Diverse metabolic systems include autotrophs, osrnotrophs and phagotrophs. All forms of symbiosis exist: mutualism, commensalism and parasitism, in both external (e.g., ectocommensalism) and internal forms (e.g., endoparasitism), and all intergradations.

FEEDING

Feeding consists of two processes: 1) the movement of food particles to the cell membrane and 2) phagocytosis.

Click here for a web site with photographs of free-living fresh-water protoctists classified according to feeding strategy.

PHAGOTROPHS

Particulate feeders. Food particle size varies widely, from bacteria to large prey. Micro- through macrophagous forrns foreshadow similar a distinction among metazoans, although a continuous gradient of prey particle size  relative to predator size exists.

So-called zoomastigophorans (heterotrophic flagellates) include both osmo- and phagotrophic forms, but few species rely on dissolved organic material. Even parasitic species are chiefly phagotrophs.

Amoeboid forms ingest at any point of the cell surface.

Ciliates and most flagellates have a more or less permanent cytostome. Most, especially ciliates, have a more or less fixed form strengthened by a pellicle.

A marked tendency exists for osmo- and phagotrophic taxa to develop in primarily autotrophic groups, sugesting repeated independent development of heterotrophy. Thus, the grouping of phototrophic flagellates together as Phytomastigophorea is artificial.

In the typically phototrophic genus Euglena, E. gracilis loses it's pigments but survives and grows in the dark, obtaining energy by oxidizing acetic acid (many flagellates can). It is thus a facultative phototroph. It has little capacity for using carbohydrates. Acetate is probably it's sole souce of organic carbon. It also can be induced to lose its chlorophyll via treatment with streptomycin.

E. pisciformis dies in the dark and is thus an obligate phototroph. However, euglenids are actually auxotrophic, that is, they need small amounts of organic compounds. E. pisciformis requires an outside source of thiamine. E. gracilis, E. viridis and many other photosynthetic (i.e., supposedly autotrophic) flagellates need vitamin B12

Some euglenids are phagotrophic instead. Peranema lacks pigments. It's rod organ, composed of microtubules and associated with the cell reservoir, is protruded for feeding on other flagellates. It attaches to stationary prey and continually reattaches in order to stuff its prey into its cytostome. It also may rasp at the prey surface. Peranema is able to digest and incorporate starch, oil and casein.

Among dinoflagellates, different species of Gyrodinium are either photo- or phagotrophic. Some individuals of the phototrophic species Ceratium hirundinella are colorless.

Choanoflagellates
A central flagellum propels water inward through a collar of 20-50 tentacles (straight fine pseudopodia), each 0.1mm thick, 0.1-0.3 mm apart. Particles are ingested by pseudopods arising from the collar margin. Diet consists of prokaryotes. Filtration rate: 10⁵ x cell vol/hr.

Ciliates
Wide diversity and specialization. Water currents are generated by zones of cilia fused together as membranelles that may also function as filters. Among oligotrichs, water is propelled away from the central mouth; particles retained on the inside of the membranelle zone accumulate at the cytostome.

Membranelles may also draw water through or against a paroral or undulating membrane, an array of motionless cilia that acts as sieve. The mechanism for rejection of unwanted particles is unknown.

RAPTORIAL FEEDING

Small flagellates
The flagellum drives a water current against  the cytostome and particles are phagocytized.

Dinoflagellates
Gyrodinium and relatives have trichocysts (threadlike extrusible organelles) that immobilize prey.

Noctiluca has an adhesive tentacle that brings algal cells, other protoctists and small crustacea to the cell for phagocytosis.

Ciliates
Didinium, a specialized feeder on Paramecium, has toxicysts, extrusible organelles for immobilizing prey. It's expandable mouth set on a slender proboscis permits ingestion of prey larger than itself. The cytostome is strengthened by microtubular rods as in Peranema.

Many raptorial ciliates exhibit specialized diets. Some have a pharyngeal basket of rods specialized for dealing with filamentous algae.

Histophagous (tissue-eating) ciliates enter wounds of damaged invertebrates. Their polymorphic life cycle includes a swarmer stage, feeding stage and cyst stage. The short-lived feeding stage (30-60 min) consumes 10x its own volume.

Sarcodines
Lobose and filose shelled and naked amoebas . Large species surround ciliates, diatoms, small invertebrates, &c. Small species feed on bacteria and microalgae.

DlFFUSION FEEDlNG

Relies on prey corning into contact with a nonmotile predator. Typical of actinopods, foraminiferans and many rhizopod amoebas.

Heliozoa
Axopods radiate in all directions. Prey are held by poorly understood extrusomes and are brought closer to the cell via axopod bending or cytoplasmic streaming. Pseudopods that arise from the cell engulf the prey. Small species feed on bacteria and flagellates. Large species ingest ciliates, rotifers, crustaceans &c.

Planktonic foraminiferans
Prey stick to reticulopodia and are then enveloped, penetrated and drawn into a vacuolated ectoplasm for digestion in food vacuoles.

Ciliate suctorians
Mature species lack cilia. Adhesive tentacles immobilize and initially penetrate prey. Prey contents are drawn through tentacles and into the cell body.

DIGESTION

All phagotrophy results in phagocytosis of food particles into a food vacuole with intracytoplasmic digestion. Phagocytosis is induced at least partly by mechanical stimulation of the cell membrane by the food particle. Food vacuole formation does not occur in particle-free water.

Amoeba takes in fluid and food at any point on its cell surface. The food vacuole first decreases in size, probably due to diffusion of fluid into the cytoplasm. Acidity increases to about 5.6, probably as a result of chemical changes associated with prey death. Later, pH increases to about 7.3, probably due to inflow of fluids and enzymes. Lysosomes containing hydrolytic enzymes fuse with the food vacuole. Enzymes have optimal activity levels within sharply defined pH limits. Dissolved breakdown products are removed from the vacuole via pinocytotic vesicles. Amoeba cannot digest starch.

Paramecium forces food into the narrow end of it's buccal cavity via ciliary pressure. The end distends to form a sac around the food under the ciliary pressure and pinches off as a vacuole. Acidity reaches 1.4 although true acid secretion from the cell itself has not been clearly demonstrated. Digestion initially occurs at about pH 7.8. Paramecium can digest both protein and fat, but is poor at starch.

Vacuoles move through the cell via cytoplasmic streaming; In ciliates, elimination is random so that the lifetime of vacuoles has an exponential distribution.

The final stage involves the transport of membrane material derived from vacuoles that fused with the cell surface as small vesicles back to the cytostome to permit formation of more vacuoles. The maximum rate of phagocytosis appears to be limited by the membrane recycling rate.

NUTRITION

Axenic studies
Culturing single species under axenic conditions, that is, free of all other organisms including bacteria, permits determination of dependence on any pre-formed organic compounds.

The freshwater ciliate, Tetrahymena pyriformis, requires 10 amino acids, 2 nucleic acid bases, 6 vitamins including thiamine and riboflavin, and several inorganic ions (e.g., Mg, K, Cu, P0₄). It is thus more specialized than many vertebrates  that can manufacture their own nucleic acid bases.

Food as a controlling factor
Among heterotrophic species, the kind and amount of food may represent the single most important factor controlling species distributions and community composition.

Some species have highly specific diets; others are very catholic.

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