PhycologyCourse handouts

ChrysophytesKalle Olli1

AbstractThe chrysophytes are unicellular or colonial algae with golden chloroplasts, therefore colloquially also names as goldenalgae. In addition to chlorophyll a, they have chlorophylls c1, c2 and fucoxanthin as the main auxiliary pigments.Chrysophytes have heterokont flagellation — two flagella, the one with tripartite hairs pointing forward anddoing all the job. The second, shorter, smooth one is trailing behind. In many species the second flagellum is reduced.Chrysophytes occur mainly as phytoplankton in temperate freshwaters, and their distribution is ecologically determinedmainly by temperature and pH.Life history includes mitotic divisions and encystment. All chrysophytes form endogeneous silcified spores, calledstomatocysts or statospores. In several species, sexuality — cell fusion followed by encystment of the zygote hasbeen observed. Stomatocysts fossilise readily and are important paleoecological markers.Cells are naked or in many cases surrounded by an envelope, e.g., of species-specific silica scales.Chrysophytes have a photoreceptor systems, which includes a swelling on the short flagellum and a correspondingstigma or eye spot in one of the chloroplasts. Reserver carbohydrate is chrysolaminaran. Many chrysophytes aremixotrophic, they take up organic food particles by phagocytosis. Some species, are heterotrophic.

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Contents

Chrysophytes in a nutshell 1Chrysophytes belong to stramenopiles . . . . . . . . 2

Chrysophyte cell structure 3Flagella . . . . . . . . . . . . . . . . . . . . . . . . 3Chloroplasts . . . . . . . . . . . . . . . . . . . . . . 4Eyespot and photoreceptor system . . . . . . . . . . 4Contractile vacuole . . . . . . . . . . . . . . . . . . 5Chrysolaminaran vesicle . . . . . . . . . . . . . . . 5Silica scales . . . . . . . . . . . . . . . . . . . . . . 5

Nutrition 7

Habitats 7Microbial biogeography . . . . . . . . . . . . . . . . 7

Life cycle 7

Ecology 8Chrysophytes as ecological indicators . . . . . . . . 8Fossil records . . . . . . . . . . . . . . . . . . . . . 8

Representatives 8Ochromonas . . . . . . . . . . . . . . . . . . . . . . 8Dinobryon . . . . . . . . . . . . . . . . . . . . . . . 8Chromulina . . . . . . . . . . . . . . . . . . . . . . 9Uroglena . . . . . . . . . . . . . . . . . . . . . . . 9Chrysamoeba . . . . . . . . . . . . . . . . . . . . . 9Synura . . . . . . . . . . . . . . . . . . . . . . . . .10Mallomonas . . . . . . . . . . . . . . . . . . . . . .10Paraphysomonas . . . . . . . . . . . . . . . . . . .10

Acknowledgments 11

References 11

The name ’Chrysophyte’ comes from theGreekword chrysos,which means gold.

Chrysophytes in a nutshell• About 1,200 species in about 200 genera have been esti-mated, but many more species will certainly be described.

• Most of the species are unicellular or colonial, with orwithout flagella. A few have a filamentous or simple mul-ticellular organisation.

• Flagellar are apical — inserted near the apex of the cell.• A typical heterocont photoreceptor is present, consistingof a swelling on the short, smooth flagellum, and an eye-spot that lies within the chloroplast.

• Chloroplasts are golden-brown. Chlorophyll a is masked

by the accessory pigment fucoxanthin.• Themost important storage product is chrysolaminarin,a polysaccharide, which is stored in the cell in aqueoussolution in special vacuoles. Chrysolaminarin is a beta-1.3 linked glucan, polymer of glycose (a type of starch,which is alpha-1.4 linked)

• Spherical, silica-walled cysts— stomatocysts—are formedwithin the cell. The silica is deposited within a specialsilica deposition vesicle.

• In several genera siliceous body scales are present, whichare formed in silica deposition vesicles.

• Chrysophytes are predominantly freshwater algae. Rela-tively few are marine.

• A few are heterotrophic—notably, the silica scaled genusParaphysomonas.

• Chrysophytes tend to be predominant in oligotrophic wa-ters. This may be due to their ability to compete success-fully under conditions of phosphorus limitation. Partlythis is due to the widely usedmixotrophy by chrysophytes.

• The scarcity of chrysophytes in highly productive lakes isdue to low competitive ability with other phytoplanktonand intense grazing by zooplankton.

• High pH values are adverse, because chrysophytes canuse only free CO2, which is present in sufficient quantitiesonly at lower pHs.

Chrysophytes belong to stramenopilesChrysophytes belong to a large eukaryotic supergroup —stramenopiles (syn. heterokonts). Stramenopiles containa variety of well known algal groups, like diatoms, brown al-gae, chrysophytes, raphidiophytes, xantophytes, eustigmato-phytes, etc., but also groups of heterotrophic protists, like bi-cosoecids, groups of ’pseudofungi, like oomycetes (includ-ing Phytophthora, which caused the Irish potato famine),labyrinthulomycetes, and others. The closest relatives ofchyrsophytes are eustigmatophytes, brown algae, and diatoms.The term stramenopiles was coined by David J. Pat-

terson, who used it without a formal Linnaean rank. Thesynonymously used term heterokonts, or formally Het-erokonta refers to the motile cell type with two differentlyshaped flagella—which I here term as heterokont flagellation(see below).Unfortunately the term heterokonts is ambiguous. It was

created asHeterocontae in 1899 byLuther for an algal grouptoday called Xanthophyceae, characterised by heterokontflagellation. Later other groups of protists found to havesimilar flagellation were included in Heterokonta, expand-ing its sense.Thus the nameHeterokonta can be confusedwith the (much

older) name Heterokontae, which is equivalent to the Xan-thophyceae, a limited subset of the Heterokonta. It is thisambiguity, why stramenopiles as a term was erected in the

Figure 1. A typical heterokont flagellate. Most heterokontmotiles have apical or near-apical insertion of their flagellawhereas this diagram shows the lateral insertion typical ofphaeophyte gametes and spores. The tinsel flagellum is anteriorlydirected and the smooth flagellum trails the cell duringswimming. Source: [Sym and Maneveldt(2011)]

Figure 2. EM image of a pleuronematic flagellum, showing theaxoneme, where the tripartite mastigonemes are attached.

first place.Heterokont flagellation. The name heterokont refers to thecharacteristic form of these cells, which typically have twounequal flagella (Fig. 1). The anterior straminipilous flag-ellum is covered with one or two rows of lateral bristlesor mastigonemes, which are tripartite (with three regionseach) (Figs. 2, 3), while the posterior flagellum is whiplike,smooth, and usually shorter, or sometimes reduced to a basalbody. The flagella are inserted subapically or laterally (Fig.1).Needless to say, the mastigonemes are not visible in light

microscope, at least EM should be used (Fig. 4).This type of flagellation defines the supergroup stremenopiles

— it is a synapomorph 1 of stramenopiles.

1Synapomorph is a derived character or trait that is shared by a taxo-nomic group and is considered to have originated in their common ancestor.

2CHRYSOPHYTES IN A NUTSHELL

Figure 3. Scheme of the heterokont mastigoneme. It has threeparts: base, tubular shaft, and terminal filaments. It is alsocovered with long and short lateral filaments.

Figure 4. EM image of a stramenopile. We see the cell body anda huge pleuronematic flagellum. The smooth flagellum isprobably behind the cell body, and is not visible.

Chrysophyte cell structureFig. 5 shows a typical scheme of a chrysophyte, perhapsOchromonas.

FlagellaTwo dissimilar flagella, dissimilar is size and structure — atypical heterokont flagellation. Chrysophyte flagella arisefrom the anterior of the cell, close to the cell apex.The longer flagellum is directed forwards during swim-

ming and is pleuronematic, being furnishedwith two rowsof short stiff hairs (sometimes termed as a flimmer on tinselflagellum). The hairs project out sideways from the flagel-lum and are about 15 nm thick, and are called mastigonemes.By contrast, the backwardly directed flagellum, which isshort and blunt, is smooth and bare.The mastigonemes of the pleuronematic flagellum each

consists of three parts (as if Figs. 2, 3):

1. a base

2. a tubular shaft

3. three terminal hairs

The pleurenematic flagellum is active during swimming.It is directed forwards and executes simultaneous undula-tory and helical movements. The shorter flagellum trails

Figure 5. Schematic diagram of a sectioned cell of Ochromonassp.: (a) contractile vacuole; (b) Golgi body; (c) nucleus(containing the nucleolus); (d) chloroplast, with four membranesand a girdle lamella enclosing the other lamellae (stacks of threethylakoids); (e) mitochondrion; (f) eyespot (as cluster of red lipidglobules containing carotenoid pigments); (g) short ‘whiplash’flagellum (note flagellar swelling); (h) flagellar basal body; (i)long ‘flimmer’ flagellum; (j) mastigonemes; (k) chrysolaminaranvesicle. Source: [Jordan and Iwataki(2012)]

3CHRYSOPHYTE CELL STRUCTURE

backwards passively, lying against the cell. It may act asa rudder to steer the cell. The helical movements of thepleuronematic flagellum causes the cell to rotate as it movesforwards.The pleuronematic flagellum undulates inwaves that travel

from its base to the tip. If the locomotory flagellum weresmooth or covered with flaccid haris, the water would bepropelled in the same direction as the waves, thus hushingthe cell in the opposite direction, with the flagellum trailingbehind the cell. This type fo flagellar locomotion is exhib-ited by protists from Opisthokonta supergroup, includingthe choanoflagellates (and indeed, the sperms of animals).In stramenopiles, however, the two rows of stiff mastigonemescause a reversal of the thrust [Cahill et al.(1996)Cahill, Cope, and Hardham].Water is therefore propelled along the flagellum from the tiptowards the base, so that the cell is towed forward, in thedirection of the flagellum.Both flagella have a typical ’9+2’ structure of two cen-

tral microtubules and nine peripheral doublets — this is theuniversal structure of all eukaryotic flagella (Fig. 9). Thebending of the flagellum is brought about by sliding betweenthe peripheral doublets of the flagellar axoneme.

ChloroplastsChrysophytes have typically one or two plate-like chloro-plasts per cell. Thylakoids are grouped into stacks of three.The chloroplast is surrounded by a four membranes (typicalfor secondary plastids). Two innermost represent the chloro-plast envelope itself, the outer pair represents a fold of endo-plasmatic reticulum— termed chloroplast endoplasmaticreticulum, which is wrapped tightly around the chloro-plast and is continuous with the nuclear envelope (Fig. 6).The chloroplasts contain chlorophylls a, c1 and , chloro-

phyll b is never present. The main accessory pigment isfucoxanthin (as in diatoms, brown algae, and many otherphototrophic stramenopiles) (Fig. 7).Pyrenoids are common in chloroplasts of the chrysophyceae.

Eyespot and photoreceptor systemPhotoreceptor systems are present in almost all motilechrysophytes. It consist of a swelling on the short flagel-lum with the photoreceptor and a stigma (or the eyespot)functioning as a screen.The eyespot is visible in light microscope and appears as a

small red spot at the anterior of the cell. The eyespot consistsof a number of red carotene lipid droplets densely arrangedjust within the chloroplast membranes. The eyespot is in oneof the chloroplasts and is positioned close to the base of theshort flagellum. Above the eyespot there is an invaginationof the chloroplast and also of the cell surface. Into thisdepression fits the swollen basal part of the short flagellum,

Figure 6. The heterokont chloroplast has the normal plastidialdouble-membrane envelope, but surrounded by a periplastidialmembrane and by rough endoplasmic reticulum (RER), whichmay be confluent with the outer membrane of the nuclearenvelope. The periplastidial membrane is considered to be theremnants of the eukaryotic endosymbiont’s (a red alga)plasmalemma. The chloroplast interior is occupied by thylakoids,the outer one/s of which is/are continuous and just beneath theenvelope — the girdle lamella. Source:[Sym and Maneveldt(2011)]

Figure 7. Fucoxanthin.

4CHRYSOPHYTE CELL STRUCTURE

Figure 8. Ochromonas swimming pattern.

Figure 9. Flagellar swelling with photoreceptor in juxtapositionto stigma bearing part of the chloroplast (Dinobryon). The 9+2microtubular structure is nicely visible. EM image x64,000.Source: [Archibald(2017)]

containing the actual photoreceptor that is responsible forthe perception of light (Fig. 9).With unilateral illumination the eyespot of the swimming

cell would shade the photoreceptor at regular intervals, sincethe cell twists around its longitudinal axis as it swims (Fig.8). The cell could therefore determine the direction of theincident light.Flagellated algae usually exhibit phototactic reactions: the

direction in which they swim is influenced by the directionof the light incident upon them. Flagellated algae gener-ally swim towards dim light (positive phototaxis), buyaway from bright light (negative phototaxis), and sothe cell must be able to determine where the light is domingfrom.Eyespot is present in great many flagellated algae and

are sometimes part of the chloroplast (as in chrysophytes),but in some groups (euglenophytes, eustigamtophytes, somedinoflagellates) lie in the cytoplasm.

Contractile vacuoleThere may be one or two contractile vacuoles per cell, lo-cated near the base of the flagella bases (Fig. 5). Each con-sists fo a small vesicle, which contracts at regular intervals,expelling its contents from the cell.Contractile vacuoles (also known as pulsatile or pulsing

vacuoles) are common organelles in various protists. Theirprimary function is to osmoregulation of the cell.The concentration of solutes in the cytoplasm is hypo-

tonic, higher inside than outside the cell. The cell membraneis semipermeable. Under these conditions, water osmosiscauses water to accumulate in the cell from the external en-vironment. The contractile vacuole removes the superfluouswater absorbed by the cell and thus prevents it bursting dueto excessive internal pressure.The growth (water gathering) and contraction (water ex-

pulsion) of the contractile vacuole are periodical. One cycletakes several seconds, depending on the species and the en-vironment’s osmolarity.Water always flows first from outside the cell into the

cytoplasm, and is only then moved from the cytoplasm intothe contractile vacuole for expulsion. Species that possess acontractile vacuole typically always use the organelle, evenat very hypertonic (high concentration of solutes) environ-ments, since the cell tends to adjust its cytoplasm to becomeeven more hyperosmotic than the environment. The amountof water expelled from the cell and the rate of contractionare related to the osmolarity of the environment. In hyper-osmotic environments, like the ocean, less water will beexpelled and the contraction cycle will be longer.

Chrysolaminaran vesicleThe storage product is chrysolaminaran, alpha-1,3 linkedglucan, supposedly found in a posterior vesicle (Fig. 5).The actual function of the chrysolaminaran vesicle may becomplex. The vesicle is much larger in organisms grownin the dark than in cells grown in the light. The oppositewould be expected if the vesicle only stored chrysolaminarin— the accumulation product of photosynthesis in the light.Supposedly the it may also function as a digestive vesicle,breaking down material taken up by the cell into buildingblocks for metabolism and growth.

Silica scalesWhile many chrysophyceans do not have a cell wall (e.g.Ochromonas), and some live in an vase-shaped organic lor-ica (e.g. Dinobryon), yet others are covered by an armourof silica scales, spines, and bristles (Fig. 10). Particularlythe order Synurales is known for silica scales.Scale structure is species specific and very complicated,

and it was understood only after electron microscopy cameinto common use. Scale pattern is species specific. To de-tect and identify many of the scale-covered species, electronmicroscopic examination is required. A scale generally con-sists of a perforated basal plate provided with ribs, spines,and other ornamentation. Several scale types are producedin the same cell and deposited on the surface in a definite

5CHRYSOPHYTE CELL STRUCTURE

Figure 10. Photomicrographs of living chrysophytes (a–b) andsynurophytes (c–f). (a) Loricate colony of Dinobryon sp., LM; (b)colony of Uroglena sp. with each cell possessing a red eyespot,LM; (c) cell ofMallomonas caudata with cell covered by scalesand bristles, LM; (d) high magnification of scales and bristles ofMallomonas caudata, SEM; (e) colony of Synura petersenii, LM;(f) colony of Synura petersenii with cells covered by siliceousscales, helically arranged, SEM. Source.

Figure 11. Archaeomonad stomatocysts, showing a variety ofsurface ornamentation (e.g. smooth, ridges, warts, perforations,spines) as well as forms with or without a collar. Scale bars=1 μm.(a, f, h) IODP 302�2A�54X�1, 32–33 cm (Middle Eocene,Arctic Ocean); (c, g) IODP 302�2A�55X�2, 142–143 cm(Middle Eocene, Arctic Ocean); (d) IODP 302�4A�11X�1,93–94 cm (Middle Eocene, Arctic Ocean); (b, e) Cesar 6,190–192 cm (Late Cretaceous, Arctic Ocean). Taken by SusumuKonno. Source.

6CHRYSOPHYTE CELL STRUCTURE

sequence.

NutritionOne of the most intriguing features of chrysophytes is thecommon occurrence of mixotrophy — utilization of dis-solved organic compounds and/or particulate food by plastid-bearing algae. Phagotrophic chrysophytes consume bacte-ria, yeasts, small eukaryotic algae. All chrysophytes (excl.Synurales) should be considered capable of mixotrophy un-der at least some conditions.Bacteria-eating (bacterivorous) chrysophytes are par-

ticularly important in oligotrophic lakes, where they are theprimary consumers of prokaryotes. These include four speciesof Dinobryon and Uroglena, both common colonial phyto-plankters.Bacterivory rates of Dinobryon can be impressively high

— on average three bacteria per cell in five minutes—- com-parable to heterotrophic flagellates that are incapable of pho-tosynthesis. In. oligotrophic lakes Dinobryon can removemore bacteria than crustaceans, rotifers, and ciliates com-bined. Populations of chrysophyceans such as Dinobryoncan occupy lake waters as deep as seven meters, forming ex-tensive metalimnetic growths. In the dimly lit metalimnion,they obtain about 80%of their total carbon from phagotrophicactivity.Phagotrophy provides not only fixed carbon, but also phos-

phorus, which is limiting to growth in oligotrophic waters—as well as vitamins. Most chrysophytes are auxotrophic,requiring several B vitamins.

Nutritional flexibility has a price. The growth ratesof mixotrophs are lower than those of specialised pho-toautotrophic algae of comparable size. Concomi-tently, metabolic costs are higher than those of spe-cialized phagotrophic flagellates.

HabitatsThe great majority of described species are found in plank-ton of fresh water, lakes and ponds. A few species, likeHydrurus are benthic, e.g., found attached to the bottom instreaming rivers.Typical freshwater chrysophyte habitats are humic, neu-

tral, or slightly acidic lakes and ponds with a moderate sup-ply of nutrients. Here the chrysophytes may constitute themain phytoplankton biomass. In more acidic, low nutrient,or alkaline waters, few species occur but sometimes at highcell numbers. Ponds surrounded by agricultural land, unlesspolluted by cattle, are often very rich in chrysophytes.

Most species have their main occurrence in spring, oftenjust after ice break. Many species are restricted to cold orcool water, thus in temperate regions occurring in spring andautumn, others prefer warmer water in summer.Small chrysophytes, togetherwith cryptomonads and prym-

nesiophytes, make up an important part of the nanoplanktonof many lakes where they are the main food for zooplankton.BecauseDinobryon has an effective phosphate-uptakemech-

anism, it is especially abundant in waters with low phos-phate concentrations. However, most species (excludingSynurales) aremixotrophic, partly covering carbon and phos-phorus demand by ingestion of bacteria. An exogenous sup-ply of organic carbon compounds, e.g., vitamins of the Bgroup, mainly B12, is also necessary. This will normallybe present in the water, either excreted by bacteria, releasedby the decomposition of algal cells, or brought by sewage.Organic compounds are also obtained by phagocytosis ofparticulate food by many species.Some chrysophytes, e.g., the genera Synura andUroglena,

may become a nuisance when they occur in great quantities,because they excrete fishy-smelling ketones and aldehydes.They may foul drinking water reservoirs.Some chrysophyte species are very common and cosmopoli-

tan, others are rare with peculiar disjunct distributions.Silica is required for scale-bearing species. Synura and

Paraphysomonas require silica in the water at a concentra-tion of at least 1 μM in order to grow well; they are ableto deplete a medium almost completely of silica. Very lowsilica content results in unstable colony structure and failureto form cysts and scales.

Microbial biogeographyThe distribution of a species is due to dispersal, mainly ofstomatocysts, by birds and by air. This has led to the ubiq-uity hypothesis advocatedmainly by Finlay andClarke (1999)that all species are everywhere, only the environment de-termines the occurrence. This everything is everywherehypothesis, inspired by the biogeography (or rather the lackof biogeography) of Paraphysomonas—a hetrotrophic, butsilica-scaled genus of chrysophytes, has been an importantdiscussion and controversy in ecology, particularly concern-ing the dispersal (and dispersal limitations) ofmicrobial species.

Life cycleAll chrysophytes form endogenous cysts (statospores, stomatocysts)during their life history. In Dinobryon cylindricum), en-cystment occurs either in the exponential phase of popula-tion growth (intrinsic, mainly sexual resting cysts) or in thestationary phase (extrinsic, induced by nutrient depletion).Two clones must be present in order to produce sexual cysts

7LIFE CYCLE

in Dinobryon cylindricum, whereas asexual cysts are pro-duced by individuals, pioneers in a new habitat. These Di-nobryon produce asexual cysts at a low rate, which graduallyslows down during the end of the growth period. They pro-duce sexual cysts rapidly during rapid growth. These twostrategies result in almost the same number of cysts. Thecysts sink into the sediment.

Ecology

Chrysophytes as ecological indicatorsBecause of their narrow ecological spectra, silica-scaled chrys-ophytes can serve as indicators for changes in trophic con-ditions, in particular of pH in lakes. Silica structures, suchas stomatocysts and scales, are used in sediment studies ingeology and limnology, often together with pollen analy-sis, to study the ecological history of lakes. Changes inpH (acidification) and anthropogenic influence can readilybe followed2. Many species have well-defined occurrenceranges regarding pH; they can thus be arranged as acidobion-tic, acidophilic, indifferent, alkaliphilic, and acidobionticspecies.

Fossil recordsThe siliceous structures of Chrysophyceae are very resistantand therefore common in many geological deposits. Stom-atocysts are more heavily silicified than scales and bristles,and so they are well the older sediments.The oldest certain fossils of chrysophyte stomatocysts are

from Tertiary or Upper Cretaceous marine deposits, South-ern Ocean sediments (ca 112 Mya), which may indicate theinitiation of silicification within chrysophyte algae. Sincethe stomatocysts are commonly found in fossil marine sedi-ments, chrysophytes are presumed to have a marine origin.Siliceous scales and bristles are generally preserved for a

shorter geologic period.The oldest records of fossilised chrysophyte scales and

bristles have been reported from the Paleogene age, ca� 60Mya, assigned to the genus Synura.According to the reconstruction of stramenopile diversi-

fication times, chrysophytes most likely originated in thePermian, ca 279 Mya.

Representatives

2Other such well preserved algal markers are the diatoms

Figure 12. Ochromonas— a wall-less mixotrophic chrysophyte.

Figure 13. Dinobryon— a colonial mixotrophic chrysophyte;each cell living in a vase-shaped organic lorica.

OchromonasOchromonas is a wall-less unicellular flagellates with oneor two golden-brown, platelike plastids. Ca 80 species.There are two unequal flagella of the typical heterokont

type. The posterior portion of the cell typically comes to apoint. Ochromonas occurs most abundantly in oligotrophicfreshwaters, but some marine forms are known. Stomato-cysts are often found. Often used as a model cell type ofchrysophytes, and also of stramenopiles in general.

DinobryonDinobryon is a unicellular or colonial organism; each cellhas two heterokont flagella that contribute to the motilityof the the cell or whole colony, which swims slowly. Eachcell is contained in a vase-shaped organic lorica (Figs. 13,14). Cells are attached to the base of their lorica by a thincytoplasmic thread. Loricas are arranged to form a dendroid(tree-shaped) colony ((Fig. 13).

Dinobryon is widely recognised as a bacteria-consumingmixotroph. Asexual reproduction occurs when cells swim

8REPRESENTATIVES

Figure 14. Dinobryon— a unicellular form. Not all species livein colonies.

Figure 15. Dinobryon—most loricas are empty — one has arecently formed statospore at the opening.

Figure 16. Chromulina— a wall-less mixotrophic unicellularchrysophyte.

out of their lor- ica and start new colonies. Sexual reproduc-tion involves isogametes and formation of statospores thatare attached to colonies (Fig. 15).Dinobryon may occur in the plankton of either soft or

hard (high alkalinity) freshwaters. Dinobryon can be verycommon in the plankton of temperate and boreal lakes thatare poor in nutrients and not very alkaline.

ChromulinaChromulina is a wall-less unicellular flagellate having onlyone emergent flagellum of the tripartite hair-bearing type(Fig. 16). One or two golden plastids are present. Thereare over 100 species, some of which are marine and othersfreshwater.

UroglenaUroglena consists of globose motile colonies of wall-lesscells held at the ends of mucilaginous threads arising fromthe colony center (Fig. 17). Cells exhibit eyespots and typi-cal heterokont flagella, as well as golden plastids. Uroglenacommonly occurs together with Dinabryon and, like Dino-bryon, consumes bacterial cells.

ChrysamoebaChrysamoeba is a globose or discoid amoeboid cell withan irregular cell outline and radiating rhizopodia that canvary in length by active extension and retraction (Fig. 18).Cells can become motile by production of a single emergentflagellum. Flagellate cells look very much like Chromulina.There are two platelike golden plastids per cell. Cells mayoccur singly or as aggregates in the plankton of freshwaterlakes and in muddy areas, but is of rare occurrence.

9REPRESENTATIVES

Figure 17. Uroglena— a colonial mixotrophic chrysophyte.

Figure 18. Chrysamoeba— amoeboid representative ofchrysophytes.

Figure 19. Synura— a colonial chrysophyte.

Figure 20. A close up of Synura. The layer of silica scales isnicely visible around single cells.

SynuraSynura is a colonial organism formed by variable numbersof cells held together at the posterior ends to form a tightlycoherent spherical colony (Fig. ). Colonies grow by addi-tion of new cells through longitudinal cell division. Newcolonies may form by fragmentation. Cells bear two (het-erokont) flagella. Each cell is covered with overlappingsilica scales. Spines are present on anterior scales. Thereare two parietal golden-brown plastids per cell.

MallomonasMallomonas is a single-celled flagellate with (usually) onlyone emergent flagellum (Figs. 21, 23). The cell membraneis covered with overlapping silica scales; at least some bearlong silica bristles. Cells contain a single deeply dividedplastid that may give the appearance of two plastids. Thereis a chrysolaminarin vacuole at the cell posterior. Mallomonasoccurs in the plankton of lakes of various types.

ParaphysomonasParaphysomonas is a genus of heterotrophic chrysophyceanflagellates. It is a single-celled flagellate whose surface iscovered with silica scales (Fig. 24, 25). There are twotypical heterokont flagella. Although it is not pigmented,there is a leucoplast (a colorless plastid), indicating that Pa-raphysomonas is derived from an ancestral form that waspigmented. It is osmotrophic or phagotrophic. There are

10REPRESENTATIVES

Figure 21. Mallomonas— single celled chrysophyte with silicascales and large silica bristles.

Figure 22. A silica bristle of Mallomonas under lightmicroscope with high magnification (100x lens, oil immersion,DIC contrast).

Figure 23. TwoMallomonas cells. On the right side, astatospore is formed within the cell.

Figure 24. Paraphysomonas— a colourless chrysophyte.

Figure 25. Paraphysomonas has just engulfed a large pray.Perhaps a diatom.

about 50 species, whose identification requires electron mi-croscopic examination of scale structure.

Acknowledgments

References[Archibald(2017)] John M. Archibald. Handbook of the pro-

tists. Springer Berlin Heidelberg, New York, NY,2017. ISBN 978-3-319-28147-6 978-3-319-28149-0978-3-319-28148-3.

[Cahill et al.(1996)Cahill, Cope, and Hardham] David M. Cahill,Michele Cope, and Adrienne R. Hardham. Thrustreversal by tubular mastigonemes: immunologicalevidence for a role of mastigonemes in forward motionof zoospores ofPhytophthora cinnamomi. Protoplasma,194(1-2):18–28, March 1996. ISSN 0033-183X,1615-6102. doi: 10.1007/BF01273164. URL http://link.springer.com/10.1007/BF01273164.

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