Crude oil bioremediation by alcanivorax borkumensis
Crude Oil Bioremediation by Alcanivorax borkumensis
Alcanivorax borkumensis is a recognized alkane-degrading organism that has the potential to be useful for crude oil spill bioremediation.
A. borkumensis Overview
Physiology and Phylogeny
A. borkumensis is a gram-negative, rod shaped, aerobic, catalase negative and oxidase positive species (1). A. borkumensis is halotolerant, which is necessary to survive in ocean salinity; optimal NaCl concentration for growth in culture is between 3-10% (1). It can use linear and branched alkanes as a primary fuel source, but cannot use aromatic alkanes for this purpose (1,2). This is an important factor in determining A. borkumensis’ role in bioremediation as discussed in section 5.2. It is classified as a γ-proteobacterium based on 16S rRNA sequencing (1). The γ-proteobacterium class includes another known alkane degrading genus, Marinobacter (1).
A. borkumensis is not typically observed in the microbial communities of uncontaminated water (4). However, it can make up 70-90% of the composition of microbial communities after contamination with crude oil, which is indicative of a competitive advantage (4). It is important to note that alkane degradation is not unique to A. borkumensis and thus, the ability to degrade alkanes alone does not explain why it is the dominant species in crude oil contaminated areas (5). It is suspected that the reason for its dominance lies in the ability to degrade branched alkanes (2).
Composition of Crude Oil
A total of three million tons of crude oil enter the ocean each year as a result of human activity and more natural mechanisms such as seepage(6). Crude oil in the ecosystem is viewed as a chronic pollutant that is toxic to most forms of life (6). Crude oil is challenging to clean up as it has over 17000 distinct compounds (3). Chief among them are saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes (3).
Crude oil can be removed from the environment through evaporation, photooxidation and microbial bioremediation (1). After a spill, the typical approach is to use booms, skimmers and adsorbents to remove as much oil from the water as possible; however, the effectiveness of this approach is very limited (7).
Structure and Function of alk operon in Alcanivorax Borkumensis
The two major alkane hydroxylases, (alkane 1-monooxygenases), are encoded by alkB1 and alkB2, each are part of separate operons (9). The presence of these genes allows A. borkumensis to degrade alkanes (9). An upstream locus, alkS, regulates the alkB1 operon (10). The operon includes alkH - an aldehyde dehydrogenase, alkJ - an alcohol dehydrogenase and alkG – a rubredoxin reductase, which reduces the electron shuttling protein rubredoxin (9,11). The alkane hydroxylases alkB1 and alkB2 preferentially degrade alkanes that are 5-12 and 8-16 carbons in length, respectively (2). This is seen in Figure 1B, which shows the preferential degradation of specific length hydrocarbons.
The genome lacks a cyclohexane monooxygenase gene (9). As a result, A. borkumensis cannot utilize aromatic alkanes as a primary fuel source (9). It is suspected that alkB1 or alkB2 are able to cometabolize cycloalkanes via oxygenase activity (9). This permits A. borkumensis to break down cycloalkanes only when there are aliphatic alkanes present, since the expression of the necessary genes is induced by the presence of aliphatic alkanes.
A. borkumensis likely has another mechanism of alkane degradation as alkB1 and alkB2 knockouts do not completely lose alkane degradation capacity (12). This is suspected to be due to the presence of three putative cytochrome P450 proteins that can break down aliphatic alkanes (12).