Effect of salinity on the formation of avermectins, odor compounds and fatty acids byStreptomyces avermitilis

Undesirable odour substances (geosmin and 2-methylisoborneol) in water environment: Sources, impacts and removal strategies

Off-flavours in fish products generated from recirculating aquaculture systems (RAS) are a major problem in the fish farming industry affecting the market demand and prices. A particular concern is the muddy or musty odour and taste in fish due to the presence of secondary metabolites geosmin and 2-methylisoborneol (2-MIB), produced by actinobacteria (mainly Streptomyces), myxobacteria and cyanobacteria. Off-flavours have deteriorated the quality of fish, rendering their products unfit for human consumption. The process of odour removal requires purification for several days to weeks in clean water; thus this leads to additional production costs. Geosmin and 2-MIB, detected at extremely low odour thresholds, are the most widespread off-flavour metabolites in aquaculture, entering through fish gills and accumulating in the fish adipose tissues. In this review, we aimed to determine the diversity and identity of geosmin- and 2-MIB-producing bacteria in aquaculture and provide possible strategies for their elimination.

Impact of water quality parameters on geosmin levels and geosmin producers in European recirculating aquaculture systems

AIMS

Geosmin is associated with off-flavour problems in recirculating aquaculture systems (RAS) and represents an economic problem for the aquaculture industry. This study aims at investigating factors influencing the composition of the bacterial microbiota, in particular the presence of geosmin producers and the environmental and farming factors favouring geosmin accumulation.

METHODS AND RESULTS

Several water quality parameters were correlated to the composition of the microbiota with special emphasis on the presence of geosmin producers within 26 different RAS from four European countries. Three novel groups of geosmin-producing bacteria were quantified to identify potential correlations with geosmin concentration.

CONCLUSIONS

The microbiome differed significantly between systems. However, phosphate levels, calcium levels and redox potential correlated to geosmin concentration in the water and the presence of the Actinomycetales geosmin-producers but not with the presence of other groups of geosmin-producing bacteria. Oxygen levels and conductivity were found to negatively correlate with geosmin concentration. A large proportion of the detected geosmin producers represented novel taxonomic groups not previously linked with this activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results improve our understanding of the diversity of microbiota in RAS and the water quality parameters favouring the populations of geosmin-producing bacteria and the production of geosmin.

Relationship between volatile odorous substances and production of avermectins byStreptomyces avermitilis

The time course of production of odorous compounds, i.e. geosmin and oxolones, and of avermectins was determined during the cultivation of S. avermitilis in flasks, 1- and 50-L fermentors. The amount of the antibiotics increased with increasing cultivation time up to more than 2 g/L while the concentration of geosmin rose to more than 4 mg/L. Cultivation without reflux condenser resulted in a lower product formation due to the higher stripping of geosmin. A relatively tight correlation was found between the production of geosmin and the production of avermectins. The production of oxolones peaked on cultivation days 3-5, the sum of oxolones being 60 microg/L. Subsequently, the production dropped below a measurable level. This can be explained as being due to the inhibition of oxolone production by geosmin.

Identification of Streptomyces odor spectrum

The chemical composition of odors produced by nine strains of Streptomyces was determined. Strains Streptomyces aureofaciens, S. avermitilis, S. cinamomensis, S. coelicolor, S. griseus, S. lividans, S. rimosus, S. spectabilis, S. virginiae (as representatives of producers of biologically active compounds) were cultivated at the same time statically in dishes and in shaken flasks at similar cultivation conditions. According to the GC-MS analysis of odor compounds, more than twenty noteworthy volatile chemical individuals were identified. As the main component of odor spectrum geosmin and homologues of oxolones (dihydrofuranones) were found; the other compounds (pyrazine derivatives, acetoin and its homologues, aromatic esters, furan derivatives, etc.) were in minority.