Amino Acid Degradation by Sulfate-Reducing Bacteria: Evaluation of Four Methods

Four methods were evaluated for estimating the proportion of dissolved free amino acids (DFAAs) metabolized by sulfate-reducing bacteria (SRB). Our main aim was to assess the problems associated with each method, each of which used molybdate (Mo) as an inhibitor of SRB activity. Mo had some side effects that clouded interpretation of the results. Mo treatment did not increase the accumulation rate of protein amino acids, but the nonprotein amino acids $[\delta-amino-valeric-acid (\delta-AVA)$, $\gamma-amino-butyric-acid (GABA)$, $\beta-amino-glutaric-acid (BAGA)$, and $unidentified amines]$ accumulated, resulting in a total amino pool increase of 6.1 nmol $cm^-3 d^-1$. There was no decrease in $NH_4^+$ production rate. SRB appeared to degrade $^14C-labeled$ aspartic acid, serine, glutamic acid, and alanine and were also involved in the degradation of $\delta-AVA$, BAGA, GABA, and taurine. Mo appeared to release DFAAs which were either bound to sediment particles or were from killed SRB. These additional DFAAs complicated comparisons between incubations with and without Mo. We suggest that in the case of Mo-amended incubations, surplus DFAAs were metabolized by fermenting bacteria, resulting in no accumulation of protein DFAA and an increased $NH_4^+$ production rate. The uninhibited rate of $NH_4^+$ production $(111 nmol cm^-3 d^-1)$ indicated that SRB were responsible for $\thicksim 5%$ of this rate.