plastoquinone-C for the synthesis of lipid X_b. Characterization of the slr2103-disruptant in Synechocystis shows that slr2103 contributes to sedimented-cell growth in a static culture, and to bloom-like structure formation and its expansion by promoting cell aggregation and floatation upon imposition of saline stress (0.3-0.6 M NaCl). These observations provide a basis for elucidation of the molecular mechanism of a novel cyanobacterial strategy to acclimatize to saline stress, and one for development of a system of seawater-utilization and economical harvesting of cyanobacterial cells with high-value added compounds, or blooming control of toxic cyanobacteria.

Diacylglyceryl-N,N,N-trimethylhomoserine-dependent lipid remodeling in a green alga, Chlorella kessleri

Membrane lipid remodeling contributes to the environmental acclimation of plants. In the green lineage, a betaine lipid, diacylglyceryl-N,N,N-trimethylhomoserine (DGTS), is included exclusively among green algae and nonflowering plants. Here, we show that the green alga Chlorella kessleri synthesizes DGTS under phosphorus-deficient conditions through the eukaryotic pathway via the ER. Simultaneously, phosphatidylcholine and phosphatidylethanolamine, which are similar to DGTS in their zwitterionic properties, are almost completely degraded to release 18.1% cellular phosphorus, and to provide diacylglycerol moieties for a part of DGTS synthesis. This lipid remodeling system that substitutes DGTS for extrachloroplast phospholipids to lower the P-quota operates through the expression induction of the BTA1 gene. Investigation of this lipid remodeling system is necessary in a wide range of lower green plants for a comprehensive understanding of their phosphorus deficiency acclimation strategies.

Requirement of the exopolyphosphatase gene for cellular acclimation to phosphorus starvation in a cyanobacterium, Synechocystis sp. PCC 6803

Polyphosphate, which is ubiquitous in cells in nature, is involved in a myriad of cellular functions, and has been recently focused on its metabolism related with microbial acclimation to phosphorus-source fluctuation. In view of the ecological importance of cyanobacteria as the primary producers, this study investigated the responsibility of polyphosphate metabolism for cellular acclimation to phosphorus starvation in a cyanobacterium, Synechocystis sp. PCC 6803, with the use of a disruptant (Δppx) as to the gene of exopolyphosphatase that is responsible for polyphosphate degradation. Δppx was similar to the wild type in the cellular content of polyphosphate to show no defect in cell growth under phosphorus-replete conditions. However, under phosphorus-starved conditions, Δppx cells were defective in a phosphorus-starvation dependent decrease of polyphosphate to show deleterious phenotypes as to their survival and the stabilization of the photosystem complexes. These results demonstrated some crucial role of exopolyphosphatase to degrade polyP in the acclimation of cyanobacterial cells to phosphorus-starved conditions. Besides, it was found that ppx expression is induced in Synechocystis cells in response to phosphorus starvation through the action of the two-component system, SphS and SphR, in the phosphate regulon. The information will be a foundation for a fuller understanding of the process of cyanobacterial acclimation to phosphorus fluctuation.

Regulatory carbon metabolism underlying seawater-based promotion of triacylglycerol accumulation in Chlorella kessleri

Chlorella kessleri accumulates triacylglycerol usable for biodiesel-fuel production to >20% dry cell weight in three days when cultured in three-fold diluted seawater, which imposes the combinatory stress of hyperosmosis and nutrients limitation. The quantitative behavior of major C-compounds, and related-gene expression patterns were investigated in Chlorella cells stressed with hyperosmosis, nutrients limitation, or their combination, to elucidate the C-metabolism for economical seawater-based triacylglycerol accumulation. Combinatory-stress cells showed repressed protein synthesis with initially accumulated starch being degraded later, the C-metabolic flow thereby being diverted to fatty acid and subsequent triacylglycerol accumulation. This C-flow diversion was induced by cooperative actions of nutrients-limitation and hyperosmosis. Semi-quantitative PCR analysis implied positive rewiring of the diverted C-flow into triacylglycerol in combinatory-stress cells through upregulation of gene expression concerning fatty acid and triacylglycerol synthesis, and starch synthesis and degradation. The information of regulatory C-metabolism will help reinforce the seawater-based triacylglycerol accumulation ability in algae including Chlorella.

Biomass production and biochemical profiles of a freshwater microalga Chlorella kessleri in mixotrophic culture: Effects of light intensity and photoperiodicity

In this work, different light conditions (light intensity and photoperiodicity) were set up to investigate the impact of light on growth, chemical compositions and fatty acid profiles of Chlorella kessleri in mixotrophic cultures. Results indicated that C. kessleri could absorb and utilize glucose rapidly when light intensity was ≤ 90 µE m^-2 s^-1, and a maximum algal biomass of 1.17 g L^-1 was obtained in the cultures with 2 g L^-1 glucose at a light intensity and light/dark (L/D) cycle of 90 µE m^-2 s^-1 and 20L:4D, respectively. Additionally, this alga would accumulate a large amount of chlorophyll a (about 30 mg g^-1) in the mixotrophic cultures under a low light intensity (≤90 µE m^-2 s^-1), and the algal chemical compositions changed with light intensity and photoperiodicity. Results of fatty acid profiles suggested that the algal biomass could be used as animal feeds or a good-quality biodiesel feedstock.

Fatty Acid Content and Composition of Triacylglycerols of Chlorella kessleri

Triacylglycerols (TAGs) are esters formed from one glycerol and three fatty acids. TAGs are induced to accumulate in algal cells under environmental stress conditions including nutrient-limitation, hyperosmosis, and low temperature, for the storage of metabolic energy and carbon, and also for the consumption of excess energy (e.g., Hirai et al., 2016 ; Hayashi et al., 2017 ). Beside their physiological significance, the commercial utilization of algal TAG has been expected for the production of biodiesel, the methyl esters of fatty acids, from the aspect of carbon-neutral conception. The amounts of TAGs can be determined through quantitative measurement of their constituent fatty acids. This protocol consists of the following three parts: the first is the extraction of total lipids from algal cells with the use of organic solvents, chloroform and methanol, according to the method of Bligh and Dyer (1959), the second is the separation of TAG from the other lipid classes by thin-layer chromatography (TLC), and the third is the production of methyl-esterified derivatives of their constitutive fatty acids and subsequent quantitation of them by capillary gas-liquid chromatography (GLC). This protocol adapted from Sato and Tsuzuki (2011) is used for TAG analysis in a green alga, Chlorella kessleri.