alkane degrading enrichment culture. The enrichments were used
for comparison supposing that a high fraction of all species are
alkane degraders. Thus, we assumed that 16S gene rRNA profiles
of the enrichments better reflected the phylogenetic richness of
alkane degraders actually present in the microcosms (being aware
of diversity biases due to the selection pressure of enrichment
conditions and the possibility of co-enrichment of an accompanying
community of non-alkane degraders [3]). The enrichment
certainly favored also species which might occur in low abundances
in the original soil samples. Thus, they potentially escaped
in fingerprinting analyses of soil DNA extracts. This also indicated
that alkane degradation might be an even more common trait in
soil than formerly expected all the more complicating the measurements
of diversity changes by diversity indices due the
enhanced community complexity [37]. Most congruent results
between the enrichment and direct soil DNA extracts were
observed for two diversity metrics (i.e. Simpson, Equitability)
which were formerly described to be relatively robust to method
variations and biases [37]. These might be best suited when
changes in complex communities were determined with molecular
fingerprinting tools.
As also advised earlier [37,43], we refrained from deriving
absolute quantification of e.g. species by richness indices from the
fingerprints. However, as suggested by Danovaro et al. [44]
applying diversity metrics to fingerprinting data gives the opportunity
for monitoring relative changes in microbial communities
to a certain extent when keeping in mind their indicated
lack in sensitivity [45]. Accordingly, we observed that diversity
changes in the alkane degrader community were reflected to a
certain extent by richness indices potentially permitting a relative
comparison among samples. Moreover, the functional and the
phylogenetic T-RFLP measured different levels of diversity
providing robustness to an ecosystem. Whereas the richness of
alkane degrading species influences the ecological range at which
the microbial ecosystem service operates, the richness of the
functional genes is likely to determine the ranges of alkanes
subject to degradation [16,18,46]. The diagnostic power of 16S
rRNA gene fingerprints might thus be regarded as complementary
to that of alkB gene fingerprints.
In conclusion, the thoroughly evaluated alkB gene T-RFLP presented
here permits to sensitively follow changes in the community
patterns of the functional guild of alkB harboring alkane degraders.
This is especially valuable for community analyses in habitats with
low substrate concentrations where alkane degraders might
contribute only a minor fraction of the total bacterial community as
could be expected in many soil environments.