Phycology classLecture handouts

EuglenidsKalle Olli1

AbstractEuglenids are a group of >1500 described species of single-celled flagellates with diverse modes of nutrition, includingphagotrophy and photoautotrophy.Almost all euglenids are free-living. The (usually) one or two emergent flagella have thick paraxonemal (paraxial) rods andoriginate in a deep pocket/reservoir, while the cell surface is almost always supported by a pellicle of parallel proteinaceousstrips underlain by microtubules.Cell wall is made of a proteinaceous pellicle — stripes ranging from apex to antapex along the cell. Cells with 4–12strips are rigid; most of those with more strips (typically ca 20–40) have them arranged helically and exhibit active celldeformation called euglenid motion or metaboly.Most phagotrophic euglenids are surface-associated bacterivores or eukaryovores that employ a flagellar gliding motility.They are abundant in marine and freshwater sediments.Photoautotrophic species (Euglenophyceae) constitute a single subclade within euglenids and have a chloroplast ofsecondary endosymbiotic origin, with three bounding membranes. The plastid is typically green, with chlorophylls a +b, and was derived from a green alga related to the Pyramimonadales. Photoautotrophic euglenids move primarily by

swimming, and most (members of the taxon Euglenales, e.g., Euglena) have a single emergent flagellum and are generallyrestricted to fresh and brackish waters.

Keywordspellicle, paramylon, gliding, euglenoid movement, paraxial rod

1EMU

Contents

Front matter 1Where do euglenids belong? . . . . . . . . . . . . . 2Nutritional diversity . . . . . . . . . . . . . . . . . . 2Euglenids are ambiregnal . . . . . . . . . . . . . . . 3Practical importance . . . . . . . . . . . . . . . . . . 4

Characteristic features in a nutshell 4

Pellicle and metaboly 5

Cell structures 6Flagella and locomotion . . . . . . . . . . . . . . . . 6Feeding apparatus . . . . . . . . . . . . . . . . . . . 7Plastids . . . . . . . . . . . . . . . . . . . . . . . . 8

Photoreception . . . . . . . . . . . . . . . . . . . . 8Mitochondrion . . . . . . . . . . . . . . . . . . . . . 9

Habitats 9

Euglenophyte examples 10Trachelomonas . . . . . . . . . . . . . . . . . . . .10Colacium . . . . . . . . . . . . . . . . . . . . . . .10Phacus . . . . . . . . . . . . . . . . . . . . . . . . .11Euglena . . . . . . . . . . . . . . . . . . . . . . . .12Eutreptiella . . . . . . . . . . . . . . . . . . . . . .12

Wrap Up 12

Front matter

Euglenids (syn. euglenoids) are a prominent group of free-living, aquatic flagellates, usually with one or two activeflagella. Most of the >1500 described species are unicellsthat are 5–50 μm in length, a few are larger.

Almost all are motile. There are three types of motion:

• Swimming• Surface-associated gliding• Euglenid motion or metaboly

Figure 1. Scanning electron micrographs showing the diversity of euglenids. (a) Petalomonad (phagotroph), (b) Ploeotiid (phagotroph),(c) Euglena (phototroph), (d) Monomorphina (phototroph), (e) Phacus (phototroph). (f–g) Lepocinclis (phototroph). Images not to scale;all cells between 10 and 100 μm. Source: [1]

Where do euglenids belong?

Euglenids, along withKinetoplastea (all heterotrophic) be-long toEuglenozoa, which is one of the sub-groups of Excavata— a super-group of eucaryotes.

The common features of Euglenozoa include:• The flagella are inserted at the base of a deep pocket (alsoknown as the reservoir).

• Active flagella are conspicuously thickened due to thepresence of paraxonemal rods.

• The mitochondrial cristae are discoidal.

Distinguishing feature of euglenids — their cell surfacearchitecture is almost always supported by a pellicle of abut-ting parallel strips of protein that lie directly under the cellmembrane (Fig. 1). Cells with many helically arrangedstrips (>20) are often capable of a characteristic squirmingor pulsing form of active cell deformation called euglenidmotion or metaboly, which is effected by sliding of adjacentstrips.

Nutritional diversity

Euglenids are notable for their diverse modes of nutrition,including:

• Phagotrophy (consumption of particles, especially othercells).

• Osmotrophy (absorbtion of organic molecules).• Photoautotrophy (photosynthesis).

Among the phagotrophs, there is a distinction drawn be-tween predominantly bacterivorous taxa, which have rigid

Figure 2. Petalomonas— a bacterivorous benthic euglenid

pellicles with 12 or fewer strips and tend to be smaller insize, and predominantly eukaryovorous taxa that have pel-licles with many strips, are usually flexible, and tend to belarger. The latter typically consume microbial eukaryotes,including unicellular algae.

The bacterivores include the petalomonads (Petalomonadida),which glidewith a forward-directed flagellum (e.g.,Petalomonas,Notosolenus) (Figs. 2, 3, 4), and ploeotiids, which glide onthe posterior/ventral flagellum (e.g., Ploeotia, Entosiphon)(Fig. 5).

The eukaryovores include some taxa that glide primarilyon a forward-directed anterior flagellum An example is thewell-known genera Peranema (Fig. 6) and Anisonema (Fig.7).

Photoautotrophic euglenids are phylogenetically less di-verse than phagotrophs, although more species have beendescribed. Most are elongate, flexible cells that swim usingone or (more rarely) two emergent flagella (e.g., Eutrep-tia, Euglena, Eutreptiella). Other commonly encounteredspecies are rigid cells with various cell shapes (e.g., Phacus)

2FRONT MATTER

Figure 3. Petalomonas— feasting on bacteria

Figure 4. Notosolenus

Figure 5. Ploeotia— a bacterivorous benthic euglenid

Figure 6. Peranema— an eukaryovorous benthic euglenid

Figure 7. Anisonema— an eukaryovorous benthic euglenid. Theone on the left has recently engulfed a diatom :(

and cells that are enclosed in an extracellular lorica but arenonetheless capable of swimming (Trachelomonas).

Among the osmotrophs, there are primary osmotrophs (e.g.Astasia), which descended from within eukaryovorous lin-eages, and secondary osmotrophs, which are a collection ofspecies that descended from various photoautotrophs.

Euglenids are ambiregnal

The co-existence of phagotrophic and photoautotrophic speciesled to euglenids being examined both as plant-like and animal-

Figure 8. Astasia— osmotrophic heterotrophic euglenid.

3FRONT MATTER

Figure 9. Euglena sanguinea. Source.

Figure 10. Euglena sanguinea bloom. Source.

like life-forms. This resulted in competing classificationschemes under the International Code of Botanical Nomen-clature and the International Code of Zoological Nomencla-ture— i.e., they are ambiregnal taxa.

Of course euglenids are neither plant nor animal, so thegroup does not fall neatly within the archaic plant-animal di-chotomy. Photoautotrophic euglenids in fact acquired pho-tosynthesis via a secondary endosymbiosis involving a prasino-phycean green alga.

Practical importance

Euglenids are not known to cause disease in humans or live-stock.

Several photoautotrophic and osmotrophic species are bloom-formers in nutrient-rich conditions and are useful indicatorsof environmental pollution.

Euglena sanguinea is a red coloured species is due tothe presence of astaxanthin and can be abundant enough tocolour water red.The pigment is used to protect the chloro-plasts from light that is too intense, but as the light levelschange the cells can take on a green colour as the red pig-ment is moved to the centre of the cells. Euglena sanguineais known to make the potent icthyotoxin euglenophycin.

Phagotrophic species are ubiquitous primary consumersand are likely to be important components of microbial food

webs, especially in sediments.

A few euglenids have been used as model systems foraddressing a wide variety of questions in basic cell biologyand physiology and as teaching aids. Euglena gracilis, forinstance, is familiar to nearly every student who has takena general biology course. Euglena gracilis can be grown ina wide range of conditions: autotrophically or heterotroph-ically on various carbon sources (or both), under a broadrange of pH values.

Characteristic features in a nutshell

• A characteristic cell wall, termed pellicle, consisting ofproteinaceous strips beneath the plasma membrane, as-sociated with microtubules. The pellicle strips are ori-ented longitudinally in bacterivorous euglenids and usu-ally helically in eukaryovorous, photoautotrophic, and os-motrophic euglenids.

Pellicle is a most clear morphological synapomorph

defining euglendids.

• Cells usually have two heterodynamic flagella that orig-inate within an anteriorly directed flagellar pocket. Oneflagellum extends anteriorly, called the dorsal flagellum;the other, the ventral flagelum, bends to run posteriorly.Inmost photoautotrophs, most osmotrophs, and a few phagotrophs,only the dorsal flagellum is emergent, while the ventralflagellum is reduced in length and confined to the flagellarpocket (or is absent altogether) (Fig. 11).

• The flagellar pocket in photoautotrophic species is mod-ified into a reservoir (equivalent to the flagellar pocketsensu stricto) (Fig. 11) and a narrower cylindrical-shapedcanal leading to the exterior of the cell. Freshwater lin-eages have contractile vacuoles associated with the reser-voir.

• The flagella are thickened, sometimes extremely so, dueto the presence of paraxonemal (paraxial) rods.

• Photoautotrophic species have green chloroplasts contain-ing chlorophylls a and b. The plastids are surrounded bythree membranes and have thylakoids in stacks of three.

• Photoautotrophic species respond to the direction and in-tensity of light using a shading stigma (eyespot) and a pho-tosensory swelling at the base of the emergent flagellum.

• Cells have a feeding apparatus consisting of a tube orpocket reinforced longitudinally by microtubules. Thefeeding apparatus in many phagotrophs is further elabo-rated by four or five electrondense vanes and reinforcedby two robust rods partly composed of microtubules. Thefeeding apparatus in photoautotrophic and osmotrophicspecies is highly reduced.

4CHARACTERISTIC FEATURES IN A NUTSHELL

Figure 11. Characteristic traits in euglenids. (a) Petalomonad (phagotroph), (b) ploeotiid (phagotroph; note thickness of theventral/posterior flagellum), (c, d) anisonemids (phagotrophs), (e) Euglena (phototroph). All cells between 20 and 50 μm. Source: [1].

Figure 12. Euglenid pellicle and the associated stripes underSEM (left) and TEM (right).

• Three modes of motility are seen, including (i) metaboly,(ii) substrate mediated gliding, and (iii) swimming.

• Mitochondria have discoidal (paddle-shaped) cristae (asin other euglenozoans).

• The nucleus has permanently condensed chromosomesand a conspicuous nucleolus.

• The main storage polymer of most euglenids (perhaps all)is paramylon, a distinctive β-1,3-glucan. Cytoplasmicparamylon granulesmay be small or very large, especiallyin some photoautotrophic species. (Fig. 11)

Pellicle and metaboly

The cell wall of of euglenids, called pellicle is quite a uniqueinvention. Commonly as strong cell wall is useful to protectthe cell. However, a strong armour makes the cell rigid.Euglenids have solved this dilemma with the pellicle — astrong cell wall, which nevertheless can retain flexibility tothat the cells can move almost like amoeba. The pellicle ofsome euglenids has secondarily become rigid.

The pellicle is comprised of parallel proteinaceous strips(Figs. 12, 15), underlain by microtubules, that run along thelength of the cell. The number of individual strips variesfrom 4 or 5 in some petalomonads to 120 in some very largeeuglenophytes. When stripes are few, they tend to be thicker

Figure 13. Large close up of an Euglena cell under lightmicroscope. Note the clearly visible pellicle stripes. Thecolourless central area is where the nucleus is. Also the whiteparamylon granules are nicely visible.

and are readily visible in light microscope. To see the moredelicate stripes, we need something else, like the electronmicroscope.

The strips are composed mostly of a family of proteinscalled articulins. The main frame of each pellicle strip is“S-shaped” in cross section and consists of an arch regionand a heel region that defines a groove (Fig. 14). Adjacentstrips articulate along their lateral margins. The strip archoverlaps with the heel of a neighbouring strip, giving thesurface of euglenid cells a striated appearance.

The articulation zones between adjacent strips allow thedynamic changes in cell shape called metaboly, euglenoidmotion, or euglenoid movement (Fig. 16). Euglenids withmore delicate strips tend to demonstrate more dramatic de-grees of metaboly, those with robust strips tend to be rigid,or nearly so.

What is the point, meaning or ecological benefit ofmetabolyremains a mystery. It is thought to facilitate the ingestionof large food particles, such as other eukaryotic cells, ineukaryovorous phagotrophs.

5PELLICLE AND METABOLY

Figure 14. A general organization of pellicle ultrastructure in flexible photoautotrophic euglenids. Left: The configuration of three stripsand associated microtubules positioned beneath the plasma membrane and subtended by tubular cisternae of endoplasmic reticulum. Right:A pellicle strip with robust toothlike prearticular projections and robust postarticular projections (e.g., some Lepocinclis). Source: [1]

Figure 15. SEM micrographs showing the diversity of pellicle structure in rigid photosynthetic euglenids (a-e) and primary osmotrophs (f).(a)Monomorphina. (b) Phacus. (c) Phacus. (d) Lepocinclis. (e) Phacus. (f) Rhabdomonas. All cells between 20 and 60 μm. Source: [1]

Figure 16. A cartoon showing the euglenoid movement

Prior to cell division, the number of pellicle strips aroundthe cell periphery doubles. Each daughter cell (usually) in-herits the same number of pellicle strips as the parent cell ina semiconservative manner.

Cell structures

Flagella and locomotion

Most euglenids possess two heterodynamic flagella that emergefrom the flagellar pocket. Some have highly reduced flag-

Figure 17. Left: three light micrographs of a cell of Astasia sp.,illustrating the process of metaboly (euglenoid movement) in thisparticularly flexible euglenid. Right: scanning electronmicrograph of Distigma sp., showing metaboly, and multipledistortions of the helical organisation of the pellicle due to slidingof adjacent strips. Source: [1]

6CELL STRUCTURES

Figure 18. Euglenid paraxonemal rods in EM (left), andschematically (right).

Figure 19. Euglenid flagellar hair, pointing towards the tip of theflagellum.

ella, giving the appearance of one or no flagella when viewedwith the light microscope.

Paraxonemal rods

Euglenids possess paraxonemal rods or paraxial rodswithinthe flagella that run alongside the 9 + 2 microtubular ax-oneme (Fig. 18). The paraxonemal rods make euglenidflagella conspicuously thick when viewed under the lightmicroscope. The thickest flagellum can approach or exceed1 μm width in many larger cells.

The paraxonemal rod in each flagellum has a differentstructure: the rod in the ventral flagellum has a lattice likestructure, and the rod in the dorsal flagellum has, at core, awhorled structure that appears tubular in transverse sections.

What is the role of the paraxonemal rods — I have noidea.

Flagella hair

Euglenid flagella characteristically have fine hairs, whichgenerally emerge in horizontal (or shallowly helical) rows oftufts associated with the flagellar axoneme and/or paraxone-mal rod (Fig. 18). The hair typically lie oriented with theirdistal ends pointing toward the distal end of the flagellum(Fig. 19).

Gliding

Gliding on a substrate is a specific way of locomotion in avariety of heterotrophic euglenids that live in the benthos.

One flagellum (i.e., the dorsal/anterior flagellum) is heldahead of the cell, while the other flagellum (i.e., the ventral,recurrent, or posterior flagellum) bends backward and ex-tends posteriorly from the cell, often within a ventral grooveor sulcus.. The hairs and paraxonemal rods of these flagellafacilitate gliding motility across substrates.

In petalomonads and peranemids, the dorsal/anterior flag-ellum is involved in gliding. During this gliding most ofthe anterior flagellum is held stiffly against the substrate,but the tip is in constant motion and functions as a sensoryapparatus. In these cells the posterior/ventral flagellum isshorter and thinner than the anterior flagellum. In somecases it lacks a paraxonemal rod, does not emerge from thereservoir, or is completely absent.

In ploeotiids and anisonemids, only the posterior flagel-lum is involved in gliding, and the whole anterior flagellumsweeps from side to side. In these cells the anterior flagel-lum is almost always thinner and usually shorter than theposterior flagellum.

Some phagotrophic euglenids also use the anterior flagel-lum like a hook to shovel prey cells into the feeding appara-tus.

Swimming

Most osmotrophic and photoautotrophic euglenids primarilymove using swimming motility. They usually possess anemergent dorsal flagellum that extends from the canal andis highly dynamic, while the reduced ventral flagellum doesnot emerge from the canal and is inactive.

The emergent flagellum beats in an organised and con-sistent pattern that takes the form of a “figure-eight” or alasso. This beat pattern pulls the euglenid cell through thewater column. By contrast, eutreptialean photoautotrophspossess two emergent flagella that both beat during swim-ming (some primary osmotrophs also have two emergentflagella).

Feeding apparatus

Phagotrophic euglenids have feeding apparatuses that rangefrom relatively simplemicrotubule-reinforced pockets or tubesto highly complex systems of rods and vanes.

Ploeotiids (e.g., Ploeotia andEntosiphon) and eukaryovoruseuglenids possess feeding apparatuses that are much morecomplex (Fig. 21). These include robust rods composed of

7CELL STRUCTURES

Figure 20. The feeding apparatus of Entosiphon sulcatum inTEM. Not the three microtubular rods.

Figure 21. Entosiphon sulcatum. The feeding apparatus is seenas a funnel like rods in the cell anterior.

ordered arrays of microtubules embedded within an amor-phous matrix (Fig. 20).

Although most phagotrophic euglenids ingest their preywhole, some euglenids (e.g., Peranema) can also feed bymy-zocytosis. This mode of feeding is vampire-like, in that thefeeding rods pierce the prey cell, allowing the cell contentsto be sucked into a phagosomal vacuole within the euglenid.

The feeding apparatuses present in photoautotrophic andosmotrophic euglenids are highly reduced, correspondingto the switch from predominantly phagotrophic modes ofnutrition to photoautotrophy and surface absorption, respec-tively.

Plastids

Photoautotrophic euglenids orEuglenophyceae, are amono-phyletic group, evolved once from eukaryovorous euglenidancestors that established a secondary endosymbiosis withgreen algal prey cells — probably a prasinophyte like Pyra-mimonas. This is reflected in the pigment composition ofeuglenids — they are green (chlorophylls a and b), as are

Figure 22. Phacus— a planktonic photosynthetic euglenid. Ahuge brigt pyrenoid is nicely visible in the middle.

Figure 23. Euglena. Plastids are numerous and elliptical, puregreen in colour like in green algae. Close to the flagella pocket isa huge orange eyespot. The nucleus is central and large (lightarea in the middle). Large elliptical O-ring shaped paramylongranules are within the cytoplasm .

green algae.

Plastids are surrounded by three membranes, and possessthylakoids in stacks of three). Most euglenid plastids con-tain a conspicuous pyrenoid (a region containing RuBisCOprotein) (Fig. 22), although the small disc-shaped plastidsof other species lack pyrenoids.

Carbohydrate storage in the form of paramylon granulesis also often associated with the pyrenoids, but is also dis-tributed throughout the cytoplasm (Fig. 23).

Some photosynthetic euglenids switch nutritional modesand survive in the dark, whereby the plastids become bleachedover time. Several different groups of photosynthetic eu-glenids include species that have independently lost photo-synthesis (e.g., Euglena quartana, Euglena longa, Lepocin-clis cyclidiopsis).

Photoreception

Euglenophytes can respond to the intensity and direction oflight and orient themselves in the water column accordingly— i.e. they are phototactic (Figs. 24, 25).

Photoreception is accomplished by an apparatus consist-ing of a photo-sensory swelling at the base of the emergentdorsal flagellum and a closely associated shading structurecomposed of orange or red carotenoids, called the stigma oreyespot (Fig. 23, 37).

8CELL STRUCTURES

Figure 24. Phototaxis in Euglena. In the darkness the swimmingpattern is random. Low light stimulates swimming towards thelight (positive phototaxis), strong light away from light (negativephototasis).

Figure 25. Photoreceptor apparatus in Euglena, consisting of anexpanded reservoir, a canal, a paraflagellar swelling near the baseof the emergent flagellum, and a shading eyespot. Source: [1].

The stigma of euglenids is positioned near the base of theflagellar pocket/reservoir. In euglenids the stigma lies in thecytoplasm, not embedded within the plastid as in many otherphotosensory algae (e.g., within the green algae, dinoflagel-lates, and chrysophyceans).

The stigma shades one side of the flagellar swelling. Asthe cell rotates through the water, the swelling can detect thedirection of the most intense light source. The behaviourof the swimming flagellum will then respond in a way thatallows the cell to maintain a position in the water columnthat is best for photosynthesis.

Mitochondrion

The mitochondria are distinctive in having stalked, paddle-shaped cristae, usually referred to as discoidal cristae (Fig.26). This feature is shared by Kinetoplastea, the sister-group of euglenids, and probably also Heterolobosea, asister-group of Euglonozoa.Sometimes the taxa having distinctive discoidmitochondi-

ral cristae are referred as a formal taxon (kingdom)—Discicristatae,within the eucaryotic super-group Excavata.The mitochondrion of Euglena gracilis forms a large retic-

ulated network. This conformationmay bewidespread among

Figure 26. Euglenid mitochondrion. Note the discoidalpaddle-shaped crista.

euglenids, although numerous separate elongated mitochon-dria are reported in some taxa.

Habitats

Phototrophic euglenidsmainly inhabit the water column offreshwater environments. Very large and vermiform specieshave reduced flagella and often inhabit the interface betweenthe sediment and water column. Common species in fresh-water lakes and ponds are Trachelomonas, Phacus, Euglena.

Only a few phototrophs inhabit the marine plankton, e.g.,the Eutreptiales. Several species are found in brackishwater and estuaries (e.g. Eutreptiella), either in sedimentsor in the water column.

Phagotrophic euglenids are widespread in marine, brack-ish, and freshwater sediments. The cells glide within thespaces between sand grains and within the narrow interfacebetween mud and the water column. They can compose upto 85% of the biomass of bacterivorous flagellates in certainaerobic freshwater, marine, and brackish sediments and arepresumably important predators in these ecosystems.

Phagotrophic euglenids are mostly raptorial feeders (i.e.,consume cytoplasm and organelles from large ruptured cells)on othermicrobial cells, although it is documented that someact as detritivores.

Phagotrophic euglenids can be divided into bacterivoresand eukaryovores, based onmorphological correlates of foodpreference. The bacterivores (petalomonads and ploeotiids)are rigid cells with few pellicular strips and tend to be rel-atively small (most are <25 μm long). The rigidity of thepellicle constrains them by gape limitation, thus they feedon small prey, primarily prokaryotes.

The eukaryvores (e.g., “peranemids” and “anisonemids”)are mostly slightly-to-highly flexible cells with unfused andmore numerous pellicular strips, and they also tend to be

9HABITATS

Figure 27. Trachelomonas showing the brownish color of thelorica. On the right, the optical focus is to the middle of the cell,showing the green colour of plastids and the red eyespot.

Figure 28. Trachelomonas, a schematic drawing and a SEMimage, nicely showing the opening in the lorica where theflagellum comes out.

larger (most are >20 μm long). As a consequence, they aretypically capable of consuming larger prey items in bothabsolute and relative terms, such as large eukaryotic cells.E.g. Peranema trichophorum can engulf whole Euglenagracilis cell, which are almost as large as themselves. Sev-eral eukaryovorous euglenids specialise in consuming ben-thic microalgae, especially pennate diatoms (e.g. Fig. 7).

Euglenophyte examples

Figs of heterotrophic egulenids were shown above; here I in-troduce some of themore common photosynthetic euglenids,termed euglenophytes (note the –phytes suffix, which refersto plant-like metabolism).

Trachelomonas

Trachelomonas produce a globular organic lorica that maybe smooth or decorated with spines. The lorica has a singleopening for the flagellum, and the cells locomote by swim-ming (Fig. 27). The primary component of the lorica is mu-cus, and during its development, the lorica slowly becomesthicker and ornamented. Iron and manganese are the mainnutrients necessary for the lorica formation, and therefore

Figure 29. The lorica of Trachelomonas is mineralised, andtherefore fragile, and can brake under mechanical pressure.

Figure 30. A scheme of Colacium, showing the overall structureof the colony, and a close-up of a terminal branch.

the cell appears dark brown under light microscope (Fig.28).

As the lorica is strongly mineralised, it becomes quitefragile and can brake under mechanical pressure (Fig. 29)

The sister group to the loricates is Colacium, which alsohas the ability to produce copious amounts of mucus.

Colacium

Colacium is one of the few colonial examples of euglenidsand is widespread in aquatic substrates. It forms mucilagi-nous stalks and dendroid colonies, where the cells occur atthe ends of branched (Fig. 30, 31, 32).

The cells attach by the anterior end, and secrete a stalkcomposed of carbohydrate. Subsequent cell division yieldsnew branched colonies. The characteristic stalk is formed ofcarbohydrate extruded from the cell anterior in the form ofGolgi-generated mucocysts. More than one hundred muco-cysts may accumulate within the anterior of each cell priorto their excretion.

Such colonies are functionally sessile, attached to a sub-strate and the cells are not motile. In the sessile stage, flag-ella are non-emergent, but individual cells may produce anemergent flagellum and swim away to start a new colonyelsewhere.

Ironically, there seems to be a tendency for the colonies toattach to bodies of planktonic mesozooplankton (Fig. 33).

10EUGLENOPHYTE EXAMPLES

Figure 31. Colacioum colonies under phase contrast lightmicroscope. In phase contrast, the stalks are nicely visible.

Figure 32. A close-up of parts of Colacioum colonies

Figure 33. A planktonic rotifer, Keratella cochlearis, is badlycovered with Colacium. Hard to imagine that the zooplanktonwould benefit anything from such behaviour.

Figure 34. Left: Phacus triqueter. The cross section illustratesthe strongly flattened cell. Right: Phacus pleuronectes, smallelliptical chloroplasts and red eyespot are nicely visible.

Figure 35. Left: Phacus tortus. The cell is distinctively twisted.

Phacus

Phacus is a common freshwater planktonic euglenid. Cellsare oval to nearly circular and are highly flattened (Fig. 34).Phacus has a rigid pellicle and metaboly is never observed.Plastids are small, elliptical and numerous.

Common Phacus tortus is distinctive with its twisted cell(Fig. 35).

Some Phacus species have a fairly rough cell surface cov-ering (Fig. 36).

Figure 36. Phacus monilatus with a highly structured cell wall.

11EUGLENOPHYTE EXAMPLES

Figure 37. Euglena— perhaos the most known photosyntheticeuglenid.

Euglena

Euglena is the best-known euglenoid genus, with >150 de-scribed species. Euglena is characterized by the presence ofa single emergent flagellum. Plastid shape is variable butoften discoidal, and cells are cylindrical (not flattened) incross section. Most Euglena species are elongated, with arounded anterior and the posterior tapered to a point (Fig.23, 37).

A dozen or so species produce red granules in numberssufficient to give cells a bright or brick-red appearance (Fig.9), resulting from large amounts of the carotenoid astaxan-thin. When large populations of cells are present, they mayform dramatic, blood-red surface scums on ponds or otherwater bodies (Fig. 10). The formation of such scums isfavored by the presence of high levels of dissolved organiccompounds and high temperatures.

Cells of some species appear red most of the time, butthose of several species can change from green to red within5–l0 minutes in response to increased light intensity such asat sunrise, then return to green coloration at the appearanceof a cloud or at sunset.

This color change involves differential positioning of thered globules and green plastids at the centre and periphery ofcells. When the red globules are at the cell periphery, cellsappear red; when the red globules occupy a central position,surrounded by green plastids, cells appear green. Euglenasanguinea is the commonest of the red species (Fig. 10).

Eutreptiella

Eutreptia and Eutreptiella are found in fresh, brackish, andmarine waters, where they sometimes forms blooms. Eu-treptia has two equal flagella, while Eutreptiella has towunequal flagella (Fig. 38). Eutreptiella is also common andforms occasional blooms in the Baltic Sea.

Figure 38. Eutreptiella

Figure 39. Through metaboly, Eutreptiella cells can change theshape from spherical to highly elongate and everything inbetween

Eutreptiella has a very flexible cell wall and can changeshape from spherical to long elongate (Figs. 38, 39). One ofthe species, Eutreptiella gymnasica, common in the BalticSea, has even a name referring to metaboly.

Wrap Up

Questions about euglenids• In which habitats euglenoids often occur?• In which way is the cell wall of euglenoids special?• Whatmeans ‘euglenoid movement’ (also named ’metaboly’)?

Important euglenid genera. If you run across thesenames, you should know they are euglenids: EuglenaEutreptiella Eutreptia Phacus Trachelomonas.

12WRAP UP

References[1] Brian S. Leander, Gordon Lax, Anna Karnkowska, and

Alastair G. B. Simpson. Euglenida. In John M.Archibald, Alastair G.B. Simpson, and Claudio H. Slam-ovits, editors, Handbook of the Protists, pages 1047–1088. Springer International Publishing, Cham, 2017.

13REFERENCES