Microbially Derived Off-Flavor from Geosmin and 2-Methylisoborneol: Sources and Remediation - Reviews of Environmental Contamination and Toxicology - page 10

they concluded that geosmin was likely a degraded sesquiterpene. Actinomycetes,
gram-positive soil bacteria Streptomyces avermitilis and S. coelicolor , produce
geosmin, and germacradienol has been identified as a precursor/cometabolite of
geosmin in streptomycetes and myxobacteria (Cane and Watt 2003). The S. aver-
mitilis gene SAV 2163 (geoA) and S. coelicolor A3 (2) SCO6073 gene encodes
germacradienol/geosmin synthase (Jiang et al. 2006; Gust et al. 2003). Among the
sesquiterpene synthases, the 2178-bp geoA gene (SAV 2163) encodes a putative
protein of 725 amino acids with a significant similarity to the S. coelicolor A3(2)
SCO6073 2181-bp gene product encoding 726 amino acids (Gust et al. 2003; Cane
and Watt 2003). Deletion of the entire SCO6073 (SC9B1.20) gene from S. coeli-
color A3(2) results in complete loss of geosmin production (Cane and Watt 2003;
Gust et al. 2003); this provides evidence that SCO6073 encodes a germacradienol
synthase, which catalyzes an essential step in the biosynthesis of geosmin.
Streptomyces avermitilis mutants with a deleted geoA were unable to produce
either germacradienol or geosmin, and biosynthesis of both compounds was
restored by introducing intact geoA gene in mutants (Cane et al. 2006).
Cane and Watt (2003) expressed a 2181-bp gene from S. coelicolor A3(2)
(SCO6703 = SC9B1.20) in Escherichia coli to give a 726-amino-acid protein and
originally proposed that formation of geosmin from germacradienol would involve
multistep biochemical redox pathways catalyzed by several hypothetical enzymes,
which has also been suggested by other researchers (Spiteller et al. 2002; Dickschat
et al. 2005). Cane and Watt (2003) and He and Cane (2004) revealed that biosyn-
thetic conversion of fernesyl diphosphate to geosmin requires a divalent cation,
preferably Mg 2+ and no other organic or inorganic cofactor is required. Recently,
Jiang et al. (2006) successfully demonstrated that a single enzyme (germacradienol
D synthase) is both necessary and sufficient to catalyze biosynthesis of geosmin
from fernesyl diphosphate without requirement of any additional enzymes and redox
cofactors, which solved the long-standing biosynthetic mystery.
Farnesol (3,7,11-trimethyl-2,6,10-dodecatrien-1-ol) is considered the universal
precursor of the sesquiterpenes (Croteau 1987). Studies conducted by Dionigi et al.
(1991) on the effect of farnesol on the growth and metabolism of the geosmin-
producing actinomycete Streptomyces tendeae revealed that farnesol can inhibit
geosmin synthesis, which in turn suppresses geosmin-producing species.

V Remediation of Off-Flavors

A Conventional Physical Methods

Management strategies for muddy and musty off-flavors are limited as geosmin and
MIB are recalcitrant to conventional water treatment (Ho et al. 2007). However,
some conventional physical techniques have been recommended. These ses-
quiterpenoids degrade over time and are purged from the fish, depending on their
concentrations, water temperature, and water quality (Tucker and van der Ploeg 1999).