Water with off-putting odors is a common nuisance in water treatment facilities and similarly presents challenges in aquaculture including Recirculating Aquaculture Systems (RAS). The primary reason for the odors is microbial activity which causes an aqueous accumulation of the metabolites Geosmin (GSM) and 2-Methylisoborneol (MIB) that are best described as carrying a musty or ‘earthy’ scent and taste.
When present in RAS these metabolites enter the fish where they rapidly accumulate at levels much higher than in the sorrounding water. This accumulation can impart the filets with a muddy off-taste, reducing their quality. The issue of off-flavour is exacerbated by the remarkable ability of humans in detecting GSM and MIB even at concentrations down to a few nanomolars (which far surpasses the ability of sharks to smell blood in water). This makes the removal of off-flavour from fish both expensive and time consuming.
PhD, Professor
⚙ Microbiology & Chemistry
⌂ Dept. Chemistry and Bioscience
A sesquiterpenoid produced mainly by actinomycete bacteria that may form biofilms in RAS.
Image credit: Geosmin (modified); Creative Commons CC0 1.0 Universal Public Domain Dedication (
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An irregular monoterpene produced mainly by actinomycete bacteria that may form biofilms in RAS.
Image credit: Rasmus Juul Jensen
The most sensitive humans can smell Geosmin down to a staggeringly low 5 ng/L concentration. This is equivalent to detecting the muddy off-flavour smell in a mixture of just 1 mL of Geosmin mixed in with 200 million liters of water. We are almost as sensitive to the smell of 2-methylisoborneol (MIB).
While Geosmin and MIB are the cause of the unpleasant off-flavour issue in aquaculture, they are also the reason for the "petrichor" phenomenon. Interestingly, petrichor is the pleasant earthy scent that can be detected after a heavy summer rain where Geosmin and MIB are released from soil bacteria into the air.
