PICOPAR · Assessing the role of parasitism in the regulation of picophytoplankton communities in open ocean environments

The oceans play a major role in determining world climate. In part this is due to the production of oxygen and the consumption of carbon dioxide (CO2) by very small, single celled photosynthetic organisms, the picophytoplankton. Picophytoplankton biomass is dominated by eukaryotes comprising cells <5 micrometres in size. These small photosynthetic eukaryotes (PPEs) hence play a critical role in global CO2 fixation but also in carbon cycling. Work to date has focused on the genetic characterisation of this PPE community which is identifying the major players involved, but there is a real dearth of information on the factors controlling PPE community structure. Given their major ecological role it is critical that we begin to understand and identify the factors controlling their diversity and distribution. While the most common consumer strategy, namely parasitism, is usually left out of aquatic trophic food web functioning, recent results highlight the potential importance of this process in ocean systems. The approaches to be developed for this research goal will couple lab-based and environmental analyses using state-of-the-art molecular ecological techniques. We will determine specific associations between parasites and their photosynthetic eukaryotic hosts using the powerful technology of flow cytometric sorting, coupled with clone library construction and fluorescent in situ hybridisation (FISH). We will also compare the effects of a more traditional loss factor, viral lysis, with that of parasitism, using electron microscopy of thin sections to assess viral presence and burst-size. Finally, we will link potential changes in parasite diversity and host specificity with environmental.Hence this project will provide essential new information on the diversity and ecological role of parasites in natural marine systems, which should allow a much greater understanding of carbon and energy flow in marine ecosystems.