Chemical characteristics of amino acid-containing dissolved organic matter in seawater

Total hydrolyzable amino acid and fluorescence properties, using 3-dimensional excitation and emission matrix (3DEEM) spectroscopy of the bulk and four molecular mass fractions (GF/F–0.1 μm, 0.1 μm–10 kDa, 10 kDa–5 kDa and <5 kDa) of dissolved organic matter (DOM), were determined in the surface (10 m) and deep (1000 m) waters of Sagami Bay, Japan. Among the molecular mass fractions, we found a successive decrease in degradation index (DI) derived from amino acid composition with decreasing molecular mass in both surface and deep waters, indicating that relatively fresh and large molecules become smaller in size along a degradation pathway. In the 3DEEM fluorescence patterns of high molecular mass DOM, only a tryptophan-like fluorophore was observed in the surface water. In contrast, both tryptophan- and tyrosine-like fluorophores were observed in the deep water, indicating that amino acid-containing DOM in the high molecular mass fractions was comprised of protein molecules or least degraded proteins (polypeptides) in surface water, but in deep water other forms of amino acids were dominant even though the molecular mass fractions were of the same size. We hypothesize that degradation products (peptides) form high molecular mass fraction in deep water DOM.