Importance, structure, cultivability, and resilience of the bacterial microbiota during infection of laboratory-grown Haematococcus spp. by the blastocladialean pathogen Paraphysoderma sedebokerense: evidence for a domesticated microbiota and its potential for biocontrol.

Industrial production of the unicellular green alga Haematococcus lacustris is compromised by outbreaks of the fungal pathogen Paraphysoderma sedebokerense (Blastocladiomycota). Here, using axenic algal and fungal cultures and antibiotic treatments, we show that the bacterial microbiota of H. lacustris is necessary for the infection by P. sedebokerense and that its modulation affects the outcome of the interaction. We combined metagenomics and laboratory cultivation to investigate the diversity of the bacterial microbiota associated to three Haematococcus species and monitor its change upon P. sedebokerense infection. We unveil three types of distinct, reduced bacterial communities, which likely correspond to keystone taxa in the natural Haematococcus spp. microbiota. Remarkably, the taxonomic composition and functionality of these communities remained stable during infection. The major bacterial taxa identified in this study have been cultivated by us or others, paving the way to developing synthetic communities to experimentally explore interactions within this tripartite system. We discuss our results in the light of emerging evidence concerning the structuring and domestication of plant and animal microbiota, thus providing novel experimental tools and a new conceptual framework necessary to enable the engineering of Haematococcus spp. microbiota toward the biocontrol of P. sedebokerense.