Abstract
Previously, Planted Fixed-Bed Reactors (PFRs) have been used to investigate microbial toluene removal in the rhizosphere of constructed wetlands. Aerobic toluene degradation was predominant in these model systems although bulk redox conditions were hypoxic to anoxic. However, culture-independent approaches indicated also that microbes capable of anaerobic toluene degradation were abundant. Therefore, we aimed at isolating anaerobic-toluene degraders from one of these PFRs. From the obtained colonies which consisted of spirilli-shaped bacteria, a strain designated 15–1 was selected for further investigations. Analysis of its 16S rRNA gene revealed greatest similarity (99%) with toluene-degrading Magnetospirillum sp. TS-6. Isolate 15–1 grew with up to 0.5 mM of toluene under nitrate-reducing conditions. Cells reacted to higher concentrations of toluene by an increase in the degree of saturation of their membrane fatty acids. Strain 15–1 contained key genes for the anaerobic degradation of toluene via benzylsuccinate and subsequently the benzoyl-CoA pathway, namely bssA, encoding for the alpha subunit of benzylsuccinate synthase, bcrC for subunit C of benzoyl-CoA reductase and bamA for 6-oxocyclohex-1-ene-1-carbonyl-CoA hydrolase. Finally, most members of a clone library of bssA generated from the PFR had highest similarity to bssA from strain 15–1. Our study provides insights about the physiological capacities of a strain of Magnetospirillum isolated from a planted system where active rhizoremediation of toluene is taking place.
Highlights
Rhizoremediation is a process in which most degradation of toxic compounds is attributed to the activity of microbes in the rhizosphere [1]
This work shows that the isolated strain 15–1 belongs to the genus Magnetospirillum
Several members of the genus Magnetospirillum have been described such as M. aberrantis, M. gryphiswaldense, M. bellicus, M. magnetotacticum MS-1, M. magneticum AMB-1, strains pMbN1 and TS-6 [17, 31,32,33,34,35,36]
Summary
Rhizoremediation is a process in which most degradation of toxic compounds is attributed to the activity of microbes in the rhizosphere [1]. Various oxygenases introduce hydroxyl groups and cleave the aromatic ring while anaerobically, toluene is converted to benzylsuccinate via the addition of its methyl group to fumarate. The latter step is catalyzed by highly oxygen sensitive benzylsuccinate synthase (BssABC) [8]. To study microbial transformations in the rhizosphere under controlled conditions, a laboratory-scale planted fixed-bed reactor (PFR) was designed. Within this PFR, a circulating flow regime prevents the formation of large chemical gradients and an efficient turnover of organic carbon occurs [13]. Their tolerance mechanism to toluene was studied in order to elucidate their potential to thrive in these wetland models
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