Abstract
BackgroundOleaginous microalgae are promising production hosts for the sustainable generation of lipid-based bioproducts and as bioenergy carriers such as biodiesel. Transcriptomics of the lipid accumulation phase, triggered efficiently by nitrogen starvation, is a valuable approach for the identification of gene targets for metabolic engineering.ResultsAn explorative analysis of the detailed transcriptional response to different stages of nitrogen availability was performed in the oleaginous green alga Monoraphidium neglectum. Transcript data were correlated with metabolic data for cellular contents of starch and of different lipid fractions. A pronounced transcriptional down-regulation of photosynthesis became apparent in response to nitrogen starvation, whereas glucose catabolism was found to be up-regulated. An in-depth reconstruction and analysis of the pathways for glycerolipid, central carbon, and starch metabolism revealed that distinct transcriptional changes were generally found only for specific steps within a metabolic pathway. In addition to pathway analyses, the transcript data were also used to refine the current genome annotation. The transcriptome data were integrated into a database and complemented with data for other microalgae which were also subjected to nitrogen starvation. It is available at https://tdbmn.cebitec.uni-bielefeld.de.ConclusionsBased on the transcriptional responses to different stages of nitrogen availability, a model for triacylglycerol and lipid hyperaccumulation is proposed, which involves transcriptional induction of thioesterases, differential regulation of lipases, and a re-routing of the central carbon metabolism. Over-expression of distinct thioesterases was identified to be a potential strategy to increase the oleaginous phenotype of M. neglectum, and furthermore specific lipases were identified as potential targets for future metabolic engineering approaches.
Highlights
Oleaginous microalgae are promising production hosts for the sustainable generation of lipid-based bioproducts and as bioenergy carriers such as biodiesel
Overview of experiments One major goal of this study was the identification of gene targets that could be promising for subsequent genetic engineering approaches with the aim to increase the microalgal triacylglycerol (TAG) accumulation
The explorative analysis of transcriptional profiles was focused on the lipid and central carbon metabolism, and the starch metabolism was included as an associated pathway (Figs. 4 and 6; Additional file 1: Figure S10b)
Summary
Oleaginous microalgae are promising production hosts for the sustainable generation of lipid-based bioproducts and as bioenergy carriers such as biodiesel. Transcriptomics of the lipid accumulation phase, triggered efficiently by nitrogen starvation, is a valuable approach for the identification of gene targets for metabolic engineering. The production of bulk bio-commodities in a sustainable way is a key target of many biotechnological processes. Owing to their phototrophic growth characteristics, microalgae have been considered to be promising candidates for the production of biofuels such as biodiesel, bioethanol, biogas, or biohydrogen (H2), as well as of high-value products such as terpenoids, polyunsaturated fatty acids (FAs), or recombinant proteins [1,2,3]. For the design of rational metabolic engineering strategies, a valuable approach is to follow the cell’s endogenous regulation of carbon partitioning under conditions of high lipid and especially triacylglycerol (TAG) productivity. The investigation of the transcriptome profiles during nitrogen limitation is an appropriate strategy for the identification of gene targets
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