Classification and characteristics of protoctists

v

Kingdom protoctista

(Greek protos, very first; ktistos, to establish)

As was noted in the five kingdom classification of Margulis and Schwartz, the protoctista is probably the most controversial group because it is the least natural. It is really a collection of all the eukaryotic organisms that do not fit neatly into the other three eukaryote kingdoms. Many are unicellular.

The protoctista contains eukaryotes that are generally regarded as identical or similar to the ancestors of modern plants, animals and fungi. It includes organisms which resembles early plants (algae), early animals (protozoa) and early fungi (oomycota). It also includes a group known as the slime moulds which produce spores like fungi but can creep slowly over surfaces and are therefore motile like animals. The earliest eukaryotes were probably unicellular organisms which moved by beating flagella.

The group is fascinating to those interested in evolution because they are the link between prokaryotes and the more advanced eukaryotes like plants and animals. For example, during the 1960s it was discovered that mitochondria, the ‘powerhouses’ of cells that provide energy in aerobic respiration, contained their DNA and ribosomes which resemble those of prokaryotes. There is now good evidence based on an examination of the base sequences in the mitochondrial DNA, that mitochondria were formerly aerobic bacteria (prokaryotes) that invaded an ancestral eukaryote cell and ‘learned’ to live symbiotically within it. Now all eukaryote cells contain mitochondria, and the mitochondria can no longer live independently.

Like mitochondria, chloroplasts, the chlorophyll containing organelles responsible for photosynthesis, also contain their own prokaryotic DNA and ribosomes. These seem to have evolved from photosynthetic bacteria that invaded the heterotrophic animal-like cells, turning them into algae which are autotrophic. It is also likely that red algae may have evolved in this way from blue-green bacteria and that green algae evolved from green bacteria known as prochlorophytes.

The theory that mitochondria and chloroplasts are the descendants of symbiotic bacteria is known as the endosymbiont theory. An endosymbiont is an organism that lives symbiotically inside (endo-) another organism.

Phylum Oomycota

Oomycotes are close relations of the fungi and have similar structure, but are now regarded as a more ancient group. Their cell walls contain cellulose, not chitin, as the strengthening material. Their hyphae are aseptate. In this phylum are a number of pathogenic organisms, including the downy mildews. One of these, Phytophthora infestans, will be studied as an example of a parasite which is generally described as obligate. Perosnospora, an obligate parasite, will also be mentioned for comparison. Finally, Pythium will be examined as a typical example of a facultative parasite. An obligate parasite is one which can only survive and grow in living cells whereas facultative parasites typically bring about the deaths of their hosts before living saprotrophically on the remains.

Phytophthora infestans

Phytophthora infestans is a pathogen of economic importance because it parasitizes potato crops, causing a potentially devastating disease known as potato blight. It does not grow independently of its host and in this respect resembles obligate parasites. It is similar in its structure and mode of attack to another member of the Oomycota, Perosnospora, which is a common, but less serious, disease of wallflower, cabbage and other members of the plant family cruciferae.

The Phytophthora mycelium overwinters in potato tubers and grows up to the leaves in spring. Blight is usually noticed in the leaves in August.

A mycelium of branched, aseptate hyphae spreads through the unicellular spaces of the leaves, giving off branched haustoria which push into the mesophyll cells and absorb nutrients from them. Haustoria are typical of obligate parasites. They are specialised penetration and absorption devices. Each is a modified hyphal outgrowth with a large surface area which pushes into cells without breaking their cell surface membranes and without killing them.

In warm, humid conditions the mycelium produces long, slender structures called sporangiophores which emerge from the lower surface of the leaf through stomata or wounds. These branch and give rise to sporangia. In warm conditions, sporangia may behave as spores, being blown or splashed by raindrops on to other plants, where further infection takes place.

A hypha emerges from the sporangium and penetrates the plant through stoma, lenticel or wound. In cool conditions, the sporangium contents may divide to form swimming spores (a primitive feature) which, when released, swim in surface films of moisture. They may encyst until conditions are suitable once more for hyphal growth, then produce new infections.

Diseased plants show individual leaflets with small, brown, dead, ‘blighted’ areas. Inspection of the lower surface of an infected leaflet reveals a fringe of white sporangiophores around the dead area. In warm, humid conditions, the dead area spreads rapidly through the whole leaf and into the stem. Some sporangia may fall to the ground and infect potato tubers. Here infection spreads very rapidly, causing a form of dry rot in which the tissues are discoloured a rusty brown in an irregular manner from the skin to the centre of the tuber.

First the base and then the rest of the plant becomes a putrid mass as the dead areas become secondarily infected with decomposing bacteria (saprotrophs). Phytophthora thus kills the whole plant, unlike its close relative Peronospora which is an obligate parasite. In this respect, Phytophthora is not a typical obligate parasite and it is sometimes described as facultative, though the distinction is perhaps not worth stressing here.

The organism normally overwinters as a dormant mycelium within lightly infected potato tubers. Except where the potato is native (Mexico, Central and South America) it is thought that the organism rarely reproduces sexually, unlike Peronospora, but under laboratory conditions it can be induced to do so. Like Peronospora, it produces a resistant resting spore, it is the result of fusion between an antheridium (male) and an oogonium (female), and a thick-walled spore is produced. This can remain dormant in the soil over winter and cause infection the following year.

In the past, Phytophthora epidemics have had serious consequences. The disease is thought to have been accidentally introduced into Europe from America in the late 1830s and caused a series of epidemics that totally destroyed the potato crops in Ireland in 1845 and in subsequent years. Widespread famine resulted and many starved to death, victims as much of complex economic and political influence as of the disease. Many Irish family migrated to North America as a result.

The disease is also of interest because in 1845 Berkeley provided the first clear demonstration that microorganisms cause disease by showing that the organism associated with potato blight caused the disease, rather than being a by-product of decay.

Controlling the potato blight disease of Phytophthora

Knowledge of the lifecycle of potato blight has since led to the methods of controlling the disease. These are summarized below:

  • Care must be taken to ensure that no infected tubers are planted.
  • New plantings must not be made in soil known to have carried the disease a year previously, since the organism can survive up to one year in the soil. Crop rotation may therefore help.
  • All diseased parts of infected plants should be destroyed before lifting tubers, for example by burning or spraying with a corrosive solution such as sulphuric acid. This is because tubers can be infected from decaying haulms (stems) and aerial parts.
  • Since the organism can overwinter in unlifted tubers, care must be taken to ensure that all tubers are lifted in an infected field.
  • The organism can be attacked with copper-containing fungicides, such as Bordeaux mixture. Spraying must be carried out at the correct time to prevent an attack, since infected plants cannot be saved. It is usual to spray at fortnightly intervals, from the time that the plants are a few centimetres high until they are well matured. Tubers intended as seed potatoes can be sterilised externally by immersion in a dilute mercury (II) chloride solution.
  • Accurate monitoring of meteorological conditions coupled with an early warning system for farmers, can help to decide when spraying should be carried out.
  • Breeding for resistance to the blight has been carried for some years. The wild potato, Solanum demissum, is known to show high resistance and has been used in breeding experiments. One great obstruction to producing the required immunity lies in the fact that the organism exists in many strains and no potato has been found to be resistant to all of them. New strains of the organism may appear as new strains of potato are introduced. This is a familiar problem in plant pathology and emphasises the need for conservation of the wild ancestors of modern crop plants as sources of genes for disease resistance.

Pythium

Unlike Phytophthora, Pythium is a relatively unspecialised parasite, attacking a great variety of plants and causing a soft rot. It causes ‘damping off’ in seedlings. It needs damp conditions since it produces swimming spores during asexual reproduction. It can grow on the living plant or on its dead remains, so is a facultative parasite. It can also live saprotrophically in wet soil. It produces extracellular enzymes which help in attack and kill its host rapidly. The first enzymes produced are pectinases which diffuse ahead of the growing fungus and digest the pectin in the middle lamellae which hold the cells together. As a result the plant tissues dissolve into a mush (soft rot). The plant collapses. Later, other enzymes are produced which digest the contents of the plant cells, but it does not produce haustoria, unlike Phytophthora. Products of digestion are absorbed by hyphae which grow between the cells.

Damping-off of seedlings is due to destruction of the shoot as it appears above the soil. Watery spots first appear on the stem at soil level. As these darken, the stem collapses. It can be a serious problem in horticulture, forestry and agriculture. Members of the cabbage family (crucifers) are particularly susceptible, especially when the seedlings are grown in crowded conditions.

Algae

The algae form a large group of protoctistans of great biological importance and significance to humans. No single characteristic is diagnostic. They are best thought of as photosynthetic eukaryotes that evolved in, and have remained in, water. A few algae have escaped to live successfully on land, these are insignificant in number compared to those in the oceans and freshwater. The bodies of algae lack true stems, roots and leaves. Such a relatively undifferentiated body is called a thallus.

The algae fall naturally into distinct groups, chiefly on the basis of their photosynthetic pigments. These groups are given the status of phyla in modern classifications. The characteristics of the algae and two of the main phyla are shown in the table below. Two examples of algae, namely Chlorella (phylum Chlorophyta) and Fucus (phylum phaeophyta) are examined in more detail below.

Classification and characteristics of two of the main groups of algae

General characteristics Almost all are specialised for an aquatic existenceGreat range of size and form, including unicellular, filamentous, colonial and thalloid forms. A thallus is a body which is not differentiated into true roots, stems and leaves and lacks a true vascular system (xylem and phloem). It is often flat.Photosynthetic eukaryotic
Phylum chlorophyta (green algae) Phylum phaeophyta (brown algae)
Dominant photosynthetic pigment is chlorophyll; therefore green in appearance. Chlorophylls a and b present (as in plants) Dominant photosynthetic pigment s brown and called fucoxanthin. Chlorophyll a and c
Store carbohydrate as starch (insoluble) Store carbohydrate as soluble laminarin and mannitol. Also store fat
Large range of types, e.g. unicellular, filamentous, colonial, thalloid Filamentous or thalloid, often large
Mostly freshwater Nearly all marine (three freshwater genera only)
e.g. Chlorella, a unicellular, non-motile alga Spirogyra, a filamentous alga Chlamydomonas, a unicellular, motile alga Ulva, a thalloid, marine alga e.g. Fucus, a thalloid, marine alga Laminaria, large thalloid, marine alga; one of the kelps

Phylum chlorophyta (green algae)

Chlorella

Chlorella is a unicellular, non-motile green alga. Its habitat is freshwater ponds and ditches. It is easily cultured and has been used as an experimental organism in research on photosynthesis as well as being an alternative source of food (single cell protein)

Phylum phaeophyta (brown algae)

Fucus

Fucus is relatively large and complex brown alga. Its body is a thallus which is differentiated into a stipe, holdfast and fronds (note these are not true stem, roots and leaves). It is a marine alga, common on rocky shores off the British coast. It is well adapted to the relatively harsh conditions of the shore, where it is alternately exposed and covered by the tides.

There are three common species and these are often found at three different levels, or zones, on the shore, phenomenon called zonation. They are principally zoned according their ability to withstand exposure to air. Their chief recognition features and positions on the shore are noted below.

F. spiralis (flat wrack) – towards high tide mark if suspended, the thallus adopts a slight spiral twist.

F. serratus (common, serrated or saw wrack) – middle zone, Edge of the thallus is serrated.

F. vesiculosus (bladder wrack) – towards low tide mark, possesses air bladders for buoyancy.

Adaptations of fucus to environment.

Before discussing the adaptation of fucus to its environment, some mention must be made of the nature of this environment, which is relatively hostile. Being intertidal, the different species are subjected to varying degrees of exposure to air when the tide recedes. Therefore they must be protected against drying out. Temperatures may change rapidly, as when a cold advances into a hot rock pool. Salinity is another factor to which the organism has adapted, and this may increase in an evaporating rock pool, or decrease during rain. The surge and tug of the tide, and the pounding of waves, are additional factors which demand mechanical strength if they are to be withstood. Large waves can pick up stones and cause great damage as they crash down.

Morphological adaptations (overall structure).

The thallus is firmly anchored by a holdfast. This forms an intimate association with its substrate, usually rock, and is extremely difficult to dislodge. In fact, the rock often breaks before the holdfast.

The thallus shows dichotomous branching (branching into two at each branch point). This minimises resistance to the flow of water which can pass between the branches. The thallus is also tough but non-rigid and its midrib is strong and flexible.

F. vesiculosus possesses air bladders for buoyancy, thus holding it fronds up near the surface for maximum interception of light for photosynthesis.

Chloroplasts are mainly located in the surface layers for maximum exposure to light for photosynthesis.

Physiological adaptations

The dominant photosynthetic pigment is the brown pigment fucoxanthin. This is because fucoxanthin strongly absorbs blue light, which penetrates water much further than longer wavelengths such as red light.

The thallus secretes large quantities of mucilage which fills spaces within the body and exudes on to its surface. This helps to prevent desiccation by retaining water.

The solute potential of the cells is higher (less negative) than that of sea water, so water is not lost by osmosis.

Reproductive adaptations

Release of gametes is synchronised with the tides. At low tide the thallus dries and squeezes the sex organs, which are protected by mucilage, out of the conceptacles. As the tide advances, the walls of the sex organs dissolve and release the gametes.

The male gametes are motile and chemotactic, attracted by a chemical secretion of the female gametes.

The zygote develops immediately after fertilisation minimising the risk of being swept out to sea.

Protozoa

Like the algae, the protozoans form a large group of protoctistans. They are unicellular, animal-like cells with heterotrophic nutrition. There are over 50 000 known species and they are found in all environment where water is present. Most are free-living and there are various methods of locomotion. Some, however, are parasites, including one (plasmodium) which causes the disease which is estimated to have killed more humans than any other, namely malaria. It is still one of the world worst killers.

Phylum ciliophora (ciliates)

Ciliates are a type of protozoan. They have the following characteristics:

  • Unicellular, heterotrophic;
  • Possession of cilia, fine hairs which beat and cause movement of water, either for locomotion or feeding;
  • A definite shape due to the presence of a thin, flexible outer region of cytoplasm, called the pellicle, which is covered by the cell surface membrane;
  • A complex cell structure with a macronucleus and a micronucleus.

A common example of ciliate is paramecium. It lives in stagnant water, or slow-flowing fresh water that contains decaying organic matter. The complexity of the cell is explained by the fact that it has to perform all the functions of a whole organism, such as feeding, osmoregulation and locomotion. The body shape is characteristic, being blunt at the front (anterior) end and tapered at the back (posterior).

Cilia occur in pairs. They run in rows diagonally across the body, causing the body to rotate as they beat and move the cell forward. Between the cilia are wholes leading into chambers called trichocysts. From these chambers, sharply tipped fine threads can be discharged which are probably used for anchorage during feeding.

Beneath the pellicle is a layer of ectoplasm, a clear, firm cytoplasm in the form of a gel. Basal bodies (identical to centrioles) are found here. They are the structures from which cilia are formed. There is also a network of fine fibres running between the basal bodies which may be involved in coordinating the beat of the cilia.

The bulk of the cytoplasm is in the form of endoplasm, which exists in a more liquid state than the ectoplasm. Here most of the organelles are found on the ventral (lower) surface near the front of the organism. It tapers back into a narrow tube-like gullet at the end of which the endoplasm is exposed to form a mouth or cytostome. Both the oral groove and gullet are lined with cilia which beat and cause a current of water to flow towards the cytostome, carrying food particles such as bacteria in suspension.

The food particles are ingested into a food vacuole formed by the endoplasm (endocytosis). The vacuoles follow a distinct pathway through the endoplasm, finishing at the cytoproct or anal pore, where undigested material is egested (exocytosis). During their movement through the cytoplasm, lysosomes add digestive enzymes to the vacuoles and products of digestion are absorbed into the surrounding cytoplasm.

Two fixed contractile vacuoles are present in the endoplasm. They are responsible for osmoregulation, that is, the maintenance of a constant water potential inside the cell. As a result of living in fresh water, water constantly enters the cell by osmosis. This water has to be pumped out by an energy-consuming active transport mechanism to prevent the cell from bursting. Around each contractile vacuole a number of canals radiate outwards and collect water before emptying it into the main vacuole.

The cell contains two nuclei. The larger, bean-shaped macronucleus is polyploid (has more than two sets of chromosomes). It controls metabolic activities apart from reproduction. The micronucleus is diploid. It controls reproduction and the formation of new macronuclei during nuclear division.

Paramecium can reproduce both asexually (by transverse binary fission) and sexually (by conjugation).

Phylum Apicomplexa

This group of protozoans also possesses  a pellicle, giving the cell a definite shape. Most, however, possess no special structures for locomotion and have limited movement. Their most distinguishing characteristic is the production of spores during asexual and sexual reproduction. An example is the parasite plasmodium  which causes malaria in humans