Bacterial influence on amino acid enantiomerization in a coastal marine sediment

The contribution of Dߚisomers of aspartate (Asp), glutamate (Glu), alanine (Ala), and serine (Ser) to the total concentrations of dissolved free amino acids (DFAA), dissolved combined amino acids (DCAA), and total hydrolyzable amino acids (THAA) was studied in a <2,300ߚyrߚold coastal sediment. Concentrations of the different amino acid pools were typically 5 (DFAA), 100 (DCAA), and 20,000 nmol cmߚ3 (THAA), but variable amounts occurred, especially in the surface layer. Dߚamino acids were identified in all three amino acid fractions. The highest concentration of Dߚisomers occurred in the THAA fraction at a depth of 10 cm, where the maximum bacterial density has been observed (Jørgensen et al. 1990; Mar. Ecol. Prog. Ser. 59: 39–54). This coincidence suggests that the Dߚamino acids originated in peptidoglycan of bacterial cell walls. The proportion of DߚAsp, DߚGlu, DߚAla, and DߚSer increased through the pools of DFAA and DCAA to THAA, suggesting a selective removal of Lߚamino acids to Dߚamino acids. The percentage of the four Dߚamino acids in the THAA pool increased with sediment depth to at least 4.3 m (∼2,150 yr). This most likely indicated that peptidoglycan THAA were retained for a longer period of time in a particulate form compared to other proteinaceous material. The contribution of THAA from peptidoglycan to the THAA pool was estimated to range from >9% at the surface to at least 18% at 4.3 m deep, depending on the assumptions used in the calculation. The distribution of amino acid isomers in Aarhus Bay sediment demonstrated that bacterial peptidoglycan contributed to the pool of organic nitrogen (the four analyzed Dߚamino acids made up about 3% of particulate organic nitrogen [PON]). Although the peptidoglycan concentration increased with sediment depth, our observations do not indicate a longߚterm accumulation of peptidoglycanߚderived amino acids.