Abstract
The availability of oxygen is often a limiting factor for the degradation of aromatic hydrocarbons in subsurface environments. However, while both aerobic and anaerobic degraders have been intensively studied, degradation betwixt, under micro- or hypoxic conditions has rarely been addressed. It is speculated that in environments with limited, but sustained oxygen supply, such as in the vicinity of groundwater monitoring wells, hypoxic degradation may take place. A large diversity of subfamily I.2.C extradiol dioxygenase genes has been previously detected in a BTEX-contaminated aquifer in Hungary. Older literature suggests that such catabolic potentials could be associated to hypoxic degradation. Bacterial communities dominated by members of the Rhodocyclaceae were found, but the majority of the detected C23O genotypes could not be affiliated to any known bacterial degrader lineages. To address this, a stable isotope probing (SIP) incubation of site sediments with 13C7-toluene was performed under microoxic conditions. A combination of 16S rRNA gene amplicon sequencing and T-RFLP fingerprinting of C23O genes from SIP gradient fractions revealed the central role of degraders within the Rhodocyclaceae in hypoxic toluene degradation. The main assimilators of 13C were identified as members of the genera Quatrionicoccus and Zoogloea, and a yet uncultured group of the Rhodocyclaceae.
Highlights
The contribution of microbes to the removal of BTEX compounds from groundwater ecosystems has been intensively investigated over the last decades (Lueders 2017)
A combination of 16S rRNA gene amplicon sequencing and terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of C23O genes from stable isotope probing (SIP) gradient fractions revealed the central role of degraders within the Rhodocyclaceae in hypoxic toluene degradation
70% of the toluene was depleted from the biotic enrichments after 3 d of incubation, while its concentration was under the detection limit by the seventh day of incubation (Fig. S1, Supporting Information)
Summary
The contribution of microbes to the removal of BTEX compounds (benzene, toluene, ethylbenzene and xylenes) from groundwater ecosystems has been intensively investigated over the last decades (Lueders 2017). The availability of oxygen is often restricted, with hydrocarbon contamination causing microoxic or anoxic conditions even in shallow aquifers. Genes for aromatic ring-cleavage dioxygenase enzymes are key to the degradation of monoaromatic compounds (El-Naas, Acio and El Telib 2014). Aerobic degraders use oxygen for respiration and as a cosubstrate for these enzymes. Kukor and Olsen (1996) suggested that a specific group of extradiol dioxygenases (subfamily I.2.C) was adapted to environments with low oxygen concentrations, hinting at their role in ring-cleavage reactions in what they called “oxygen-requiring, but nitrate-enhanced” hypoxic degradation. It has to be noted that ring-cleaving dioxygenases belonging to the same subfamily may show different oxygen affinities as this was observed in case of chlorocatechol 1,2dioxygenases (Balcke et al 2008)
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