Dynamics of triacylglycerol and EPA production in Phaeodactylum tricornutum under nitrogen starvation at different light intensities

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Continuous versus batch production of lipids in the microalgae Acutodesmus obliquus

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The role of starch as transient energy buffer in synchronized microalgal growth in Acutodesmus obliquus

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Evaluation of diurnal responses of Tetradesmus obliquus under nitrogen limitation

Tetradesmus obliquus is an oleaginous microalga with high potential for triacylglycerol production. We characterized the biochemical composition and the transcriptional landscape of T. obliquus wild-type and the starchless mutant (slm1), adapted to 16:8 h light dark (LD) cycles under nitrogen limitation. In comparison to the nitrogen replete conditions, the diurnal RNA samples from both strains also displayed a cyclic pattern, but with much less variation which could be related to a reduced transcription activity in at least the usually highly active processes. During nitrogen limitation, the wild-type continued to use starch as the preferred storage compound to store energy and carbon. Starch was accumulated to an average content of 0.25 g·gDW−1, which is higher than the maximum observed under nitrogen replete conditions. Small oscillations were observed, indicating that starch was being used as a diurnal energy storage compound, but to a lesser extent than under nitrogen replete conditions. For the slm1 mutant, TAG content was higher than for the wild-type (average steady state value was 0.26 g·gDW−1 for slm1 compared to 0.06 g·gDW−1 for the wild-type). Despite the higher TAG content in the slm1, the conversion efficiency of photons into biomass components for the slm1 was only half of the one obtained for the wild-type. This is related to the observed decrease in biomass productivity (from 1.29 gDW·L−1·day−1 for the wild-type to 0.52 gDW·L−1·day−1 for the slm1). While the transcriptome of slm1 displayed clear signs of energy generation by degrading TAG and amino-acids during the dark period, no significant variation of these metabolites could be measured. When looking through the diurnal cycle, the photosynthetic efficiency was lower for the slm1 mutant compared to the wild-type especially during the second half of the light period, where starch accumulation occurred in the wild-type.publishedVersionPaid Open Acces

Photosynthetic efficiency in microalgal lipid production

Microalgae can contain large amounts of lipids which make them a promising feedstock for sustainable production of food, feed, fuels and chemicals. Various studies, including pilot-scale, have been performed and the knowledge on microalgal processes has advanced quickly. Unfortunately, current production costs for cultivation are still too high for bulk lipid production from microalgae. One of the major causes for the high costs of bulk lipid production is the reduced solar-to-lipid conversion efficiency. Current research, however, does not provide sufficient insight to identify optimization targets. Therefore, in this thesis we have studied the lipid production in microalgae in depth. Different TAG-accumulation strategies were investigated from a process engineering and metabolic point of view. The combination of all findings were used in the general discussion to thoroughly evaluate the microalgal lipid accumulation strategies. Current phototrophic microalgal lipid yields are still 10 times lower than the theoretical maximum. There is, however, still an enormous potential for further improvements. Future research should focus on (genetically) improved strains and advanced cultivation strategies, including adaptation to fluctuating outdoor weather conditions. This thesis was performed within the EU FP7 FUEL4ME project under grand agreement No 308938. Objective of this program is to develop a sustainable and scalable process for biofuels from microalgae and to valorize the by-products.</p

Can We Approach Theoretical Lipid Yields in Microalgae?

Can we approach theoretical lipid yields in microalgae? Yes: we can substantially reduce the gap between current and theoretical maximum yield. A realistic maximum is approximately 0.5. g triacylglycerol (TAG) per mol photons, about five times higher than what is currently achieved in outdoor cultivation. Achieving this realistic maximum will require several breakthroughs. First, outdoor operation typically has low yields, mainly caused by fluctuating insolation. Future adaptive control models will help increase these yields. Additionally, the lipid production capacity of currently used strains needs to increase. Powerful strain-specific molecular toolboxes are being developed, shifting the bottleneck towards understanding metabolism and identifying target genes. Finally, strains and processes should be improved concurrently to fully exploit the potential lipid production from microalgae. To fulfill the necessity for improved yields in microalgal lipid production, current research is moving towards advanced cultivation strategies, including adaptation to fluctuating outdoor weather conditions.Recent developments of targeted gene-editing technology (e.g., CRISPR/Cas9) and improved high-throughput screening techniques in microalgae contribute to fast and automatized strain improvement approaches.Industrial needs and higher oil productivities of microalgae compared with plants will foster the development of tailored microalgal oils

Impact of five different incident light intensities on (A) the biomass concentration, (B) F_v/F_m ratio and (C) the TAG content over time.

<p>Light was supplied at intensities of 60 μmol m^-2 s^-1 (blue circles), 100 μmol m^-2 s^-1 (red squares), 250 μmol m^-2 s^-1 (green triangles), 500 μmol m^-2 s^-1 (black circles) and 750 μmol m^-2 s^-1 (open diamonds). Sufficient nitrogen was added to reach biomass concentrations of 1.2 g L^-1, thereafter cells were exposed to nitrogen starvation. Average values of duplicate reactor runs are shown, except for the cultures exposed to 250 and 750 μmol m^-2 s^-1 (n = 1). Error bars indicate the standard deviation of both cultures from the average value.</p

Average fatty acid composition of <i>P</i>. <i>tricornutum</i> in the TAG and ML lipid fraction for five different light intensities: 60, 100, 250, 500 and 750 μmol m^-2 s^-1.

<p>Values are averaged over the entire time course of the experiment (n = 7). Standard deviations are shown between brackets.</p

Comparison of the maximum TAG and total fatty acid content obtained in this study to literature values.

<p>*TAG contents were estimated from the total fatty acid content, assuming 8% of membrane lipids per dry weight.</p

Time averaged TAG yield on light for five different incident light intensities over time.

<p>Incident light intensities of 60 μmol m^-2 s^-1 (blue circles), 100 μmol m^-2 s^-1 (red squares), 250 μmol m^-2 s^-1 (green triangles), 500 μmol m^-2 s^-1 (black circles) and 750 μmol m^-2 s^-1 (open diamonds). Sufficient nitrogen was added to reach biomass concentrations of 1.2 g L^-1, thereafter cells were exposed to nitrogen starvation. Error bars indicate the standard deviation of both cultures from the average value.</p