Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 167

desirable reaction because it generally results in completely mineralized end products
(inorganic compounds). In contrast, photochemical and other nonmetabolic processes
usually result in only slight modifications in the parent compound. 156
Biodegradation is the process by which aerobic or anaerobic microorganisms
break down organic chemicals to either a higher- or a lower-molecular-weight
chemical(s) called a biodegrade(s). Soil and aquatic microorganisms can detoxify
chemicals, but can also sometimes form more toxic chemical compounds. Typical
degrading microorganisms are heterotrophic bacteria, actinomycetes, autotrophic
bacteria, fungi, and protozoa. 45 Organisms responsible for biodegradation display a
spectrum of interactions, ranging from predation to synergism. The presence/absence
of predators, parasites, or hosts causes microbial population to fluctuate over a wide
range.
Guidelines used to predict whether biodegradation can occur include chemicals
that are highly water soluble can biodegrade, but those with low water solubility
will not; chemicals that adsorb in soil will usually not biodegrade, but those that do
not adsorb can; chemicals with a high K ow will usually not biodegrade, but those
with a low K ow can; and chemicals that leach in soil will usually biodegrade, but
those that do not leach usually will not. 137
Two general types of biodegradation are recognized: growth metabolism and
cometabolism. Growth metabolism occurs when the organic compound serves as a
food source for bacteria. Adaptation times from 2 to 20 days were suggested by
Mills et al. 165 Adaptation may not be required for some chemicals or in chronically
exposed environments. Adaptation times can be lengthy in environments with a low
initial density of degraders. 165 For cases where biodegradation is limited by the
degrader population size, adaptation is faster in high density microbial populations
and slower in the low initial density populations. Following adaptation, biodegrada-
tion proceeds at the first-order rate. Co-metabolism occurs when the organic com-
pound is not a food source for the bacteria. Here, adaptation is seldom necessary,
and the transformation rates are slow compared to growth metabolism.
Biodegradation also encompasses the broad and complex processes of enzymatic
attack by organisms on organic chemicals. Dehalogenation, dealkylation, hydrolysis,
oxidation, reduction, ring cleavage, and condensation reactions are all known to
occur either metabolically or via organisms that are not capable of utilizing the
chemical as a substrate for growth. 78 It also encompasses a number of distinctly
different processes denoted by 45

• Mineralization—the conversion of an organic compound to inorganic
products;
• Detoxication—the conversion of a toxicant to innocuous byproducts;
• Co-metabolism—the metabolism of a compound that organisms cannot
use as a nutrient that does not result in mineralization (organic metabolites
remain);
• Activation—the conversion of a nontoxic substance to a toxic one or an
increase in a substance's toxicity, due to microbial action;

• Defusion—the conversion of a potential toxicant into a harmless metab-

olite before its potential is realized.