No abstract

We studied the effects of a bacterium (Pseudomonas chlororaphis) and a bactivorous protozoan (Uronema sp.) on transformations of labile dissolved organic carbon (DOC). In 36-day time series experiments, bacteria were grown on glucose both with and without protozoa. We measured bulk organic carbon pools and used electrospray ionization mass spectrometry to characterize dissolved organic matter on a molecular level. Bacteria rapidly utilized glucose, depleting it to nondetectable levels and producing new DOC compounds of higher molecular weight within 2 days. Some of these new compounds, representing 3 to 5% of the initial glucose-C, were refractory and persisted for over a month. Other new compounds were produced and subsequently used by bacteria during the lag and exponential growth phases, pointing to a dynamic cycling of organic compounds. Grazers caused a temporary spike in the DOC concentration consisting of labile compounds subsequently utilized by the bacteria. Grazing did not increase the complexity of the DOC pool already established by the bacteria but did continually decrease the particulate organic carbon pool and expedited the conversion of glucose-C to CO 2 . After 36 days, 29% of initial glucose-C remained in pure bacteria cultures, while only 6% remained in cultures where a grazer was present. In this study the bacteria were the primary shapers of the complex DOC continuum, suggesting higher trophic levels possibly have less of an impact on the qualitative composition of DOC than previously assumed.Dissolved organic matter (DOM) comprises the largest, yet least-characterized reservoir of reduced organic carbon in aquatic systems, estimated at 700 ϫ 10 15 g C (11). DOM is important in the carbon and nitrogen cycles, the scavenging and solubilization of trace contaminants, and biogeochemical cycles of other elements (3,14). Heterotrophic bacteria process and reprocess some of this DOM (2), channeling about one-half of oceanic primary production through the microbial loop (8).The role of bacteria in the rate and extent of DOM mineralization and their production of (semi)refractory DOM have received less attention. Some studies indicate that bacteria produce refractory DOM that is resistant to further utilization (5,12,37,39). Ogawa et al. (26) showed that a natural inoculum of marine bacteria (and undoubtedly nanoflagellates and viruses) growing on labile compounds (glucose and glutamate) produce new DOM compounds that appear to be refractory for at least a year. It was not known if a single strain of bacteria could produce similar refractory material. Bacterioplankton can also be a source of photoreactive C-DOM that is refractory to a natural bacteria assemblage following photochemical alteration (18). What kinds and how many different compounds make up the refractory DOM pool are largely unknown.In aquatic ecosystems, bacteria are consumed by protozoa and other zooplankton, which in turn release DOM as colloidal matter (17, 40) and macromolecular organic complexes (24). A substantial portion (Ͼ50%...

No abstract

No abstract

No abstract