Hydrocarbon Biodegradation in a Sub-Antarctic Soil: Response to Varying Oxygen Levels.

A microcosm experiment utilizing a respirometry system and ¹⁴C-labeled hexadecane was conducted to investigate the effects of varying oxygen regimes on hydrocarbon degradation in contaminated soil from sub-Antarctic Macquarie Island.  Soil was fertilized with NH₄NO₃ prior to incubation.  Measurements were made of oxygen consumed, carbon dioxide produced, and ¹⁴C evolution over 14 weeks and residual petroleum hydrocarbons, and soil nitrate and ammonium concentrations were measured at the conclusion of incubation. Target O₂ levels during incubation ranged from 0 to 20.9%.  The microbial community structure at the start of the experiment and after 4, 8 and 12 weeks incubation was also explored.  The amount of ammonium utilized in the aerobic microcosms was significantly higher than in the anaerobic microcosms which utilized mainly nitrate, suggesting that hydrocarbon degradation under low oxygen concentrations was coupled to denitrification.  Residual hydrocarbon concentrations were lowest in soil incubated with a target O₂ of 10.5% and 20.9%.  ¹⁴CO₂ evolution was also slower in the anaerobic than aerobic microcosms although there was no discernable lag-phase in either. The microbial communities in the aerobic microcosms had fewer total bacteria, fewer denitrifying bacteria, and a smaller proportion of microbes had genes for hydrocarbon degradation than those in the anaerobic soils.  The microbial community in the soil from Macquarie Island is adapted to ambient low oxygen concentrations, however it possesses the ability to utilize available oxygen and hydrocarbon degradation is favored by O₂ levels of approximately 10% or greater.