CENSUS – Bacterial Groups Table

Redox conditions and microbial populations are intrinsically coupled in that availability of electron acceptors influences the microbial community composition and microbial processes in turn impact site geochemistry.  The link between site microbiology and geochemistry then plays a governing role in the microbial metabolism and therefore the processes responsible for contaminant biodegradation.  In the most general terms, microbial metabolism is an oxidation/reduction reaction where oxidation of one compound (electron donor) is coupled to reduction of another compound (electron acceptor).  Depending on the contaminant of concern and the biodegradation process involved, site contaminants can serve as either an electron donor or electron acceptor.  Compounds that are already in a reduced state like BTEX and PAHs are more readily oxidized and typically serve as electron donors.  Oxidation of BTEX, however, must be coupled to reduction of a terminal electron acceptor such as oxygen (DO) or biodegradation will be limited.  Conversely, highly oxidized compounds like PCE and TCE can serve as electron acceptors provided a suitable electron donor is present.  However, reductive dechlorination of PCE and TCE can be limited by the presence of competing electron acceptors (DO, nitrate, etc.) that may also be present in the subsurface.  While quantification of electron acceptor concentrations provides valuable information, analysis of site geochemistry can at times be convoluted.  CENSUS analysis provides direct quantification of the bacterial groups responsible for dominant terminal electron accepting processes that compliments geochemical analysis and CENSUS quantification of contaminant degrading bacteria.

CENSUS – Bacterial Group Targets include:

Target

MI Code

Relevance / Data Interpretation

Total Eubacteria qEBAC  Targets universal regions of bacterial 16S rRNA genes to provide a broad index of total bacterial biomass.
Denitrifying Bacteria qDNF  Under anoxic conditions, nitrate reduction can be a dominant terminal electron accepting process. The DNF assay quantifies the two types of nitrite reductase genes (nirS and nirK, respectively) encoding the second key step in denitrification.  While important in any situation where nitrate is serving as the dominant electron acceptor, quantification of denitrifying bacteria using the DNF assay can be critical at perchlorate impacted sites.  Many, but not all, perchlorate reducing bacteria will utilize nitrate as an electron acceptor potentially limiting perchlorate reduction.
Archaeal Denitrifying Bacteria
qADNF  Similar to the qDNF assay, qADNF quantifies the two types of nitrite reductase genes (nirS and nirK) found in archaeal organisms.
Geobacter qGEO  This assay quantifies iron reducing Geobacter species often implicated in biological metals reduction.
Sulfate Reducing Bacteria qAPS  Sulfate reduction can be an extremely important terminal electron accepting process in a number of field applications.  For example, addition of sulfate-containing solids (e.g. gypsum) is sometimes used to promote anaerobic BTEX biodegradation.  At sites impacted by chlorinated compounds, extensive sulfate reduction can be a competing electron accepting process limiting reductive dechlorination.  The qAPS assay targets the dissimilatory adenosine-5′-phosphosulfate (APS) gene to quantify sulfate reducing bacteria.
Methanogens qMGN  Under extremely reducing conditions, methanogens utilize hydrogen and CO2 to produce methane.  Depending on the type of contaminant and the desired microbial process, quantification of methanogens can provide valuable information.  For example, methanogens can compete with dechlorinating bacteria such as Dehalococcoides for available hydrogen.  Conversely, maintenance of methanogens can be an important aspect of bioremediation at some sites.  Methanogens can produce methane to support co-metabolic degradation of chlorinated solvents by methanotrophic (methane-oxidizing) bacteria.