Cyanosphere of commercial spirulina (Limnospira spp.) cultures and characterization of a dominant cyanobacterial contaminant (Sodalinema sp.)

Various studies, including morpho-taxonomic identifications, have indicated that commercial spirulina cultures (Limnospira spp.) can be contaminated by other cyanobacteria without establishing a connection with the potential presence of cyanotoxins. The taxonomy and nomenclature of cyanobacteria has changed significantly since the usage of phylogenetic methods and reclassification could lead to confusion in the designation of taxa previously identified as toxin producers. It therefore appears necessary to characterize the cyanosphere of commercial spirulina cultures in the light of these recent developments. In a collection of six spirulina cultivars used in commercial biomass production, we detected a recurrent dominant cyanobacterial contaminant using 16S rRNA gene amplicon sequencing. Further, Next-Generation Sequencing as well as genetic and morphological characterization pointed towards the genus Sodalinema. A comprehensive analysis of five Sodalinema available genomes as well as our novel metagenome-assembled genome did not reveal any genes involved in the synthesis of cyanotoxins, which suggested that this contaminant is harmless. Still, to avoid valuable biomass related financial loss, we took advantage of spirulina capacity to utilize various forms of nitrogen to set up a procedure to control the proliferation of Sodalinema sp. Finally, a glimpse into heterotrophic contamination of those cultivars indicated that the bacterial composition is very diverse but still several phyla appeared to be very common in open-pond cultivation of spirulina in the Mediterranean region.