sea, are the heterotrophic nanoflagellates (“HNAN”), ranging from 2 to 20 μm in cell
size. Most are “heterokont”, that is they have two functionally and often anatomically
different flagellae. Bacterivorous protists come from a wide variety of phylogenetic
groups and vary in shape, flagella arrangement, and feeding mode. Sherr and Sherr
(2000) list the groups of more common forms as chrysomonads, bicosoecids,
pedinellids, choanoflagellates, bodonids, and small ciliates. All of these eat bacteria,
including autotrophic bacteria (cyanobacteria), and many also eat small phytoplankton
up to sizes approaching their own. In addition, some flagellated phytoplankton,
thought of as primarily autotrophic, may eat bacteria. The prymnesiophytes (e.g.
Coccolithus), prasinophytes (e.g. Micromonas) and dinoflagellates include such
“mixotrophs”.
(^) Bacterivorous protists appear to face a fairly severe problem finding bacteria in
typical ocean water. Although 10^6 bacteria ml−1 is a large number, at ∼0.6 μm
diameter they are only 0.1 parts per million by volume, so lots of water must be
processed by relatively small sensing and ingesting structures to generate significant
nutrition from captured bacteria. Protists do that either by swimming and shifting
water in the oral area, achieving clearance of bacteria from up to 10^5 body volumes
per hour (Hansen et al. 1997) or by actively searching for food particles following
chemical cues. The exact mechanics of particle capture and chemical sensing are still
conjectural (Strom 2000).
(^) Several methods are available for estimating rates of bacterivory by single cells and
by the whole protist assemblage. The most widespread technique is the use of
fluorescently labeled bacteria. (See Box 5.2). Total bacterivory can also be estimated
by serial dilution experiments. Seawater samples are diluted at several different
fractions with bacteria-free water. Net per-bacterium increase rates (growth − grazing)
rise as dilution increases, since diluted grazers must search more water to get prey.
Grazing rates can be calculated from the relative increase at greater dilutions (at lower
bacterial and grazer concentrations). Bulk grazing on the order of 3–5% of water
volume per hour is commonly measured (Vaqué et al. 1994). This amounts to 25–
100% of the daily bacterial production.
Box 5.2 Grazer consumption rates
(^) Single grazer consumption rates can be determined by providing fluorescently labeled bacteria
(FLB), usually dead or rendered non-dividing (Sherr et al. 1987). After a short incubation, single
protists are examined by epifluorescence microscopy, and the labeled bacteria within them counted.
Averages for large numbers of grazer cells give I = bacteria ingested per grazer per time. Supplied
FLB concentration, [FLB] = FLB ml−1, can be used to estimate clearance rates: C = I/[FLB], ml per
grazer per time. If it can be assumed that natural bacteria are cleared at the same rate, then the result
can be applied to the community. However, it has been shown that grazers do select for live motile
particles over heat-killed non-motile particles (Gonzalez et al. 1993). Live bacteria can be engineered
to express a green fluorescent protein and then used as a cultured tracer to measure ingestion rates
(Fu et al. 2003).