very small) and from chemicals dispersing from prey. Sensed chemicals can be quite
simple, like those amino acids with small side-chains (e.g. proline). Strom et al.
(2007) have shown that the simpler amino acids can be strong inhibitors of feeding in
the tintinnid ciliate Favella at nanomolar concentrations. The adaptive value of this is
unclear, but it indicates that complex systems for interpreting extremely dilute
chemical information are used by protists, their prey, and their predators.
(^) Protists grapple with food particles by several mechanisms. At the smallest scales
(microns), e.g. nanoflagellates feeding on picoplankton (both auto- and
heterotrophic), hydrophobic forces associated with cell surfaces of both predator and
prey can pull prey out of the streamlines around a moving predator, creating contact
leading to ingestion (Monger et al. 1999). Some flagellates, like the colorless
cryptophyte Katablepharis (Lee et al. 1991) have modestly elaborate “mouths”,
which can manipulate and engulf food using microtubular organelles that extend into
the cell as a digestive tract. Others (choanoflagellates) have feeding collars coated
with sticky material to which nanoparticulate food adheres, followed by eventual
ingestion at a localized site. Ciliates move to particulate food items, ingesting them
with mouth-like organelles specialized for phagocytosis. Ciliate feeding mechanisms
are reviewed in Hausmann et al. (1996).
(^) Three techniques are often applied to determine the feeding rates of marine protists,
often referred to as “microheterotrophs”.
(^1) As for mesozooplankton, prey in a suitable container can be counted
before and after a timed interval of feeding by counted protists. Prey are
assumed to be decrease exponentially, allowing calculation of both water-
clearance rate (volume per grazer per time) and grazer ingestion rates.
2 The FLB (fluorescently labeled bacteria) technique (Box 5.2; Sherr et al.
1987) can be applied to algae (as “FLA”). Fluorescently labeled cells are
visible, glowing with fluorescence, in food vacuoles inside the protists. The
rate of water clearance, F, by a cell is given by:
(^) Given suitable averaging and a density estimate for protists from the same
slide, an overall rate (volume cleared time−1 ) of microheterotrophy can be
obtained. This is most useful for grazing rates on Synechococcus and
Prochlorococcus.
(^3) The dilution-series technique (Landry & Hassett 1982; Box 7.5) has been
more widely applied than FLB, perhaps because some versions do not
require extensive microscopy.