Abstract
The regulation of carbon metabolism in the diatom Phaeodactylum tricornutum at the cell, metabolite, and gene expression levels in exponential fed-batch cultures is reported. Transcriptional profiles and cell chemistry sampled simultaneously at all time points provide a comprehensive data set on carbon incorporation, fate, and regulation. An increase in Nile Red fluorescence (a proxy for cellular neutral lipids) was observed throughout the light period, and water-soluble glucans increased rapidly in the light period. A near-linear decline in both glucans and lipids was observed during the dark period, and transcription profile data indicated that this decline was associated with the onset of mitosis. More than 4,500 transcripts that were differentially regulated during the light/dark cycle are identified, many of which were associated with carbohydrate and lipid metabolism. Genes not previously described in algae and their regulation in response to light were integrated in this analysis together with proposed roles in metabolic processes. Some very fast light-responding genes in, for example, fatty acid biosynthesis were identified and allocated to biosynthetic processes. Transcripts and cell chemistry data reflect the link between light energy availability and light energy-consuming metabolic processes. Our data confirm the spatial localization of processes in carbon metabolism to either plastids or mitochondria or to glycolysis/gluconeogenesis, which are localized to the cytosol, chloroplast, and mitochondria. Localization and diel expression pattern may be of help to determine the roles of different isoenzymes and the mining of genes involved in light responses and circadian rhythms.
Highlights
The regulation of carbon metabolism in the diatom Phaeodactylum tricornutum at the cell, metabolite, and gene expression levels in exponential fed-batch cultures is reported
Biochemical pathways involved in carbon metabolism appeared to be tightly regulated, with processes such as photosynthesis and carbon assimilation dominating during the day and catabolic utilization of energy reserves occurring mainly during the night (Dron et al, 2012)
Our results show that genes encoding enzymes involved in processes such as the tricarboxylic acid (TCA) cycle and fatty acid biosynthesis are highly coordinated over the light/dark cycle, and the transcriptional data indicate activity that may result in the synthesis of carbon skeletons in the chloroplast during daytime, and the breakdown of carbon-rich compounds in the cytosol and mitochondria and cell division during the night
Summary
The regulation of carbon metabolism in the diatom Phaeodactylum tricornutum at the cell, metabolite, and gene expression levels in exponential fed-batch cultures is reported. The synthesis of polymers (that must be reduced to monomers again) is considered a way to reduce the large oscillations in cellular Glc levels that would otherwise occur over the light/dark cycles (Vårum et al, 1986) Another fraction of the fixed carbon is incorporated via acetyl-CoA into fatty acids. Major cellular processes in microalgae, such as photosynthesis, nutrient uptake and incorporation, and cell growth and division, show diel regulation that follow the light/ dark periods (Sanchez et al, 2009; Radakovits et al, 2010) This general pattern may be altered in species with high growth rates, where some cells undergo more than one cell division per day (Granum et al, 2009). Similar analyses of eukaryotic plankton are rare, but about 94% of the expressed genes in the prasinophyte Ostreococcus tauri were differentially expressed during a light/dark cycle (Monnier et al, 2010)
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