Interactions of bactivorous grazers and heterotrophic bacteria with dissolved organic matter

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Abstract
Ciliated Protozoa (Euplotes sp. and Uronema sp.) and natural microzooplankton assemblages (10-200 .mu.m), collected from Los Angeles Harbor [USA], a coastal lagoon and San Pedro Channel, were fed bacteria previously labeled with 14C(U)-glucose. Dissolved organic C-14 (DO14C) released during grazing was collected and subsequently fractionated by AMICON ultrafiltration into 5 nominal MW (NMW) classes: > 105, 104 to 105, 103 to 104, 5 .times. 102 to 103 and < 5 .times. 102 daltons. Release of labeled organic compounds with low (< 5 .times. 102 daltons) and intermediate NMW (103-104 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 DO14C which was released during grazing experiments. Bacterioplankton used 196% d-1 .+-. 100 more of the DO14C released in presence of cultured ciliates or microzooplankton than DO14C released in their absence. Low (< 5 .times. 102 daltons)and intermediate NMW (103-104 daltons) labeled fractions were incorporated and respired more rapidly by bacteria than the other 3 NMW labeled fractions. Fractionation of DO14C after bacterial incubation showed a decrease in all NMW classes and a shift toward a dominance of low NMW labeled fractions in the DO14C 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 C source for bacterioplankton and may thereby influence bacterioplankton metabolism and growth activity.