Nitrogen Starvation Induced Oxidative Stress in an Oil-Producing Green Alga Chlorella sorokiniana C3

Hui Chen,Qiang Wang,Yun-Ming Zhang,Vasu D. Appanna,Chen-Liu He

doi:10.1371/journal.pone.0069225

Abstract

Microalgal lipid is one of the most promising feedstocks for biodiesel production. Chlorella appears to be a particularly good option, and nitrogen (N) starvation is an efficient environmental pressure used to increase lipid accumulation in Chlorella cells. The effects of N starvation of an oil-producing wild microalga, Chlorella sorokiniana C3, on lipid accumulation were investigated using thin layer chromatography (TLC), confocal laser scanning microscopy (CLSM) and flow cytometry (FCM). The results showed that N starvation resulted in lipid accumulation in C. sorokiniana C3 cells, oil droplet (OD) formation and significant lipid accumulation in cells were detected after 2 d and 8 d of N starvation, respectively. During OD formation, reduced photosynthetic rate, respiration rate and photochemistry efficiency accompanied by increased damage to PSII were observed, demonstrated by chlorophyll (Chl) fluorescence, 77K fluorescence and oxygen evolution tests. In the mean time the rate of cyclic electron transportation increased correspondingly to produce more ATP for triacylglycerols (TAGs) synthesis. And 0.5 d was found to be the turning point for the early stress response and acclimation of cells to N starvation. Increased level of membrane peroxidation was also observed during OD formation, and superoxide dismutase (SOD), peroxide dismutase (POD) and catalase (CAT) enzyme activity assays suggested impaired reactive oxygen species (ROS) scavenging ability. Significant neutral lipid accumulation was also observed by artificial oxidative stress induced by H2O2 treatment. These results suggested coupled neutral lipid accumulation and oxidative stress during N starvation in C. sorokiniana C3.

Highlights

Extensive utilization of fossil fuels has led to global climate change, environmental pollution, health problems and an energy crisis, associated with irreversible depletion of traditional sources of fossil fuels [1]
0 d, 0–0.5 d, 0.5–2 d and 2– 8 d after N starvation were defined as the control stage (Cs), pre-oil droplet formation stage (PDFs), oil droplet formation stage (ODFs), and late-oil droplet formation stage (LDFs), respectively, in further tests
It has been reported that N starvation enhances TAGs accumulation in many microalgal strains such as Neochloris oleoabundans, Chlorella vulgaris and Chlorella protothecoides [43,44,45]

Summary

Introduction

Extensive utilization of fossil fuels has led to global climate change, environmental pollution, health problems and an energy crisis, associated with irreversible depletion of traditional sources of fossil fuels [1]. One of the most commonly used biofuels, has attracted much attention in recent years and is recognized as an ideal renewable energy carrier, and as a possible primary energy source [4]. A variety of biolipids can be used to produce biodiesel, and vegetable oils are a renewable and potentially inexhaustible source of energy with an energy content close to diesel fuel [5]. Extensive use of vegetable oils may cause significant problems such as starvation in developing countries, and it is important that productive and cultivated land is used for food instead of fuel production. In recent years, renewed interest in producing biodiesel from microalgae has arisen, as microalgae can grow rapidly and convert solar energy into chemical energy via CO2 fixation and are considered as one of the most promising sources of oil for making biodiesel [6,7]

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Results

Conclusion