Off-flavours in water:: hydroxyketones and β-ionone derivatives as new odour compounds of freshwater cyanobacteria

Cyanobacteria essentially contribute to the off-flavours of drinking water and freshwater fish. They are primary nuisance microorganisms and their control in lakes and ponds used for the production of drinking water and fish farming is expensive. We describe new odour compounds of freshwater cyanobacteria that are fermentation products and norcarotenoids. These may contribute to the off-flavours frequently observed with cyanobacteria. The fermentation products were represented by 2,3-pentandione, 2,3-hexandione, 2-hydroxy-3-pentanone, 3-hydroxy-2-pentanone, 2-hydroxy-3-hexanone, 3-hydroxy-2-hexanone and 2,3-hexandiol. These compounds are important for odour quality in microbially-processed dairy food, caused by the metabolism of fungi and heterotrophic bacteria. Their cyanobacterial synthesis was shown by C-13-labelling, growth kinetics and biotransformation experiments. The labelled mass spectra of the hydroxyketones showed unequivocally the cyanobacterial rather than bacterial origin of the compounds. Norcarotenoids of the beta-ionone-type (beta-ionol, beta-ionone-5,6-epoxide, dihydro-beta-ionone, dihydro-beta-ionol, tetrahydroionone, 4-oxo-beta-ionone and 2,4,4-trimethyl-3-(3-oxobutyl)-2-cyclohexen-1-one) were another important group of compounds that were found in axenic cyanobacterial cultures and a monoxenic culture of Phormidium sp. The results implicate the presence of further oxygenases in cyanobacteria. Except for tetrahydroionone, the syntheses of the beta-ionone derivatives were established by biotransformation experiments applying the precursor beta-ionone. The norcarotenoid 6-methyl-5-hepten-2-one, which is regarded as a precursor compound of 1,3,3-trimethyl-2,7-dioxabicylo(2,2,1)heptane (TDH), was C-13-labelled, while TDH did not show any labelling. There is evidence that TDH arises from the analytical procedure and is an artefact rather than a biogenic compound produced by the cyanobacteria. Copyright (c) 2005 John Wiley & Sons, Ltd.