Changes in the D- and L-content of aspartic acid, glutamic acid, and alanine in a scleractinian coral over the last 300 years

The D- and L-contents of aspartic acid (Asp), glutamic acid (Glu), and alanine (Ala) together with absolute and relative concentrations of 17 amino acids (AAs) were determined in samples of aragonite skeletons of a scleractinian coral from the Caribbean through the last 308 years. Regular patterns of increasing D/L ratios with increasing age are seen for the last 300 years. High and linear racemization rates occur through the first 250 years in Asp, the first 150 years in Glu, and the first 250 years in Ala. An evaluation of the utility of the D/L ratios of these amino acids as a chronological tool through the last 250 years yields standard errors for individual age estimates ranging between 1.7 and 6.4 years. Thereafter the racemizations slow down and in the 308-years old terminal sample the D/L values are discordant. A rapid decrease in the total amino acid concentration of 33% over approximately the last hundred years is identified. The rapid rate of decrease in Asp and Glu may be responsible for this decrease in the total AA concentration. The relative concentrations of Asp, Glu, and Ala (g AA/100 g protein) show a general pattern of decrease, although not significant, through the time interval analyzed. A regularity of the decrease in L-contents and an irregularity of the decrease in D-contents are observed. This may imply that, in addition to possible diagenetic alterations and/or leaching of amino acids, contamination of D-amino acids explains the discordant D/L values. The D-amino acids could originate from peptidoglycans in bacterial cell walls.