Protozoa abundance, growth, and bacterivory in the water column, on sedimenting particles, and in the sediment of Halifax Harbor

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Abstract
The role of protozoan grazing in controlling bacterial populations was examined in four microbial habitats in Halifax Harbor, Canada: the water column, setting particles, the sediment–water interface, and the sediment. Large numbers of protozoans were found in all habitats although most (>56%) were small (20 μm were never observed. Protozoans were also observed to a depth of 9 cm below the sediment surface although efforts to culture viable protozoa failed except for the top 1 cm. The use of the metabolic inhibitor cycloheximide with and without colchicine to selectively inhibit eucaryotic metabolism was shown to severely affect procaryotic metabolism in sediment (and presumably particle and water) samples. Using fluorescently labelled bacteria as food, and under optimum conditions, up to 42% of the Protozoa population exhibited active grazing within 7 h. Using protozoan and bacterial community sizes and doubling times, it was calculated that each protozoan in Halifax Harbor would have to consume 13–118 bacteria per hour for the enumerated nanoplanktonic (<20 μm) Protozoa to be the sole control of the size of the bacterial community. Key words: marine, Protozoa, bacterivory, particles, bacteria.

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