Production and transformation of dissolved neutral sugars and amino acids by bacteria in seawater

L Jørgensen,M Middelboe,R Benner,C A Stedmon,O J Lechtenfeld

doi:10.5194/bg-11-5349-2014

Abstract

Abstract. Dissolved organic matter (DOM) in the ocean consists of a heterogeneous mixture of molecules, most of which are of unknown origin. Neutral sugars and amino acids are among the few recognizable biomolecules in DOM, and the molecular composition of these biomolecules is shaped primarily by biological production and degradation processes. This study provides insight into the bioavailability of biomolecules as well as the chemical composition of DOM produced by bacteria. The molecular compositions of combined neutral sugars and amino acids were investigated in DOM produced by bacteria and in DOM remaining after 32 days of bacterial degradation. Results from bioassay incubations with natural seawater (sampled from water masses originating from the surface waters of the Arctic Ocean and the North Atlantic Ocean) and artificial seawater indicate that the molecular compositions following bacterial degradation are not strongly influenced by the initial substrate or bacterial community. The molecular composition of neutral sugars released by bacteria was characterized by a high glucose content (47 mol %) and heterogeneous contributions from other neutral sugars (3–14 mol %). DOM remaining after bacterial degradation was characterized by a high galactose content (33 mol %), followed by glucose (22 mol %) and the remaining neutral sugars (7–11 mol %). The ratio of D-amino acids to L-amino acids increased during the experiments as a response to bacterial degradation, and after 32 days, the D/L ratios of aspartic acid, glutamic acid, serine and alanine reached around 0.79, 0.32, 0.30 and 0.51 in all treatments, respectively. The striking similarity in neutral sugar and amino acid compositions between natural (representing marine semi-labile and refractory DOM) and artificial (representing bacterially produced DOM) seawater samples, suggests that microbes transform bioavailable neutral sugars and amino acids into a common, more persistent form.

Highlights

700 petagrams (1015 g) of carbon in the ocean are in the form of dissolved organic matter (DOM) and consist of a broad range of different chemical compounds spanning a continuum of sizes and reactivities (Hansell, 2013; Siegenthaler and Sarimento, 1993)
The molecular compositions of combined neutral sugars and amino acids were investigated in Dissolved organic matter (DOM) produced by bacteria and in DOM remaining after 32 days of bacterial degradation
Results from bioassay incubations with natural seawater and artificial seawater indicate that the molecular compositions following bacterial degradation are not strongly influenced by the initial substrate or bacterial community

Summary

Introduction

700 petagrams (1015 g) of carbon in the ocean are in the form of dissolved organic matter (DOM) and consist of a broad range of different chemical compounds spanning a continuum of sizes and reactivities (Hansell, 2013; Siegenthaler and Sarimento, 1993). All organic matter originates from organisms, only about 6.6 % of surface DOM and 2 % of deep ocean DOM is identified as specific biomolecules such as neutral sugars and amino acids (Benner, 2002). Despite their low concentrations, the rapid turnover of simple biomolecules suggests that they play an important role in the cycling of carbon and nitrogen in the ocean (Rich et al, 1997, 1996; Skoog et al, 1999). This is in part due to the low concentration of individual

Methods

Results

Conclusion