Investigation into the metabolic control of lipid accumulation in the marine diatom Phaeodactylum tricornutum

Abstract

The uncontrolled consumption of fossil fuels over the last two centuries has resulted in unprecedented rates of CO 2 (greenhouse gas) accumulation in the atmosphere, which caused an increased average global temperature and climate shifts. Replacing current fuel demands with a renewable and carbon neutral biofuel can alleviate some of the negative consequences of fossil fuel emissions. Algal biofuel can be that renewable energy source. Microalgae are unicellular photosynthetic organisms that can grow on CO 2 as the sole source of carbon by harvesting light energy. Under nutrient limited conditions microalgae will store carbon as oil or triacylglycerides (TAGs). Algal bio-oil can be harvested and converted to biodiesel, which can be put into any current diesel engine without modification. The objective of this dissertation is to characterize bio-oil accumulation in the marine diatom, Phaeodactylum tricornutum by transcriptomic profiling and to investigate metabolic control points during light and dark photoperiods. Extensive growth analysis in tandem with RNA sequencing provided insight into the metabolism of TAG accumulation in microalgae. Phosphate and nitrate depletion initiated lipid accumulation independently, but nitrate depletion resulted in greater lipid yields. Nutrient depletion caused cell cycle arrest, which is controlled by the differential expression diatom specific cyclins. During times of decreased inorganic carbon levels P. tricornutum will employ carbon-concentrating mechanisms to maintain carbon flow for photosynthesis during the light cycle. During the dark period up-expression of CCM genes is still maintained for replenishing carbon intermediates of the TCA and urea cycle. Acetyl-CoA carboxylase is the rate-limiting step in fatty acid biosynthesis, but during the light period it is not highly over-expressed. However, central carbon metabolism pathways are highly expressed throughout growth and during the light and dark cycle. It was postulated that elevated concentrations of precursor carbon pools are driving lipid accumulation without a highly expressed acetyl-CoA carboxylase. Thus, enhancement of lipid accumulation is controlled by upstream carbon flow from pathways like the urea and TCA cycle to fatty acid biosynthesis.