Assessing
the biodegradation of organic compounds is a frequent
question in environmental science. Here, we present a sensitive, inexpensive,
and simple approach to monitor microbial mineralization using reverse
stable isotope labeling analysis (RIL) of dissolved inorganic carbon
(DIC). The medium for the biodegradation assay contains regular organic
compounds and 13C-labeled DIC with 13C atom
fractions (x(13C)DIC) higher
than natural abundance (typically 2–50%). The produced CO2 (x(13C) ≈ 1.11%) gradually
dilutes the initial x(13C)DIC allowing to quantify microbial mineralization using mass-balance
calculations. For 13C-enriched CO2 samples,
a newly developed isotope ratio mid-infrared spectrometer was introduced
with a precision of x(13C) < 0.006%.
As an example for extremely difficult and slowly degradable compounds,
CO2 production was close to the theoretical stoichiometry
for anaerobic naphthalene degradation by a sulfate-reducing enrichment
culture. Furthermore, we could measure the aerobic degradation of
dissolved organic carbon (DOC) adsorbed to granular activated carbon
in a drinking water production plant, which cannot be labeled with 13C. Thus, the RIL approach can be applied to sensitively monitor
biodegradation of various organic compounds under anoxic or oxic conditions.
Biological activated carbon (BAC) filters are frequently used in drinking water production for removing dissolved organic carbon (DOC) via adsorption of organic compounds and microbial degradation. However, proper methods are still missing to distinguish the two processes. Here, we introduce reverse stable isotope labelling (RIL) for assessing microbial activity in BAC filters. We incubated BAC samples from three different BAC filters (two granular activated carbon- and one extruded activated carbon-based) in a buffer amended with
13
C-labelled bicarbonate. By monitoring the release of
12
C–CO
2
from the mineralization of DOC, we could demonstrate the successful application of RIL in analysing microbial DOC degradation during drinking water treatment. Changing the water flow rates through BAC filters did not alter the microbial activities, even though apparent DOC removal efficiencies changed accordingly. Microbial DOC degradation activities quickly recovered from backwashing which was applied for removing particulate impurities and preventing clogging. The size distributions of activated carbon particles led to vertical stratification of microbial activities along the filter beds. Our results demonstrate that reverse isotope labelling is well suited to measure microbial DOC degradation on activated carbon particles, which provides a basis for improving operation and design of BAC filters.
Many phage genes lack sequence similarity to any other open reading frame (ORF) in current databases. These enigmatic ORFan genes can have a tremendous impact on phage propagation and host interactions but often remain experimentally unexplored. We previously revealed a novel interaction between phage P22 and its Salmonella Typhimurium host, instigated by the ORFan gene pid (for phage P22 encoded instigator of dgo expression) and resulting in derepression of the host dgoRKAT operon. The pid gene is highly expressed in phage carrier cells that harbor a polarly located P22 episome that segregates asymmetrically among daughter cells. Here, we discovered that the pid locus is fitted with a weak promoter, has an exceptionally long 5′ untranslated region that is instructive for a secondary pid mRNA species, and has a 3′ Rho-independent termination loop that is responsible for stability of the pid transcript.
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