Distribution and alteration of amino acids in bulk DOM along a transect from bay to oceanic waters

A systematic survey of the concentrations and composition of total hydrolyzable amino acids (THAA) in bulk dissolved organic matter (DOM) was conducted at 11 stations along the 137°E transect from bay to oceanic areas in the northwestern Pacific. Concentrations of THAA and their contributions to dissolved organic carbon were high in the bay and coastal areas, declined toward the oceanic area and decreased with depth in the water columns. From the distribution patterns of the relative abundances of amino acids along the transect, individual amino acids were divided into four groups. One group included tyrosine, valine, isoleucine, phenylalanine, leucine and tryptophan, and was considered to represent easily degradable THAA, while glycine and alanine belonged to a more biorefractory group of THAA. Principal components analysis (PCA) was conducted to quantitatively differentiate patterns of amino acid composition. Amino acid groups based on PCA agreed with the groups classified by distribution patterns, indicating that first principal component scores reflected the degree of degradation of THAA in DOM, and were defined as a degradation index (DI). Two amino acids, glycine and alanine, increased in relative abundance with increasing DI, while valine, isoleucine, phenylalanine and leucine decreased with decreasing DI. The agreement indicated that the degradation process was the key factor controlling the quantity and quality of THAA in bulk DOM.