Metabolic and gene expression changes triggered by nitrogen deprivation in the photoautotrophically grown microalgae Chlamydomonas reinhardtii and Coccomyxa sp. C-169

Article (PDF Available)inPhytochemistry 75:50-9 · March 2012with269 Reads
DOI: 10.1016/j.phytochem.2011.12.007 · Source: PubMed
Microalgae are emerging as suitable feedstocks for renewable biofuel production. Characterizing the metabolic pathways involved in the biosynthesis of energy-rich compounds, such as lipids and carbohydrates, and the environmental factors influencing their accumulation is necessary to realize the full potential of these organisms as energy resources. The model green alga Chlamydomonas reinhardtii accumulates significant amounts of triacylglycerols (TAGs) under nitrogen starvation or salt stress in medium containing acetate. However, since cultivation of microalgae for biofuel production may need to rely on sunlight as the main source of energy for biomass synthesis, metabolic and gene expression changes occurring in Chlamydomonas and Coccomyxa subjected to nitrogen deprivation were examined under strictly photoautotrophic conditions. Interestingly, nutrient depletion triggered a similar pattern of early synthesis of starch followed by substantial TAG accumulation in both of these fairly divergent green microalgae. A marked decrease in chlorophyll and protein contents was also observed, including reduction in ribosomal polypeptides and some key enzymes for CO₂ assimilation like ribulose-1,5-bisphosphate carboxylase/oxygenase. These results suggest that turnover of nitrogen-rich compounds such as proteins may provide carbon/energy for TAG biosynthesis in the nutrient deprived cells. In Chlamydomonas, several genes coding for diacylglycerol:acyl-CoA acyltransferases, catalyzing the acylation of diacylglycerol to TAG, displayed increased transcript abundance under nitrogen depletion but, counterintuitively, genes encoding enzymes for de novo fatty acid synthesis, such as 3-ketoacyl-ACP synthase I, were down-regulated. Understanding the interdependence of these anabolic and catabolic processes and their regulation may allow the engineering of algal strains with improved capacity to convert their biomass into useful biofuel precursors.

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Available from: Edgar Cahoon, Sep 13, 2014
    • "Some studies have shown that lipid content in algae can be increased by nutrient depletion[1,4,25262728. Among the macronutrients (nitrogen, phosphate, and sulfur), nitrogen deprivation is widely used for stress experi- ments[26,27,293031323334. However, such stresses lower the growth rate and productivity of the system[1,35], which is a major bottleneck for producing biofuels and byproducts on commercial scales, and therefore some studies have addressed solutions using genetic engineering[36,37]. "
    [Show abstract] [Hide abstract] ABSTRACT: Background: Algae have attracted attention as sustainable producers of lipid-containing biomass for food, animal feed, and for biofuels. Parachlorella kessleri, a unicellular green alga belonging to the class Trebouxiophyceae, achieves very high biomass, lipid, and starch productivity levels. However, further biotechnological exploitation has been hampered by a lack of genomic information. Results: Here, we sequenced the whole genome and transcriptome, and analyzed the behavior of P. kessleri NIES-2152 under lipid production-inducing conditions. The assembly includes 13,057 protein-coding genes in a 62.5-Mbp nuclear genome. Under conditions of sulfur deprivation, lipid accumulation was correlated with the transcriptomic induction of enzymes involved in sulfur metabolism, triacylglycerol (TAG) synthesis, autophagy, and remodeling of light-harvesting complexes. Conclusions: Three-dimensional transmission electron microscopy (3D-TEM) revealed extensive alterations in cellular anatomy accompanying lipid hyperaccumulation. The present 3D-TEM results, together with transcriptomic data support the finding that upregulation of TAG synthesis and autophagy are potential key mediators of the hyperaccumulation of lipids under conditions of nutrient stress.
    Full-text · Article · Dec 2016
    • "One proposed mechanism for the rapid chlorophyll degradation is that microalgae channel N from chloroplastic proteins in order to reduce stress from light [73]. This reshuffling of the photocentres appears to be consistent with a number of studies [20,72] and suggests that N is an important element in chloroplastic processes, particularly for the synthesis of pigments and photosynthetic efficiency as well as for carbon fixation. In contrast to the decreased abundance of proteins of the photosynthetic machinery, there was a 2.1-fold increase in mitochondrial ATPase, F0 complex, subunit B (p = 0.037). "
    [Show abstract] [Hide abstract] ABSTRACT: The microalga Nannochloropsis oculata is a model organism for understanding intracellular lipid production, with potential benefits to the biofuel, aquaculture and nutraceutical industries. It is well known that nitrogen deprivation increases lipid accumulation in microalgae but the underlying processes are not fully understood. In this study, detailed proteomic and biophysical analyses were used to describe mechanisms that regulate carbon partitioning in nitrogen-deplete N. oculata. The alga selectively up- or down-regulated proteins to shift its metabolic flux in order to compensate for deficits in nitrate availability. Under nitrogen deprivation, proteins involved in photosynthesis, carbon fixation and chlorophyll biosynthesis were all down-regulated, and this was reflected in reduced cell growth and chlorophyll content. Protein content was reduced 4.9-fold in nitrogen-deplete conditions while fatty acid methyl esters increased by 60%. Proteomic analysis revealed that organic carbon and nitrogen from the breakdown of proteins and pigments is channeled primarily into fatty acid synthesis. As a result, the fatty acid concentration increased and the fatty acid profile became more favorable for algal biodiesel production. This advancement in microalgal proteomic analysis will help inform lipid accumulation strategies and optimum cultivation conditions for overproduction of fatty acids in N. oculata.
    Full-text · Article · Nov 2016
    • "Thus, the focus of the scientific community has been on understanding and maximizing the lipid production in C. reinhardtii and other algal strains [11, 13]. In C. reinhardtii, several genes coding for diacylglycerol:acyl-CoA acyltransferases, catalyzing the acylation of diacylglycerol (DAG) to TAG, displayed increased transcript abundance under nitrogen depletion but, counter intuitively, genes encoding enzymes for de novo fatty acid synthesis, such as 3-ketoacyl-ACP synthase I, were down-regulated in the same condition [7,14,15]. A chloroplast pathway for the de novo biosynthesis of TAG in C. reinhardtii employs a distinct pathway that uses DAG (derived almost exclusively from the chloroplast) to produce TAG [16]. "
    [Show abstract] [Hide abstract] ABSTRACT: Chlamydomonas reinhardtii has recently emerged as a viable alternative source of fossil fuel. However, the metabolic flow of carbon in C. reinhardtii towards making the carbon reserve is not yet understood. In addressing this issue, we have monitored the assimilation of 13C-labeled acetate by the wild-type (cw15) and the starch deficient (sta6) strains of C. reinhardtii using 13C NMR. The dynamics of starch and lipid reserves was studied under nitrogen starvation and supplement conditions. The starch was found to accumulate and mobilize faster than triacylglycerol (TAG) through the formation of CO2aq and bicarbonate. Addition of exogenous acetate significantly slowed down mobilization of both labeled starch and TAG reserves, suggesting that cells preferred incoming acetate to internal reserve as a source of carbon. In parallel, we undertook the analyses of cellular free amino acid pool changes using LC/MS to investigate how specific amino acid changes contribute towards “nitrogen sparing effect” during nitrogen starvation. Bulk of remainder amino acids do not exhibit any discernible changes in their steady state pool sizes, suggesting the possibility that carbon flow leading to starch, followed by lipid is homeostatically balanced during nitrogen starvation such that the amino acid carbon levels in the cells stay largely regulated. Taken all together, this study thus describes an excellent cellular system for probing the nitrogen starvation, nitrogen uptake and the consequent changes in carbon flow in relation to both starch/lipid accumulation and their re-mobilization and the changes in free amino acid pools in C. reinhardtii cells.
    Full-text · Article · Sep 2016
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