The role of Actinobacteria in taste and odour episodes involving geosmin and 2-methylisoborneol in aquatic environments

The secondary metabolites geosmin and 2-methylisoborneol (2-MIB) provide soil with its characteristic earthy-musty odour, being notably produced by the abundant spore-forming filamentous bacterial genus Streptomyces, among other Actinobacteria. Taste and odour (T&O) problems attributed to these compounds affect drinking water supplies worldwide, often occurring sporadically and untraced to their biological origins. A number of prokaryotic and eukaryotic organisms are recognised geosmin and 2-MIB producers in aquatic environments. However, the focus of this paper is to assess the potential contribution of Actinobacteria to this water quality issue. To date, the aquatic ecology of these bacteria remains poorly understood and debate surrounds whether they exist solely as dormant spores of terrestrial origin or are capable of growing and biosynthesising these odourous compounds in aquatic environments. The Actinobacteria which are known to produce geosmin and 2-MIB are identified and a critical assessment of habitats within aquatic environments in which they may be metabolically active residents and thus potential sources of T&O is provided. Current understandings of the chemical ecology and biosynthetic pathways of geosmin and 2-MIB, as well as the conditions under which these secondary metabolites are produced by Streptomyces, are reviewed.