Description: The degradation of marine dissolved organic matter (DOM) is an important control variable in the global carbon cycle. For our understanding of the kinetics of organic matter cycling in the ocean, it is crucial to achieve a mechanistic and molecular understanding of its transformation processes. A long-term microbial experiment was performed to follow the production of non-labile DOM by marine bacteria. Two different glucose concentrations and dissolved algal exudates were used as substrates. We monitored the bacterial abundance, concentrations of dissolved and particulate organic carbon (DOC, POC), nutrients, amino acids, and transparent exopolymer particles (TEP) for two years. The molecular characterization of extracted DOM was performed by ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) after 70 days and after ~2 years of incubation. Although glucose was quickly degraded, a non-labile DOC background (5-9% of the initial DOC) was generated in the glucose incubations. Only 20% of the organic carbon from the algal exudate was degraded within the 2 years of incubation. The degradation rates for the non-labile DOC background in the different treatments varied between 1 and 11 µmol DOC L-1 yr-1. TEP, which are released by microorganisms, were produced during glucose degradation but decreased back to half of the maximum concentration within less than three weeks (degradation rate: 25 µg xanthan gum equivalents L-1 d-1) and were below detection in all treatments after 2 years. Additional glucose was added after two years to test whether labile substrate can promote the degradation of background DOC (co-metabolism; priming effect). A priming effect was not observed but the glucose addition led to a slight increase of background DOC. The molecular analysis demonstrated that DOM generated during glucose degradation differed appreciably from DOM transformed during the degradation of the algal exudates. Our results led to several conclusions: (i) Based on our experimental setup, higher substrate concentration resulted in a higher concentration of non-labile DOC; (ii) TEP, generated by bacteria, are degraded rapidly, thus limiting their potential contribution to carbon sequestration; (iii) The molecular signatures of DOM derived from algal exudates or glucose after 70 days of incubation differed strongly from refractory DOM. After 2 years, however, the molecular patterns of DOM in glucose incubations were more similar to deep ocean DOM whereas the degraded exudate was still different.

Description: Transparent exopolymer particles (TEP) exist abundantly in oceans and lakes and have been found to play an important role for sedimentation and biochemical cycling of matter. But the origin of these particles and the factors regulating their formation are not well understood. This study examined several strains of algae and bacteria with respect to their production of TEP or TEP-precursors. The formation rate of TEP in batch cultures of algae varied widely between species, and interspecies variability between diatoms was as large as that between species belonging to different classes or even divisions. Species, growth phase and environmental factors acted in concert in determining the accumulation of TEP in algal cultures and no general rules or patterns could be derived. The concentration of TEP during the growth phase of algal batch cultures, mesocosm or natural phytoplankton blooms was a significant function of chl. a, confirming the significance of phytoplankton for the formation of TEP. Experiments with three bacterial strains and a natural bacteria population indicated, that bacteria are also able to generate TEP, but the role of bacterial derived TEP for in situ TEP concentrations remains unclear.