Occurrence, Vertical Variations and Degradation Behavior of Dissolved Amino Acids in Seawater of the Northern Yap Trench

Amino acids are widely present as intermediates in marine nitrogen cycle. However, amino acid distributions in deep-sea seawater, especially in abyssal and hadal zones, are very limited. This study investigated the occurrence, vertical variations, and degradation behavior of dissolved free amino acids (DFAA), dissolved combined amino acids (DCAA), and total hydrolyzable amino acids (THAA) in seawater from the sea surface to the hadal zone of the northern Yap Trench. The results showed that concentrations of ΣDFAA, ΣDCAA and ΣTHAA ranged from 0.09 to 1.78, 0.99 to 17.69 and 1.18 to 18.01 μmol/L in the study area, respectively. In the seawater from the trench, glycine was the predominant DFAA, while the DCAA and THAA was dominated by threonine. Mean concentrations of DFAA, DCAA and THAA in different water layers were in the order of: mesopelagic>euphotic>abyssal>bathyal. The concentrations of DCAA and THAA in the sediment-seawater interface were higher than those from the euphotic to abyssal layer. The organic matter (OM) in the seawater of the stations near the Yap Islands were older, while the OM in the stations near the Yap Trench axis was relatively fresh above 1000-m depth. The OM in the sediment-seawater interface is older, especially on the west side of the trench. This is the first systematic survey of DCAA and THAA in the Yap Trench, providing insights into the vertical variations and degradation behaviors of amino acids from the sea surface to the hadal environment.