Predator-prey interactions of nanozooplankton and bacteria in an oligotrophic marine environment l

Population interactions between nanoplankton and bacteria were investigated by means of long (ca, 5 day) incubations of prescreened seawater samples. Evidence is presented to show that population development is controlled by the predation of larger organisms on smaller ones. The development of the bacterial populations seems to be totally controlled by predation by nano-sized organisms (flagellates and ciliates) and by picoflagellates. It is possible, though less certain, that picoflagellatc development is controlled by nanociliate predation. It has been known for many years that bacteria will proliferate in seawater that has been simply left in a container (ZoBell and Anderson 1936) or has been filtered (Sheldon et al. 1967), but diverse opinions have been expressed as to whether similar growth would occur in the natural environment. Various workers have concluded, often from indirect evidence, that bacteria in the sea are dormant (Stevenson 1978), actively growing (Azam and Hodson 1977), attached to particles of detritus (Seki 1972), or free living (Ferguson and Rublee 1976). There is also a considerable difference of opinion as to whether the suspended particulate material < 1 ,um is mainly detrital (Herbland and LeBouteiller 198 1) or living (Johnson and Sieburth 1979). Furthermore, it has recently been suggested that a good proportion of oceanic primary production is due to organisms < 1 pm (Li et al. 1983). The situation, at least with regard to heterotrophic bacteria, has been clarified somewhat by the work of Ammerman et al. (1984). They have shown that bacteria in the sea form a definite free-living population. In this respect they are no different from other planktonic populations, and one would expect them to be subject to similar regulatory mechanisms. The production of ’ Research supported in part by grant CNRS-PIRO AIP 953 146-GRECO P4 (Centre National de la Recherche Scientifiquc). these free-living bacteria can represent a significant fraction of the total pelagic production (Linley et al. 1983), but to maintain equilibrium in the system this production must be controlled by predation or by some other means of removal. Certain of the larger gelatinous net-feeding zooplankton, such as Oikopleura, can take bacteria and these can form a substantial part of their diet (King ct al. 1980). But the more common microzooplankton, such as tintinnids and naked ciliates, cannot effectively remove particles from suspension that are <2-4 pm (Rassoulzadegan 1978, 1982; Capriulo and Carpenter 1980; Rassoulzadegan and Etienne 1981). If we consider pelagic populations from the point of view of a linear biomass spectrum (Sheldon et al. 1972, 1977) they can be organized as a series of predator-prey links of increasing size, with the predators about 10 times larger than the prey. On this basis one would expect that if there is predation on bacterial populations they might be controlled by predators in the size range around 5 pm. Heterotrophic flagellates are the obvious candidates, although microciliates (~30 pm) could also be involved (Rivier et al. 1985; Sherr et al. 1986). Fenchel(l982, 1983) and Andersen and Fenchel (1985) have shown that heterotrophic flagellates not only consume bacteria but may also be capable of exerting predation control on the development of the bac-