Diatom triacylglycerol metabolism: from carbon fixation to lipid droplet degradation

Diatoms are a prominent microalgae family that has attracted interest for its production of molecules of biotechnological interest. Their fatty acid profile is rich in health-beneficial omega-3 eicosapentaenoic acid (EPA). Furthermore, under nutrient-deprived conditions, many diatom species, notably Phaeodactylum tricornutum, Thalassiosira pseudonana and Fistulifera solaris, can accumulate a high proportion of their biomass as neutral lipids, mostly triacylglycerol (TAG), that have adequate properties for conversion into biodiesel. Compared to commonly studied green and red microalgae, diatoms have specific metabolic characteristics linked to their unique evolutionary history. Although the physiology and biochemistry of diatoms have been well studied, an in-depth exploration of metabolism has been possible only in the last two decades with the help of "-omics" approaches and genetic engineering technologies. In this review, a global landscape of diatom TAG metabolism is presented. The pathways leading to the production of TAG are considered first, followed by the characteristics and dynamics of the lipid droplets that serve as TAG storage, and then by TAG degradation pathways. We end with an overview of future research perspectives, with emphasis on yet understudied metabolism aspects.