Abstract
Marine chlorophytes of the genus Chlorella are unicellular algae capable of accumulating a high proportion of cellular lipids that can be used for biodiesel production. In this study, we examined the broad physiological capabilities of a subtropical strain (C596) of Chlorella sp. “SAG-211-18” including its heterotrophic growth and tolerance to low salt. We found that the alga replicates more slowly at diluted salt concentrations and can grow on a wide range of carbon substrates in the dark. We then sequenced the RNA of Chlorella strain C596 to elucidate key metabolic genes and investigate the transcriptomic response of the organism when transitioning from a nutrient-replete to a nutrient-deficient condition when neutral lipids accumulate. Specific transcripts encoding for enzymes involved in both starch and lipid biosynthesis, among others, were up-regulated as the cultures transitioned into a lipid-accumulating state whereas photosynthesis-related genes were down-regulated. Transcripts encoding for two of the up-regulated enzymes—a galactoglycerolipid lipase and a diacylglyceride acyltransferase—were also monitored by reverse transcription quantitative polymerase chain reaction assays. The results of these assays confirmed the transcriptome-sequencing data. The present transcriptomic study will assist in the greater understanding, more effective application, and efficient design of Chlorella-based biofuel production systems.
Highlights
As the market for biofuels expands, photosynthetic microalgae can be potentially employed in third-generation biofuel production at commercial scales [1]
The majority of the transcripts identified in the photosynthesis sub-category (34 down, 6 up) and the photosynthesis— antenna proteins sub-category (14 down, 0 up) within the Energy Metabolism Category are down-regulated. These results suggest that strain C596 is scaling back the photosynthetic apparatus when substantially enhancing lipid and carbohydrate metabolism
The heterotrophic growth rate achieved by this strain when grown in the dark rivaled that of the specific growth rate obtained using inorganic nutrients under photosynthetic light conditions, allowing for a potential industrial practice of supplementing photoautotrophic growth with carbon substrates to increase growth and possibly to enhance TAG productivity
Summary
As the market for biofuels expands, photosynthetic microalgae can be potentially employed in third-generation biofuel production at commercial scales [1]. Single-celled algae have received heightened attention because of their fast growth rates, high photosynthetic efficiencies, and rapid lipid accumulation [2, 3]. Many algal species are capable of biosynthesizing and storing. Monitoring the Transcriptome of Chlorella during Lipid Accumulation neutral lipids composed primarily of triacylglycerides (TAGs) [4,5,6], which can be extracted and industrially transesterified into biodiesel [7]. Algae bioaccumulate TAGs as a means to store excess energy when cells experience a non-carbon nutrient limitation that prevents cell doubling (i.e., phosphate or nitrate limitation) [8, 9]. The accumulation of TAGs is decoupled from the exponential growth of the organism, and genes encoding enzymes contributing to TAG accumulation are tightly controlled [10,11,12,13]
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