Phycospheric versus free-living microbiomes: the case of two co-dominant phytoplanktonic species in an extreme hypersaline lake (Dziani Dzaha, Mayotte)
Microbial organisms are key contributors of aquatic biogeochemical cycles, but details regarding their microscale ecology remain largely unexplored. Lake Dziani Dzaha (Mayotte) is a unique thalassohaline ecosystem also considered as a modern analogue of some Precambrian environments. It presented characteristics features such as being an original hypersaline and alkaline environment with a strictly microbial network, where photosynthetic biomass is permanently co-dominated by two phytoplanktonic microorganisms, Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (picoeukaryotic green microalgae).  A. fusiformis and P. salinarum phycosphere diversity (i.e. Bacteria, Microeukaryotes and Archaea) was assessed in cell-sorted fractions collected by using flow cytometry coupled with Illumina sequencing. As expected, bacterial dataset was dominated by A. fusiformis sequences and microeukaryotic dataset by P. salinarum sequences, in the A. fusiformis and P. salinarum cell-sorted fractions, respectively, making thus bacterial and microeukaryotic diversity analyses tendencies rather than strict patterns. However, this work presented a great interest in exploring archaeal diversity distribution among phycospheres and free-living fractions. Phycospheric microbiomes richness differed between the two photosynthetic species with notably P. salinarum phycosphere enriched in Woesearchaeales OTUs while A. fusiformis intimate zone was enriched in methanogenic OTUs. Archaeal phylogeny highlighted the presence of phylogenetically close Woesearchaeales OTUs retrieved only in the free-living fraction suggesting peculiar adaptations dedicated to the free-living lifestyle. Overall, our work clearly showed that the microbial scale study is a powerful approach to reveal unsuspected biodiversity patterns in aquatic ecosystems that might have functional consequences.