Vol. 23: 129-141. 1985 MARINE ECOLOGY - PROGRESS SERIES Mar. Ecol. Prog. Ser. Published May 16 Interactions of bactivorous grazers and heterotrophic bacteria with dissolved organic matter Gordon T. Taylor*, R. Iturriaga & C. W. Sullivan Department of Biological Sciences, University oi Southern California, University Park, Los Angeles, California 90089-0372, USA ABSTRACT: Ciliated Protozoa (Euplotes sp, and Uronema sp.) and natural microzooplankton assem- blages (10 to 200 pm) - collected from Los Angeles Harbor, a coastal lagoon, and San Pedro Channel - were fed bacteria previously labeled with 14C(U)-glucose. Dissolved organic carbon-l4 (D014C) released during grazing was collected and subsequently fractionated by AMICON ultrafiltration into 5 nominal molecular weight (NMW) classes: > 105, 104 to 105, 103 to 104, 5 X 102 to 103, and < 5 X 102 daltons. Release of labeled organic compounds with low (< 5 X 102 daltons) and intermediate NMW (103 to 104 daltons) was most rapid in experimental and control samples and was enhanced in the presence of grazers. Unlabeled bacterioplankton assemblages were incubated 24 h with either fractionated or unfractionated DO14Cwhich was released during grazing experiments. Bacterioplank- ton utilized 196 % d-I + 100 more of the DO14C released in presence of cultured ciliates or microzoo- plankton than D014C released in their absence. Low (< 5 X 102 daltons) and intermediate NMW (103to 104daltons) labeled fractions were incorporated and respired more rapidly by bacteria than the other 3 NMW labeled fractions. Fractionation of D014C after bacterial incubation revealed a decrease in all NMW classes and a shift toward a dominance of low NMW labeled fractions in the D0I4C released in the presence of grazers, but not in the controls. Bactivorous ciliates and microzooplankton contribute quantitatively and qualitatively to the dissolved organic matter pool in nearshore planktonic systems. Dissolved organic matter released in the presence of bactivores is a readily available carbon source for bacterioplankton and may thereby influence bacterioplankton metabolism and growth activity. INTRODUCTION The microheterotrophically-mediated flow of carbon in marine systems is a topic of keen interest to marine microbiologists. Microheterotrophs, particularly bac- teria, have an essential role in regulating the dynamics of carbon flux in aquatic systems (Pomeroy 1974). Recent information - reviewed in Azam et al. (1983), Ducklow (1983), and Sieburth (1984) - suggests that bactivory (ingestion of bacteria) by microzooplankton (20 to 200pm in diameter, primarily ciliated Protozoa and Micrometazoa) and nanozooplankton (2 to 20 pm in diameter), primarily heterotrophic mastigophoran and ciliated protozoans, may regulate standing stocks, species composition, and metabolic activity of bac- Present address: Department of Oceanography/Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA O Inter-Research/Printed in F. R. Germany terioplankton. By regulating bacterial activity, bacti- vores thereby indirectly impact dissolved carbon flux dynamics. This study examines the hypothesis that bactivorous microzooplankton may also directly influ- ence dissolved carbon flux by releasing, or stimulating release, of dissolved organic matter (DOM). Ciliated protozoans and microzooplankton (operationally defined here as phagotrophic organisms 10 to 200 pm in diameter) were the focus of this study because they can be cultured and collected relatively easily, they are abundant in nearshore planktonic systems, and their trophic role is not well understood. The ocean represents a large reservoir of DOM. According to Bada & Lee (1977),oceanic waters con- tain 0.5 to 1.5mg 1-' dissolved organic carbon. The chemical composition of this DOM pool, however, is poorly known. Bada & Lee report that the vast majority of the DOM pool is composed of complex, refractory materials and a small, but significant portion of this