Contribution of Bacterial Cell Wall Components to DOM in Alkaline, Hypersaline Mono Lake, California

Abstract

Soda lakes are often characterized by high densities of prokaryotes and high concentrations of dissolved organic carbon. Since bacterial cell walls are less degradable than most other cell constituents, accumulation of cell wall material may occur in these lakes and contribute to the DOM pool, but composition of DOM in soda lakes has rarely been examined. Here we report concentrations of DOM components likely originating from bacterial cell walls, including D amino acids, glucosamine (GluA) and muramic acid (MurA), in depth profiles of stratified, alkaline, hypersaline Mono Lake, CA. Concentrations of cell wall components were related to total pools of dissolved free and combined amino acids (DFAA and DCAA), and bacterial density and production. In the free pool, total DFAA ranged from 50 to 3250 nM and typically increased with depth, while GluA (5 to 140 nM) and MurA (< 0.5 nM and only detected in 2005) fluctuated with depth. In the combined pool, DCAA varied between 5000 and 15000 nM and did not show clear depth-related trends. GluA ranged from 1000 to 5000 nM and tended to increase in the hypolimnion, while MurA varied between 25 and 75 nM. Free D isomers in the DFAA pool either made up < 13% (Asp and Ser) or varied from 10 to 57% (Glu and Ala). In the combined pool, D isomers of Asp, Glu, Ser and Ala made up 24-48% of these DCAA and typically showed minor changes with depth. In 2005, lysozyme activity had highest rates in the surface and correlated negatively with most D isomers among the combined amino acids. Our observations demonstrate that the pool of dissolved combined amino compounds in the lake was about 5-fold higher than in other eutrophic lakes and that a substantial portion of these amino compounds originated from bacterial cell walls.