Rates of Microbenthic and Meiobenthic Bacterivory in a Temperate Muddy Tidal Flat Community

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Rates of bacterivory in micro- and meiobenthic species were determined by an improved technique in a muddy tidal flat community in Boston Harbor, Mass. The predominant grazers of bacteria were identified, and their rates of grazing were measured in the top 1 cm of the sediment. Grazing rates were measured by a fluorescence-labeled bacteria (FLB) technique. A mixture of two Enterococcus spp. isolates and two isolates of Escherichia coli were prepared as FLB, and they were added to intact sediment cores by replacing the pore water in the upper centimeter of the core. A standard FLB procedure was modified by filtering sediment dilutions onto cellulose membrane filters and processing the filters to render them optically transparent while preserving the physical integrity of the micro- and meiobenthic organisms. Thus, it was possible, on the same microscopic field, to switch from light microscopy for identification of grazers to epifluorescence microscopy for counting FLB present in the gut contents of the same grazers. The majority of benthic organisms present in these sediments consumed FLB, but their consumption rates varied widely. Two ciliate species, a Prorodon sp. and a Chlamidodon sp., and a nematode, a Metoncholaimus sp., consumed fluorescence-labeled coliforms at the highest rates, 126 to 169 FLB per individual per h. Other ciliates and nematodes, as well as microflagellates and harpacticoid copepods, consumed fluorescence-labeled coliforms at lower rates, 1.2 to 26 FLB per individual per h. Foraminiferans and gastrotriches did not contain FLB. Some ciliate grazers discriminated between enterococci and coliforms, consuming the rod-shaped fluorescence-labeled coliforms at 74- to 155-fold-higher rates than did the coccus-shaped fluorescence-labeled enterococci. Other ciliates did not select between fluorescence-labeled enterococci and fluorescence-labeled coliforms. The high rates of bacterivory by some ciliates and nematodes indicated intensive grazing. However, at their low extant densities, the grazers consumed only a small portion of the bacterial standing stock. Major bacterial grazers, e.g., microflagellates, ciliates, and nematodes, could potentially consume, per day, only 0.2, 0.1, and 0.03%, respectively, of the bacterial standing stock (7.5 × 108 bacteria per cm3).

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