Download NUTRITIONAL DIVERSITY (CONTINUED) 2. Chemoheterotrophs

NUTRITIONAL DIVERSITY (CONTINUED)
2. Chemoheterotrophs: selected examples
a) Aerobes and facultative anaerobes:
example: bioremediation of oil spills: 10E: pp. 674-676; 597-598; 9E: pp. 696-698; 632-634
• Petroleum is a complex mixture of predominantly hydrocarbons (containing only carbon and
hydrogen) with a low proportion of other molecules containing O, N, and S. Because of this
complexity, a consortium of microbes is needed to biodegrade petroleum.
• Two classes of chemicals are more biodegradable than the others:
the “saturates” (or “aliphatics”) fraction and the “aromatics” fraction.
Saturates, including n-alkanes, are degraded by a variety of microbes
Aromatics, including benzene and polycyclic aromatics, can be toxic and may persist in
the environment because they are harder to degrade (more recalcitrant) than the saturates.
• The larger the molecule, or the more chemical substitutions, the more recalcitrant it is. Some
petroleum components cannot serve as a C source but they can be degraded by “co-metabolism”,
in which another compound serves as the C and energy source and the co-metabolized compound
is attacked fortuitously, i.e. as a side-reaction.
• After biodegradation, there is almost always a residue of undegraded hydrocarbons: those that
are recalcitrant to biodegradation and remain in the environment, and those that might be
biodegradable but have “escaped” because of low concentration or location (e.g. in soil pores).
• Hydrocarbon-degrading bacteria are ubiquitous, but typically increase in numbers after an oil
spill, through “enrichment”. After the oil is degraded, their proportion in the normal flora of the
environment decreases again.
• The most significant biodegradation processes are aerobic, although anaerobic degradation of
some hydrocarbons can occur very slowly.
• For efficient biodegradation, the microbes require addition of N and P (e.g. as fertilizer) to
balance the high load of Carbon.
• If there are insufficient numbers of oil-degraders in the environment for clean-up, two methods
are available: bioaugmentation (addition of microbes to the spill site, e.g. commercial products)
or biostimulation (addition of nutrients [fertilizers] to stimulate the natural degrading population.