- UV-induced transformation of dissolved
organic matter (DOC) is often accompanied by reduction
of molecular weight and aromaticity and an increase of
low-molecular weight (LMW) matter that can be utilized as
a substrate by heterotrophic bacteria. Moreover, the generation
of reactive transients and mineralization of DOC
occurs. For a better understanding of the modification that
starts after irradiation and to distinguish between possible
chemical and biological modifications, we selected different
natural organic matter (NOM) from Norway and
Germany. The aqueous solutions were treated by UV
irradiation and divided into two aliquot samples. NaN3
anti-bacterial treatment was applied to one sample, and
high-pressure size-exclusion chromatography (HPSEC)
analysis was used for both. In all samples, we found typical
modifications of NOM after UV irradiation. Incubation
(>7 days) of UV-irradiated NOM samples resulted in
lower levels of LMW matter and increased aromaticity.
Parallel to theseUV-induced transformation of dissolved
organic matter (DOC) is often accompanied by reduction
of molecular weight and aromaticity and an increase of
low-molecular weight (LMW) matter that can be utilized as
a substrate by heterotrophic bacteria. Moreover, the generation
of reactive transients and mineralization of DOC
occurs. For a better understanding of the modification that
starts after irradiation and to distinguish between possible
chemical and biological modifications, we selected different
natural organic matter (NOM) from Norway and
Germany. The aqueous solutions were treated by UV
irradiation and divided into two aliquot samples. NaN3
anti-bacterial treatment was applied to one sample, and
high-pressure size-exclusion chromatography (HPSEC)
analysis was used for both. In all samples, we found typical
modifications of NOM after UV irradiation. Incubation
(>7 days) of UV-irradiated NOM samples resulted in
lower levels of LMW matter and increased aromaticity.
Parallel to these changes of carbon fractions, an increase in
bacterial cell numbers was observed. Addition of NaN3 to
NOM, however, inhibited the reduction of LMW matter,
indicating that microbial activity accounted for the
observed changes in NOM. Analysis of the bacterial
community composition by denaturing gradient gel electrophoresis
(DGGE) of the amplified 16S rRNA genes
revealed that bacterial communities of non-irradiated and
UV-irradiated NOM were different and that UV selected
for specific members of α-proteobacteria, β-proteobacteria,
and Bacteriodetes. Our results imply that after UV-irradiation
of NOM, specific bacterial members are well adapted
to low pH, high LMW DOC concentrations, and oxidative
stress, and therefore thrive well on UV-irradiated humic
matter.…
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