Alanine enantiomeric ratio in the combined amino acid fraction in seawater

Abstract

In deep ocean waters the concentrations of free amino acids are very close to blank levels, while surface waters contain only very low quantities^1,2. However, the concentration of ‘combined’ amino acids (whoch occur at all ocean depths in significant quantities) is roughly 10 times that of free amino acids^1,2. These combined amino acids are bound in some sort of polymeric material and are released only on hydrolysis in strong acid. The combined amino acid material apparently has a substantial D-aspartic acid content; the D/L aspartic acid ratio in the combined amino acid fraction, isolated from water from the Sargasso Sea, is ∼0.4 at all depths². It has been suggested that combined amino acids were not actually part of the dissolved organic material in the oceans but were components of bacteria that passed through the filter used in the initial seawater-processing step^2,3. Although in most organisms only the L-enantiomers of the amino acids are present, becateria contain several D-amino acids in then cell-wall proteins⁴. To verify the bacterial explanation we have carried out investigations into the enantiomeric composition of amino acids, besides aspartic acid, in the combined fraction. We measured the alanine enan-tiomeric ratio in seawater samples from three stations in the Pacific Ocean. Alanine was chosen because it is one of the more abundant components of the combined amino acid fraction in the oceans^1,2, and because D-alanine is a major component of bacterial cell-wall proteins⁴.