Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 151

susceptible to microbial destruction under ambient conditions. Some compounds widely considered to
be nonbiodegradable under ambient conditions can be degraded under enhanced conditions, which
may be created in ex situ bioreactors or in situ by injection of a substrate, inoculation of acclimated
microbes, temperature control, or adding nutrients essential to sustaining microbial populations.
The by-products of microbial metabolism can sometimes produce compounds that are more
toxic and regulated at a lower threshold than the original contaminant. The complete mineralization
of the contaminant may involve multiple metabolic steps, possibly by different groups of microbes
and at different metabolic rates. Biodegradation may be limited by the toxicity of the subject com-
pound to microbes. The metabolic by-product may also be toxic to microorganisms, limiting further
biodegradation after an initial step. “Complete biodegradation” occurs in six half-lives, after which
only approximately 1.5% of initial contaminant concentration remains; after seven half-lives, less
than 1% remains.
Testing for biodegradability often involves using pure cultures of a single strain of microbe to
establish the identity of microbes responsible for biodegradation mechanisms and testing for
intermediate and end products of biodegradation. Laboratory studies of biodegradation by mixed
cultures of microbes are more relevant for determining biodegradability under ambient conditions;
however, the conditions under which tests are performed are usually not representative of ambient
conditions (Boethling et al., 1994). For example, temperatures in laboratory microcosm studies are
elevated, l ow rates are increased, growth media and nutrient solutions are enhanced, and high con-
centrations of the target contaminant are used to accelerate the process and complete the study
within a reasonable time frame. When chemicals are present at trace concentrations, microorgan-
isms may not assimilate carbon from the chemicals, and they will not grow or produce the large,
acclimated populations needed for enhanced biodegradation (Alexander, 1985). Tests representing
ambient conditions may take a long time to complete. Data from different studies may not be com-
parable where different environmental conditions are used.
Microorganisms in groundwater are more often found attached to grains of aquifer material than
in l owing groundwater, and most microbes are larger than the smallest pores in most clays; conse-
quently, hydrophilic contaminants that have diffused into very small pores or intraparticle voids
may not be subjected to biodegradation. Microbes generally mediate biotransformation that is ener-
getically favorable. Biotransformation reactions produce a net decrease in the free energy of the
chemical system, and the microbes harvest some of the released energy for their own use, stimulat-
ing their growth (Hemond and Fechner, 1994).
Microorganisms responsible for biodegradation include heterotrophic bacteria, including actino-
mycetes, some autotrophic bacteria, fungi including yeasts and basidiomycetes, and some protozoa.
Fungi are less able to utilize recalcitrant compounds as growth substrates than are the bacteria,
which possess greater metabolic capacity. Fungi function only aerobically, whereas bacteria may
evolve to metabolize only in aerobic systems, only in anaerobic systems, or in both environments
(i.e., facultative organisms) (Hemond and Fechner, 1994).
Most compounds become usable as an energy or nutrient source only after a period of acclima-
tion during which the population capable of metabolizing the substance grows to a threshold where
appreciable degradation becomes noticeable. The acclimation of a microbial community to a chemi-
cal may not occur below a threshold concentration; low concentrations of contaminants may be
resistant to biodegradation (Alexander, 1994).
Acclimation also involves existing microbes adapting through induction of enzymes that cata-
lyze biodegradation. The number of steps in the biodegradation sequence for a compound will affect
the rate of biodegradation (Scow, 1982). Induction is the series of cellular processes by which
microbes produce enzymes specii c to a particular substrate when that substrate is actually present
(Chappelle, 1993). Products generated during the catabolism * of one substrate may repress the
* Catabolism is the metabolic process that breaks down molecules into smaller units.