Trophic interactions between heterotrophic Protozoa and bacterioplankton in estuarine water analyzed with selective metabolic inhibitors

Abstract

We estimated In situ grazing rates of heterotrophic nanoplankton (HNAN) on bactenoplankton in a salt marsh estuary using a select~ve metabolic inhibitor technique. Seasonal experiments with prokaryohc inhibitors showed a significant, positive relation between bacterial abundance and HNAN grazing rates. On average, only 40 to 4 5 '/o of bacterioplankton produchon in the estuary appeared to be consumed by phagogrophic Protozoa <20 pm in size. Eukaryotic inhibitor experiments suggested that protozoan grazlng can have a positive feedback effect on bacterioplankton production, and that nitrogen regeneration by Protozoa may be of particular importance to bacterial growth. Although eukaryotic inhibitors did not prove to be rouhnely useful to estimate protozoan grazing rates on bacterioplankton, they may contribute to understanding qualitahve control mechanisms in the microbial food web.

Size fractionated metabolism of coastal microbial plankton

“…It is unlikely that the respiration enhancement is due to an increase in microbial biomass during incubation in the bottles following predator removal. As discussed previously, the linear nature of oxygen consumption over time in the < 1 lim fraction suggests that microb~al biomass remained constant for at least 24 h. Furthermore, experiments by Sherr, B. F. et al (1986), using the same water that we collected from the upper Duplin River station in February and August, showed that bacterial numbers were constant for at least 12 h (the longest time examined) following the addition of eukaryotic inhibitors to 10 pm screened water.…”

“…Ogura 1975, Fuhrman & Azam 1980. Williams (1981) reported that the rate of respiration of Loch Ewe plankton, either fractionated or unfractionated, did not vary for over 48 h, and Pomeroy & Deibel (1986) showed linear respiration rates of plankton incubated at 4.2 "C for over 20 d. In our water samples, respiration rates were statistically constant for 24 h, and Sherr, B. F. et al (1986) tvith subsamples of water used here found no bacterial growth during the first 12 h interval in samples tvith and without eukaryotic inhibitors present. In contrast to these results however, there have been several reports of substantial increases in bacterial numbers in samples incubated for long (h) periods (Ferguson et al 1984 and references therein).…”

“…The maximum potential increase in substrate concentration following the complete disruption of all bacteria in our samples could be only about 50 pg C 1-' (Bratbak & Dundas 1984, Sherr pers, comm.). This could support microbial respiration for a maximum of 3 to 4 h. It could not sustain microbial respiration at rates in excess of 180 % of rates for 208 pm filtered water for more than 24 h. As the biomass of organisms between 1 and 10 pm in size is less than that of organisms passing a 1 ym screen (Sherr, B. F. et al 1986), potential enhancement from their disruption would be even less than that from microbes smaller than 1 pm. It is unlikely that the respiration enhancement is due to an increase in microbial biomass during incubation in the bottles following predator removal.…”

Seasonal patterns of bacterivory by flagellates, ciliates, rotifers, and cladocerans in a freshwater planktonic community

“…Cladocerans consume heterotrophic flagellates and large, pliable colonies, including those of the mixotroph Dinobryon (Tappa 1965;Sanders and Porter 1987). Although copepods may have a significant impact on ciliates (Porter et al 1979), they are less efficient at cropping particles in the size range of flagellates (E. Sherr et al 1986;Bogdan and Gilbert 1987). In many lakes, cladocerans are an important food source for predatory fishes and invertebrates.…”

“…The microbial loop is now recognized as a dynamic component of pelagic marine food webs (Pomeroy 1974;Azam et al 1983; E. Sherr et al 1986). In it, heterotrophic and autotrophic picoplankton and their protistan grazers recycle particulate and soluble nutrients released by the classical pelagic food chain of algae, crustacean zooplankton, and fish.…”