Contribution of bacterial cells to lacustrine organic matter based on amino sugars and d-amino acids

Abstract

Amino sugars (ASs), d-amino acids (d-AAs), and bacterial cell counts were measured in two Swiss lakes to study the contribution of bacterial cells to organic matter (OM) and the fate of ASs and bacterial amino biomarkers during OM degradation. Concentrations of individual ASs (glucosamine, galactosamine, muramic acid, and mannosamine) in the particulate and total OM pools were analyzed in water-column profiles of Lake Brienz (oligotrophic and oxic throughout the entire water column) and Lake Zug (eutrophic, stratified, and permanently anoxic below 170m) in spring and in fall. Generally, carbon-normalized AS concentrations decreased with water depth, indicating the preferential decomposition of ASs. For Lake Brienz the relative loss of particulate ASs was higher than in Lake Zug, suggesting enhanced AS turnover in an oligotrophic environment. AS ratio changes in the water column revealed a replacement of plankton biomass with OM from heterotrophic microorganisms with increasing water depth. Similar to the ASs, highest carbon normalized d-AA concentrations were found in the upper water column with decreasing concentrations with depth and an increase close to the sediments. In Lake Zug, an increase in the percentage of d-AAs also showed the involvement of bacteria in OM degradation. Estimations of OM derived from bacterial cells using cell counts and the bacterial biomarkers muramic acid and d-AAs gave similar results. For Lake Brienz 0.2–14% of the organic carbon pool originated from bacterial cells, compared to only 0.1–5% in Lake Zug. Based on our estimates, muramic acid appeared primarily associated with bacterial biomass and not with refractory bacterial necromass. Our study underscores that bacteria are not only important drivers of OM degradation in lacustrine systems, they also represent a significant source of OM themselves, especially in oligotrophic lakes.