Improving photosynthetic conversion efficiency in marine microalgae

Abstract

Marine photosynthetic microalgae have great potential in biotechnology. They have huge genetic diversity and naturally make an array of metabolites that are precursors in high value products such as fuels and pharmaceuticals. They do not compete with agriculture for land or fresh water and can be used to reduce industrial carbon-emissions. In order to realize this potential however much work needs to be done to overcome the engineering challenges of growing microalgae on large scales and developing the genetic tools required to increase the yield and diversity of products synthesized by cells. Irrespective of the type of microalgal species selected for growth, the efficiency at which light energy is converted into product, the 'photosynthetic conversion efficiency' sets a fundamental limitation on the potential yield. In natural systems as much as 90% of absorbed light energy is re-emitted as heat or fluorescence, representing a major loss in overall efficiency. In this thesis a high-throughput pipeline using random mutagenesis and live single cell sorting has been used to isolate two cell-lines of the eukaryotic microalgae Dunaliella tertiolecta with reduced chlorophyll content (termed lca1 and lca2). As there is no published genome for D. tertiolecta and the species is difficult to transform, this approach represents a feasible method to develop improved cell-lines from any microalgal species. These cell lines are characterized physiologically and shown to increase the maximum rate of chlorophyll-normalized photosynthesis Pmax by 289 (lca1) and 131% (lca2) respectively. The molecular basis of these random mutations characterized by transcriptomics using next-generation sequencing approaches, helps define the differences in regulation in light-harvesting and photosynthesis between the lca1, lca2 and wild-type. The approaches applied in this thesis therefore show how microalgal strains with poor genetic characterization can rapidly be selected for biotechnological applications, and providing new gene targets and valuable insights into the fundamental mechanisms of photosynthesis.

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ORCID for Matthew Terry: orcid.org/0000-0001-5002-2708

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