Identification of geosmin biosynthetic gene in geosmin-producing colonial cyanobacteria Coelosphaerium sp. and isolation of geosmin non-producing Coelosphaerium sp. from brackish Lake Shinji in Japan

Geosmin is an earthy-muddy smelling compound produced in aquatic ecosystems by microorganisms including cyanobacteria. An increase in geosmin levels affecting the local fishery occurred in May 2007 in Lake Shinji, Japan, and geosmin-producing colonial cyanobacterium, Coelosphaerium sp. G2, was isolated from a water sample from the lake and identified. Cyanobacteria Coelosphaerium sp. is commonly found in Lake Shinji; however, prior to 2007, earthy-muddy odors were not a frequent issue. Further, there was no information regarding the geosmin biosynthetic gene in colonial cyanobacteria. Here, the geosmin biosynthetic gene (geoA) in strain G2 was identified and its nucleotide sequence was determined. It was found that geoA had 79% and 78% identity with geoA from filamentous geosmin-producing cyanobacteria Fischerella sp. PCC 9431 and geoA2 from Phormidium sp. P2r, respectively. The deduced amino acid sequence of GeoA consisted of two domains that were annotated as terpene cyclase. In 2015, geosmin non-producing Coelosphaerium sp. S3C5 was isolated from Lake Shinji and identified by morphological and genetic analyses. There was no difference in morphology or nucleotide sequences of 16S rRNA and 16S-23S internal transcribed spacer (ITS) between geosmin-producing and non-producing strains, which are therefore closely related and can exist in Lake Shinji. Distinguishing the two strains by observation under a microscope and sequencing of 16S rRNA and 16S-23S ITS have proven difficult. Inconsistency between the appearance of Coelosphaerium cells and the detection of the odor in water samples could therefore be attributed to dominance by the geosmin-producing strain or the non-producing strain. The increase in earthy smell is assumed to be caused by an increase in the geosmin-producing strain in Lake Shinji. Genetic analysis of geoA in Coelosphaerium sp. and the relative abundances of geosmin-producing and non-producing Coelosphaerium strains in Lake Shinji can be used to mitigate the economic damages caused by geosmin. Development of a molecular method to monitor the geosmin-producing strain in water ecosystems is equally important to alleviate the earthy smell caused by this particular strain.