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1 UFZ, Helmholtz Centre for Environmental Research, Department of Environmental Microbiology, Permoserstr. 15, D-04318 Leipzig, Germany
2 Aquatic Biotechnology, Biofilm Centre, University Duisburg-Essen, Geibelstr. 41, D-47057 Duisburg, Germany
Correspondence
Roland H. Müller
r.mueller{at}ufz.de
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Two supplementary figures showing the influence of temperature and pH on the growth of A. tertiaricarbonis on MTBE are available with the online version of this paper.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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), leading to pollution by unnoticed leakages and accidental spills (Baehr et al., 1999; Klinger et al., 2002; Schmidt et al., 2002; Squillace et al., 1996). They threaten water resources by their unpleasant odour and taste and suspected carcinogenicity (McGregor, 2006). Consequently, much effort is put into studying the environmental fate of these compounds and developing measures against fuel oxygenate pollution. Microbial degradation has been considered in both respects (Deeb et al., 2000; Fayolle et al., 2001; Schmidt et al., 2004). However, MTBE and structurally related compounds were initially found to withstand microbial attack. This was thought to be mainly due to the ether bond in these compounds and the presence of a tertiary carbon atom.
Generally, it has proved difficult to isolate strains from enrichment cultures using MTBE as sole carbon and energy source. Attempts for more than 15 years were of limited success. At present, there are only a few strains capable of growing solely on oxygenates. These include Methylibium petroleiphilum PM1 (Nakatsu et al., 2006), Methylibium sp. R8 (Rosell et al., 2007), Hydrogenophaga flava ENV735 (Hatzinger et al., 2001), Mycobacterium austroafricanum IFP2012 and IFP2015 (François et al., 2002, 2003; Lopes Ferreira et al., 2006), Variovorax paradoxus CL-8 (Zaitsev et al., 2007), and other strains described in little detail (Hernandez-Perez et al., 2001; Lin et al., 2007; Okeke & Frankenberger, 2003; Pruden & Suidan, 2004). In addition, we have recently isolated strain L108 from a polluted site in Germany, which is able to grow on MTBE, ETBE and TAME (Rohwerder et al., 2004, 2006) and, like Methylibium petroleiphilum PM1, belongs to the Ideonella–Leptothrix–Rubrivivax branch of the β-proteobacteria. Strain L108 was classified as a species of a new genus that was named Aquincola tertiaricarbonis (Lechner et al., 2007).
Growth rates on oxygenates are in general low. A mixed culture with V. paradoxus CL-8 as the MTBE-degrading entity exhibited a growth rate of 0.012 h–1 (Zaitsev et al., 2007); a rate even one magnitude lower was reported for another mixed culture (Lin et al., 2007). Mycobacterium austroafricanum IFP2012 (François et al., 2002) and IFP2015 (François et al., 2003), Hydrogenophaga flava ENV735 (Hatzinger et al., 2001) and Methylibium petroleiphilum PM1 (Hanson et al., 1999) appeared to utilize MTBE in batch degradation experiments, but specific rates were hardly accessible or even impossible to derive from the published data and indicated difficulties in sustaining growth. Here we report on an examination of the potential of strain L108 to grow on MTBE, ETBE and TAME as sole source of carbon and energy under batch conditions. We also investigated productive, i.e. growth-coupled, degradation of the primary metabolites, viz. tert-butyl alcohol (TBA), tert-amyl alcohol (TAA) and 2-hydroxyisobutyrate (2-HIBA) to identify possible metabolic bottlenecks.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The strain was inoculated into 200 ml mineral salts solution to give an initial biomass concentration of about 25 mg dry mass l–1. Incubation was performed in 600 ml bottles, closed with gas-tight butyl rubber stoppers, on a rotary shaker at 150 r.p.m. at 30 °C. The initial pH was 7.0 and was not corrected throughout the experiment. Liquid samples were taken at various times by a sterile syringe puncturing the butyl rubber stopper. Specific growth rates were derived from the linear part of semi-logarithmic plots of the biomass concentration versus time by regression analysis. The kinetic parameters were derived from double-reciprocal plots of rates versus substrate concentrations to obtain the apparent, experimentally relevant values or by nonlinear regression (Haldane equation) to obtain kinetically based parameters according to
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Analytics.
Biomass was determined as OD700. OD700 multiplied by a factor of 0.54±0.03 was found to give the dry mass in mg ml–1. MTBE, ETBE, TAME, TAA and TBA were determined by head-space gas chromatography as previously described (Rohwerder et al., 2006). 2-HIBA was determined by gas chromatography as the methyl ester according to a procedure given elsewhere (Rohwerder et al., 2006). Fructose, lactate, pyruvate and succinate were measured by HPLC at an oven temperature of 70 °C using an Ion 300 OA column (Macherey Nagel). The mobile phase was 0.005 M H2SO4 fed with a rate of 0.6 ml min–1; detection was by refractive index (RID). The compounds measured by the various methods were detected with 95 % confidence.
MTBE, ETBE, lactate, pyruvate and succinate (sodium salts) and fructose were purchased from Merck; TAME and other chemicals used were obtained from Fluka.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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and 2, respectively. With MTBE, the maximum specific growth rate, µmax, was reached at a concentration around 2.5 mM. Data treatment at this concentration range in a double-reciprocal plot revealed a µmax of 0.047 h–1 and Ks of 0.59 mM. Higher concentrations resulted in inhibition that levelled off at about 50 % of the maximum rate above 6 mM. The above values correspond to the apparent but finally effective kinetic parameters. Inclusion of nonlinear regression analysis by using equation 1 resulted in kinetically more sound parameters. With MTBE as a substrate these amounted to µmax=0.097 h–1, Ks=1.86 mM and Ki=7.0 mM. With ETBE, an effective µmax of around 0.06 h–1 was obtained, with only slight inhibition appearing at higher concentrations (Fig. 1). Nonlinear regression revealed µmax=0.065 h–1, Ks=0.089 mM and Ki=35.0 mM. Due to the quotient of µmax/Ks (specific affinity; Healey 1980), ETBE (µmax/Ks=0.73 h–1 mM–1) was a significantly better substrate for strain L108 than MTBE (µmax/Ks=0.052 h–1 mM–1). TAME was apparently a better growth substrate than MTBE also. The µmax of 0.055 h–1 was reached already at a TAME concentration of 0.35 mM. The growth rate levelled off between 5 and 10 mM before slightly decreasing at higher concentrations until approaching zero at 32 mM (data not shown). Due to the higher affinity to TAME, our rate data did not allow calculation of reliable Ks values with this substrate. Because of problems in verifying growth at these low concentrations, experiments were not performed at such substrate levels.
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) gave a µmax of 0.08 h–1 at a substrate concentration of 1 mM and this decreased only slightly to 0.07 h–1 towards the highest tested concentration of 20 mM. The affinity to TBA was the lowest among all tested growth substrates although exact affinities could not be derived from the present data. A µmax of 0.1 h–1 was obtained at about 35 mM (Fig. 2). At higher concentrations the activity decreased to about half of the maximum rate at about 75 mM and stopped at 200 mM (not shown). The fastest growth was obtained with 2-HIBA. Double-reciprocal data analysis revealed an apparent µmax of 0.19 h–1, obtained at around 5 mM substrate, followed by rather pronounced inhibition to 20 % of µmax at 29 mM. The apparent Ks for 2-HIBA derived in the same way was 1.05 mM. With nonlinear regression we obtained µmax=0.305 h–1, Ks=1.6 mM and Ki=7.0 mM by using equation 1.
Table 1 lists all values of µmax together with the corresponding yield coefficients, Y, that were derived from biomass formation after substrate exhaustion and maximum specific substrate consumption rates, qs=µ Y–1. Values obtained with the common substrates pyruvate, succinate, lactate and fructose were included as references for comparison. Yields with the oxygenates and their metabolites were around or above 0.5 g biomass (g substrate)–1, with the highest yields of around 0.8 g g–1 obtained with TAME and TAA. These values are about twice as high as those obtained with the reference substrates and reflect the more reduced state of the former. The rates of growth and consumption of oxygenates and metabolites were between 2 and 10 times slower than those with the reference substrates. Particularly, growth with 2-HIBA approached the rates obtained with the reference substrates. There was a general trend that the oxygenate metabolites were consumed faster than the mother compounds. This seems to indicate that later stages of the metabolism did not control the rates of oxygenate consumption. Nevertheless, during growth on MTBE and ETBE, TBA concentrations between 3 and 5 µM were always released into the cultivation broth. In addition, similar amounts of the TBA precursor tert-butyl formate were detected when strain L108 was cultivated on MTBE. In contrast, the corresponding esters tert-butyl acetate and tert-amyl formate were not detectable in cultures growing on ETBE and TAME, respectively. In summary, it appears that oxygenate intermediates such as TBA and 2-HIBA, which were previously supposed to be recalcitrant, are substrates almost as good as succinate or fructose for strain L108.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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) is very effective in the productive degradation of all three common fuel oxygenates MTBE, ETBE and TAME. An effective maximum growth rate of about 0.045 h–1 with MTBE and up to 20 % higher ones with ETBE and TAME were found. Growth rates on the MTBE and ETBE metabolites TBA and 2-HIBA even amounted to 0.1 h–1 and 0.17 h–1, respectively. These high growth rates approach rates obtained with common substrates such as fructose or succinate, indicating that fuel oxygenate metabolism in strain L108 is well established.
Hence, the strain has attained a metabolic capacity that is clearly beyond the level of fortuitous enzyme activity but suggests elevated substrate specificity. The former statement refers to the observation that the conversion of a pollutant is often catalysed by defined enzymes in an unspecific manner, resulting in low rates due to the xenobiotic structure of the compounds (Janssen et al., 2005). Although the oxygenate degradation pathway(s) have not been elucidated completely, three important steps have already been identified in strain L108. The initial monooxygenase reaction attacking both the methyl and ethyl groups of MTBE and ETBE, respectively, is catalysed by a cytochrome P450-type enzyme encoded by the ethABCD genes (Breuer et al., 2007). A similar monooxygenase has been previously detected in Rhodococcus ruber IFP2001 growing on ETBE (Chauvaux et al., 2001). A different oxygenase system, which has similarity to phthalate dioxygenase, is likely responsible for hydroxylating TBA in strain L108 (Schäfer et al., 2007). The metabolite 2-HIBA is converted by a novel cobalamin-dependent mutase to 3-hydroxybutyrate (Rohwerder et al., 2006). Thus far, this combination of specific enzymes has not been found in other oxygenate-degrading strains. Recently, a monooxygenase of the AlkB type has been described for Mycobacterium austroafricanum IFP2012 (Lopes Ferreira et al., 2007), supposed to hydroxylate both MTBE and TBA. Hydroxylation of the latter is inhibited by MTBE, resulting in the accumulation of TBA in strain IFP2012 (François et al., 2002, 2003). For Methylibium petroleiphilum PM1, a phthalate dioxygenase-like enzyme and a mutase very similar to the enzymes found in strain L108 have been recently proposed for TBA hydroxylation and 2-HIBA isomerization, respectively (Hristova et al., 2007). However, ethABCD is not present in the genome of strain PM1 (Kane et al., 2007). Consequently, at least the initial steps in MTBE degradation deviate in strains PM1 and L108, obviously resulting in different capacities for growth on ether oxygenates such as MTBE and ETBE. The pathway for TAME degradation has not been elucidated so far. Proposing similarity with the pathways for MTBE and ETBE, tert-amyl formate and TAA would be intermediates. The latter could be converted to 3-hydoxyisovaleric acid and then split into acetone and acetyl-CoA (Nemecek-Marshall et al., 1999). However, due to the inability of strain L108 to grow on acetone (Lechner et al., 2007), a cobalamin-dependent route via 2-hydroxy-2-methylbutyrate, corresponding to the TBA intermediate 2-HIBA, is more likely.
A comparison of the capacities of various described strains to degrade MTBE and some key intermediates shows that strain L108 is apparently effective (Table 2). Only Variovorax sp. strain JV-1 appears to have a similar potential for the degradation of MTBE (Uotila & Zaitsev, 2003). Obviously, this strain is equipped with a set of specific enzymes as efficient as the one found in strain L108. For most of other strains besides those included in the table, reported information did not allow derivation of qs values for MTBE. Although not always obvious, this may indicate that MTBE degradation was at best weakly coupled to growth in these bacteria, e.g. strains IFP 2012, PM1 and ENV735. With TBA as the substrate, the specific degradation rates of strain L108 were in each case significantly above those found with the other strains (Table 2). This was also true for 2-HIBA except for strain CIP I-2052, which achieved similarly high rates. For strains L10 and CIP I-2052, it has been demonstrated that growth rates on TBA and 2-HIBA significantly decreased when cobalamin was replaced by cobalt ions in the growth medium (Table 2). This phenomenon may be due to the effort in synthesizing cobalamin required for the mutase pathway, converting 2-HIBA into 3-hydroxybutyrate (Rohwerder & Müller, 2007). Although the cobalt dependency of strain IFP2012 for degrading MTBE and its metabolites (François et al., 2002) indicates 2-HIBA mutase activity also in this strain, slower consumption of TBA and 2-HIBA in the presence of cobalamin may be caused by incapacity of taking up the externally added vitamin.
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). Experimental yields of 
0.5 g g–1 prove the effective use of these substrates for the growth and multiplication of strain L108. A similar yield coefficient of 0.49 g g–1 was obtained with Variovorax paradoxus CL-8 (Zaitsev et al., 2007). For Mycobacterium austroafricanum IFP2012, yields of 0.44 g g–1 (MTBE), 0.61 g g–1 (TBA) and 0.42 g g–1 (2-HIBA) were reported (François et al., 2002). Biomass synthesis may, however, be nil or even negative at very low rates of substrate consumption (Müller et al., 2007). The enrichment of MTBE-degrading cultures under such conditions indicates that the maintenance requirements are modest in these strains, which may be either an intrinsic property of such strains or an adaptation of productive MTBE degraders. The difficulty of enriching degradative strains in the early years of MTBE research or the need to provide them with auxiliary substrates such as n-alkanes (Haase et al., 2006; Steffan et al., 1997) may be an indication of the recent acquisition of the regulatory and enzymic capacity to use MTBE as sole carbon and energy source.
Interestingly, the ether-related metabolites, namely TBA, 2-HIBA and TAA, are better substrates than the mother compounds. This raises questions about the evolution of the degradation pathways. MTBE and related ethers have been only recently introduced into nature, since their massive use as fuel additives began only in the late 1980s (Squillace et al., 1997). In contrast, oxygenate-independent sources are known for the tertiary alcohols and 2-HIBA (Fig. 3). 2-HIBA is a by-product of the classical methacrylate synthesis process via 2-hydroxyisobutyronitrile (Rohwerder et al., 2006), which started in the mid-1930s. This may explain why Holowach and coworkers reported already in 1994 the isolation of 2-HIBA-converting bacterial strains from the wastewater of a methacrylate-producing plant (Holowach et al., 1994). In addition, 2-hydroxyisobutyronitrile is a degradation product of the plant cyanoglycoside linamarin (Forslund et al., 2004) and can form 2-HIBA in the presence of nitrilase or nitrile hydratase and amidase activity (Banerjee et al., 2002). A third ether-independent 2-HIBA source could be the conversion of isobutene via the corresponding 1,2-epoxide and 2-hydroxy-2-methylpropanol (Rohwerder & Müller, 2007). Likewise, Hyman and co-workers have recently questioned whether MTBE and ETBE are the only source for TBA (Hyman et al., 2007). Indeed, it has been reported several times that the activity of methane monooxygenase and other alkane monooxygenases on isobutane, isopentane and homologous hydrocarbons not only resulted in the corresponding primary and secondary alcohols but also formed the tertiary ones (Dubbels et al., 2007; Imai et al., 1986; Onodera et al., 1990; Patel et al., 1982). Hence, although growth on 2-HIBA and tertiary alcohols was not reported before man started ether oxygenate production, their degradation pathways could have evolved far earlier and totally independent of ether oxygenate contamination (Fig. 3).
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ACKNOWLEDGEMENTS |
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Edited by: H. L. Drake
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Received 21 October 2007;
revised 14 February 2008;
accepted 18 February 2008.
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