AMINO-ACID DEGRADATION BY SULFATE-REDUCING BACTERIA - EVALUATION OF 4METHODS

Four methods were evaluated for estimating the proportion of dissolved free amino acids (DFAAs) metabolized by sulfate-reducing bacteria (SR B). Our main aim was to assess the problems associated with each metho d, each of which used molybdate (Mo) as an inhibitor of SRB activity. Mo had some side effects that clouded interpretation of the results. M o treatment did not increase the accumulation rate of protein amino ac ids, but the nonprotein amino acids [delta-amino-valeric-acid (delta-A VA), 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,+ prod uction rate. SRB appeared to degrade C-14-labeled aspartic acid, serin e, glutamic acid, and alanine and were also involved in the degradatio n 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 wi th and without Mo. We suggest that in the case of Mo-amended incubatio ns, surplus DFAAs were metabolized by fermenting bacteria, resulting i n no accumulation of protein DFAA and an increased NH4+ production rat e. The uninhibited rate of NH4+ production (111 nmol cm(-3) d(-1)) ind icated that SRB were responsible for similar to 5% of this rate.