During the initial 24 months of the project, we successfully established an extensive culture collection of terrestrial algae from the Eifel National Park, Germany. This collection includes a wide representation of chlorophyte and streptophyte algae, encompassing environmental relatives of the model organism Chlamydomonas reinhardtii and some of the closest extant relatives of land plants. Using these isolates, we developed innovative mesocosm systems designed to study the assembly of microbial communities (phycosphere microbiota) around algae under controlled laboratory conditions.
Through detailed microbial profiling (bacterial, fungal, and other eukaryotic groups), we discovered that algae significantly shape their microbial communities, and these communities differ distinctly based on algal evolutionary relationships. Our research highlighted that algae actively release specific photosynthetically derived compounds and other metabolites that selectively foster beneficial microbial communities, leading to generally mutualistic interactions.
To investigate these interactions further, we conducted advanced transcriptomic and metabolomic analyses on Chlamydomonas reinhardtii grown with soil-derived bacteria. Our findings revealed substantial changes in algal gene expression, notably increased photosynthesis and nutrient transport, coupled with higher release of amino acids that support bacterial growth. This active physiological adjustment by algae, without signs of stress, fundamentally alters the existing perspective that release of photosynthates is primarily an overflow mechanism to sustain the efficiency of the algal photorespiratory mechanism and suggest that the host can perceive the presence of their microbiota member and adjust its responses in order to engage in mutualistic associations.
In collaboration with expert partners, we established the groundwork for genetic screens using a newly developed mutant library of C. reinhardtii. These future genetic screens are aimed at identifying algal genes critical for microbiome interactions, further deepening our understanding of the genetic mechanisms underlying these relationships.
Finally, we established an advanced long-term synthetic ecosystem experiment, continuously cultivating algae and microbial communities under highly controlled laboratory conditions using photobioreactors. This setup, running uninterrupted for over two years, provided unique insights into how environmental factors (such as light intensity) influence community stability and microbial dynamics, validating long-standing ecological theories with empirical data.
