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

Abstract

Microalgae can accumulate a large fraction of reduced carbon as lipids under NaCl stress. This study investigated the mechanism of carbon allocation and reduction and triacylglycerol (TAG) accumulation in microalgae under NaCl-induced stress. Micractinium sp. XJ-2 was exposed to NaCl stress and cells were subjected to physiological, biochemical, and metabolic analyses to elucidate the stress-responsive mechanism. Lipid increased with NaCl concentration after 0-12 hr, then stabilized after 12-48 hr, and finally decreased after 48-72 hr, whereas TAG increased (0-48 hr) and then decreased (48-72 hr). Under NaCl-induced stress at lower concentrations, TAG accumulation, at first, mainly shown to rely on the carbon fixation through photosynthetic fixation, carbohydrate degradation, and membrane lipids remodeling. Moreover, carbon compounds generated by the degradation of some amino acids were reallocated and enhanced fatty acid synthesis. The remodeling of the membrane lipids of NaCl-induced microalgae relied on the following processes: (a) Increase in the amount of phospholipids and reduction in the amount of glycolipids and (b) extension of long-chain fatty acids. This study enhances our understanding of TAG production under NaCl stress and thus will provide a theoretical basis for the industrial application of NaCl-induced in the microalgal biodiesel industry.

An appraisal of the variable response of microalgal lipids to culture salinity

“…138 In freshwater microalgae, PG increased upon salt stress in Synechococcus sp. 153 and Micractinium sp., 115 while it decreased in C. zofigiensis 27 and C. nivalis. 140 PG decrease was closely linked to photosynthesis inhibition after salt stress.…”

“…A similar view could be deduced from the SQDG decrease with salinity in the less salt-tolerant C. zofigiensis 27 and Micractinium sp. 115 Nevertheless, the SQDG moderate increase with salinity in the cyanobacterium Synechococcus sp. 146 recommends caution in this kind of interpretation.…”

“…138 In Micractinium sp., salt stress induced production of phosphatidylcholine and phosphatidylethanolamine. 115 Given the analogous function of betaine lipids and phospholipids in microalgal cell membranes, it can be deduced that maintaining or slightly rising the levels of both types of membrane lipids is the most frequent response to high salt exposure in microalgae. Higher polar lipids in the plasma membrane have been described to prevent influx of toxic Na + cations.…”

“…141 Recent studies associate some betaine lipids with the saline adaptation of microalgae on the basis that a marine strain of Synechocystis contained DGTS while a freshwater counterpart did not. 142 Although salt stress can increase the pool of phospholipids in cell membranes and decrease the pool of chloroplast glycolipids in microalgae, 115 specific changes can be detected within the latter when the response of different glycolipid classes is observed. In microalgal photosynthetic membranes, the neutrally charged galactolipids monogalactosyl-diacyllglycerol (MGDG) and digalactosyl-diacylglycerol (DGDG) are the most abundant lipids.…”

“…66 In freshwater microalgae, particular sensitivity to salt stress could be suspected in the cyanobacterium Synechococcus sp., 146 and the chlorophytes P. kessleri, 75 Micractinium sp. 115 and Chromochloris zofigiensis 27 as a consequence of their respective significant decrease in MGDG and DGDG in culture media with added salt. On the contrary, MGDG increased twofold in Scenedesmus sp.…”