Genomics of mutualistic interactions between phytoplankton and bacteria : to a better understanding of marine symbiosis

Interactions between phytoplankton and heterotrophic bacteria play major roles at the basis of marine food webs. Through the balance between photosynthesis and respiration as well as nutrient recycling, they impact marine biochemistry via global carbon, oxygen, nitrogen and other nutrient cycles. By analogy to the terrestrial rhizosphere, phytoplankton and bacteria are known to interact at a microscale, in the microenvironment surrounding phytoplankton cells called the “phycosphere”. Despite its ecological significance, the diversity of bacterial species within the phycosphere, along with their nutritive and/or chemical communication with phytoplankton cells, remain poorly understood. 

We aim to characterize these mutualistic interactions by investigating long-term stable associations between bacteria and phytoplankton, cultured in our laboratory - where phytoplankton cells have been surprisingly surviving for over 3 years without any addition of fresh medium. Using genomic and metagenomic analyses, we assess the bacterial diversity within these phytoplankton-associated microbiomes and explore the metabolic pathways involved in these mutualistic interactions. Co-culture experiments will allow us to identify specific phytoplankton-bacteria symbioses. Microbiome transplant experiments across various phylogenetically diverse phytoplankton strains are performed to test the hypothesis of a universal core microbiome. 

Our findings may reveal key bacteria for phytoplankton survival, providing new insights into marine symbiosis as well as new marine genetic resources paving the way for improving high-biomass algal production in bioreactors, biofuels and algal growth for biotechnology.