Classification and characteristics of protoctists

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Kingdom protoctista

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

As was noted in the five kingdomclassification of Margulis and Schwartz, the protoctista is probably the mostcontroversial group because it is the least natural. It is really a collectionof all the eukaryotic organisms that do not fit neatly into the other threeeukaryote kingdoms. Many are unicellular.

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

The group is fascinating to thoseinterested in evolution because they are the link between prokaryotes and themore advanced eukaryotes like plants and animals. For example, during the 1960sit was discovered that mitochondria, the ‘powerhouses’ of cells that provideenergy in aerobic respiration, contained their DNA and ribosomes which resemblethose of prokaryotes. There is now good evidence based on an examination of thebase sequences in the mitochondrial DNA, that mitochondria were formerlyaerobic bacteria (prokaryotes) that invaded an ancestral eukaryote cell and‘learned’ to live symbiotically within it. Now all eukaryote cells containmitochondria, and the mitochondria can no longer live independently.

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

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

Phylum Oomycota

Oomycotes are close relations of the fungiand have similar structure, but are now regarded as a more ancient group. Theircell walls contain cellulose, not chitin, as the strengthening material. Theirhyphae are aseptate. In this phylum are a number of pathogenic organisms,including the downy mildews. One of these, Phytophthorainfestans, will be studied as an example of a parasite which is generallydescribed as obligate. Perosnospora, anobligate parasite, will also be mentioned for comparison. Finally, Pythium will be examined as a typicalexample of a facultative parasite. An obligate parasite is one which can onlysurvive and grow in living cells whereas facultative parasites typically bringabout the deaths of their hosts before living saprotrophically on the remains.

Phytophthorainfestans

Phytophthorainfestans is a pathogen of economic importancebecause it parasitizes potato crops, causing a potentially devastating diseaseknown as potato blight. It does not grow independently of its host and in thisrespect resembles obligate parasites. It is similar in its structure and modeof attack to another member of the Oomycota, Perosnospora, which is a common, but less serious, disease ofwallflower, cabbage and other members of the plant family cruciferae.

The Phytophthoramycelium 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 hyphaespreads through the unicellular spaces of the leaves, giving off branched haustoria which push into the mesophyllcells and absorb nutrients from them. Haustoria are typical of obligateparasites. They are specialised penetration and absorption devices. Each is amodified hyphal outgrowth with a large surface area which pushes into cellswithout breaking their cell surface membranes and without killing them.

In warm, humid conditions the myceliumproduces long, slender structures called sporangiophoreswhich 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 splashedby raindrops on to other plants, where further infection takes place.

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

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

First the base and then the rest of theplant becomes a putrid mass as the dead areas become secondarily infected withdecomposing bacteria (saprotrophs). Phytophthorathus 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 describedas facultative, though the distinction is perhaps not worth stressing here.

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

In the past, Phytophthora epidemics have had serious consequences. The diseaseis thought to have been accidentally introduced into Europe from America in thelate 1830s and caused a series of epidemics that totally destroyed the potatocrops in Ireland in 1845 and in subsequent years. Widespread famine resultedand many starved to death, victims as much of complex economic and politicalinfluence as of the disease. Many Irish family migrated to North America as aresult.

The disease is also of interest because in1845 Berkeley provided the first clear demonstration that microorganisms causedisease by showing that the organism associated with potato blight caused thedisease, rather than being a by-product of decay.

Controlling the potato blight disease of Phytophthora

Knowledge of the lifecycle of potato blighthas since led to the methods of controlling the disease. These are summarizedbelow:

  • Care must be taken to ensurethat no infected tubers are planted.
  • New plantings must not be madein soil known to have carried the disease a year previously, since the organismcan survive up to one year in the soil. Crop rotation may therefore help.
  • All diseased parts of infectedplants should be destroyed before lifting tubers, for example by burning orspraying with a corrosive solution such as sulphuric acid. This is becausetubers can be infected from decaying haulms (stems) and aerial parts.
  • Since the organism canoverwinter in unlifted tubers, care must be taken to ensure that all tubers arelifted in an infected field.
  • The organism can be attackedwith copper-containing fungicides, such as Bordeaux mixture. Spraying must becarried out at the correct time to prevent an attack, since infected plantscannot be saved. It is usual to spray at fortnightly intervals, from the timethat the plants are a few centimetres high until they are well matured. Tubersintended as seed potatoes can be sterilised externally by immersion in a dilutemercury (II) chloride solution.
  • Accurate monitoring ofmeteorological conditions coupled with an early warning system for farmers, canhelp to decide when spraying should be carried out.
  • Breeding for resistance to theblight has been carried for some years. The wild potato, Solanum demissum, is known to show high resistance and has beenused in breeding experiments. One great obstruction to producing the requiredimmunity lies in the fact that the organism exists in many strains and nopotato has been found to be resistant to all of them. New strains of theorganism may appear as new strains of potato are introduced. This is a familiarproblem in plant pathology and emphasises the need for conservation of the wildancestors of modern crop plants as sources of genes for disease resistance.

Pythium

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

Damping-off of seedlings is due todestruction of the shoot as it appears above the soil. Watery spots firstappear on the stem at soil level. As these darken, the stem collapses. It canbe a serious problem in horticulture, forestry and agriculture. Members of the cabbagefamily (crucifers) are particularly susceptible, especially when the seedlingsare grown in crowded conditions.

Algae

The algae form a large group ofprotoctistans of great biological importance and significance to humans. Nosingle characteristic is diagnostic. They are best thought of as photosyntheticeukaryotes that evolved in, and have remained in, water. A few algae haveescaped to live successfully on land, these are insignificant in numbercompared to those in the oceans and freshwater. The bodies of algae lack truestems, roots and leaves. Such a relatively undifferentiated body is called a thallus.

The algae fall naturally into distinctgroups, chiefly on the basis of their photosynthetic pigments. These groups aregiven the status of phyla in modern classifications. The characteristics of thealgae and two of the main phyla are shown in the table below. Two examples ofalgae, namely Chlorella (phylumChlorophyta) and Fucus (phylumphaeophyta) are examined in more detail below.

Classification and characteristics of two of the maingroups 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-motilegreen alga. Its habitat is freshwater ponds and ditches. It is easily cultured andhas been used as an experimental organism in research on photosynthesis as wellas being an alternative source of food (single cell protein)

Phylum phaeophyta (brown algae)

Fucus

Fucus is relatively large and complex brownalga. Its body is a thallus which is differentiated into a stipe, holdfast andfronds (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 therelatively harsh conditions of the shore, where it is alternately exposed andcovered by the tides.

There are three common species and theseare often found at three different levels, or zones, on the shore, phenomenoncalled zonation. They areprincipally zoned according their ability to withstand exposure to air. Theirchief recognition features and positions on the shore are noted below.

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

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

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

Adaptations of fucus to environment.

Before discussing the adaptation of fucusto its environment, some mention must be made of the nature of thisenvironment, which is relatively hostile. Being intertidal, the differentspecies are subjected to varying degrees of exposure to air when the tiderecedes. Therefore they must be protected against drying out. Temperatures maychange rapidly, as when a cold advances into a hot rock pool. Salinity is anotherfactor to which the organism has adapted, and this may increase in an evaporatingrock pool, or decrease during rain. The surge and tug of the tide, and thepounding of waves, are additional factors which demand mechanical strength ifthey are to be withstood. Large waves can pick up stones and cause great damageas they crash down.

Morphological adaptations (overall structure).

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

The thallus shows dichotomous branching(branching into two at each branch point). This minimises resistance to theflow of water which can pass between the branches. The thallus is also toughbut 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 lightfor photosynthesis.

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

Physiological adaptations

The dominant photosynthetic pigment is thebrown pigment fucoxanthin. This isbecause fucoxanthin strongly absorbs blue light, which penetrates water muchfurther than longer wavelengths such as red light.

The thallus secretes large quantities ofmucilage which fills spaces within the body and exudes on to its surface. Thishelps 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 thetides. At low tide the thallus dries and squeezes the sex organs, which areprotected by mucilage, out of the conceptacles. As the tide advances, the wallsof the sex organs dissolve and release the gametes.

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

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

Protozoa

Like the algae, the protozoans form a largegroup of protoctistans. They are unicellular, animal-like cells withheterotrophic nutrition. There are over 50 000 known species and they are foundin all environment where water is present. Most are free-living and there arevarious methods of locomotion. Some, however, are parasites, including one (plasmodium) which causes the diseasewhich 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 havethe following characteristics:

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

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

Cilia occur in pairs. They run in rowsdiagonally across the body, causing the body to rotate as they beat and movethe 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 foranchorage during feeding.

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

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

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

Two fixed contractile vacuoles are present in the endoplasm. They areresponsible for osmoregulation, that is, the maintenance of a constant water potentialinside the cell. As a result of living in fresh water, water constantly entersthe cell by osmosis. This water has to be pumped out by an energy-consumingactive transport mechanism to prevent the cell from bursting. Around eachcontractile vacuole a number of canals radiate outwards and collect waterbefore emptying it into the main vacuole.

The cell contains two nuclei. The larger,bean-shaped macronucleus ispolyploid (has more than two sets of chromosomes). It controls metabolicactivities apart from reproduction. The micronucleusis diploid. It controls reproduction and the formation of new macronucleiduring nuclear division.

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

Phylum Apicomplexa

This group of protozoans alsopossesses  a pellicle, giving the cell adefinite shape. Most, however, possess no special structures for locomotion andhave limited movement. Their most distinguishing characteristic is theproduction of spores during asexual and sexual reproduction. An example is theparasite plasmodium  which causes malaria in humans

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