Identification of conditions underlying production of geosmin and 2-methylisoborneol in a recirculating system

Geosmin and 2-methylisoborneol (MIB) are semi-volatile terpenoid compounds produced as secondary metabolites by benthic and planktonic cyanobacteria, several genera of fungi, and various actinomycetes. These off-flavor compounds pose a heavy economic burden in the aquaculture industry rendering fish unmarketable unless purified by purging with large quantities of clean water. In the present study, the presence of off-flavor compounds was examined in a recirculating aquaculture system (RAS) for tilapia culture. In this zero-discharge system, where water from the fish basins is recirculated through parallel aerobic (drum filter and a trickling filter) and anaerobic treatment loops (sedimentation/digestion basin), concentrations of geosmin and, in particular, MIB were highest in the aerobic treatment loop. Lowest concentrations were detected in the anaerobic treatment loop. This latter finding pointed toward a possible reduction of these compounds in this basin. Two bacterial strains of the streptomycetes family were isolated from the aerobic, organic-rich, drum filter and the nitrifying trickling filter. In vitro tests with these isolates, closely related to Streptomyces roseoflavus and Streptomyces thermocarboxydus, revealed that MIB production exceeded geosmin production under all conditions tested and was significantly higher under aerobic than under anoxic conditions. Under the latter conditions, with nitrate as an electron donor, the S. roseoflavus-like isolate was capable of denitrification. Based on the results obtained in this study, it was concluded that aerobic, organic-rich conditions stimulate the growth of actinomycetes and subsequent production of geosmin and MIB in the system. The observed reduction of these compounds in the anaerobic water treatment component may serve in designing treatment steps aimed at alleviating the problem of geosmin and MIB accumulation in recirculating systems.