4396 MINIREVIEW APPL. ENVIRON. MICROBIOL. isms in odor outbreaks in raw or posttreatment water is an important issue which has yet to be addressed. Photoautotrophic producers. As noted above, cyanobacteria are considered to be the major sources of geosmin and 2-MIB in aquatic environments where photosynthetic growth is pos- sible (31, 45, 53, 65, 110). More than 200 studies have made considerable advances in our knowledge of the biochemis- FIG. 1. Structure of geosmin and 2-MIB. try, taxonomy and ecology of some of the cyanobacteria which produce these VOCs. Yet even though cyanobacteria are now considered to be the chief sources of geosmin and 2-MIB, the number of species (here we refer to the phylogenetic specieswhile cyanobacteria were not detected at that time. Sugiura concept, since cyanobacteria are asexual ) which are signif-and Nakano (90) concluded that elevated 2-MIB levels in Lake icant producers is unknown, for several reasons. Fewer than 50Kasumigaura (Japan) were generated by actinomycete activity of the Ͼ2,000 species classiﬁed to date (according to the In-in aerobic sediment layers, based on in vitro observations of ternational Code of Botanical Nomenclature) have been di-isolates from this habitat. Jensen and coworkers (37) traced the rectly conﬁrmed as producers, while the majority have yet to beannual spring outbreaks of earthy-musty odor in tap water investigated for their production of these and other VOCsderived from a major Canadian river to actinomycetes, which (100). Unsightly and highly visible surface blooms are usuallywere introduced into these surface waters during snowmelt and considered to be primary sources of source water odor, but inrunoff. Klausen et al. (47) concluded that actinomycetes were fact many of the known cyanobacterial producers are non-responsible for low concentrations of geosmin and 2-MIB in planktonic (ϳ30%), while the remainder are benthic or epi-streams ﬂowing past trout breeding aquaculture operations, phytic, with a single isolate from soil (Table 2). Interestingly,because isolated strains of Streptomyces from these habitats geosmin and 2-MIB production appears to be limited to ﬁla-were able to synthesize geosmin and 2-MIB, while cyanobac- mentous cyanophytes and to date is unknown among chroo-teria were not present in large numbers. It is important to note, coccalean taxa. Furthermore, to our knowledge no marinehowever, that most of these studies did not investigate benthic cyanobacterium has been identiﬁed as a producer of eitheror littoral bioﬁlms, which can be a major source of these compound to date; in fact, geosmin has been proposed as acompounds (see below). Furthermore, while persuasive correl- chemical land mass homing signal for anadromous ﬁsh (94).ative evidence was presented in each of these studies, none On the other hand, this may simply reﬂect the fact that fewprovided a direct demonstration that actinomycetes were pri- people rely on marine sources for drinking water rather thanmarily responsible for the malodor. Overall, however, the con- any relationship with halotolerance. Persson (64) observed thattribution of actinomycetes to taste-and-odor outbreaks will among four fresh and brackish water isolates of Oscillatoriacontinue to be extremely difﬁcult to assess until methods todifferentiate in situ producing and nonproducing strains are aghardhii (syn. Planktothrix aghardhii), only brackish wateravailable. clones produced geosmin and 2-MIB, indicating that salinity It is well known that odor can originate downstream of water per se does not preclude the biosynthesis of these terpenoids.treatment as a result of heterotrophic biological activity in Along the same lines, geosmin and 2-MIB outbreaks are com-distribution pipes or ﬁltration beds, and posttreatment produc- mon problems in many areas of the North American prairies,tion of geosmin has also been documented in several studies. where the presence of an ancient seabed results in highly salineFor example, heterotrophic eukaryotes such as fungi, which and productive surface waters (46, 99).colonize bioﬁlms in activated ﬁlters and distribution pipes, cangenerate potent musty-smelling metabolites such as trichloro-anisole (68). In a recent case, geosmin in treated drinkingwater was traced to the disturbance of thick bioﬁlms that had TABLE 1. Actinomycetes and other noncyanobacterial taxa thatdeveloped on the pipe surface of a distribution system from a produce geosmin (GE) and 2-MIBgroundwater-supplied treatment plant (F. Juttner, unpub- ¨ VOC(s) Taxon Referencelished data). After a change in water treatment processes in theplant to remove iron from the iron-rich source water, the 2-MIB, GE Penicillium and Aspergillus species 78bioﬁlms degraded and were sloughed off, releasing high levels GE P. expansum 17 GE Streptomyces albidoﬂavus 92of geosmin and leading to consumer complaints. In another GE S. avermitilis 73case, biological activity in poorly maintained ﬁltration media GE S. citreus 69was considered to be the most likely cause of high geosmin GE S. griseus 105levels downstream of ﬁlter beds in a small rural treatment plant GE, 2-MIB S. griseofuscus 1 GE S. halstedii 82(S. B. Watson, unpublished data). The biological sources were GE S. psammoticus 37not identiﬁed in these last two cases, and it is not known if they GE S. tendae 17were actinomycetes, fungi, and/or other microorganisms. For GE, 2-MIB S violaceusniger 78example, myxobacteria (83, 111) and molds such as Penicillium GE, MIB Streptomyces spp. Variousare known proliﬁc sources of earthy odors in sediments and GE Symphyogyna brongniartii (liverwort) 89 GE Vannella sp. (heterotrophic amoeba) 24foodstuffs (51, 78, 81), and the potential role of these organ-
VOL. 73, 2007 MINIREVIEW 4397 TABLE 2. Cyanobacteria producing geosmin (GE) and 2-MIB, listed by current taxonomic names, past synonyms, and primary habitata Production (ϩ) Taxon Synonym Habitatb of: Comments GE 2-MIBGeitlerinema splendidum Oscillatoria splendida BEN ϩJaaginema geminatum Oscillatoria geminata BEN ϩLeibleinia subtilis Lyngbya subtilis BEN ϩLyngbya aestuarii BEN ϩOscillatoria curviceps BEN ϩOscillatoria tenuis var. levis BEN ϩOscillatoria variabilis BEN ϩPhormidium allorgei Lyngbya allorgei BEN ϩPhormidium amoenum Oscillatoria amoena BEN ϩPhormidium breve Oscillatoria brevis BEN ϩ ϩ Strain speciﬁcPhormidium chalybeum Oscillatoria chalybea BEN ϩPhormidium cortianum Oscillatoria cortiana BEN ϩPhormidium favosum BEN ϩPhormidium formosum Oscillatoria formosa BEN ϩPhormidium strain LM689 BEN ϩPhormidium simplissimum Oscillatoria simplicissima BEN ϩPhormidium sp. strain NIVA 51 BEN ϩ ϩPhormidium tenue Oscillatoria tenuis BEN ϩPhormidium uncinatum BEN ϩPhormidium viscosum BEN ϩPlanktothrix proliﬁca Oscillatoria proliﬁca BEN ϩPorphyrosiphon martensianus Lyngbya martensiana BEN ϩSymplocastrum mulleri ¨ Schizothrix mulleri ¨ BEN ϩ Actinomycete contaminantTychonema bornetii Oscillatoria bornetii BEN ϩ Strain speciﬁcTychonema granulatum Oscillatoria f. granulata BEN ϩ ϩHyella sp. EPI ϩMicrocoleus sp. EPI ϩAnabaena circinalis PL ϩAnabaena crassa PL ϩAnabaena lemmermannii PL ϩAnabaena macrospora PL ϩAnabaena solitaria PL ϩAnabaena viguieri PL ϩAphanizomenon ﬂos-aquae PL ϩAphanizomenon gracile PL ϩOscillatoria limosa PL ϩPlanktothrix agardhii Oscillatoria agardhii PL ϩ ϩ Strain speciﬁcPlanktothrix cryptovaginata Lyngbya cryptovaginata PL ϩPlanktothrix perornata Oscillatoria perornata PL ϩPlanktothrix perornata var. attenuata Oscillatoria perornata var. attenuata PL ϩPseudanabaena catenata PL ϩ ϩPseudanabaena limnetica Oscillatoria limnetica PL ϩSymploca muscorum SL ϩ a Data from reference 99. b BEN, benthic; PL, planktonic; EPI, epiphytic; SL, soil. BIOSYNTHESIS OF GEOSMIN AND 2-MIB Initially, successful labeling experiments to trace the biosyn- thetic pathway(s) of geosmin were difﬁcult to perform with strep- The conclusion that 2-MIB is a monoterpene and geosmin tomycetes, and they still have not been conducted successfullyan irregular sesquiterpene (Fig. 1) dates back to early labeling with cyanobacteria. Farnesyl diphosphate is the immediate pre-experiments with Streptomyces conducted by Bentley and cursor of cyclic sesquiterpenes (11), but early attempts showedMeganathan (4). These authors found that both compoundswere labeled when radioactive acetate was administered to that farnesol addition to cultures inhibited the growth of bacteriacultures, and they interpreted this as an indication of isoprene (16, 88) and cyanobacteria (44), and this compound therefore hadsynthesis. 2-MIB showed labeling when methionine with a la- to be rejected as a tool to study the biosynthesis of terpenoids.beled methyl group was added. The authors concluded that More recently, considerable progress has been made using other2-MIB is a methylated monoterpene and geosmin a sesquiter- precursors. Experiments showed that labeled geosmin was pro-pene that has lost an isopropyl group. To our knowledge, no duced by Streptomyces when labeled 1-deoxy-D-xylulose (88) wasfurther experiments have been conducted to elucidate the bio- added, while labeled mevalolactone and leucine were appliedsynthesis of 2-MIB. Geosmin, however, has received consider- successfully with the myxobacteria Myxococcus xanthus and Stig-able recent interest, and much progress in the knowledge of its matella aurantica (14). These studies, together with other labelingbiosynthetic pathways has been made. and genetic work, have provided evidence that in fact several
4398 MINIREVIEW APPL. ENVIRON. MICROBIOL. FIG. 2. Simpliﬁed biosynthetic scheme (suggested or proven) for the formation of 2-MIB and geosmin in streptomycetes and myxobacteria.different biosynthetic pathways of isoprenoid synthesis exist in based on the production of labeled geosmin and 2-MIB frommicroorganisms, one or more of which may lead to the produc- labeled acetate. However, more recent studies failed to ﬁndtion of geosmin by different taxa (Fig. 2). similar labeling using this precursor (88), and a plausible ex- Of particular importance is the 2-methylerythritol-4-phos- planation for this apparent inconsistency is that streptomycetesphate (MEP) pathway. This pathway was only recently discov- are capable of gluconeogenesis and can use acetate and etha-ered and has been now completely elucidated on the genetic nol as sole C sources (35). When no exogenous sugar is sup-and enzymatic levels in higher plants (75). Work with Strepto- plied, acetate would be metabolized in long-term experimentsmyces showed that labeled geosmin was produced when deu- to sugar compounds, and labeling can then be introduced intoterated [5,4-2H2]1-deoxy-D-xylulose, but not [4,4,6,6,6-2H5]- the MEP pathway. Nevertheless, there is some evidence thatmevalolactone, was administered, demonstrating the predominance some streptomycetes may use both pathways but at differentof the MEP isoprenoid pathway over the mevalonate (MVA) growth stages, with the MEP pathway providing the predomi-route in these organisms (88). The genes coding for the MEP nant route during active growth and the MVA pathway pro-pathway have been found in the cyanobacterium Synechocystis viding the predominant route in the stationary phase (84, 85).sp. strain PCC 6803 (48), although members of this genus (and However, rigorous proof that the MVA pathway is a wide-other chroococcales) do not produce geosmin. However, this spread and signiﬁcant route in geosmin biosynthesis is stillsuggests that the same isoprenoid pathway may also function in lacking, and there is a need for the development of morethe geosmin-producing cyanobacterial taxa. speciﬁc tracing techniques. For example, 3-hydroxy-3-methyl- The above discussion suggests that the MEP pathway is the glutaryl-CoA reductase is often used as a key enzyme indicat-major biosynthetic isoprenoid route in many bacterial groups; ing the presence of the MVA pathway (22), but this enzymenevertheless, there is some evidence that the MVA pathway is can also function in a catabolic pathway, the breakdown ofalso used. The latter pathway may function exclusively in the MVA. Its presence is therefore not necessarily indicative of ansynthesis of geosmin and other isoprenoids in some groups MVA pathway to geosmin.such as myxobacteria (14) and contribute to geosmin produc- For both geosmin pathways, there has been much recenttion in the stationary growth phase of streptomycetes (84, 85). interest in the elucidation the ﬁnal steps: the cyclization ofArchaea use the MVA pathway exclusively for isoprenoid syn- farnesyl diphosphate to a mono- or dicyclic sesquiterpene.thesis (50), but geosmin producers have yet to be found among Pollack and Berger (69) found that a strong geosmin-produc-these taxa. Myxobacteria also use the MVA pathway as a major ing strain of Streptomyces citreus also produced measurableroute to synthesize a range of isoprenoid compounds, includ- quantities of the sesquiterpene alcohol (4S,7R)-germacra-ing geosmin. In these microorganisms a minor pathway starts 1(10)E,5E-diene-11-ol, and they postulated that farnesylwith L-leucine and feeds label via dimethylacrylyl coenzyme A diphosphate cyclization yields germacradienol as the immedi-(CoA) into 3-hydroxy-3-methylglutaryl-CoA, a precursor of ate precursor of geosmin. More recently germacradienol/MVA (14). The original results reported by Bentley and Mega- germacrene D synthases of Streptomyces coelicolor (23, 38) andnathan (4) favored the MVA pathway for streptomycetes, Streptomyces avermitilis (11) were cloned, and the recombinant
VOL. 73, 2007 MINIREVIEW 4399enzyme was studied in more detail. The enzyme was shown to vide insight into the relative capacities of different taxa tocatalyze the formation of (4S,7R)-germacra-1(10)E,5E-diene- produce these VOCs or allow comparison between different11-ol, (Ϫ)-(7S)-germacrene D, and geosmin from farnesyl studies. Few studies have differentiated between total and cell-diphosphate. Magnesium ions supported the reaction; other bound production capacities, which can differ considerably andcofactors were not necessary (11, 38). However, in these ex- are imprecisely estimated by most current extraction and ana-periments, germacradienol was the major product and geosmin lytical protocols (see below). Environmental factors (such asproduction was minor, while under natural conditions, the light intensity, temperature, ion concentrations, etc.) havecellular concentrations of geosmin are much higher than those been shown to modulate the production rate of odor com-of germacradienol. This suggests that the enzymatic environ- pounds for both cyanobacteria (57, 58, 59, 76, 79, 95, 102) andment may strongly affect the formation of these two products. actinomycetes (1, 15, 17, 82, 92, 105), but these alone cannotOur understanding of the genetic coding and of the sesquiter- explain the substantial differences in concentrations often ob-pene synthase involved in the Mg-dependent conversion of served in surface waters under natural conditions (Table 3farnesyl diphosphate to germacradienol was further advanced gives an overview of typical environmental levels).by two recent independent studies. Gust and coworkers (23) Actinomycetes. In general, techniques currently used toused PCR-targeted gene replacement to identify the gene measure actinomycete abundance typically yield poor correla-(Sco6073) (cyc2), which codes for a synthase with two domains, tions between these estimates and geosmin and/or 2-MIB con-with one required for geosmin biosynthesis. In a parallel study, centrations in surface waters, for several reasons. First, acti-Cane and Watt (10) used PCR ampliﬁcation of this gene to nomycetes are difﬁcult to enumerate. They are ﬁlamentousdetermine the identity of this synthase as a protein encoded by (hyphal), spore-producing organisms which fragment when2,181 bp (designated SC9B1.20), and they concluded that pro- plated, with each fragment or spore yielding a CFU. Highlyduction of germacradienol represents the committed step in selective media are needed to identify and enumerate actino-the geosmin biosynthetic pathway. mycetes; different studies use different media, and these media Several studies used precursors with deuterium labeling in vary in the biomass they will generate (18). Estimates usingdifferent positions to investigate the cyclization process leading these traditional methods suggest that the abundance of acti-to the formation of geosmin. A number of different mecha- nomycetes in open waters is generally low, ranging from belownisms and intermediates were postulated by the authors to detection to ϳ1.4 ϫ 106 cells ⅐ literϪ1 (64, 113, 114). Immu-explain the labeling patterns observed (10, 11, 14, 25, 38, 88). noﬂuorescence techniques have been employed extensively inHowever, some key questions remain unresolved, notably, medical and soil sciences to investigate these and other mixedwhich of the postulated intermediates occur in which taxa and microbial assemblages but are often limited by their nonspe-under which conditions they are released and contribute to the ciﬁc nature, and without appropriate controls, these methodsodor bouquet of a particular microorganism. Importantly, for can yield estimates that include many (other) bacterial taxacyanobacteria it has not been resolved which pathway is used, and which differ signiﬁcantly from CFU. Recent work, how-while for streptomycetes, it has yet to be established whether ever, has made considerable advances in sensitive enumerationall strains use the same pathway or whether both pathways are techniques using ﬂuorescence in situ hybridization (FISH),used for geosmin under different growth conditions. This in- FISH with catalyzed reporter deposition, and combined micro-sight is essential to explain why geosmin producers and non- autoradiography. These techniques indicate that abundancesproducers, and low and high producers, are found among dif- of active streptomycetes may be considerably higher in someferent species and cospeciﬁc strains of both cyanobacteria and waters, reported at levels of 0.4 ϫ 108 to 3.7 ϫ 108streptomycetes. A second, molecular level of understanding is cells ⅐ literϪ1 in oligotrophic and eutrophic streams and ﬁshalso missing, which is fundamental to these questions: the ponds (60). In essence, because of the large discrepanciescomplete knowledge of the genes and enzymes responsible for among these methods, our knowledge of the actual abun-the synthesis of geosmin and other sesquiterpenes and their dances of active cells in different environments is almost com-regulating mechanisms. pletely lacking. A further key consideration is that not all streptomycetes TRACING GEOSMIN AND 2-MIB PRODUCERS produce geosmin or 2-MIB (13), yet plate counts and other enumeration methods estimate a total count, i.e., both odor In the following section, we elucidate several key factors producers and non-odor producers. Studies using FISH andwhich may explain why many efforts to trace the biological FISH with catalyzed reporter deposition have demonstratedsources of geosmin and 2-MIB have been unsuccessful. Above that active actinomycetes (which metabolize and/or incorpo-all, a reliable method to distinguish between the contributions rate labeled thymidine) are ubiquitous (60) and can be impor-of cyanobacteria and actinomycetes (or other potential pro- tant components of the plankton even during periods whenducers) to geosmin and 2-MIB in surface waters has not been there is little or no odor (47, 49). Authors have reported a widedeveloped, and a considerable number of taste-and-odor out- range in the proportion of potentially odorous actinomycetesbreaks caused by geosmin and/or 2-MIB remain unsolved (7). in different environments: between approximately 20% andImportantly, there is a general lack of standardized units 70% of isolates from river and lake samples have been foundamong papers reporting production and production rates, to be active (in vitro) producers of geosmin and/or 2-MIB (37,which should be expressed per volume/weight of cells or per 47, 113, 114).unit chlorophyll (or any other cell-speciﬁc parameter). Fre- In addition to the above points, it is important to note thatquently, however, these measures are given as geosmin and there is considerable variation in the cell-speciﬁc capacity to2-MIB concentrations per culture volume, which does not pro- produce geosmin or 2-MIB among active individual isolates.
4400 MINIREVIEW APPL. ENVIRON. MICROBIOL. TABLE 3. Typical concentrations of geosmin (GE) and 2-MIB in different habitats, microorganisms, and drinking water Source Details Concna ReferenceCultures Cyanobacteria Oscillatoria brevis, light limited 0.35 g GE mg (dry wt)Ϫ1 ϭ 58 g GE mg Chl aϪ1 57 Anabaena lemmermannii, P limited, total GE 0.15 g GE mg (dry wt)Ϫ1 ϭ 670 g GE mg Chl aϪ1 Wab A. lemmermannii, P limited, cell-bound GE 0.14 g GE mg (dry wt)Ϫ1 ϭ 500 g GE mg Chl aϪ1 Wa A. lemmermannii, P replete, total GE 0.5 g GE mg (dry wt)Ϫ1 ϭ 850 g GE mg Chl aϪ1 Wa A. lemmermannii, P replete, cell-bound GE 0.41 g GE mg (dry wt)Ϫ1 ϭ 700 g GE mg Chl aϪ1 Wa Streptomycetes Streptomyces tendae, grown on different agars 0.25–33 (52)c ng GE mg (dry wt)Ϫ1 18Lake water Temperate/subpolar Oligotrophic (various; Canada) 0–5 ng GE literϪ1 Wa Mesotrophic; Lake Zurich (depth and seasonal ¨ 2.7–23 ng GE literϪ1 19 differences) Subtropical Eutrophic; Central Europe, shallow lake 600 ng GE literϪ1 45 Eutrophic; Lake Kasumigaura, Japan Up to 900 ng literϪ1 2-MIB and 700 ng literϪ1 GE 110 Various reservoirs in South Africa 150–3,170 ng GE literϪ1 106 Spatial/temporal Central Europe (eutrophic) Oxic epilimnion 50 ng GE literϪ1 26 Anoxic hypolimnion 950 ng GE literϪ1 26 Winter turnover 3 ng GE literϪ1 26Running water Australia (Carcoar Dam) 4,000 ng GE literϪ1 39 Canada/United States (St. Lawrence River) 2–40 ng 2-MIB literϪ1; 5–40 ng GE literϪ1 102 Central Europe (streams; Zurich, Switzerland) ¨ 3–7 ng GE literϪ1 (mostly dissolved) 43 Japan (Sakagawa River, polluted; Tokyo) 3,600 ng MIB literϪ1; 30 ng GE literϪ1 53 Mediterranean area (Llobregat River) 15–20 (200) ng GE literϪ1 97 Tropical Africa (Senegal River) 47 ng GE literϪ1 7 Dammed river (Ruhr River, Germany) 2-MIB (ϽMDL); 35 ng GE literϪ1 42Bioﬁlms Benthic cyanobacteria, aqueduct water Up to 78 ng 2-MIB literϪ1; 48 ng GE literϪ1 36 (California) Attached bioﬁlms (Tokyo, Japan) 100 ng GE mg (dry wt)Ϫ1 ϭ 6,300 ng GE cmϪ2 53 Canada (oligotrophic; N. Alberta streams) ϽMDL up to 18 ng GE cmϪ2 Wa Floating bioﬁlms (Spain) 1,000 ng GE mg (dry wt)Ϫ1 ϭ 36,500 ng GE cmϪ2 97 Aquatic GE concn from ﬂoating bioﬁlms 10–100 ng GE literϪ1 8Drinking water Customer complaints United States 46–88 ng GE literϪ1 8 Canada (5) 10–120 ng GE literϪ1 Wa Odor episodes France 2–10 ng GE or 2-MIB literϪ1 7 a Chl a, chlorophyll a; MDL, minimum detection level. b Wa, S. B. Watson, unpublished data. c Values in parentheses represent single minimum or maximum values observed which were not included in the more typical range shown.For example, up to 200-fold differences have been reported for enrichment such as found in sediment material and plant de-strains of Streptomyces (46). Zaitlin et al. (114) found that bris. In a later study, Sugiura and coworkers (91) reported thatϳ60% of 41 Lake Ontario isolates were producers, with ca. sedimented cyanobacteria and diatom cells (Microcystis aerugi-100- and 300-fold variation in geosmin and 2-MIB production, nosa, Anabaena spiroides, and Synedra acus) also providedrespectively; these included both Streptomyces and nonstrepto- good substrates for VOC production by benthic streptomycetemycete (i.e., nonhyphal) genera, but high production was seen isolates (we note here that in cases where this involved theonly among the former taxonomic group. mineralization of geosmin/2-MIB-producing cyanobacteria, Not surprisingly, the growth and VOC production of an odor could originate from active streptomycete metabolismindividual strain also varies with the environment and, in many and/or the release of cell-bound VOCs from the decaying cyano-cases, is not well supported in the pelagic zone. As early as bacteria cells [see below]). As a further signiﬁcant complica-1985, Wood and coworkers (107) demonstrated that natural tion, optimal conditions vary among taxa, and in some casesreservoir water did not support geosmin production by Strep- even nutrient-poor environments can support VOC productiontomyces albidoﬂavus and other isolates but required a source of by some actinomycetes. For example, Zaitlin and coworkers
VOL. 73, 2007 MINIREVIEW 4401(113, 114) observed considerable differences in the capacity to cyanobacteria, however, is that geosmin is constitutively pro-produce geosmin and 2-MIB among three streptomycete and duced and its induction (as observed for streptomycetes) hasnonstreptomycete actinomycetes grown with enriched media yet to be conclusively shown.or sterile/nonsterile (unenriched) river water, with one isolate From the above discussion, it is clear that the capacity forshowing equivalent production on agar and river water. Geos- geosmin and 2-MIB production is a complex phenomenonmin production has been induced in cultures of Streptomyces which varies considerably among and within different taxa.tendae under stationary-phase conditions (as frequently found Thus, a simplistic approach to source tracking (for example bywhen antibiotics are applied) (18). However, it has not yet measuring total cyanobacteria or actinomycetes in a waterbeen clariﬁed whether growing or nongrowing streptomycetes body) is unlikely to succeed (see below). Both taxonomicare more active geosmin producers under natural conditions. groups (and other potential VOC producers) show consider- Cyanobacteria. Because of their link with poor water quality, able diversity in their biochemistry, morphology, and habitat,the seasonal dynamics of cyanobacteria are monitored more and this should be carefully considered in any study. Above all,frequently than those of actinomycetes in freshwaters, allowing in order to understand the fundamental drivers behind themore opportunity for correlation analyses between these taxa variability in geosmin and 2-MIB production (as with any otherand volatile compounds such as geosmin. In contrast to the secondary metabolite) at the different organizational levelscase for actinomycetes, there is often a strong relationship (i.e., cell, species, population, and environment), the intrinsicbetween these measures. For example, the seasonal concentra- molecular control mechanisms need to be fully characterizedtions of geosmin were well correlated with the abundance of in future work.Aphanizomenon gracile in a eutrophic freshwater lake (45) and,in another study of the Australian Hay Weir Dam and Carcoar INTRA- AND EXTRACELLULAR VOC FRACTIONSDam, with Anabaena (39). Nevertheless, cyanobacteria can be as challenging and enig- As elucidated below, it is essential to recognize that geosminmatic as actinomycetes. For example, between 1999 and 2006, and 2-MIB occur in surface waters as cellular (cell-bound) andannual late summer peaks of geosmin in western Lake Ontario dissolved fractions and that the differentiation between these(ranging between 5 and 200 ng ⅐ literϪ1) have shown little two fractions is key to the effective management of waterconsistent relationship with the abundance of the large bundle- quality control and treatment. Apart from the treatment-re-forming cyanobacterium Anabaena lemmermannii in the sur- lated issues, the dynamics of cell-bound and dissolved fractionsface waters, although this organism has been the only likely also have an important effect on the sensory assessment ofcandidate source identiﬁed to date (101). Similarly, substantial odor (and hence any related drinking water monitoring tech-geosmin peaks (up to 2,000 ng ⅐ literϪ1) in raw water in a niques) (71), because the bound VOC fraction does not followreservoir supplying the city of Tulsa, OK, have been poorly Henry’s law.related to the abundance of the dominant planktonic cya- Unambiguous evidence for any organism can be obtainednobacterium Anabaena circinalis (62) even though this partic- only by comparing cell counts to cell-bound VOC concentra-ular taxon is known to be a major cause of taste and odor in tions. If the dissolved component, which can sometimes rep-other source waters (87, 96). resent the majority, is not differentiated from the particulate Even though they are often more visible, considerable ex- fraction, the correlation with biomass may be obscured. Fur-pertise is required to identify cyanobacteria microscopically, a thermore, the proportions of intra- and extracellular produc-factor that is often overlooked. Their taxonomy is continually tion cannot be assumed to be constant, as these vary amongevolving, as the traditional emphasis on morphological traits as taxa and with physiological state (growth phase and environ-key criteria for positive classiﬁcation is increasingly integrated mentally induced stress) (76) (Table 3).with biochemical and molecular data. As a result, many species Intracellular dissolved and protein-bound geosmin and(again we refer to the phylogenetic species concept, since 2-MIB. Work by Wu and Juttner (108, 109) demonstrated ¨cyanobacteria are asexual ) identiﬁed as geosmin or 2-MIB clearly that particulate geosmin occurs in cyanobacterial cellsproducers by early studies have since been renamed, leading to as two distinct intracellular fractions, one which is dissolved inconfusion among many workers (Table 2), particularly since the aqueous cytosol and a second which is bound to proteins.there is considerable plasticity in the morphology of many Using polar solvents, these authors showed that this secondodor-producing (and non-odor-producing) cyanobacteria (27). geosmin fraction is bound to membrane proteins and not dis-Similar to the case for actinomycetes, there are low- and high- solved in the aqueous cytosol, as is also seen with chlorophyllsVOC-producing strains of (apparently) the same cyanobacte- and carotenoids, which are integral to the macromolecularrial species, for example, as seen for Calothrix parietina, Oscil- protein-pigment photosystem units. In fact, the phycobilin pro-latoria limosa, Anabaena lemmermannii, and Fischerella (29; teins are attached to the surface of the thylakoids and may beJuttner and Watson, unpublished data), which contributes to ¨ the macromolecules to which geosmin and 2-MIB are boundthe overall ambiguity, together with the fact that the morpho- by hydrogen bonds and van der Waals forces.types and a large portion of the genotypes of producers and The presence of bound and dissolved intracellular geosminnonproducers may be the same. A further and very important fractions has important implications for protocols commonlyconsideration is that some cyanobacterial species identiﬁed as employed for extraction and analysis, because these fractionsgeosmin and/or 2-MIB producers may in fact not be the source behave differently. Stripping analysis using freeze-thaw, soni-of these compounds. One of the essential steps in the identi- cation, grinding, or nearly saturated NaCl (20%, wt/vol) toﬁcation of a producer is the veriﬁcation of VOC production by disrupt cells (e.g., as with the increasingly popular headspaceisolated strains of the study organism. A positive feature of all solid-phase microextraction technique) primarily determines
4402 MINIREVIEW APPL. ENVIRON. MICROBIOL.cytosol-solubilized geosmin. The recovery of intracellular frac- means that reported cell densities of these bacteria in open-tions may improve where these methods include heating, but water environments vary considerably among studies and likelythis needs to be carefully veriﬁed. In many cases it is likely that misrepresent the true levels of active cells (see above). How-the proportion of protein-bound geosmin is underestimated by ever, it appears that most cells are largely bound to suspendedsuch techniques, since a suitable polar solvent is required to sediment particles (114). The contribution of their intracellularcleave the bonding forces and access this fraction, which often fractions to particle-bound geosmin (and likely also 2-MIB)represents the predominant geosmin moiety in a healthy cyano- may therefore be generally rather small but seasonally variable,bacterial cell. In Fischerella muscicola, for example, separate for example, during storm events or periods of high runoff (37).water- and solvent-based extractions and analyses showed that The occurrence of these different VOC fractions has impor-82% of the geosmin was protein bound (109). In a typical tant implications for the interpretation of published data. Mostprocedure, cells are ﬁltered onto a glass ﬁber ﬁlter and treated workers continue to overlook the fundamental differencesbrieﬂy with methanol. Once separated, geosmin is not readily among cell-bound and dissolved geosmin and 2-MIB fractions,again sorbed by the protein when water is added and can be even though much of this knowledge was available almost 20easily stripped by a conventional method. As a nonpolar com- years ago, and a simple ﬁltration step easily allows their sep-pound, geosmin is lipophilic. The fact that treatment with aration and analysis (19). Thus, since there are a number ofwater does not lead to the rebinding of this compound to different analytical procedures for geosmin and 2-MIB, in eachcellular constituents supports the premise that the lipid mem- case it is essential to know which method has been employed.branes (thylakoids) themselves are not the primary binding Conversion of cell-bound to dissolved geosmin fractions.sites for geosmin. Since the thylakoids are the most ubiquitous The presence of separate particulate and dissolved VOC fractionsand abundant components in the cell lipid phase, if they were in source waters has other important implications, both for thethe primary sites, much of the geosmin should again be bound interpretation of ﬁeld data and for the choice of optimal waterto these structures after dilution of the aqueous phase. treatment processes (discussed below). As previously noted, the On the other hand, considerable research is still required relative amounts of these fractions can vary among and withinto elucidate 2-MIB fractions in cyanobacteria. In a study of species, while cell-bound VOCs can be transferred rapidly intopigments and intra- and extracellular 2-MIB synthesis rates the dissolved form. One major mechanism for this is via cellin Oscillatoria perornata (syn. Planktothrix perornata) and degradation by heterotrophic microorganisms (which could in-Pseudanabaena articulata, Zimba and coworkers (115) found clude both producers and nonproducers, e.g., fungi and strepto-that the size of the intracellular 2-MIB pool was independent mycetes [90, 91]). This process, for example, liberates geosminof lipophilic and phycobilin pigment production. However, from the cyanobacterial cell protein matrix. Much of the cell-since it is still unknown if, similar to the case for geosmin, there bound material can be transferred into the dissolved form by thisare two intracellular pools of 2-MIB, we recommend that the process because geosmin itself is much more slowly degraded bytwo extraction methods described above also be applied for most bacteria than other cell components.analysis of this terpenoid. This phenomenon is observed frequently in water layers near The cellular pools of geosmin and 2-MIB in actinomycetes, the sediment, and the resulting elevated concentrations of dis-which lack the extensive thylakoid membrane complex, have solved geosmin are readily visible in depth proﬁles (19). Thesimilarly not been examined. Combined evidence points to the VOC release takes place when geosmin-containing microorgan-important effects of growth conditions on total VOC produc- isms sediment into deep oxic or anoxic waters and are mineralizedtion and its allocation to intra- versus extracellular fractions. In by one or more of several processes: the catalytic enzymes of botha study of the growth, sporulation and geosmin production by the producers themselves and other microbial ﬂora and, in someStreptomyces tendae, Diogini et al. (18) observed that a large cases, benthic grazers (26, 29). In the pelagic zones of lakes,fraction of geosmin was stored in the cells relative to that particularly the epilimnion, grazing may be a more importantreleased into the environment. This is consistent with what is liberating mechanism. Experiments with the geosmin-producingexclusively observed with cyanobacteria. However, those au- cyanobacterium Aphanizomenon gracile showed that when grazedthors also found that growth, sporulation, and geosmin pro- by the crustacean Simocephalus or Daphnia magna, the cell-duction varied signiﬁcantly among different agar growth media bound geosmin was almost completely transferred into the dis-and that VOC yield was highest in sporulating cultures. This solved form (19). This process is typically overlooked by mostsuggests that geosmin production is intensiﬁed either in these taste-odor studies and yet is likely to be a signiﬁcant modiﬁer inpropagules or in both these and the accompanying somatic many water bodies. Crustaceans can turn over a large portion ofcells. In contrast, Pollack and Berger grew S. citreus in a liquid the phytoplankton in short time periods, and thus it is likely thatbroth media and showed that the majority of this compound relative proportions of dissolved and cell-bound geosmin inwas extracellular over much of the growth cycle (69). Those source waters are often related to grazing activity. 2-MIB mayauthors did not examine the cultures for spore production. show similar patterns, although this is yet to be examined. Simi-Both studies applied suitable extraction methods to recover larly, as noted below, the role of this VOC release mechanism incell-bound fractions, and taken together, their results indicate bioﬁlms is unknown but is also likely to be signiﬁcant.that extra- versus intracellular production may vary accordingto whether these organisms are suspended or on a substrate IN SITU AND ALLOCHTHONOUS PRODUCTION OF(and that differences in sporulation may be the reason), which GEOSMIN AND 2-MIBhas implications for suspended versus sediment-associatedcells and treatment efﬁcacy. The existence of major differences Many running waters investigated to date have containedin streptomycete abundances estimated by different techniques geosmin (8, 41, 43), 2-MIB (31), or both odor compounds (36,
VOL. 73, 2007 MINIREVIEW 440397, 102) (Table 3). VOC concentrations can range consider- concentrations were reduced to ϳ50 ng ⅐ literϪ1, but at theably, and in these lotic systems the major fractions of both same time a substantial increase from ϳ190 to 950 ngcompounds are often in the dissolved form. For example, Jutt- ¨ geosmin ⅐ literϪ1 was observed in the hypolimnion. Maximumner (43) found 4 to 7 ng ⅐ literϪ1 geosmin in some minor levels were found at the 10-m depth just above the sediment.streams near Zurich (Switzerland), with the particle-bound ¨ The total geosmin amount in the lake was nearly constant overfraction being between 3.5 and 22.2% of the total. Hoson and that same 1-month observation period. Although cell-boundcoworkers also found that the dissolved 2-MIB fraction was geosmin was not determined at that time, the most likely causelarger than the cell-bound fraction (32). In some cases, alloch- of these temporal and spatial patterns was the sedimentationthonous soil runoff may be important (33, 113). Bioﬁlms can of particle-bound geosmin. The organic seston fraction wasalso be very signiﬁcant sources, even in large, fast-ﬂowing riv- readily digestible under anoxic conditions, but geosmin degra-ers. For example, Watson and Ridal (102) concluded that most dation was very low under these same conditions, and most wasof the fairly signiﬁcant levels of geosmin and 2-MIB (up to ϳ70 therefore transferred from the cell-bound into the water-solu-ng ⅐ literϪ1) sustained during late summer every year in the ble fraction. It is also important to note here that to date,Saint Lawrence River are produced by shoreline bioﬁlms geosmin (or 2-MIB) production has not been described for(cyanobacteria, actinomycetes, and others) on rocks, macro- anoxic environments.phytes, and mussels. With a ﬂow of ϳ8,000 m3 ⅐ sϪ1 of the Vertical transport caused by water or particle movementsriver, this represents substantial in situ production. Vilalta and can be an effective vector of VOCs. For example, the extensiveSabater (98) found a very strong correlation between geosmin transport of offshore particle-bound geosmin to deep-waterlevels in the open water and those in attached and ﬂoating treatment plant intakes occurs each summer in Lake Ontariocyanobacterial mats in the Llobregat River (Spain). Baker and as a result of large-scale wind-induced downwelling (70, 101).coworkers (2) traced high 2-MIB levels in the Murray River to Detachment of benthic and epipelic VOC-producing cyano-the proliferation of a toxic benthic mat of Phormidium. Under bacterial populations (e.g., Gloeotrichia echinulata andnormal growth conditions, cyanobacteria (5, 109) and strepto- Oscillatoria limosa) and their subsequent ﬂotation can introducemycetes (18) excrete very little of these compounds into the these compounds into the pelagic zone (8, 98, 99). Gases thatmedium, and in many ﬂowing waters, their liberation into open accumulate in small bubbles in the sediment below cyanobac-water from these attached or ﬂoating producers is likely caused terial mats can increase their buoyancy sufﬁciently to loosenby a combination of grazing and environmental stressors, such large bioﬁlm patches, which then rise and transport cell-boundas desiccation and photooxidation, that can lead to the disin- geosmin to the water surface (87). Such a mechanism wouldtegration of these cells and release of cell contents during explain, for example, the observed surface maxima of boundsloughing off or water level changes. In addition, the concerted geosmin in Lake Zurich (19). ¨interaction of actinomycetes and cyanobacteria in bioﬁlmscould contribute to VOC release in a different way than when PROACTIVE CONTROL OF GEOSMIN AND 2-MIB INthese organisms act independently (90, 91). WATER SOURCES: TREATMENT REMOVAL AND Alternatively, plankton may be primary sources of geosmin LONG-TERM MANAGEMENTand 2-MIB in some slow-moving turbid rivers. For example,this was observed in the Murrumbidgee River (Australia), Geosmin and 2-MIB are relatively stable to chemical (66,where high geosmin levels were caused by the buoyancy-regu- 104) and biological degradation and can persist in the openlating cyanobacterium Anabaena circinalis (5), and in the Yodo water in the dissolved form for some time. This is an importantRiver (Japan), where the stagnation of dammed river water factor to consider when attempting to understand and trace thepromoted the growth of geosmin-producing Anabaena mac- distribution, transport, and fate of these VOCs in aquatic sys-rospora (28). Investigations of several other dammed rivers tems and their response to water treatment. Dissolved geosminhave shown elevated geosmin concentrations near to the dam is slowly degraded by microorganisms in oxic freshwater (19),(42), for example, in the Ruhr River (Westfalia, Germany), but little is known about the fate of this compound underwhere this was most likely produced by the liberation of geos- anoxic conditions. Similarly, to our knowledge, little researchmin from microorganisms undergoing accelerated sedimenta- has been carried out on natural 2-MIB degradation. Grazingtion in the stagnant waters beside the dam. by crustaceans, the major herbivores in lakes and reservoirs, Spatial distribution of geosmin and 2-MIB in lakes. In both does not appear to change the total amount of geosmin signif-lotic and lentic systems, three zones should be investigated as icantly, as shown by Durrer and coworkers (19), who measuredpossible sources of geosmin and 2-MIB: the epilimnetic water this process in closed vessels to prevent volatilization (whichand associated plankton; the hypolimnetic water (oxic or can be a major source of losses).anoxic), which is often below the light compensation point for Most taste-and-odor outbreaks are unanticipated, and thusoxygenic photoautotrophic microorganisms; and the littoral/ there is a heavy reliance on water treatment plants to controlbenthic zone. Efﬁcient transport of geosmin and 2-MIB among their impairment of drinking water. An extensive review of thethese compartments is mediated primarily by particles (cyano- treatment removal of geosmin and 2-MIB from raw water isbacteria and other geosmin-producing microorganisms) that beyond the scope of this paper, which focuses on microbialcarry these odor compounds. As long as these VOCs remain processes. For overviews of the numerous chemical and pho-cell-bound, sedimentation can introduce a rapid and efﬁcient tooxidative techniques widely in use, the reader is referred todisplacement. Henatsch and Juttner (26) measured epilimnetic ¨ the numerous studies dealing with these processes and theirgeosmin concentrations of up to 190 ng ⅐ literϪ1 in Lake effectiveness for different source waters (52, 54, 61, 77). TheseSchleinsee (Germany) in July. By September, surface water techniques are generally combined with various ﬁltration
4404 MINIREVIEW APPL. ENVIRON. MICROBIOL.media such as sand, activated carbon, and, more recently, tions about their biological and spatial origins and their storagemembrane systems (12, 34, 72). and release by cells. In particular, (i) it is often assumed that Because of their stability, both compounds are difﬁcult to planktonic or benthic aquatic cyanobacteria and streptomy-oxidize with the conventional processes applied during water cetes are the major sources of these VOCs, while eukaryotespuriﬁcation (54). More advanced treatments, such as granular (e.g., fungi), and terrestrial and distribution system sources areor particulate activated carbon, ozonation, and membrane ﬁl- disregarded; (ii) discrete cell-bound and dissolved VOC frac-tration, can be applied with variable success; their effectiveness tions respond very differently to treatment, and their relativeis modiﬁed by factors such as age of ﬁlter beds, type of carbon proportions have major implications for data interpretation,used, levels of source water dissolved organic material, and yet these fractions are rarely considered and, furthermore, areproportion of dissolved/particulate geosmin or MIB (12, 34). incompletely measured by most current analytical methods;Because the bulk of the two compounds is often present in the (iii) key physical and biological processes (e.g., treatment,particulate form, treatments such as the application of chem- grazing, and bacterial digestion) may interconvert these frac-ical oxidants or algicides that disrupt cells and liberate the tions and thus affect source water quality and treatment, andcell-bound material prior to their removal should be avoided yet the importance of these processes is rarely addressed; and(67). Ideally, removal of intact particulate material should be (iv) problems associated with many of the methods currentlyachieved (e.g., by dissolved air ﬂoatation, ﬁltration, and sedi- used to identify and enumerate source biota may obscure anymentation) prior to disinfection and other oxidative processes correlations with VOC levels and thus confound tracking ef-(103). forts. In comparison, the microbial treatment methods applied by Clearly, many intriguing questions still need to be addressed.the water industry (river bank ﬁltration and activated ﬁlter These include the spatial and temporal analysis of cell-boundbeds) are few in number (63), but some salient considerations and dissolved geosmin and 2-MIB; the molecular regulation ofare relevant to our discussion. In fact, microbial removal offers their production; the major physiological, biochemical, anda potentially highly effective means to treat geosmin and molecular traits which determine which species/strains are low2-MIB in source water, particularly when integrated with other and high odor producers; the physiological and ecological rolestreatment processes. For example, recent bench experiments (i.e., chemical ecology) of these compounds; and the mecha-with granular carbon (20) and sand columns (30) have dem- nisms which trigger their extracellular release (i.e., whetheronstrated active microbial degradation activity for these odor this occurs primarily as a result of cell disruption and decay orcompounds under deﬁned conditions. River bank ﬁltration and via a transporter that is activated/induced under certain con-slow sand ﬁltration have been used in treatment facilities ditions). Future research into these and other related issuesthroughout Europe for some time and offer a very effective will likely provide insight into much of the current ambiguitymeans to obtain high elimination rates. Juttner (42), for exam- ¨ surrounding many outbreaks of these major odor compounds.ple, reported that a slow sand ﬁltration unit (ﬂow rate of 420liters mϪ2 dayϪ1) achieved excellent rates of elimination of REFERENCESgeosmin and other terpenoid alcohols. That study found that 1. Aoyama, K., B. I. Tomita, and K. Chaya. 1993. Inﬂuence of incubation temperature on production of earthy-musty odour substances by actinomy-geosmin was not detectable in the upper layers of the sand cetes. Jpn. J. Toxicol. Environ. Health 39:207–212.ﬁlter when this material was removed and chemically ex- 2. Baker, P. D., D. A. Steffensen, A. R. Humpage, B. C. Nicholson, I. R.tracted, indicating the efﬁcient degradation of this compound Falconer, B. Lanthois, K. M. Fergusson, and C. P. Saint. 2001. 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