review · Microorganisms
Cyanobacteria frequently produce both hazardous cyanotoxins and volatile compounds that generate unpleasant tastes and odours in water supplies, including geosmin, 2-methylisoborneol, beta-ionone and beta-cyclocitral. Although both classes of metabolites are well documented individually, their co-occurrence has received little collective review. These compounds can originate from identical or differing cyanobacterial strains, as well as non-cyanobacterial species. Current environmental data remain insufficient to determine whether taste and odour compounds co-vary with cyanotoxins or stimulate their production. As a result, taste and odour compounds cannot be relied upon as early warning surrogates for toxic events. While available evidence suggests that taste and odour metabolites present low direct toxicological risks, specific concerns such as beta-cyclocitral inhalation require further investigation. Crucially, the biological impacts of simultaneous exposure to both metabolite classes remain unknown, leaving the overall health risks of co-occurring blooms unresolved.
Water managers and consumers often associate bad tastes and smells in water with dangerous contamination. However, unpleasant odours do not reliably indicate whether dangerous cyanotoxins are present. Understanding the distinct dynamics and potential combined effects of these cyanobacterial metabolites is vital for ensuring public safety, protecting drinking water quality, and designing accurate monitoring protocols for affected freshwater bodies.
The findings highlight that water utilities and environmental monitoring organisations cannot use simple taste or odour detection as an operational proxy for cyanotoxin risks. Instead, sensor developers and testing service providers must deploy dedicated, direct detection tools for toxins alongside odour monitoring. This work represents early-stage foundational review research, indicating a need for multi-analyte water monitoring systems before combined risk assessments can be operationalised.
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Cyanobacteria commonly form large blooms in waterbodies; they can produce cyanotoxins, with toxic effects on humans and animals, and volatile compounds, causing bad tastes and odors (T&O) at naturally occurring low concentrations. Notwithstanding the large amount of literature on either cyanotoxins or T&O, no review has focused on them at the same time. The present review critically evaluates the recent literature on cyanotoxins and T&O compounds (geosmin, 2-methylisoborneol, β-ionone and β-cyclocitral) to identify research gaps on harmful exposure of humans and animals to both metabolite classes. T&O and cyanotoxins production can be due to the same or common to different cyanobacterial species/strains, with the additional possibility of T&O production by non-cyanobacterial species. The few environmental studies on the co-occurrence of these two groups of metabolites are not sufficient to understand if and how they can co-vary, or influence each other, perhaps stimulating cyanotoxin production. Therefore, T&Os cannot reliably serve as early warning surrogates for cyanotoxins. The scarce data on T&O toxicity seem to indicate a low health risk (but the inhalation of β-cyclocitral deserves more study). However, no data are available on the effects of combined exposure to mixtures of cyanotoxins and T&O compounds and to combinations of T&O compounds; therefore, whether the co-occurrence of cyanotoxins and T&O compounds is a health issue remains an open question.
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DOI: 10.3390/microorganisms11040872
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