Processing of ingested matter in Strombidium sulcatum, a marine ciliate (Oligotrichida)

We examined the hypothesis that different types of ingested matter may be processed at different rates by a marine microzooplankter. Ciliates were exposed to a prey item and then diluted at 1: 100. Disappearance or digestion of inert (fluorescent microspheres) and digestible prey items (heatkilled and fluorescently labeled Synechococcus, natural Synechococcus, and Isochrysis galbana) were determined from changes in cell contents using epifluorescence microscopy. Ingested prey declined exponentially, and prey analogues were processed like natural prey. There were no significant differences in rates of disappearance or decay constants for the different prey items or in feeding vs. nonfeeding ciliates or between ciliates cultured on (or naive to) natural Synechococcus. The overall average tlh of cell contents at 22°C was 75 min. The exponential declines in cell contents indicated that some ingested chlorophyll a, whether in Synechococcus or Zsochrysis, can be long-lived. The relatively invariant decay constant for ingested matter opens the possibility of using a gut passage approach to estimate instantaneous rates of grazing for natural populations of oligotrich ciliates. Ciliate microzooplankton are generally considered an important component of planktonic food webs and are assigned the role of grazers on ultraplankton, i.e. cells between 0.5 and 5 ,um in size (e.g. Sherr et al. 1991). In recent years, the grazing activity of ciliates has been evaluated, for the most part, using two major approaches: monitoring changes in phytoplankton pigment concentrations in dilution experiments or using prey analogues. An alternative technique, used occasionally in the past, is the gut contents approach; it relies on estimates of cell contents combined with data on digestion rates (Goulder 1972, 1973; Fenchel 1975; Kopylov and Tumantseva 1987; Dolan and Coats 1991). With regard to the dominant group of marine planktonic ciliates, oligotrichs, this particular approach has not been attempted and the process of digestion has received little attention. To our knowledge, the existing data concern disappearance rates of fluorescently labeled bacteria (Sherr et al. 1988). Although the need for digestion rate data is obvious if a gut contents approach is to be attempted, knowledge of how food items are processed may be useful regardless of the technique used to investigate grazing. The use of fluorescently labeled bacteria (Sherr et al. 1987) or algae (Rublee and Gallegos 1989) as prey analogues has become common (Putt 1991). Grazing rates are estimated either via increases of prey analogues inside grazers (e.g. Simek et al. 1995) or via declines in bulk concentrations of prey analogues (e.g. Fuhrman and Noble 1995). When prey analogues are used to estimate ingestion, it is overtly assumed that analogues appear in the predator (via ingestion) at a rate similar to natural prey items (e.g. Simek et al. 1995), but there is an underlying assumption that prey analogues disappear (i.e. are digested or evacuated) at the same rate as natural prey. To our knowledge, this assumption has never been examined. There are reasons to suspect that some prey analogues could be digested at rates different from natural prey. For example, heat-killing the bacterium Staphylococcus, a process used in the preparation of fluorescently labeled bacteria (Sherr et al. 1987), coagulates cytoplasm, thereby rendering the cells resistant to digestion by the ciliate Paramecium (Mehlis et al. 1990). Thus, it is worth considering that heat-killed prey may be processed slower in the digestive vacuoles of ciliates compared to natural prey. Here we report on the processing of ingested matter by Strombidium sulcatum, which is considered typical of marine planktonic ciliates and is the subject of previous studies concerning feeding and nutrient excretion (Allali et al. 1994). We examined in a set of laboratory experiments the disappearance of different types of ingested matter in S. sulcatum: inert fluorescent microspheres, heat-killed and fluorescently labeled Synechococcus cells, native marine Synechococcus, and cultured Isochrysis galbana. Our data support studies that reported an exponential decay of ingested matter in ciliates (Goulder 1972; Fenchel 1975; Berger and Pollock 1981; Fok and Schockley 1985; Fok et al. 1982;