Concentrations And Sources Of 2-MIB & Geosmin In Freshwater Fishponds And The Influencing Factors

Many aquatic products in ponds have different degrees of earthy-musty odor,which decreases their taste and edible value and threatens the cultivation efficiency.Existing research has demonstrated that earthy-musty odor,are caused by geosmin and MIB excreted by planktonic algae and actinomycetes.This study was conducted to evaluate the present condition,the primary source and the major limiting factor of geosmin and MIB in intensive cultivation ponds.This information was then used to select an effective technology for the elimination of earthy-musty odor and to improve the level of quality and safety of aquatic products.The results of this study were as follows.1.The head space solid-phase microextraction-gas chromatography-mass spectrophotometer(HSPME-GC-MS) method can be used to effectively measure the content of geosmin and MIB in water.Filtering algae had a significant impact on the levels of geosmin,but little effect on the concentrations of MIB.In addition,the results of this study enabled establishment of a sensory evaluation method for freshwater fish.In this method,the earthy-musty odor of fish was divided into five levels:normal,mild, clear,heavy and very heavy.The MIB content in the fish could be estimated based on the level of earthy-musty odor.The human olfact value for MIB in fish meat was 500 ng/kg.2.Geosmin was found to be the primary element responsible for the earthy-musty odor of fish in freshwater ponds in Beijing.Furthermore,geosmin was present in almost all ponds,as indicated by concentrations of 1.22～35.58 ng/L.MIB was present in some fishponds,with average concentrations of 1.39-6.00 ng/L being observed.Additionally, the geosmin content in fishponds was found to be related to the gross biomass of algae in the ponds.Cyclotella striata and Melosira granulata were found to be the primary algae responsible for the production of geosmin in freshwater fishponds in summer and autumn in Beijing.The MIB and geosmin content were not related to actinomycetes.Geosmin and MIB were found to be widespread in saline water ponds in Tianjin.In these ponds, MIB was the main factor responsible for off-flavor and was present at concentrations of 0.33-5302.70 ng/L.However,the concentration of geosmin in Tianjin was relatively low,as indicated by observed values of 0.29-12.10 ng/L.The MIB content was positively correlated with actinomycetes in fishponds in Tianjin.Water salinity was found to have an important impact on the concentration of actinomycetes and MIB.3.When separately cultivated using Agar No.1,the biomass of actinomycetes in fishponds in Beijing was found to be(1-27)×10⁶/L,while that of actinomycetes in fishponds in Tianjin was(0.01-1.50)×10⁶/L.The dominant species of actinomycetes in both areas was Streptomyces.The biomass of actinomycetes in fishponds in Beijing was higher that of fishponds in Tianjin.However,mathematical analysis revealed that the higher biomass of actinomycetes in the fishponds in Beijing was not related to the concentrations of earthy-musty odor elements(geosmin and MIB),while the relatively low biomass of actinomycetes in fishponds in Tianjin was related to the MIB.This may have been due to the presence of saline water ponds in Tianjin(salinity 1 to 5),because salinity may cause actinomycetes to produce odorous substances such as MIB.4.In this paper,sequencing of the 16SrRNA combined with evaluation of the colony morphology was used to identify the species of actinomycetes in the fishponds in Tianjin, and a phylogenetic tree diagram was constructed.Four genera and 9 species of common and dominant actinomycetes were identified,with Streptomyces being the dominant genus.The most common genera in Tianjin were Streptomyces,Nocardioides,and Micromonospora,which were present in concentrations of(0.007-0.575)×10⁶/L, (0.0033-0.07)×10⁶/L;and(0.00133-0.4776)×10⁶/L,respectively.The total biomass of actinomycetes was(0.01-1.50)×10⁶ / L.5.Six types of Streptomyces were observed in fishponds in Tianjin.Furthermore,the earthy-musty odor production among Streptomyces was found to differ greatly. Specifically,the ability of the various species of Streptomyces found in fishponds in Tianjin to produce MIB were as follows:Sp2(5570.0 ng/ml or 2×10⁶ ng/g),Sp3(776.0 ng/ml or 8.5×10⁴ ng/g),Sp5(72.4 ng/ml or 2.6×10⁴ ng/g),Sp4(19.8 ng/ml or 7.1×103 ng/g),Sp1(14.3 ng/ml or 5.8×103 ng/g),Sp6(2.6 ng/ml or 7.6×102 ng/g).Sp2 and other types of Streptomyces were found to be the primary cause of off-flavor in saline water ponds in Tianjin,China.6.After observation of the morphological and cultivation features and molecular analysis,Actinomycete SP2 was identified as Streptomyces praecox.This species was found to produce much more MIB than geosmin,and both of these compounds were found to primarily exist outside of the cells.Temperature and salinity were also important factors that affected the production of MIB and geosmin by Streptomyces praecox. Specifically,25℃-30℃was found to be the mutation region for Streptomyces praecox, at which they produced earthy-musty odor substances.Additionally,Streptomyces praecox produced the greatest amounts of MIB and geosmin when cultivated at salinities of 2 and 1,respectively.NH₄⁺,pH and their interactions had a significant impact on the ability of Streptomyces praecox to produce MIB,while NH₄⁺,PO₄^3-,and pH,and any interaction between these compounds had a significant effect on its production of geosmin.Extracellular metabolites produced by Scenedesmus obliquus may accelerate the production of MIB and Streptomyces praecox to produce MIB and geosmin.7.In this study,a variety of techniques for reducing earthy-musty odor were tested. The results revealed that the use of clear water,flowing water and active carbon can eliminate earthy-musty odor of fish to some extent.For example,15 h of treatment with flowing water and the use of active carbon adsorption treatment for 12 h were found to effectively remove earthy-musty odor associated with Cyprinus carpio.In addition,the application of photosynthetic bacteria was also found to reduce or eliminate earthy-musty odor of John Dory,grass carp and fresh water white sharks,but this required a longer treatment than the aforementioned treatments.