Microbial processes affecting alanine and glutamic acid in anoxic marine sediments

The utilization of glutamic acid and alanine in anoxic marine sediments was studied using sediments collected from a tidal flat on the Atlantic Ocean side of Virginia's Eastern Shore (Bordens-take Bay) and a site in the mid-Chesapeake Bay. At both sites volatile fatty acids (VFAs) were produced as intermediates in the catabolism (or oxidation) of both amino acids. In contrast, methylated amines were not produced as metabolic intermediates from either amino acid. The addition of 20 mM molybdate to sediment slurries (to inhibit bacterial sulfate reduction) led to the continuous production of VFAs from both amino acids, indicating that the majority of this VFA production from these amino acids occurred by fermentative processes. Non-catabolic uptake of these amino acids (presumably into bacterial bio-mass) also appeared to be an important process in removing alanine and glutamic acid from these sediment slurries. A kinetic model used to analyze these data indicated that ∽ 85% of the alanine catabolism occurred by fermentative processes, with remineralization by sulfate reducing bacteria accounting for the difference. In contrast, all of the glutamic acid catabolism appeared to occur by fermentation. Calculations using data on VFA and ΣCO2 production in molybdate inhibited sediments also suggested that acetate and formate were the predominant VFAs produced by the fermentation of alanine, and perhaps glutamic acid as well. The oxidation of dissolved, free amino acids appeared to account for a significant fraction of the ammonium production in these anoxic marine sediments, although amino acids represented less than ∽ 2% of the carbon sources/electron donors used by sulfate reduction. These observations suggest that the general pathway of amino acid utilization in anoxic sediments involves their oxidation by fermentative bacteria to produce compounds such as VFA or H2 which are then themselves used as substrates by either sulfate reducing or methanogenic bacteria. As such, dissolved free amino acids appear to play an important role as intermediates in carbon and nitrogen cycling in these environments.