PhD Studentship - Evolution and Spread of a Bacterial Gene for Organic Nitrogen Acquisition in Marine Phytoplankton and its Biogeochemical Impact

Questions: Ocean nitrogen budgets rest almost entirely on nitrate, ammonium and urea. We have recently found that marine phytoplankton also grow on a small nitrogen compound that many bacteria expel as a toxin, using an enzyme acquired from bacteria by horizontal gene transfer. If this capability is widespread, a nitrogen pool missing from current ecosystem models supports primary production in coastal seas, changing how nutrient budgets and eutrophication risk are assessed. The Doctoral Researcher will ask how this capability spread across plankton lineages and ocean regions, how the compound's concentration varies seasonally in coastal water, and how much including it changes modelled nitrogen cycling. Methods: combining global metagenome and metatranscriptome archives (Tara Oceans, Bio-GO-SHIP) with phylogenomics to map where the trait is used, ocean by ocean; LC-MS/MS quantification of the compound in coastal time-series samples; isotope-labelled growth experiments on wild-type and CRISPR knockout diatoms to derive uptake and yield parameters; and implementation in the trait-based ecosystem models developed, then validation experiments. Training: At Exeter the Researcher will learn phylogenomics, axenic culturing and molecular genetics with Dr Monier, and metabolite quantification and stable-isotope methods with Professor Smirnoff, a plant and algal biochemist. An extended placement in Bristol adds modelling, high-performance computing and parameter estimation with Professor Monteiro. Sequencing and mass spectrometry are funded by the supervision team's NERC grant on phytoplankton interactions with nitrogen-fixing bacteria. A patent application on the enzyme as a selection marker for synthetic biology also shows the Researcher how discovery becomes technology. Collaboration: All co-supervisors already work on that grant. Bristol's modelling turns Exeter's measurements into nitrogen budgets, and the Researcher will spend an extended period in Bristol. After a supervised first-year survey, they will choose which strand to lead, in analytical chemistry, comparative genomics or global modelling, and which taxa and ocean regions to focus on. Funding For eligible students the studentship will cover home tuition fees plus an annual tax-free stipend.