PHYCOSPHERES · Structure and functions of terrestrial phycospheres

Microscopic algae release organic compounds to the region immediately surrounding their cells, known as the phycosphere, constituting a niche for colonization by heterotrophic bacteria. These bacteria consume algal photoassimilates and provide beneficial functions to their host, in a process that resembles the establishment of microbial communities associated with the roots and rhizospheres of land plants. Phycosphere communities have been well studied in aquatic environments, where they are known to play important roles in nutrient and energy fluxes. For many species of algae, interactions with their associated phycosphere bacteria can also provide beneficial functions, often mediated by metabolic exchanges. Despite the known importance of these associations in aquatic environments, their role in terrestrial ecosystems and parallels with the root microbiota of land plants have not yet been resolved.The goal of this project is to employ a newly developed reductionist host-microbiota system based on the model chlorophyte alga Chlamydomonas reinhardtii to study the structure and functions of terrestrial phycospheres and identify the core ecological principles that explain the overlap between the root and phycosphere microbiota. Using a variety of gnotobiotic systems and synthetic communities composed of phycosphere bacteria, we will study the genetic and molecular mechanisms that drive host-microbiota interactions in C. reinhardtii and as well as their evolutionary origins and conservation in other lineages of green algae and land plants. Finally, we will design and build synthetic phototrophic microcosms based on terrestrial phycospheres using bioreactors to explore the ecological and molecular mechanisms underpinning microbiota diversity, stability and functions. This project has the potential to reveal fundamental, quantitative principles governing the dynamic behaviour of host-associated microbial communities.