degrade alkanes, but they attack the alkanes in very different ways. If we used odd-numbered alkanes, C-15 and C-17, AK-01 formed cellular fatty acids that are odd numbered. If we used odd-numbered alkanes for the other organism, the resulting cellular fatty acids were even numbered. The opposite also occurred, i.e., even-numbered alkanes generated even-numbered fatty acids in strain AK-01, and they yielded odd-numbered fatty acids in the other strain. This pattern led us to hypothesize that these two organisms, though cousins, appear to have different mechanisms for alkane degradation. Using substrates that were unlabeled or deuterated or C13-labeled alkanes, we found that the two different strains have very different ways of attacking the alkane under anaerobic conditions.

Strain AK-01 carries out a carbon addition at the subterminal C-2 position of the alkane chain. As a consequence, the terminal carbon then swings down so it forms the methyl group of the C-2 carbon of this fatty acid. Once this occurs, the organism can carry out normal beta-oxidation. It can carry out chain elongation to form larger fatty acids as well. The other strain has a very different attack. It uses inorganic carbonate from solution as the carbon donor. This inorganic carbon is added to the C-3 position of the alkane, and the two terminal carbons are released as acetate. We then end up with two carbons removed and one carbon added to the original alkane so that the resulting fatty acid ends up as an odd-numbered fatty acid. That description is really just the tip of the iceberg because these are only two organisms that have been investigated under anaerobic conditions.

To further contemplate this area of study, consider the following questions:

  • Are microorganisms from terrestrial or freshwater systems similar to those found in marine sediments? We know too little about the diversity of these types of degradative anaerobes.

  • Are competent organisms actually present? This question is not as straightforward. When we looked for anaerobic toluene and benzene degradation in anoxic-contaminated sediments or in anoxic pristine sediments, our data indicated that the toluene loss and benzene loss occur in the contaminated sediments, but not under the same conditions as in the pristine sediments (Figure 5). This difference suggests that organisms competent for degrading these contaminants are not present in the pristine environments. If they are not there, bioaugmentation may be a viable application for adding organisms where needed.

Let me also point out that there are many pathways for aerobic degradation of PAHs. There are also many aerobic organisms able to carry this out. This kind of diversity has yet to be tapped for the anaerobic microbial community.

We also know about the aerobic degradation of alkanes in terms of

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