Quantification of novel geosmin-producing bacteria in aquaculture systems

Earthy off-flavour, caused by geosmin, constitutes an economic problem in recirculating aquaculture systems (RAS). Cyanobacteria, Streptomyces and myxobacteria are thought to be the main geosmin-producing bacteria in aquatic systems. In this study, the phylogeny of the gene involved in the geosmin production, geosmin synthase gene (geoA), was investigated. The results revealed an absence of multiple horizontal gene transfer, and therefore, the gene might be suited as a molecular marker to investigate the diversity of putatively geosmin-producing bacteria. Degenerate primers targeting the major phylogenetic groups of geoA were used to describe the geosmin-producing populations in water and biofilm. Gene libraries from six RAS, collected in Scotland and Denmark, were investigated. This revealed at least two novel, deep-branching lineages. The most dominant geoA-containing bacteria present in the investigated RAS affiliated with myxobacteria, Sorangium and Actinobacteria. TaqMan qPCR approaches were designed to quantify four novel groups of geosmin-producing bacteria. The results showed that geosmin-producing bacteria are present in the biofilters and water phase, and although they constitute only 0.007%–0.9% of the total bacteria, they cause significant off-flavour problems. This study provides an insight into the phylogeny and quantities of geosmin-producing microbiota in full-scale recirculated aquaculture systems.