Polycyclic Aromatic Hydrocarbon Degradation by Anaerobic Bacteria from the Great Artesian Basin

Abstract

The Great Artesian Basin (GAB) is a vast subterranean thermal aquifer system underlying over 20% of the Australian continent. Substantial reserves of oil, gas and minerals exist within the GAB and combined with industrial activities can often contaminate the groundwaters. The current study investigated the bacterial ecology of bore waters that were in or close to oil deposits. Three sites were selected, two in the Quilpie/Eromanga region of Queensland (Naretha bore registered number 4022 and Adavale bore registered number 305), and the other in the Moomba oil field of South Australia (Moomba bore 9). A wide diversity of bacteria was detected across all of the samples collectively, including members of 32 bacterial phyla. There was greater diversity in the water samples from bores 4022 and 305 compared with Moomba 9, which was likely due to its closer association with oil. The most dominant bacterial taxa were similar in bores 4022 and 305, families Rhodobacteraceae, and Xanthomonadaceae and the order Bacillales. In contrast, the dominant taxa from Moomba 9 were the family Oxalobacteriaceae and the genus Agrococcus. Three different water samples were tested from bore 4022, from the source, 100 m and 250 m downstream in the runoff channel water. The bacterial diversity increased the further away the water flowed from the bore, due to the cooler water and contamination from the surrounding environment. A comprehensive anaerobic thermophilic enrichment program revealed that bacteria grew on a wide variety of organic substrates and a range of heavy metals as terminal electron acceptors. Isolation yielded 164 bacteria capable of using substrates from sugars and extracts through to polycyclic aromatic hydrocarbons (PAH) and reducing the metals iron(III), vanadium(V), cobalt(III) and manganese(IV). Seven pure iron(III) reducing polycyclic aromatic hydrocarbon (PAH) degrading bacteria, designated strains RN40AT, RN40BT, RN40CT, RN40DT, RN305AT, RN305BT and MBA9BT, were selected for further studies. The rates of degradation Polycyclic Aromatic Hydrocarbon Degradation by Anaerobic Bacteria from the Great Artesian Basin and PAH preference varied significantly between the isolates. Strain MBA9BT showed the highest extent (97.6%) of anthracene degradation while strain RN305BT was the lowest (9.7%). Again, strain MBA9BT also showed the highest extent (91.2%) of pyrene degradation and strain RN40DT showed the lowest (16.1%). Phenanthrene degradation was highest in strain RN40BT and the lowest in strain MBA9BT (2%). PAH degradation in all the isolates showed a direct dependence on Fe(III) reduction. A measurable decrease in the surface tension of the medium suggested the production of biosurfactants by all isolates when grown with PAHs as substrates. Genomic analysis of the isolates revealed subsystems of different pathways that are commonly found in metal acquisition, transport and utilisation as well as those for aromatic hydrocarbon degradation. Strain RN40DT had the highest number of genes involved in the metabolism of PAHs, though all of the isolates had genes for the metabolism of central aromatic intermediates, especially those related to catechol, salicylate and homogentisate degradation. The isolates had varying levels of genes involved in the metabolism and resistance to iron, copper, chromium, zinc, molybdenum, manganese and mercury. Strains RN40AT, RN305AT RN305BT, RN40BT and RN40DT contained the most genes for iron acquisition and metabolism including those for siderophores, hemin transporters and ferric ABC transport systems. The results in the current study have built upon our understanding of bacterial diversity in the GAB, particularly with insights into waters associated with oil deposits. Bacteria that degrade polycyclic aromatic hydrocarbons with the simultaneous reduction of Fe(III) under anaerobic conditions were isolated for the first time and may have significant potential in bioremediation of contaminated groundwaters.