As a typical secondary metabolite of cyanobacteria, the earthy-musty smelling geosmin is considered as one of the problematic compounds responsible for many water episodes in freshwater supplies.  

The ability to synthesize geosmin is widely distributed in various groups of cyanobacteria. Though the biochemical mechanisms of geosmin synthase gene (geo) in several cyanobacteria species have been elucidated in recent years, the diversity and evolutionary lineage of geo in cyanobacteria is unknown.   

The research groups led by Prof. LI Yeguang from Wuhan Botanical Garden of the Chinese Academy of Sciences (CAS) and Prof. LI Renhui from Institute of Hydrobiology of CAS explored the genetic diversity, phylogeny, and evolution of geo in cyanobacteria on the basis of 16 geosmin-producing cyanobacterial species representing 11 genera in Nostocales and Oscillatoriales. The study was published in Science of the Total Environment. 

Results revealed that geo was highly conserved in the examined cyanobacterial species with DNA sequence identities over 0.72. Phylogenetic reconstruction and codon bias analysis of geo in geosmin-producing microorganisms suggested that cyanobacterial geo formed a monophyletic branch and shared a common ancestor with actinomycetes and myxobacteria.   

Besides, the evolutionary analysis of cyanobacterial geo sequences showed that no site with ω > 1 was observed. Further studies on global selection pressures revealed that the global ω value of cyanobacterial geo was 0.125, indicating strong purifying selection of this gene. 

Interestingly, Phylogenetic topology comparison of geo with housekeeping genes 16S rDNA and rpoC in cyanobacteria revealed significantly incongruent topologies. This finding clearly indicated that geo was acquired by cyanobacteria and then a mixture of co-evolution and HGT (horizontal gene transfer). 

This study enhances the fundamental understanding of cyanobacterial geo in diversity and evolution and highlight the importance of geo and its product geosmin in cyanobacteria.