Transporters at the root of organic carbon acquisition and metabolism in eukaryotic microalgae - current knowledge and engineering potential

Microalgae play a key role in the global carbon cycle both as primary producers of photosynthetic assimilates and also as consumers of reduced carbon sources which some species can use to sustain hetero- and mixotrophic growth. Acquisition of reduced carbon and the partitioning of intermediates derived from its metabolism between different intracellular compartments requires the function of membrane transporter proteins positioned at the plasma membrane and the membranes of various organelles. Despite their important role in controlling the strength and direction of carbon fluxes, relatively little is known about microalgal membrane transporters of organic carbon substrates. Here we review state-of-the-art knowledge on microalgal transporters involved in acquisition of reduced carbon sources and intracellular carbon partitioning. Furthermore, we explore how this knowledge can be applied in engineering of novel microalgal strains with improved properties for use in various biotechnological fields. • Provides exhaustive repository of described plasma membrane transporters of organic substrates in different microalgal groups • Highlights the role of intracellular metabolite transporters in hetero- and mixotrophic growth • Explores transporter proteins as viable engineering targets in developing microalgal cell factories • Focus on trophic conversion and enhanced lipid production as examples of successfully applied transport engineering strategies