3.1.1. Molecular evidence Ammonium was previously shown to degrade in the subsurface of a treatment wetland at metabolically significant levels (Powell,2010). The same study also indicated the potential to couple the ammonium oxidation to TCE biodegradation by aerobic cometabolism in shallow vegetated subsurface. Of interest then was the question of whether ammonium-oxidizing bacteria (AOB) are present in association with wetland plant roots in the shallow subsurface.Two approaches were used to gather evidence of AOB presence and activity. The molecular biology approach yielded evidence based on DNA presence. PCR primers were chosen to target the gene encoding the a-subunit of the amoA enzyme (Junier et al., 2008; Park et al., 2008). PCR results demonstrated that amoA gene was present in DNA extracts from the reactors described above(Fig. S1 in SM). As expected, an abundance of the amoA target gene was present in the three live microcosms at both sampling times,while no amoA was detected in the triplicate control microcosms.This provides substantial evidence that AOB were present in live microcosms and inhibited in control microcosms, as intended. A similar approach was taken by Bollmann and Laanbroek (2001),who amplified 16S rDNA from ammonium-oxidizing bacteria cultures,and followed PCR with other techniques to quantify the diversity and microbial community structure. In the present study,techniques other than PCR were not employed since determining the presence or absence of the amoA gene was the desired outcome.By targeting the functional gene (amoA) responsible for ammonium oxidation rather than 16S, as is often done, the results could be relied upon to positively demonstrate the presence of ammonium oxidizers.