Prevalence of Actinobacteria in the production of 2-methylisoborneol and geosmin, over Cyanobacteria in a temperate eutrophic reservoir

• Identification of freshwater bacteria during a taste-and-odor outbreak. • Abundances of 16S genes highlight the dominance of Actinobacteria. • Streptomyces species identified as prevalent producers of both geosmin and MIB. • Cyanobacteria involved in geosmin biosynthesis. Commonly detected in surface waters worldwide, odorous compounds such as geosmin (GSM) and 2-methylisoborneol (MIB) pose important issues in drinking water and aquaculture industries. Impaired waters are frequently plagued by severe Cyanobacterial blooms and seasonal occurrences of Actinobacteria due to increased anthropogenic stressors as well as climate change that negatively impact the quality of water resources globally. Historically, studies have focused on the identification of key environmental factors and organisms involved in the production of GSM and MIB. This past decade, fast-evolving molecular techniques have greatly contributed to document the structure of bacterioplankton communities. Eagle Creek Reservoir, a eutrophic temperate reservoir, is frequently impacted by seasonal odorous episodes. During the spring 2013 outbreak of MIB and GSM (120.9 and 51.4 ng L −1 , respectively), a shotgun metagenomics approach illustrated the dominance of Actinobacteria, Proteobacteria, Firmicutes, Bacteroidetes and Cyanobacteria in the reservoir waters. A network analysis highlighted interrelationships between bacterial taxa and environmental factors, and revealed two major clusters of bacteria: Cluster I driven by temperature and nitrate nitrogen and, Cluster II reflective of elevated concentrations of MIB and GSM. Analysis of 16S-based sequences recovered prevalently MIB- and GSM-encoding gene sequences belonging to several Actinobacteria (genus Streptomyces ) rather than Cyanobacteria, formerly assumed to be the main producers. The use of genetics shifted our perception and stressed the role of Actinobacteria in the biosynthesis of odorous metabolites through the non-mevalonate pathway; as well as the detection of non-producing Arthrobacter potentially involved in the degradation of these compounds.