This article builds on a recent European Aquaculture Society webinar featuring Theresa Afi of Dalhousie University and Jan-Eric Haagensen of Moleaer, exploring new research on geosmin and the broader potential of nanobubble technology in RAS.
An eight-month study at Dalhousie University found 82% lower mean geosmin concentrations in an Atlantic salmon RAS using Moleaer oxygen nanobubbles compared with a control using conventional oxygen diffusers. Beyond the result itself, the research points to a compelling opportunity for RAS producers: managing more of the geosmin challenge within the production system.
Geosmin is a persistent challenge in recirculating aquaculture systems (RAS).
Geosmin is a naturally occurring compound produced by certain microorganisms that can accumulate in RAS water and be absorbed by fish, creating the earthy taste and odor associated with off-flavor.
It isn't considered a fish or human health concern. But it can become a product quality issue at extremely low concentrations.
“The issue with geosmin is more of a consumer rejection,” explained Theresa Afi, a researcher at Dalhousie University's Centre for Water Resources Studies.
People can detect geosmin at concentrations measured in parts per trillion. Afi puts the sensory threshold at approximately 4–10 nanograms per liter.
That makes the results from Dalhousie particularly significant.
During the eight-month study, the RAS using Moleaer oxygen nanobubbles had 82% lower mean geosmin concentrations than the control system using conventional oxygen diffusers.
For Moleaer's Jan-Eric Haagensen, the result points to an important opportunity for RAS producers.
“If you can significantly reduce geosmin, you can also significantly reduce the amount of purging that you need.”
Today, producers can manage geosmin through depuration, or purging, before harvest.
“The solution right now is to purge the fish in clean water,” Afi explained.
Fish are moved into clean water and held there while accumulated geosmin is eliminated. The process works, but it takes time, clean water and production resources.
In the nanobubble RAS, geosmin was mostly below the detection limit. When detected, concentrations were approximately 3–5 ng/L.
For producers, that creates a compelling next question: if fish are exposed to substantially lower geosmin concentrations during production, could that ultimately reduce the resources required to manage geosmin before harvest?
Afi noted that reducing purging could mean “less resources and less time.”
The Dalhousie study did not measure purge time or geosmin accumulation in fish tissue, so those outcomes will require further research. But the 82% difference in mean geosmin concentration provides a strong foundation for investigating them.
The result also raises an important scientific question: why was geosmin so much lower in the nanobubble RAS?
The Dalhousie research team explored several possible pathways, including biological degradation and oxidative processes.
One clue emerged after the systems were treated with formalin.
Following treatment, researchers recorded a significant increase in nitrite in the control RAS. Nitrite levels in the nanobubble RAS remained relatively stable.
The difference led researchers to look more closely at the microbial communities in the two systems.
Geosmin is biologically produced, and previous research has also identified microorganisms associated with its degradation. The researchers found differences in some bacterial groups between the two RAS, providing another direction for deeper investigation.
The initial microbial analysis wasn't designed to identify the specific species or strains responsible for the geosmin result, so more targeted work is needed to establish the precise biological pathway.
But the result aligns with a broader observation Moleaer has seen across water treatment applications: nanobubbles can influence physical, chemical and biological processes in water.
As Haagensen explained during the webinar:
“We don't believe that there is one single mechanism.”
Instead, the research is opening new questions about how nanobubbles interact with microbial activity, biological degradation, oxidation and other water quality processes within RAS.
This broader view of Moleaer nanobubbles is important. In aquaculture, nanobubbles are often first considered as an oxygenation technology. But Moleaer's work in commercial RAS has shown measurable effects beyond oxygen delivery.
At a commercial RAS in Norway, Moleaer nanobubbles were associated with a 30% reduction in turbidity after 50 days, a 67% reduction in ozone demand and an increase of more than 60% in biofilter nitrification rates.
At a commercial post-smolt RAS in Chile, total suspended solids decreased from approximately 17 mg/L to less than 5 mg/L, while sulfide concentrations decreased from approximately 20 mg/L to less than 10 mg/L. Operators also observed reduced foam and improved water clarity.
The Dalhousie geosmin research adds an important new result to that growing body of evidence.
As Haagensen summarized during the webinar, nanobubbles can have “a very broad impact on the system.”
That's increasingly what the data is showing.
For RAS producers, the significance of the Dalhousie research goes beyond one 82% result. It creates an opportunity to rethink when geosmin is managed.
Rather than relying solely on managing accumulated geosmin at the end of production, reducing geosmin concentrations within the RAS could create another line of defense earlier in the production cycle.
The next step is to understand what that means at commercial scalem, and they now start from a strong result: over eight months, the Atlantic salmon RAS using Moleaer oxygen nanobubbles had 82% lower mean geosmin concentrations than the conventional diffuser control.
For an industry continually looking for better ways to manage water quality, resource use and production efficiency in increasingly closed systems, that's an exciting finding — and an important area for continued research.
Hear directly from Theresa Afi of Dalhousie University and Jan-Eric Haagensen of Moleaer as they discuss the new research, what the results could mean for RAS producers, and findings from commercial RAS facilities in Norway and Chile.
Watch the full webinar: Nanobubble Technology in RAS: New Research and Insights from the Field
https://youtu.be/0jm7iF75ZZA?si=oSdu8XpfyWcp7Sch