Bacterial release of dissolved organic matter during cell growth and decline: Molecular origin and composition

Heterotrophic bacterial growth and the chemical composition of dissolved organic matter (DOM) produced by bacteria from freshwater and marine environments were monitored during experiments with artificial media containing glucose as the sole carbon source. Glucose was quickly consumed, and DOM was released during bacterial growth. Percentages of extracellular release of DOM from bacteria ranged from 14% to 31%, indicating that bacterial production and growth efficiency are underestimated when only cellular carbon is measured. Relatively high concentrations of D-alanine (D-Ala) were observed in DOM released during exponential growth, whereas the concentrations of muramic acid and other D-amino acid components of peptidoglycan were not detected or were in low concentration. The selective release of D-Ala occurred during cell growth and division when peptidoglycan is cleaved and newly synthesized subunits are incorporated into the cell wall via transpeptidation. Most of the D-Ala released during exponential growth was rapidly consumed. Following exponential growth, bacterial abundance decreased due to grazing and possibly viral lysis. The DOM remaining in the incubations after one or more months included a mixture of D-amino acids commonly found in peptidoglycan and the amino sugars glucosamine and galactosamine, which were highly resistant to decomposition. The percentage of D-amino acids was much higher in DOM than in cells due to the preferential release of D-amino acids and decomposition of L-amino acids. The final concentrations of dissolved organic carbon (DOC) ranged from 20 to 30 mmol L21 regardless of the initial concentration of glucose or the source of inoculum. The observed abundances of D-amino acids and amino sugars in DOM from diverse aquatic environments indicate a bacterial source and common decomposition processes. Heterotrophic bacteria have long been recognized for their critical roles in the transformation and mineralization of organic matter in aquatic and terrestrial environments. It is estimated about half of the photosynthetic production in the ocean is processed by heterotrophic bacteria in the microbial loop (Ducklow 2000). Heterotrophic bacterial production is often 10–20% of primary production in aquatic environments (Cole et al. 1988; Ducklow 2000), but relatively few studies have considered heterotrophic bacteria as an important source of dissolved organic matter (DOM), the most abundant form of organic carbon in most aquatic ecosystems (Hedges 1992; Wetzel 1992). Given the global abundance and production of heterotrophic bacteria (Whitman et al. 1998), it is logical they would be a major source of DOM, but it is difficult to distinguish and quantify the biological origins of DOM in natural environments. The biochemical composition of bacteria is largely the same as other organisms, but several biomolecules that are unique to bacteria can be used to trace their contributions to DOM. Specific bacterial membrane proteins, such as porin P (Tanoue 1995), and specific lipid components of membrane lipopolysaccharides (Wakeham et al. 2003) are unique to Gram negative bacteria and are widely distributed in the ocean. Gram negative and Gram positive bacteria synthesize a unique cell wall biopolymer, peptidoglycan, which consists of glycan strands of a repeating disaccharide (N-acetyl-glucosamine and N-acetyl muramic acid) that are cross-linked by small peptides consisting of both Land D-enantiomers of specific amino acids (Schleifer and Kandler 1972). Muramic acid (Benner and Kaiser 2003) and the D-enantiomers of amino acids in peptidoglycan (Lee and Bada 1977; McCarthy et al. 1998) have also been identified in DOM. These studies demonstrate the ubiquitous nature of bacterially derived DOM (bacterial DOM) in aquatic environments. The mechanisms for the release of DOM from bacteria are poorly understood. Experiments by Ogawa et al. (2001) demonstrated that heterotrophic bacteria rapidly consume labile compounds (glucose or glutamate) and release DOM with a complex chemical composition. The rapid (,48 h) release of bacterial DOM in these experiments is consistent with a mechanism linked to cell division during exponential growth. Grazing of bacteria by protozoans (Strom et al. 1997) and viral lysis (Middelboe and Lyck 2002) could release DOM from bacteria, but most grazers were removed from the incubations by filtration, and both grazing activity and viral lysis would be expected to lag behind bacterial growth. Phylogenetic studies using 16S rRNA (ribosomal ribonucleic acid) show that bacterial community structure varies significantly among aquatic environments (Pernthaler et al. 1998; Crump et al. 1999). There are several studies indicating that biodegradation by heterotrophic bacteria plays an important role in shaping the composition of DOM (Brophy and Carlson 1989; Stoderegger and Herndl 1998), but the role of bacterial community structure in 1 Corresponding author ([email protected]). Acknowledgments We thank Karl Kaiser and Xiaquon Wang for assistance with amino sugar and amino acid analyses. This research was supported by grants EAR 0120579 and OCE 0080782 from the National Science Foundation. Limnol. Oceanogr., 51(5), 2006, 2170–2180 E 2006, by the American Society of Limnology and Oceanography, Inc.