Chemical reactions and stability of biomarkers and stable isotope ratios during in vitro biodegradation of petroleum

Abstract

Petroleum from the Gifhorn Trough (Lower Saxony, F.R.G.) and the Maracaibo Basin (Venezuela) were analyzed for microorganisms capable of biodegradation. Genetically related oils were inoculated with the isolated microorganisms and the degradation of the oils was followed by chromatographic techniques. Parameters important for the reactions (pH, oxygen concentration, nitrogen, and phosphorus, reaction medium) were monitored and optimized. The degradation of n-alkanes was followed closely. It was shown that the microorganisms responsible for degradation (yeasts and bacteria) could easily survive a period of inactivity due to missing nutrients and could be reactivated within hours to degrade newly added petroleum. Under conditions in which oxygen or nitrogen was limited, selectivity was observed: medium-chain n-alkanes (C 18, C 20) were degraded faster than long-chain (C 30) or short-chain (C 14) n-alkanes. Branched-chain alkanes were also degraded. The physical parameters of the oils (e.g. density, viscosity, average molecular weight) were altered as expected during degradation. In vitro degraded oil is very similar to oil degraded in nature. It was found that biomarker molecules are resistant to degradation under the conditions used. Pyrolysis-GC analysis of asphaltenes revealed no significant changes in the composition of pyrolyzates during biodegradation. A small shift in the carbon isotope ratios in the aromatic hydrocarbon fraction was observed between biodegraded and unaltered samples.