Impact of bacteria and zooflagellates on the composition of sinking particles: an in situ experiment

Multiple sediment trap (MULTITRAP) arrays were deployed at 3 hydrographically and biologically distinct stations in the eastern North Pacific Ocean and used to evaluate experimentally the hypothesis that protozoan grazers are important in the diagenesis of sinking biogenic materials in the ocean. Measurements were made of adenosine triphosphate (ATP) concentrations and the abundances of bacteria and zooflagellates associated with sinking particles collected from in situ preserved and 2 sets of unpreserved (live) traps: one containing a selective eukaryote inhibitor (thiram) and the second without thiram which served as a live-control. Particulate organic carbon and nitrogen (POC and PON), NH:, NO;, NO;, dissolved organic nitrogen (DON), diatoms, and fecal pellets were quantified in live-control and live-thiram traps. In situ microbial production was estimated by measuring RNA and DNA synthesis in live-control and live-thiram traps pre-charged with 3H-adenine. Trap-collected bacteria and zooflagellates accounted for a low percentage of the total POC (2 to 11 % ) but a high percentage of the total microbial biomass (an estimated 25 to 2 100 % of total ATP). Zooflagellates were the principal protozoans collected and accounted for 2 to 25% of the total microbial biomass. Synthesis of RNA and DNA was observed at all stations and depths, indicating microbial growth in the live traps. ATP and microscopic analyses, however, independently confirmed that death rates exceeded growth rates on the sedimenting particles. Differences between the livecontrol traps (coupled microbial assemblage) and live-thiram traps (uncoupled assemblage, grazers inhibited) indicated slower decomposition and nutrient regeneration in the uncoupled system. In general, POC was higher and in the shallowest traps, NH: was lower, and DON was higher in the livethiram traps. Zooflagellates appear to strongly influence decomposition and nutrient recycling within the microbial assemblage associated with sedirnenting particles.