The occurrence of actinomycetes, cyanobacteria and microalgae in surface water of the Arno River, used for water supplies in Florence, was investigated in this study on taste and odour compounds (such as geosmin and 2-methylisoborneol (MIB)). Throughout the 12-month monitoring period, raw water samples were collected and analysed for the number and genera of actinomycetes, cyanobacteria and microalgae.

The research has shown that actinomycetes (Streptomyces, Nocardia, Micromonospora) are numerous in autumn and winter and decrease in spring and summer; in contrast, diatoms (Melosira, Navicula), green algae (Chlorella, Pediastrum) and cyanobacteria (Anabaena, Oscillatoria, Nostoc) present a completely opposite trend and are numerous in spring and summer and reduced in autumn and winter. The monitoring of the two odour-causing metabolites gives rise to a trend that is more similar in time to that of cyanobacteria than that of actinomycetes. The constant occurrence of geosmin may be a sign of a synergy between microalgae, cyanobacteria and actinomycetes, while MIB could derive from the activity of cyanobacteria or other species of actinomycetes present in a low concentration.

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