Page 455 - Corrosion Engineering Principles and Practice

422 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 423

hydrogenase enzyme that is produced by bacteria able to use
hydrogen as an energy source. The test is usually performed on
sessile samples by exposing a sample to an enzyme-extracting
solution and monitoring the degree of hydrogen oxidation in an
oxygen-free atmosphere after adding a dye [19].
Performance of several of these kits has been assessed by field
personnel in round robin tests. Correlation of activity assays and
population estimates is variable. In general these kits have a narrower
range of application than growth-based assays and it is therefore
important to select a kit with a detection range appropriate to the
problem under consideration [25].

Metabolites Adenosine triphosphate (ATP) is present in all living
cells but it disappears rapidly on death. A measure of ATP may thus
quantify the presence of living material. The ATP can be measured
using an enzymatic reaction, which generates flashes of light that are
detected by a photomultiplier [19]. Commercial instruments are
available that measure the release of light by the firefly luciferin/
luciferase with ATP. The method is best suited to clean aerobic
aqueous samples since suspended solids and chemical quenching
can affect the results. Detection of metabolites such as organic acids
in deposits or gas compositions including methane or hydrogen
sulfide by routine gas chromatography can also indicate biological
involvement in industrial problems [9].
Cell Components Biomass may be generally quantified by assays for
protein, lipopolysaccharide, or other common cell constituents but
the information gained is of limited value. An alternate approach is to
use cell components to define the composition of microbial populations
with the hope that the insight gained may allow future damaging
situations to be recognized and managed. Fatty acid analysis and
nucleic acid sequencing provide the basis for the most promising
methods in this category.

Fatty Acid Profiles Analyzing fatty acid methyl esters derived from
cellular lipids can fingerprint organisms rapidly and, provided perti-
nent profiles are known, organisms in industrial and environmental
samples may be identified with confidence. The immediate impact of
events such as changes in operating conditions or the application of
biocides can be monitored by such analysis. Problem populations of
certain organisms may also be identified in order to implement appro-
priate management responses in a timely fashion.
Nucleic Acid–Based Methods Specific DNA probes have been
developed to detect segments of genetic material coding for known
enzymes. A gene probe developed to detect the hydrogenase enzyme
which occurs broadly in SRB from the genus Desulfovibrio has been
tested on samples from an oilfield waterflood plagued with iron-sulfide
related corrosion problems. The enzyme was detected with this probe