Page 454 - Corrosion Engineering Principles and Practice

420 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 421

(MPN) [21] are made. The test can show some results in a few days,
but the usual incubation period for the test is 14 to 28 days [19].
However, despite the common use of these assays, only a small
fraction of wild organisms actually grow in commercially available
artificial media. For example, estimates for SRB in marine sediments
indicate that only one in a thousand of the organisms present actually
show up in standard growth tests [22].
Activity Assays

Whole Cell The conversion of a radioisotopically labeled substrate
can be used to assess the potential activity of microbial populations in
field samples. The technique depends on bacterial growth for detection,
but it generates results in about two days and is specific to SRBs. The
sample is incubated with a known trace amount of radioactive-labeled
sulfate. (SRBs reduce sulfate to sulfide.) After incubation, the reaction
is terminated by adding an acid to kill the cells and the radioactive
sulfide is fixed with zinc acetate for evaluation. This is a highly
specialized technique, involving expensive laboratory equipment and
the handling of radioactive substances [19].
The radiorespirometric method, however, can use field samples
directly without the need to separate organisms and it is very
sensitive. Selection of the radioactively labeled substrate is key to
interpretation of the results. The method can provide insights into
factors limiting growth by comparing activity in native samples with
supplemented test samples under various conditions. Oil degrading
organisms, for example, can be assessed through the mineralization
of C labeled hydrocarbon to carbon dioxide.
14
Radioactive methods are not routinely used by field personnel.
However, they have been particularly useful in a number of
applications including biocide screening programs, identification of
nutrient sources, and assessment of key metabolic processes in
various corrosion situations [9].

Enzyme-Based Assays An increasingly popular approach is the use
of commercial kits to assay the presence of enzymes associated with
microorganisms suspected to cause problems. For example, kits are
available for the sulfate reductase enzyme common to SRBs [23]. This
technique takes advantage of the fact that SRBs reduce sulfate to
sulfide through the APS-reductase enzyme, common to all SRBs.
Measurement of the amount of APS-reductase in a sample gives an
estimate of SRBs present. The test does not require bacterial growth
and the entire test takes only 15 to 20 minutes [19].
Another example is the hydrogenase enzyme implicated in the
acceleration of corrosion through rapid removal of cathodic
hydrogen formed on the metal surface [24]. The test analyzes for the