Dynamics and Characterization of Refractory Dissolved Organic Matter Produced by a Pure Bacterial Culture in an Experimental Predator-Prey System

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%...

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

“…Goldberg et al, 2011;Kaiser and Benner, 2012) or via experimental degradation approaches (e.g., Gruber et al, 2006;Kawasaki and Benner, 2006;Davis et al, 2009). The spatiotemporal variability of biomolecules and net removal rates of DOC or distinct fractions (e.g., Amon and Benner, 1994;Loh et al, 2004;Kaiser and Benner, 2009;Hansell, 2013) corroborated the concepts of age-, size-and reactivity continua in which the microbial carbon pump (Jiao et al, 2010) plays a major role in transforming fresh into refractory organic matter.…”

Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol

“…Associated with organic matter-producing autotrophic microbes (e.g., phytoplankton) through the microbial loop [Pomeroy et al, 2007], heterotrophic bacteria and their associated enzymes serve as the primary reactive surface responsible for chemical transformation of DOM [Azam and Malfatti, 2007;Jiao et al, 2010]. While the molecular-level details have not been thoroughly characterized to date, bacterial activity is known to alter the chemical composition of DOM [Gruber et al, 2006;Coble, 2007;Kujawinski et al, 2009;Flerus et al, 2012]. In stark contrast with laboratory sea spray experiments that use phytoplankton exudates as an OM sea chemical proxy [Wex et al, 2010a;Fuentes et al, 2011], the results presented in this study demonstrate the importance of including a more holistic representation of the biological complexity of marine microbial assemblages.…”

“…[20] Marine bacteria are known to chemically alter the organic matter composition of seawater [Ogawa et al, 2001;Gruber et al, 2006;Coble, 2007;Jiao et al, 2010]; therefore, an evaluation of the impact of bacterial processes on SSA composition and CCN activity must be made by continuously monitoring changes in aerosol properties, along with changes in biological metrics in the seawater. Coincident time-resolved measurements of single particle chemical composition, CCN, chlorophyll concentration, and bacterial abundance in the seawater during the mesocosm experiment are presented in Figures 1a-c, demonstrating the interplay between marine biogeochemistry, aerosol composition, and CCN activity.…”

Inefficient microbial production of refractory dissolved organic matter in the ocean

“…The conceptual framework of the microbial carbon pump 8 summarizes the not yet fully understood biological processes leading to RDOM formation. The microbial transformation of simple substrates produces DOM that persists further degradation for months to years, as demonstrated based on bulk parameters and concentrations of selected compound classes like amino acids and carbohydrates [9][10][11][12][13][14] . However, o5% of oceanic RDOM consists of these molecularly defined compound classes 15 .…”