Julia Södergren

Despite advances in the operation of recirculating aquaculture systems (RAS), accumulation of earthy-muddy off-flavors in the fish remains a potential risk. Myxobacteria (Myxococcota) are reported to be among the most abundant geosmin synthase-harboring groups in RAS, but previous isolation attempts have been unsuccessful, limiting the knowledge of their role in off-flavor production. For the first time, we successfully isolated two geosmin-producing myxobacteria from RAS: Myxococcus virescens AT3 and Corallococcus exiguus AT4. Cell-specific geosmin production varied with the nutrient content in different media but was highest in a low-nutrient medium and when cultivated in water from RAS. Cultivation in RAS water also stimulated the production of other volatile organic compounds (VOCs). Newly identified potential off-flavor compounds included 4-methyl-2-heptanone (“forest” odor), 3-methyl-1-butanol (“medicinal” and “chemical”), and a presumptive sesquiterpenoid described as “musty,” “earthy,” and “flowery.” The previously known off-flavor compound dimethyl sulfide was also detected. Myxobacteria have previously been proposed as keystone bacteria in the environment due to their predatory lifestyle. In predation assays using isolated bacteria from RAS, M. virescens AT3 and C. exiguus AT4 could successfully feed on 15 of 16 tested strains, suggesting a large influence on the biology of RAS microbiomes. The combination of predatory behavior and potent production of geosmin and other VOCs underscores the ecological and sensory impact of these bacteria in RAS. Understanding their behavior and metabolic outputs is critical to developing strategies for mitigating off-flavors in RAS. IMPORTANCE: Issues with off-flavored fish in recirculating aquaculture systems (RAS) due to the presence of the earthy-musty smelling compounds geosmin and 2-MIB are considered as one of the industry’s most economically significant challenges. Knowledge of conditions that affect off-flavor production is essential information in the development of viable solutions for its mitigation. Little is known about the function of these microbially produced compounds or the conditions that trigger their production, especially in the underexplored myxobacteria. Investigation of natural isolates is crucial to determine the function of the genes involved and their differential expression in response to environmental cues. While myxobacteria in RAS have been previously shown to harbor the geosmin synthase gene in molecular studies, the present study is the first attempt to isolate these bacteria from RAS and quantify their geosmin production under various nutrient conditions. Through cultivation-based methods, we demonstrate their production of both known and novel compounds with earthy attributes.

Despite advances in the operation of recirculating aquaculture systems (RAS), accumulation of earthy-muddy off-flavors in the fish remains a potential risk. Myxobacteria (Myxococcota) are reported to be among the most abundant geosmin synthase-harboring groups in RAS, but previous isolation attempts have been unsuccessful, limiting the knowledge of their role in off-flavor production. For the first time, we successfully isolated two geosmin-producing myxobacteria from RAS: Myxococcus virescens AT3 and Corallococcus exiguus AT4. Cell-specific geosmin production varied with the nutrient content in different media but was highest in a low-nutrient medium and when cultivated in water from RAS. Cultivation in RAS water also stimulated the production of other volatile organic compounds (VOCs). Newly identified potential off-flavor compounds included 4-methyl-2-heptanone (“forest” odor), 3-methyl-1-butanol (“medicinal” and “chemical”), and a presumptive sesquiterpenoid described as “musty,” “earthy,” and “flowery.” The previously known off-flavor compound dimethyl sulfide was also detected. Myxobacteria have previously been proposed as keystone bacteria in the environment due to their predatory lifestyle. In predation assays using isolated bacteria from RAS, M. virescens AT3 and C. exiguus AT4 could successfully feed on 15 of 16 tested strains, suggesting a large influence on the biology of RAS microbiomes. The combination of predatory behavior and potent production of geosmin and other VOCs underscores the ecological and sensory impact of these bacteria in RAS. Understanding their behavior and metabolic outputs is critical to developing strategies for mitigating off-flavors in RAS. IMPORTANCE: Issues with off-flavored fish in recirculating aquaculture systems (RAS) due to the presence of the earthy-musty smelling compounds geosmin and 2-MIB is considered one of the industry’s most economically significant challenges. Knowledge of conditions that affect off-flavor production is essential information in the development of viable solutions for its mitigation. Little is known about the function of these microbially produced compounds or the conditions that trigger their production, especially in the underexplored myxobacteria. Investigation of natural isolates is crucial to determine the function of the genes involved and their differential expression in response to environmental cues. While myxobacteria in RAS have been previously shown to harbor the geosmin synthase gene through molecular studies, the present study is the first attempt to isolate these bacteria from RAS and quantify their geosmin production under different nutrient conditions. Through cultivation-based methods, we demonstrate their production of both known and novel compounds with earthy attributes.

The consumption of seafood is driven by flavour, yet achieving its high quality remains a challenge for many species reared in recirculating aquaculture systems (RAS). A comprehensive knowledge of off‐odour sources in aquatic foods is indispensable in ensuring flavour quality standards. At the beginning of the production chain, early post‐harvest lipid oxidation products develop into endogenous off‐odours and accumulate over time. These malodours add to those already absorbed exogenously, namely from the water and feeds, although the information on the interactions between these sources is currently scarce. Despite geosmin and 2‐methylisoborneol receiving significant attention in relation to fish off‐flavour, only limited knowledge on the molecular and ecological mechanisms driving their production in aquatic environments has been reported. Moreover, RAS‐hosted bacteria have been associated with a wide range of other odour‐active compounds, such as pyrazines, terpenoids, and other degradation byproducts that are frequently overlooked when studying flavour taint in fish. The influence of aquaculture feeds on the flavour of fish flesh has been underestimated, too, both as a source of off‐odours but also as a novel modulator strategy to achieve desirable aquatic food flavours. Finally, the influence of water treatment processes widely used in RAS operations, such as advance oxidation process, ozone, ultraviolet and hydrogen peroxide disinfections, is greatly underexplored with respect to odour quality. This article reviews the current scientific evidence with supporting data on the chemical diversity of off‐odours found in aquaculture fish worldwide and their potential sources and highlights knowledge gaps that should be addressed in future research.