METABOLISM AND FUNCTION OF STORAGE RESERVES IN DIATOMS

Diatoms are a dominating and biodiverse group of phytoplankton accounting for some 10 billion tonnes of carbon fixed per year, a significant proportion of marine primary productivity. Recent genome sequencing of two diatoms has revealed many unique features, including major differences in primary metabolism from what is known in plants and green algae, and it now allows studies of gene expression and protein abundance which would not previously have been possible. Accumulation and depletion of various storage reserves, such as carbohydrates, protein and lipids is intimately linked to growth and survival in the changing environment of diatoms. In addition to predictable diel changes, a major problem facing diatoms is that their environment is inherently unstable. Phytoplankton of temperate and boreal regions can experience frequent deep-mixing due to strong winds and thermal convection, which can expose them to prolonged periods of darkness. In addition, diatom productivity is often limited by available nutrients. Both of these factors affect the accumulation of storage reserves and these can, in turn, affect factors such as buoyancy. Despite the importance of storage reserves in diatoms, there are no comprehensive studies of partitioning between different categories of storage compound (carbohydrates, lipids and protein) in a single diatom species under different environmental conditions. This project will study the metabolism of the carbohydrate, chrysolaminaran, a glucose polymer, in the diatom, Thalassiosira pseudonana, which has had its genome sequenced. We will ask three questions. First, how do contents of storage products (carbohydrate, lipid, amino acids and protein) and carbon partitioning to storage products change on a diurnal basis, and with light or nutrient depletion and resupply? Second, which metabolic pathways are used to synthesise and mobilise chrysolaminaran in diatoms, and are they regulated at the gene transcript and/or protein level? Third, what is the nature of stored protein? To answer these questions we will use a combination of molecular and biochemical techniques. We will measure algal contents of storage products under different environmental conditions. We will assess potential metabolic pathways by 14C-labelling and by measuring relevant gene transcripts and corresponding enzyme abundances and/or activities. Finally, we will use metabolomics to identify intermediates in the synthesis and mobilisation of storage reserves.