Occurrence and degradation of peptidoglycan in aquatic environments
Niels O.G. J0rgensen, a Ramunas Stepanaukas,b Anne-Grethe
U. Pedersen,c Michael Hansen,a Ole Nybroea
aDepartment of Ecology, Royal Veterinary and Agricultural
University, Thorvaldsensvej 40, DK-1871 Frederiksberg C. Denmark
bDepartment of Ecology/Limnology, Lund University, S-22362
LWid. Sweden
cDepartment of Microbial Ecology, University of Aarhus. Ny
Munkegade. DK-8000 Aarhus C. Denmark
Received 1 December 2002; received in revised form 21 March 2003; accepted
26 April 2003
First published online 27 August 2003
Abstract
Mechanisms controlling microbial degradation of dissolved organic matter
(DOM) in aquatic environments are poorly understood, although microbes
are crucial to global nutrient cycling. Bacterial cell wall components
may be one of the keys in understanding the presence of slowly degrading
DOM in nature. We found that dominant components of bacterial cell walls
(D-atnino acids (D-AA), glucosamine (GluA) and diatninopimelic acid (DAP))
comprised up to 11.4% of the dissolved organic nitrogen in 50 diverse
rivers entering the Baltic Sea. Occurrence of DAPA, a characteristic component
of Gram-negative (G-) bacteria, in the rivers suggests that G- bacteria
rather than Gram-positive (G+) were the major source of the cell wall
material. In laboratory studies, the degradation of whole bacterial cells,
cell wall material and purified peptidoglycan was studied to characterize
degradation of cell wall material by natural aquatic bacteria. Addition
of whole killed G- and G+ bacteria to cultures of estuarine bacteria demonstrated
fragmentation and loss of cell structure of the G+ bacteria, while the
G- bacteria maintained an intact cell shape during the entire 69-day period.
In another experiment, estuarine bacteria degraded 39-69% of GluA, D-AA
and DAPA in added cell wall material of a representative G- baCterial
species during 8 days, as compared to a 72-89"/0 degradation of GluA,
D-AA and DAP A in cell material of a G+ bacterial species. When cultures
ofestuarlne bacteria were enriched with purified G- and G+ peptidoglycan
(1 mg 1-1), at least 49% (G-) and 58"/0 (G+) of D-AA in
the peptidoglycan was degraded. No major changes in GluA were obvious.
Interpretation of the results was difficult as a portion of the purified
peptidoglycan was of sitnilar size to the bacteria and could not be differentiated
from cells growing in the cultures. Addition of the purified peptidoglycan
stimulated the bacterial growth, and after 6 days the cell density in
the enriched cultures was 4-fold higher than in the controls. A regrowth
of bacteria after addition ofL-broth at 105 days caused a 50- to 75-fold
increase in dissolved D-AA and GluA. Most of the D-AA and GluA were taken
up during the following 10 days, indicating that cell wall constituents
are dynamic compounds. Our results show that a variable portion of peptidoglycan
in G- and G+ bacteria can be degraded by natural bacteria, and that peptidoglycan
in G- bacteria is more resistant to bacterial attack than that in G+ bacteria.
Thus, the presence of cell wall constituents in natural DOM may reflect
the recalcitrant nature of especially G- peptidoglycan.
Keywords: Peptidoglycan; Bacterial cell wall; Baltic
river; Degradation; D-amino acid; Glucosamine; Diaminopimelic acid
SREL Reprint #2783
Jørgensen, N. O. G., R. Stepanauskas, A. U. Pedersen, M. Hansen and O. Nybroe. 2003. Occurrence and degradation of peptidoglycan in aquatic environments. FEMS Microbiology Ecology 46:269-280.
