Diagenesis of amino compounds in water column and sediment of Lake Baikal

Abstract

The diagenesis of amino compounds in Lake Baikal was studied in sediment trap material (18 different water depths between 50–1350m) and underlying sediment (0–40 cm). Total hydrolysable amino acids (THAA), the amino acid (AA) composition including d- and l-enantiomers and amino sugars were analyzed. The study provides information on early diagenesis (<1 year) of settling material as well as on longer-term diagenesis (up to 600 years) occurring in the sediment. AA based diagenetic indicators were successfully applied to reveal changes at the different diagenetic stages. With increasing water and sediment depth, consistent decreases were found in: (1) the contributions of AA to total organic carbon (%TAAC, from 20 to 7%) and total nitrogen (%TAAN, from 54 to 20%); (2) the ratios of protein AA and their respective non-protein degradation products (aspartic acid:β-alanine, from 15 to 2; glutamic acid:γ-aminobutyric acid, from 28 to 2; arginine:ornithine, from 19 to 2); and (3) the AA based degradation index (from 0.7 to −1.1). There was multiple evidence that diagenesis went along with a progressive transformation of phytoplankton and zooplankton into bacterial organic material. Glycine, which is enriched in bacterial peptidoglycan, increased in relative abundance and bacteria-derived d-AA preferentially accumulated over proteinaceous l-AA. Molar ratios of glucosamine and galactosamine decreased towards a ratio of 1.0 ± 0.1 in the sediment, similar to ratios previously reported for detrital marine organic material. The general pattern of amino compound diagenesis in Lake Baikal resembles the pattern observed in marine systems, indicating a universal character of the processes controlling diagenesis in aquatic environments.