The genetic diversity and evolution of diatom-diazotroph associations highlights traits favoring symbiont integration

Diatom diazotroph associations (DDAs) are a widespread marine planktonic symbiosis between several diatom genera and di-nitrogen (N-2)-fixing bacteria. Combining single cell confocal microscopy observations and molecular genetic approaches on individual field collected cells, we determined the phylogenetic diversity, distribution and evolution of the DDAs. Confocal analyses coupled with 3-D imaging re-evaluated the cellular location of DDA symbionts. DDA diversity was resolved by paired gene sequencing (18S rRNA and rbcL genes, 16S rRNA and nifH genes). A survey using the newly acquired sequences against public databases found sequences with high similarity (99-100%) to either host (18S rRNA) or symbiont (16S rRNA) in atypical regions for DDAs (high latitudes, anoxic basin and copepod gut). Concatenated phylogenies were congruent for the host and cyanobacteria sequences and implied co-evolution. Time-calibrated trees dated the appearance of N-2 fixing planktonic symbiosis from 100-50Mya and were consistent with the symbiont cellular location: symbioses with internal partners are more ancient. An ancestral state reconstruction traced the evolution of traits in DDAs and highlight that the adaptive radiation to the marine environment was likely facilitated by the symbiosis. Our results present the evolutionary nature of DDAs and provide new genetic and phenotypic information for these biogeochemically relevant populations.