RELEASE OF MACROMOLECULAR ORGANIC-COMPLEXES BY HETEROTROPHIC MARINE FLAGELLATES

We examined whether or not the heterotrophic marine flagellate Paraphysomonas imperforata releases macromolecular dissolved organic matter (DOM). H-3-bacterial prey (Vibrio spp.) were added to flagellate cultures and the accumulation of H-3-DOM was measured. Flagellates released a large amount of macromolecular DOM (6 to 57 % of H-3-DOM) that was rich in lipids (lipid : macromolecular ratio = 0.20 : 0.92). One component of the macromolecular DOM was a digestive enzyme (acid phosphatase) of the flagellate. The flagellate acid phosphatase was not degraded by proteases, but it was susceptible to proteases after treatment with polymyxin B, which destroys bacterial membranes. These data suggest that bacterial membranes protected the released acid phosphatase from proteolytic attack. Ultracentrifugation showed that the sedimentation coefficient of the phosphatase was 15 to 42 S, consistent with the hypothesis that the phosphatase is part of a macromolecular complex. We hypothesize that flagellates release their own digestive enzymes and incompletely digested membranes and probably other cellular components from bacterial prey and that these compounds form liposome-like organic complexes. These complexes may be important for understanding the chemical nature and turnover of DOM in the oceans.