Alkane oxidation and the emulsification of crude oil by a Rhodococcus species

Abstract

The objective of this work was to study the physiology of one crude oil emulsifying bacterium, with particular emphasis on its emulsifying system. There is a potential use of oil emulsifying bacteria (OEB) for the clean up of oil- contaminated natural environments, and of oil transport and storage systems. Knowledge about OEB is important for optimisation of such processes.

The bacterium chosen for this study was isolated from the Trondheimsfjord, Norway and was the best oil emulsifier out of the 99 bacteria isolated from globally distributed seawater samples. It was found to belong to the genus Rhodococcus. This genus is often found to dominate in waters with chronic petroleum pollution.

Rhodococcus sp. strain 094 makes very stable oil in water emulsions from many different crude oils types with variable content of aliphatic and aromatic compounds by utilising the C11-C33 η-alkanes as carbon and energy sources. The utilisation of crude oil as growth substrate by Rhodococcus sp. strain 094 may be viewed as a sequence of there events, adhesion of the cells to the oil, oxidation of the alkanes leading to exponential growth, and utilisation of accumulated oxidation products giving linear growth.

The initial growth of crude oil by Rhodococcus sp. strain 094 was exclusively associated with the oil phase. This bacterium formed a hydrophobic cell surface when exposed to crude oil, which ensured a very strong adhesion to the oil thereby establishing an efficient uptake of the hydrophobic substrate by a direct contact mechanism. A phase of exponential growth then followed.

In mid-exponential phase the whole cell population was adhered to oil droplets. During exponential growth, 70% of the alkanes in the crude oil disappeared, but only 30% of the total biomass was found. At the same time there was a very pronounced accumulation of acidic products.

The exponential phase was followed by a longer period of linear growth, during which approximately 70% of the total cell biomass was produced. Despite the relatively large biomass production, only 30% of the alkanes were oxidised. During the shift from exponential to the linear growth phase there was a shift from net acid production to net acid consumption, and at the same time an increasing percentage of the cell population could be found in the water phase. The results strongly suggest that the cell growth during this period is mainly based on consumption of acidic products accumulated during the exponential growth period.