Jpn. J. Protozool. Vol. 35, No. 2. (2002)

85

ReviewFood acquisition, food ingestion and food digestion by protists
Klaus Hausmann
Arbeitsgruppe Protozoologie, Institut für Biologie / Zoologie
Freie Universität Berlin, Königin-Luise-Str. 1-3, D-14195 Berlin, Germany

Food acquisition, food ingestion and fooddigestion are crucial for the survival of all living
creatures, including, of course, unicellular organisms. The following article will concentrate onthese three topics with respect to free-living protists.

I. Food acquisition (Fig. 1)
In general, two different types of food acquisition can be discerned in free-living protists: filterfeeding and predation (Fig. 1) (Hausmann and
Hülsmann, 1996).

General types of food acquisition

Fig. 1. Synopsis of general types of food acquisition in protists.

86

Jpn. J. Protozool. Vol. 35, No. 2. (2002)

Prey capture by Actinophrys sol

Fig. 2. Prey capture in the heliozoon A. sol (b) involves a high number of extrusomes (a') which are seen in
the electron microscope in the process of transformation and discharge (b', arrows).

Filter-feeding is food acquisition in which suspended particles are concentrated through the action of flagella or cilia in the areas of the cell at
which endocytosis can take place. The flagella /cilia direct a current of water to the cell and - especially in many ciliates - towards a filter device
which concentrates food particles before their enclosure within a food vacuole. Well-known examples of filter feeders are ciliates such as paramecia(Ishida et al., 2001) as well as numerous flagellates(Boenigk and Arndt, 2000; Boenigk et al., 2001).
Predatorial feeding, in contrast, is characteristic for those protists which obtain their food throughthe action of special cell organelles, generally
called extrusomes. These are cell elements withintravacuolar and dischargeable contents, derived
from typical dictyosomes (Hausmann, 1978). In thecontext of this article the function of extrusomes inheliozoans and of toxicysts in ciliates will be discussed.
Extrusomes of Actinophrys sol (Fig. 2)
The capture of prey by the heliozoon Actinophrys sol follows the random contact between theaxopods of the passively floating heliozoon and theactively swimming ciliate prey. The ciliate adheresto the predator despite the active beating of its cilia
and numerous avoidance responses (Fig. 2 b). In theregion of contact, individual cilia may be seen toadhere to axopodia or to each other (Patterson and
Hausmann, 1981). A very similar situation has beenreported for another heliozoon, Echinosphaerium
nucleofilum (Suzaki et al., 1980).
It can be clearly seen using an electron micro-

Jpn. J. Protozool. Vol. 35, No. 2. (2002)87

Predatory Homalozoon vermiculare

Fig. 3. The oral bulge of the ciliate H. vermiculare is filled with hundreds of toxicysts which appear in lightmicrographs as rood like structures (a + b) and in ultra-thin cross sections as organelles with a circular
to oval profile (d) which are filled with more-or-less concentric structures (e).

scope that direct contact between the prey andpredator is minimal, if it occurs at all. However,much extracellular material is typically found between the two cells (Fig. 2 b’). Obviously this material connects the two cells. At least a part of it
originates from the discharged content of the numerous extrusomes of A. sol which can be recognised easily by light microscopy (Fig. 2 a'). The
functional role of this discharged material has been
investigated in great detail up to the molecular level
by Sakaguchi et al. (2001) (see also article by M.
Sakaguchi in this issue, pp. 35-39). In addition,
large amounts of small pieces of membrane, arised
from the plasmalemma of the predator, were alsopresent. The membranes exist as fine threadlike
pseudopodia extending from the surface of the
predator (Hausmann and Patterson, 1982). Again,

the extrusomes seem to play an essential role also inthis process. It has not yet been clarified whether or
not these morphologically more-or-less identicalorganelles represent different types of extrusomeswith different functions.
In Rhaphidiophrys contractilis, a centrohelid
heliozoon, so-called kinetocysts are implicated in
the process of prey adhesion. The posterior part ofthe discharged kinetocyst is always attached to theplasma membrane of the heliozoon, while the anterior end is directed towards the prey, permitting
close contact with the food organism (Sakaguchi etal., 2002) (see also article by M. Sakaguchi in thisissue, pp. 35-39).
Toxicysts of Homalozoon vermiculare (Figs. 3-7)

Homalozoon vermiculare is a predatory ciliate

88Jpn. J. Protozool. Vol. 35, No. 2. (2002)

Fig. 4. The first step of food capture by H. vermiculare
is mechanical contact with the prey ciliate Colpidium (a + b). Within milliseconds extruded
toxicysts are detectable (c, arrow).

which feeds on other ciliates, e.g. on Parameciumor Colpidium (Fig. 4). Its oral bulge is filled withseveral hundred toxicysts (Fig. 3) (Kuhlmann et al.,1980). The resting toxicysts are detectable under alight microscope as rod-like elements (Fig. 3 a + b);
in cross section they appear in electron micrographs
as round to spherical organelles (Fig. 3 d + e)
(Kuhlmann and Hausmann, 1980).
Toxicysts are used to immobilize and to kill thenormally fast-swimming prey (Hausmann, 1978).
The explosive extrusion of toxicysts occurs due tomechanical contact with the food organism andlasts only milliseconds (Fig. 4). In all likelihood,chemical recognition of the prey is also involved inthis event. The tubules are forced into the prey or-

ganisms like hypodermic needles.
Resting toxicysts are morphologically complex
organelles which contain fully developed structuresinside a capsule which is located inside an extrusomal vesicle. Tubes lying within the capsule arefilled with an ordered material which is probably atoxin. Furthermore, an amorphous substance is
detectable in the vicinity of the capsule wall (Fig. 5
a - c, Fig. 6, resting stage).
The first step in the expulsion process of toxicysts is the fusion of the toxicyst membrane withthe plasma membrane (Fig. 5 d), after which a tubule is discharged via evagination (Fig. 6, discharging stage). Alternatively, for certain ciliates a
telescopic discharge of a tube is reported (Fig. 7).
During or near the end of toxicyst expulsion a toxic
material is secreted by the tubule (Fig. 7, toxifying).
Only wild speculations exist about the force driving
toxicyst ejection. In this context, however, the apparent similarities in structure, effect, and mode offunction between the toxicysts of protozoa and thenematocysts of cnidarians should be mentioned,
although the structures differ significantly in size.
Using darkfield microscopy, Krüger, the nestorof extrusomal research, postulated already in 1936
that three different categories of toxicysts exist:
capsule, tubule and tube. He based his scheme on
the ratio of the length of the different parts of theejected extrusomes (compare Fig. 7). Any deviationfrom these ratios, if they exist, have not been reported so far.
There are still important questions which need
to be answered for a better understanding of toxicyst function:
What triggers the ejection of toxicysts?
● What is the origin of the forces needed forthe expulsion of toxicysts?
● What is the nature of the extruded toxic
agent?
● How are toxicysts synthesized in the cytoplasm and rearranged in the oral bulge?
●

Jpn. J. Protozool. Vol. 35, No. 2. (2002)89

Ultrastructural appearance of toxicysts

Fig. 5. Resting toxicysts at higher magnifications (a - c) revealing complex internal structures a part of which
are inverted tubules. The first step of exocytotical toxicyst expulsion (d) is the fusion of the toxicyst
membrane (Tm) with the plasma membrane (Pm).

II. Food ingestion (Fig. 8)
Food ingestion, the internalization of externalmaterial into a cell, is ruled by certain necessities.
One of these is the fact that under normal, natural
conditions, everything which is engulfed by a cellneeds to be enclosed by a membrane, e.g. in thecase of protistan phagocytosis by a food vacuolar
membrane. The origin of this membrane is of greatinterest. From the light microscopical point of view,in some cases it seems to be trivial, in others merely
mystical. For instance, Amoeba proteus easily engulfs prey organisms by an invagination of its
plasma membrane. However, what occurs in Actinophrys sol, whose prey is enclosed in a giant foodvacuole, the origin of which cannot be the plasma
membrane at all, 10-15 minutes after the first contact (Fig. 8 a + b) (Linnenbach et al., 1983)?

Electron microscopical investigations of A. sol
revealed that during the development of the foodvacuole, extrusomes expand and fuse with eachother and eventually with the plasma membrane(Fig. 8 c + d). Their investing membrane is thereby
made available as food vacuole membrane(Hausmann and Patterson, 1982).
At first glance, food ingestion by ciliophora
seems to be rather diverse. But on closer inspection,
it obeys rules which seem to be generally valid forall phagocytosing ciliates (Radek and Hausmann,
1996): Food vacuoles are formed at the cytostome,
i.e., the region in the oral apparatus bounded onlyby the plasma membrane, which is lacking alveoli
and infraciliature. The membrane for the growing
food vacuole is not derived from the plasma membrane but is delivered by pre-formed cytoplasmic

90

Jpn. J. Protozool. Vol. 35, No. 2. (2002)

Expulsion of toxicysts

Fig. 6. Sequence of the expulsion of a toxicyst illustrated by selected stages of extrusion.

Relative length of capsule, tubule and tube

Fig. 7. Currently known length ratios of components of
extruded toxicysts.

vesicles which fuse with the plasma membrane in
the cytostomal region. De novo synthesis would be
far too slow to make the large amounts of required
membrane available within a relatively short periodof time, i.e., in few seconds or minutes. The vesicles are transported towards the cytostome alongribbons of microtubules. In all likelihood, cytoplasmic dynein, a minus-end-directed microtubulebased motor, is responsible for the movement.
However, in most cases due to the small size ofthese vesicles, their transport cannot be analyzed
using light microscopy. Electron microscopicalanalyses of these events were needed.
In Paramecium these vesicles are very flat andwithout visible contents. In other ciliates such as
Climacostomum, Eufolliculina, and Euplotes, thevesicles contain an electron-opaque material (atleast in some cases digestive enzymes) which isJpn. J. Protozool. Vol. 35, No. 2. (2002)

91

Food ingestion by Actinophrys sol

Fig. 8. Actinophrys sol with completed food vacuoles surrounding the prey (a + b). During food ingestion,compact resting extrusomes (c) transform to expanded stages (d) which fuse readily with each otherand the plasma membrane, thus supplying sufficient membrane material for the nascent food vacuole.

released when the vesicles fuse with the cytostomal
membrane.
The gulper Pseudomicrothorax dubius (Figs. 9-10)

A rather impressive example of ciliate phagocytosis is the food uptake by the gulper Pseudomicrothorax dubius. This ciliate is highly specialized
to feed on cyanobacterial filaments (Fig. 9).
A complex cytopharyngeal basket of rods aswell as sheets of microtubules and associatedstructures was developed to move algal filaments
into a growing food vacuole at very high speeds(Fig. 10) (Hausmann and Peck, 1978). During fooduptake, the membrane of the food vacuole increases
rapidly at rates of up to 270 µm2 per second. Vacuole growth results - again - from the fusion of
membrane-bound, spherical vesicles (1 µm in diameter) which are similar to the flat vesicles seen inother ciliates. During phagocytosis, a fast streamingof these relatively large vesicles can be observed
using light microscopy in the cytoplasm surrounding the basket (Fig. 11). The vesicles enter the lumen of the basket at its anterior end, in a zone
where the wall of the basket is perforated(Hausmann and Peck, 1979).

III. Food digestion (Fig. 11)
The process of food digestion has been studiedin great detail in ciliates like Tetrahymena (Nilsson,1979) and particularly in Paramecium (Allen andFok, 1993, 2000; Fok and Allen, 1993). Their general digestion processes correspond to those known
for multicellular organisms.
However, at least for the ciliate Pseudomicrothorax dubius there exists a two-step digestive scenario. The second step seems to be comparable with

92

Jpn. J. Protozool. Vol. 35, No. 2. (2002)

Food ingestion by the ciliate P. dubius

Fig. 9. Starved P. dubius cells start food uptake immediately after supply with cyanobacteria (left micrograph, arrows). The filaments (bga) are engulfed rapidly by a specially constructed ingestion apparatus, the cytopharyngeal basket (cb, right micrographs) ( fvm – food vacuole membrane).

the well-known processes of unicellular and multicellular organisms, but the first step is very different. Due to the fusion of the aforementioned vesicles with the growing food vacuole membrane (Fig.
11 b), lysosomal hydrolases (AcPase) are liberated
into the food vacuole. They are detectable in the
cyanobacterial cytoplasm within less than one second after AcPase's entry into the food vacuole, andwithin a few seconds, the destruction of the wall of
the cyanobacterial filament can be observed.
The benefits to P. dubius of rapid destructionof the cell walls of Oscillatoria are:softening of the rigid algal filament andtherefore faster food uptake.
● fast local extracellular digestion of the
cyanobacterial cell wall and subsequentfolding of algal filaments before uptake and●consequently ingestion of twice as muchfood.
● possibility of chemically “biting off” thealgal filament when the filament is not
driven into the cell but arrested in the cytopharyngeal basket; here the cell wall isdestroyed due to the activity of the digestive enzymes and the remnant algal filament is pulled out of the oral apparatus.
Pseudomicrothorax dubius ingests a large volume of filamentous blue-green alga within a shorttime span (about one minute). Immediately after
this rapid ingestion, the food is enclosed in a single,
extremely large food vacuole, which fills up theciliate almost entirely. During the following hour
this giant food vacuole vesiculates. Finally, numerous small vacuoles 1-2 µm in diameter are formed.

Jpn. J. Protozool. Vol. 35, No. 2. (2002)

93

Cytopharyngeal basket of
Pseudomicrothorax dubius

Fig. 10. The 3D-reconstruction of the cytopharyngeal basket of P. dubius (a) is based on numerous ultra-thinsections (b).

Simultaneously the content of the vacuoles is noticeably condensed. Only at this time does the digestion of the food start, as is indicated by numerous, highly active dictyosomes which now surroundthe periphery of the food vacuoles (Hausmann,
1980).

at the same time, especially in ciliates, act as sieves
for retaining food particles. Complex cytostomal
apparatuses, sometimes in combination with a special food-finding behaviour, have been evolved forthe ingestion of large food organisms. As a generalrule, the food vacuole grows by fusion of small
vesicles, partly extrusomes, with the cytostomal
area.

Conclusions
In the natural environment the nutrition of freeliving protists mainly depends on the uptake ofsolid food. A variety of devices for food-trapping
such as extrusomes has been developed in responseto the kind of food preferred. For example, small
particles such as bacteria are filtered out by the useof flagella and cilia which create water currents andAcknowledgements
This review is based on a plenary lecture heldin the workshop Prey-predator interaction in protozoa on the occasion of the Annual Meeting of theJapanese Society of Protozoology (JSP), November
16-18, 2001, in Kobe, Japan. The author is gratefulto the JSP for generously funding the travelling

94

Jpn. J. Protozool. Vol. 35, No. 2. (2002)

Rapid destruction of cyanobacterial cell walls by Pseudomicrothorax dubius

Fig. 11. Feeding P. dubius (a) and schematic representation of this process (b). The small arrows indicate thestreaming of vesicles opposite to the direction of the ingestion of the blue-green alga (bga, big arrow).
The vesicles enter the lumen of the cytopharyngeal basket (cb), fuse with the nascent food vacuole andsupply the material for the increase of the membrane of this vacuole (fvm) and simultaneously secrete
digestive enzymes into the food vacuole for the rapid destruction of the cyanobacterial cell walls.

expenses. The research of K. H. was supported by
grants from the Deutsche Forschungsgemeinschaft,Bonn, Germany.
REFERENCES
Allen, R. D., Fok, A. K., (1993) Endosomal membrane traffic of ciliates. In: Advances in celland molecular biology of membranes, vol. 2,
Membrane traffic in protozoa. Plattner, H.
(ed.). JAI Press, Greenwich, pp. 57-83.
Allen, R. D., Fok, A. K. (2000) Membrane trafficking and processing in Paramecium. Int.
Rev. Cytol. 198, 277-318.
Boenigk, J., and Arndt, H. (2000) Particle handlingduring interception feeding by four species ofheterotrophic nanoflagellates. J. Eukaryot.
Microbiol. 47, 350-358.
Boenigk, J., Arndt, H., and Cleven, E.-J. (2001) The
problematic nature of fluorescently labelledbacteria (FLB) in Spumella feeding experiments - an explanation by using video microscopy. Arch. Hydrobiol. 152, 329-338.

Fok, A. K., Allen, R. D. (1993) Membrane Flow inthe digestive cycle of Paramecium . In: Advances in cell and molecular biology ofmembranes, vol. 2, Membrane traffic in protozoa. Plattner, H. (ed.). JAI Press, Greenwich, pp. 85-111.
Hausmann, K. (1978) Extrusive organelles in protists. Int. Rev. Cytol. 52, 197-276.
Hausmann, K. (1980) Zur Digestion bei Pseudomicrothorax dubius Mermod - Nahrungsvakuolen-Vesikulation im Anschluß an die
Phagocytose. Zoomorphology 96, 231-241.
Hausmann, K., Hülsmann, N. (1996) Protozoology,
2nd ed.. Georg Thieme Verlag, Stuttgart.
Hausmann, K., Patterson, D. J. (1982) Pseudopod
formation and membrane production during
prey capture by a heliozoon (feeding by
Actinophrys, II). Cell Motil. 2, 9-24.
Hausmann, K., Peck, R. K. (1978) Microtubules
and microfilaments as major components of aphagocytic apparatus: the cytopharyngeal
basket of the ciliate Pseudomicrothorax

Jpn. J. Protozool. Vol. 35, No. 2. (2002)

dubius. Differentiation 11, 157-167.
Hausmann, K., Peck, R. K. (1979) The mode offunction of the cytopharyngeal basket of theciliate Pseudomicrothorax dubius. Differentiation 14, 147-158.
Ishida, M., Allen, R. D. and Fok, A. K. (2001)
Phagosome formation in Paramecium: Rolesof somatic and oral cilia and of solid particles
as revealed by video microscopy. J. Eukaryot. Microbiol. 48, 640-646.
Krüger, F. (1936) Die Trichocysten der Ciliaten im
Dunkelfeldbild. Zoologica 91, 1-83.
Kuhlmann, S., Hausmann, K. (1980) Untersuchungen zu Nahrungserwerb und Nahrungsaufnahme bei Homalozoon vermiculare, Stokes
1887. 2. Mucocysten, Konocysten, Toxicysten. Protistologica 16, 125-134.
Kuhlmann, S., Patterson, D. J., Hausmann, K.
(1980) Untersuchungen zu Nahrungserwerb
und Nahrungsaufnahme bei Homalozoon
vermiculare, Stokes 1887. 1. Nahrungserwerb und Feinstruktur der Oralregion. Protistologica 16, 39-55.
Linnenbach, M., Hausmann, K., Patterson, D. J.
(1983) Ultrastructural studies on the foodvacuole cycle of a heliozoon (feeding by
Actinophrys, III). Protoplasma 115, 43-51.
Nilsson, J. R. (1979) Phagotrophy in Tetrahymena.

95

In: Biochemistry and physiology of protozoa,
2nd. ed.. Levandowsky, M., Hutner S. H.
(eds.). Academic Press, New York, pp. 339 379.
Patterson, D. J., Hausmann, K. (1981) Feeding by
Actinophrys sol (Protista, Heliozoa): 1 Light
microscopy. Microbios 31, 39-55.
Peck, R., Hausmann, K. (1980) Primary lysosomesof the ciliate Pseudomicrothorax dubius:
cytochemical identification and role inphagocytosis. J. Protozool. 27, 401-409.
Radek, R., Hausmann K. (1996) Phagotrophy ofciliates. In: Ciliates: cells as organisms.
Hausmann, K., Bradbury, P. C. (eds.). Gustav Fischer Verlag, Stuttgart, 197-219.
Sakaguchi, M., Murakami, H. and Suzaki, T (2001)
Involvement of a 40-kDa glycoprotein infood recognition, prey capture, and inductionof phagocytosis in the protozoon Actinophryssol. Protist 152, 33-41.
Sakaguchi, M., Suzaki, T., Kamal Khan, S. M. M.,
Hausmann, K. (2002) Food capture by kinetocysts in the heliozoon Rhaphidiophrys
contractilis. Europ. J. Protistol. 37, 453-458.
Suzaki, T., Shigenaka, Y., Watanabe, S., Toyohara,a. (1980) Food capture and ingestion in thelarge heliozoan, Echinosphaerium nucleofilum. J. Cell Sci. 42, 61-79.