We have developed a procedure for preparing monodispersed, fluorescently labeled bacteria (FLB), which may be used to measure virtually instantaneous rates of protozoan bacterivory in natural waters. FLB can be prepared both from natural bacterioplankton assemblages and from clonal isolates and can be stored in frozen suspension or freeze-dried without apparent loss of fluorescence intensity. They are not toxic to protozoa and can be metabolized to support bacterivorous protozoan growth rates equal to those on the same strain of unstained, viable bacteria. In experiments comparing uptake of FLB with uptake of fluorescent latex microspheres by protozoan assemblages in a salt marsh tidal creek, we found that both pelagic oligotrichous ciliates and phagotrophic flagellates ingested FLB with a frequency 4to 10-fold greater than they ingested the microspheres. Consequently, it appears that the use of latex microspheres leads to underestimation of protozoan bacterivory and that the FLB technique is superior for estimating instantaneous rates of in situ protozoan grazing on bacterioplankton.

Research on "microbial loop" organisms, heterotrophic bacteria and phagotrophic protists, has been stimulated in large measure by Pomeroy's seminal paper published in BioScience in 1974. We now know that a significant fate of bacterioplankton production is grazing by < 20-µm-sized flagellates. By selectively grazing larger, more rapidly growing and dividing cells in the bacterioplankton assemblage, bacterivores may be directly cropping bacterial production rather than simply the standing stock of bacterial cells. Protistan herbivory, however, is likely to be a more significant pathway of carbon flow in pelagic food webs than is bacterivory. Herbivores include both < 20-µm flagellates as well as > 20-µm ciliates and heterotrophic dinoflagellates in the microzooplankton. Protists can grow as fast as, or faster than their phytoplankton prey. Phototrophic cells grazed by protists range from bacterial-sized prochlorophytes to large diatom chains (which are preyed upon by extracellularly-feeding dinoflagellates). Recent estimates of microzooplankton herbivory in various parts of the sea suggest that protists routinely consume from 25 to 100% of daily phytoplankton production, even in diatom-dominated upwelling blooms. Phagotrophic protists should be viewed as a dominant biotic control of both bacteria and of phytoplankton in the sea.

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Simultaneous measurements were made of bacterioplankton productivity ([3~]thymidine assay) and of bacterial mortality due to protozoan grazing (measured via uptake of fluorescently labeled bacterioplankton, FLB). Water samples were taken from a salt marsh tidal creek and from an estuarine sound near Sapelo Island, Georgia, USA at low tide over a 2 wk period in late summer. In control experiments performed to test the extent of selectivity of estuarine bacterivorous protozoa for or against FLB compared to natural bacterioplankton, we found no evidence for consistent discnmination, Rates of bacterial production and of protozoan bacterivory were greater in the tidal creek than in the open sound. Ciliates were responsible for the largest fraction of total protozoan consumption of bacteria in tidal creek water, and colorless flagellates in open estuary water. Bacterial production and protozoan bacterivory were not always in balance in individual samples, with the largest discrepancies in the open estuary. Estimated bacterivory was, on average, 80 % of bacterial production in the tidal creek and 50 "/o of production in the open estuary. Explanations for the measured shortfall in bacterial mortality include methodological problems with the assays used or alternate fates of bacterial production besides protozoan grazing.