Distribution Characteristics of Dissolved Amino Acids in Seawater of the Yap Trench

Abstract: Amino acids are widely present as intermediates in marine nitrogen cycle. However, the information of amino acid distribution in deep-sea seawater, especially in abyss and hadal zone, is very limited. In the present study, we determined the concentrations of dissolved free amino acids (DFAA), dissolved combined amino acids (DCAA) and total hydrolysable amino acids (THAA) in the seawater from sea surface to hadal zone of the Yap Trench. Their occurrence, vertical variations and degradation behavior in the Yap Trench were investigated. 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, respectively, in the study area. In the seawater samples from the trench, Glycine (Gly) was the predominant DFAA, while the DCAA and THAA was dominated by threonine (Thr). Concentration of chlorophyll a (chl a) was not significantly correlated with any amino acids (DFAA, DCAA and THAA) in the euphotic layer. Mean concentrations for DFAA, DCAA and THAA in different water layers were in the order of: mesopelagic > euphotic > abyssal > bathyal. The concentrations of DCAA and THAA in sediment-seawater interface were higher than those from the euphotic to abyssal layer. Stations near the Yap Islands have the older organic matter (OM), while the organic matter in stations near the Yap Trench axis was relatively fresh above 1000-m depth. Degradation behavior of OM in the mesopelagic and abyssal layer are affected by transport of dissolved organic matter with water current. The organic matter in the sediment-seawater interface are older, especially on the west side of the trench. This is a survey of amino acids (DFAA, DCAA and THAA) in the Yap Trench, which provides information on the vertical variations and degradation behavior of amino acids from sea surface to hadal environment, and will be helpful to understand marine organic nitrogen cycle.