PhycologyCourse handouts

ChlorophytesKalle Olli1

Abstract

Chlorophyta are commonly known as the green algae or chlorophytes, because they appear bright grass green, as domost plants. This is because the chlorophylls a and b of green algae are usually not concealed by large amounts ofaccessory pigments.Indeed, plants have evolved from green algae, and in addition so similar pigmentation, there are other structuralsimilarities between terrestrial plants and chlorophytes. Firstly, the architecture of chloroplasts is very similar. Also,both use cellulose as the structural material to build cell walls.The poor set of accessory makes chlorophytes relatively inefficient light users, and in many aquatic habitatschlorophytes grow well in the well illuminated layers.Chlorophytes form starch with the chloroplast (as do plants), usually in association with a pyrenoid (which plants donot have). The Chlorophyta thus differ from the rest of the eukaryotic algae in forming the storage product in thechloroplast instead of in the cytoplasm.The Chlorophyta are primarily freshwater; only about 10% of the algae are marine, whereas 90% are freshwater.Some orders are predominantly marine (Caulerpales, Dasycladales, Siphonocladales), whereas others are predominantlyfreshwater (Ulotrichales, Coleochaetales) or exclusively freshwater (Oedogoniales, Zygnematales).The freshwater species have a cosmopolitan distribution, with few species endemic in a certain area. In the marineenvironment, the green algae in the warmer tropical and semitropical waters tend to be similar everywhere in theworld. This is not true of the Chlorophyta in the colder marine waters; the waters of the Northern and Southernhemispheres have markedly different species. The warmer waters near the equator have acted as a geographical barrierfor the evolution of new species and genera.

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Contents

Introduction to green algae 2Morphological diversity . . . . . . . . . . . . . . . . 2Calcifying chlorophytes . . . . . . . . . . . . . . . . 3Oil algae chlorophytes . . . . . . . . . . . . . . . . 3Biotechnology . . . . . . . . . . . . . . . . . . . . . 3

Cell structure 3Cell wall . . . . . . . . . . . . . . . . . . . . . . . . 3Pigmentation . . . . . . . . . . . . . . . . . . . . . 4Eyespot and phototaxis . . . . . . . . . . . . . . . . 4

Phototaxis by the secretion of mucilage . . . . 4

Reproduction 4Asexual reproduction . . . . . . . . . . . . . . . . . 4Sexual reproduction . . . . . . . . . . . . . . . . . . 5

Where do green algae belong and classification 5Subdivision of chlorophytes . . . . . . . . . . . . . 5

Streptophyta 5Charophyceae . . . . . . . . . . . . . . . . . . . . . 5

Thallus . . . . . . . . . . . . . . . . . . . . . 5

Reproduction . . . . . . . . . . . . . . . . . . 7Habtiats . . . . . . . . . . . . . . . . . . . . . 7

Zygnematophyceae . . . . . . . . . . . . . . . . . . 7Chloroplasts . . . . . . . . . . . . . . . . . . . 7Photomovement of chloroplasts . . . . . . . . 8Practical use . . . . . . . . . . . . . . . . . . . 9

Habitats . . . . . . . . . . . . . . . . . . . . . . . . 9Reproduction . . . . . . . . . . . . . . . . . . . . . 9

Chlorophyta 9Prasinophyceae . . . . . . . . . . . . . . . . . . . . 9

Pyramimonas . . . . . . . . . . . . . . . . . .10Ostreococcus . . . . . . . . . . . . . . . . . .10

Ulvophyceae . . . . . . . . . . . . . . . . . . . . .10Ulotrichales . . . . . . . . . . . . . . . . . . .10Ulvales . . . . . . . . . . . . . . . . . . . . .11Cladophorales . . . . . . . . . . . . . . . . . .11Dasycladales . . . . . . . . . . . . . . . . . .11Caulerpales . . . . . . . . . . . . . . . . . . .11Siphonocladales . . . . . . . . . . . . . . . . .12

Acknowledgments 12

Starch

Two membrane envelope

Stacked thylakoids

Figure 1. Scheme of a green algal chloroplast. The chloroplasthas a two membrane enevelope, thylakoids are stacked, reservepolysacharide — starch — accumulates within the chloroplasts.

Introduction to green algaeChlorophytes include a wide diversity of unicellular flagel-lates (and some complex colonial forms), non-flagellatedunicells and colonies, filamentous forms, and some morecomplex macroalgae, including green seaweeds. Unicellu-lar and filamentous green algae are significant componentsof freshwater planktonic, and periphytic communities. Verytiny green algae such as Ostreacoccus, which at less than1 μm in diameter is barely visible with the light or fluores-cence microscope, are members of the extremely abundantand productive marine picoplankton.Tropical nearshore waters are frequently dominated by

green seaweeds having very unusual bodies composed of gi-ant, multinucleate cells, known as siphonalean forms. Someof these, notably species of the genus Caulerpa, form veryserious and extensive nuisance growths in theMediterraneanand other parts of the world.The best known subgroups include theChlorophyceae (e.g.,

Chlamydomonas, Volvox),Ulvophyceae (marinemacroalgae),andTrebouxiophyceae. There are several additional distinctlineages, mostly of small scaly unicellular flagellates, thatcollectively are referred to as ‘prasinophytes’.Chlorophytes contain at least one plastid, and most of the

green algae are considered to be autotrophic. Some of theprasinophycean green algae feed on particles and thereforeexhibit phagotrophy and mixotrophy.Features that are common to nearly all chlorophytes in-

clude:

1. Flagella, commonly occurring in pairs ormultiples of two,that are of approximately equal length and without tri-partite, tubular hairs. Fibrillar hairs (Chlamydomonas)and Golgi-produced scales (Pyramimonas), are present insome genera.

2. Chloroplasts bound by a two-membrane envelope (withno enclosing periplastidal endoplasmic reticulum).

3. Chlorophylls a + b.

Figure 2. Two unicellular chlorophytes, representing flagellatedmonad (Clamydomonas, left) and non-flagellated coccoid(Chlorella, right; Source.).

4. Chloroplast thylakoids occurring singly or in stacks ofvariable numbers.

5. Production and storage of starch (α-1,4-linked polyglu-cans) inside the chloroplasts.

Production and storage of the photosynthetic reserve in-side the plastid is unique to the green algae (Fig. 1). In otheralgae the photosynthetic storage product, whether starch orsome other material, is found primarily in the cytoplasm.Plastidal starch of green algae is reminiscent of cyanophycean

glycogen storage within cyanobacterial cells.The presence and plastidal location of green algal starch

can be visualised by treating cells with a solution of I2KI,which stains starch a dark blue-black. Staining for starch isone of the most helpful ways to distinguish green algae fromsimilar-appearing forms belonging to other algal groups.Chloroplasts of green algae may or may not contain eye-

spots and pyrenoids. If eyespots are present in green algalcells, they are always located inside the chloroplast, neveroutside it as in euglenoids, some dinoflagellates, and eustig-matophyceans.Among the green algae, chloroplasts are very variable in

shape and number per cell, but are typically uniform withingenera. As a result, chloroplast shape and number are of-ten useful taxonomic characters, more so than is typical forother groups of eukaryotic algae.

Morphological diversityMorphological diversity of the green algae ranges from tinyflagellates to multicellular macroscopic organisms. I thinkall known cell types and life forms are present in the greenalgae: unicellular flagellates, non-flagellate unicells (Fig.2), motile colonies, nonmotile colonies (Fig. 3), coloniesof regular size and shape known as coenobia (Fig. 4), un-branched filaments, branched filaments (Fig. 5), tissue likecellular sheets, and multinucleate coenocytes (Fig. 6).

https://toddcaldecott.com/herbs/chlorella/

Figure 3. Two colonial chlorophytes, representing non-motilecolony (Palmella, left; Source.) and motile colony (Eudorina,right; ). Palmella colony lives within a gelatinous bag; the cellsareuniformly arranged at the peripheral matrix; 2-4 cells form asmall subset. Eudorina colony is ellipsoidal (sometimes nearlyspherical), 60-200 μm long, consisting of 32 or 16 or 64 cells,each 18-20 μm in diameter.

Figure 4. Coenobial colonies — Pediastrum (left), a four-cellcoenobium of Scenedesmus (right). Coenobium is a colonycontaining a fixed number of cells, with little or no specialization.

Figure 5. Cladophora — a branched filamentous green algae(left) Source., and Spirogyra— unbranced filaments. Source.

Figure 6. Multicellular green algae — Ulva (left) andmulti-nucleate single cell coenotcytic macrophyte —Acetabularia.

Figure 7. Calcifying green algae — Halimeda incrassata fromGulf of Mexico (left; Source.). Calcium carbonate is deposited inits tissues, making Halimeda inedible to most herbivores.Oil-algae — Botryococcus braunii (right; Source.) — a greenalgae that has hydrocarbons typically around 30–40% of their celldry weight, making it a potential candidate for biotechnology.

Calcifying chlorophytesA variety of tropical green macroalgae, in particular Hal-imeda, precipitate calcium carbonate onto their bodies (Fig.7). When they die, these algae contribute substantially tothe production of carbonate sand, and over geological timesuch calcareous algae have generated important carbonatedeposits.

Oil algae chlorophytesThe microscopic green alga Botryococcus (Fig. 7) producesvery large amounts of lipid and is also the source of somepetroleums. This alga is a potential modern-day source ofrenewable energy-rich compounds.

BiotechnologyDunaliella (Figs. 4-11, 20-37) and Haematococcus (Fig.4-12, 4-13).are widely cultivated for production of usefulorganic compounds, while Chlorella (Figs. 4-1, 19-2) isgrown for use as a human food supplement. Selenastrumis a single-celled green alga that is widely used in bioassaysof water quality (Fig. 4-6, 20-42).

Cell structure

Cell wallCell walls usually have cellulose as the main structuralpolysaccharide, although xylans or mannans often replacecellulose in the Caulerpales.The primitive algae in the Prasinophyceae have extracel-

lular scales, or a wall derived from interlacing scales, com-posed of acidic polysaccharides.

Volvocales have walls composed of glycoproteins.

3CELL STRUCTURE

http://protist.i.hosei.ac.jp/PDB/Images/Chlorophyta/Palmella/sp_5.htmlhttp://protist.i.hosei.ac.jp/PDB/Images/Chlorophyta/Cladophora/sp_3.htmlhttp://protist.i.hosei.ac.jp/PDB/Images/Chlorophyta/Spirogyra/group_D/sp_04.htmlhttp://cfb.unh.edu/phycokey/Choices/Chlorophyceae/siphonous_greens/Bryopsidales/HALIMEDA/Halimeda_Image_page.htmhttps://alchetron.com/Botryococcus-braunii

PigmentationChloroplast pigments are similar to those of higher plants— chlorophyll a and b are present. The main carotenoid islutein.However chlorophytes may not always have green col-

oration and therefore are sometimes difficult to recognise asgreen algae. Widely encountered examples includeTrentepohlia,which often forms dramatic orange-red growths on cliff facesand other terrestrial substrates, the flagellate Haematococ-cus, which colours bird baths and other such structures red(Fig. 17-1), and Chlamydomonas nivalis, which can coloursnow red (see Fig. 1-11, Text Box 20-1).InTrentepohlia andChlamydomonas nivalis the red colour

is due toβ-carotene1, which accumulates between thylakoidsin the chloroplast. InHaematococcus the red colour is due toastaxanthin, which accumulates in lipid globules outsidethe chloroplast.

Hematochrome is a general term for these carotenoids.Accumulation of hematochromes colours the cells orange orred, with hematochrome accumulating up to 8–12% of thecellular contents in Dunaliella.Accumulation of carotenoids occurs under conditions of

nitrogen deficiency, high irradiance (Trentepohlia, Chlamy-domonas nivalis) or high salinity (Dunaliella). The largeamounts of carotenoid pigments obscure chlorophylls andserves a photoprotective function.Animals can not synthesise these carotenoids and they ac-

quire the pigments through the food chain from primary pro-ducers. Hematochromes are responsible for the colouring infish, crustaceans and birds (such as the pink in flamingos).

Eyespot and phototaxisMost of the flagellated cells that show phototactic move-ment have an eyespot. In chlorophytes, the eyespot is al-ways in the chloroplast, usually in the anterior portion nearthe flagella bases. The eyespot consists of lipid droplets,usually between the chloroplast envelope and the outermostband of thylakoids. The eyespot is coloured orange-red fromthe carotenoids in the lipid droplets.The photoreceptor in Chlamydomonas is in the plasma

membrane above the eyespot and consists of a chromophore(colored substance) linked to a protein — opsin, that isembedded in the plasma membrane.The chromophore is 11-cis-retinal (the aldehyde of

vitamin A). Light excitation causes isomerization of 11-cis-retinal into trans, triggering a conformational change thatinitiates the signalling process.The chromophore 11-cis-retinal and the protein opsin pro-

duce a rhodopsin, a general class of compounds that ab-sorb light maximally around wavelengths of 500 nm.

1The same pigment is responsible for the orange colour of carrots.

The eyespot filters light by reflecting blue and green lightback onto the photoreceptor in the plasma membrane as thealga swims through the medium. This results in changes inmembrane potential involving rhodopsin. Entry of calciuminto the cell is affected by the membrane potential of theplasmamembrane, and, in turn, the concentration of calciumions in the cytoplasm affects the rate of beating of the flag-ella.The swimming direction of the cell is affected by the rate

of beating, because at one concentration of calcium ions,each flagellum beats differently. Therefore, changing the cy-toplasmic calcium concentration differentially changes thebeat of each flagellum, causing the cell to swim in a differentdirection.

Phototaxis by the secretion of mucilageA second type of phototactic movement in the chlorophytesuses secretion ofmucilage in desmids. Already in1848 desmidmovement on a surface of mud brought to the lab was de-scribed, and presumed to be due to the stimulus of light.Penium (a desmid chlorophyte) aligned their long axis

and moved toward the light, accumulating on the lightedside of the culture vessel they were growing in. The move-ment is brought about by the extrusion of slime through cellwall pores in the apical part of the cell.

ReproductionThe high diversity within chlorophytes also translates into ahigh diversity of reproductive strategies

Asexual reproductionMost common in unicellular organisms is simple cell divi-sion.For colonial forms the simplest is fragmentation of colonies

into two or more parts, each part becoming a new colony.Further, zoosporogenesis commonly occurs, usually

induced by a change in the environment of the alga. Inthe chlorophytes, zoospores are normally produced in veg-etative cells, and only in a few cases are they formed inspecialised sporangia.Next, aplanospores are non-flagellated and have a wall

distinct from the parent cell wall. Aplanospores are consid-ered to be abortive zoospores and have the ability to form anew plant on germination.Next, autospores are aplanospores that have the same

shape as the parent cell, and are common in the Chlorellales(e.g. Chlorella).Next, coenobia are colonies with a definite number of

cells arranged in a specific manner (e.g., Volvox, Pedias-trum, Scenedesmus). Generawith colonies arranged in coeno-

4REPRODUCTION

Figure 8. Isogamous, anisogamous, and oogamous sexualreproduction.

bia form daughter colonies with a certain number of cells. Inmaturation of the daughter coenobia, there is enlargementbut no division of vegetative cells in the coenobia.

Sexual reproductionSexual reproduction in theChlorophyceaemay be isogamous,anisogamous, or oogamous, with the general line of evolu-tion occurring in the same direction (Fig. 8).If the species is isogamous or anisogamous, the gametes

are usually not formed in specialized cells although in theoogamous species, gametes are normally formed in special-ized gametangia (e.g. Coleochaete). Whereas most chloro-phytes form motile flagellated gametes (zoogametes), inthe Zygnematales aplanogametes or amoeboid gametesare formed.

Where do green algae belong andclassificationThe green algae and land plants, collectively known asChloro-bionta or Viridiplantae, form a monophyletic group withinArchaeplastida2.

Subdivision of chlorophytesThe green algae are divided into two major clades, the strep-tophytes and chlorophytes sensu stricto (Fig. 9). The formerincludes land plants, as well as many green algae. Strepto-phyte green algae are often referred to as ‘charophytes’, andthe best studied groups are the Zygnematophyceae, whichare unicellular or filamentous freshwater algae, and theCharo-phyceae, which are truly multicellular freshwater algae.Despite the similarity in complexity betweenCharophyceae

and land plants, phylogenetic evidence indicates that landplants are more closely related to Zygnematophyta.

2The Archaeplastida (meaning ‘ancient plastids’; sometimes alsocalled Plantae) is an eukaryotic supergroup whose plastids/chloroplastswere acquired directly through a symbiosis with a cyanobacterium.

StreptophytaThe Streptophyta include land plants (embryophytes) andtheir closest green algal relatives.The evolutionary relationship of charophytes to embryophytes

remains unresolved. There is strong support for the hypoth-esis that conjugating green algae (Zygnematohyceae) con-stitute the sister group to embryophytes, but it can also beColeochaetophyceae.

CharophyceaeThe charophytes, or stoneworts, are a group of large, parenchy-matous green algae with six extant genera in one family.They are distributedworldwide in freshwater ponds and lakesand occasionally in brackish water, including the Baltic Sea.The genus Chara was erected by Vaillant in 1719 for sev-

eral living species of this genus and formally recognised byLinnaeus (1753) as one of several genera of algae.

Thallus

The charophyte thallus is composed of basal rhizoids, withan upright main axis consisting of alternating internodes andnodes (Figs. 10, 11). The rhizoids grow downward, an-choring the thallus axis in the sediment, and the axes growupward.Charophytes are relatively large for green algae and can

grow up to a half meter or more in height. Some generaaccumulate calcium carbonate externally.The charophyte axis has a distinctive node-internode struc-

ture (Fig. 11).Internodes consist of giant cells, which are multinucleate,

and with numerous ellipsoidal plastids distributed in the cy-toplasm surrounding a large central vacuole. The cytoplasmstreams actively lengthwise around the cell periphery.Nodes comprise several, smaller, uninucleate cells that

give rise towhorls of leaflike organs of limited growth called‘branchlets’, and secondary axes (branches of unlimited growth),which also exhibit the node-internode construction. A singleapical meristematic cell occurs on each axis tip.Growth occurs through division of an apical cell at the tips

of the main axes or secondary branches. A single cuttingface of the apical cell produces an alternation of internodalcells and nodal initials.Due to their large size and apparent complexity, charo-

phytes may be mistaken for bryophytes or certain aquaticangiosperms (e.g., Ceratophyllum) in the field.The plantlike structures of charophytes, complex asym-

metric sperm, and large, protected egg cells led earlier work-ers to see them as intermediate in complexity between greenalgae and embryophytes.

5STREPTOPHYTA

Figure 9. Overview phylogeny of the green lineage. Source: [?]

Figure 10. Charophyte thallus morphology. (a) Charadrummondii; (b) Nitella haagenii; (c) Lamprothamniummacropogon; (d) Tolypella polygyra. Source: [?]

Figure 11. Main external features of Chara. Source.

6STREPTOPHYTA

http://www.biologydiscussion.com/algae/life-cycle-algae/chara-occurrence-structure-and-reproduction-algae/21135

Figure 12. General thallus structure of Chara. showing thelocation of gametangia — oogonium and antheridium. Source.

Reproduction

Asexual reproduction. occurs through growth of erect axesfrom nodes on the rhizoids, and through contracted starch-filled branches, and tubercular, starch-filled outgrowths ofthe rhizoids called bulbils, which may fall away and germi-nate separate from the thallus.Sexual reproduction. is oogamous. Oogonia and antheridiaare the female and male gametangia, respectively, whichinclude gamete-producing cells and associated vegetativecells (Fig. 12).Each oogonium contains a single large egg cell, whereas

sperm are produced in filaments with numerous antheridialcells, packed inside a spherical antheridium (Fig. 12).Oogonia and antheridia occur on the branchlets at nodes

and may be associated with small sterile cells. The oogoniaare oblong, 200–1000 μm long by 200–600 μm wide. Maleantheridia are spherical and range from 200 to 1500 μm indiameter, often bright orange in colour. Sexual structuresare easily visible with a hand lens or even with the nakedeye.Sperms have two flagella attached slightly below the apex

of an asymmetric, helically twisted cell reminiscent of spermcells in mosses and liverworts.

Habtiats

Charophytes are primarily freshwater organisms, but are oc-casionally abundant in brackish waters. They occur in quietor gently flowing waters, from very shallow (several cm) todeep (>10 m), as long as light levels are adequate. Habitats

are typically alkaline (hard water).Stands of charophytes provide habitat for invertebrates

and structural refuges for juvenile vertebrates (fish and frogs).Charophytes are often early colonisers and water clarifiers.Practical applications for charophytes include managementofwater quality through encouragement of charophyte coloni-sation. Nutrients are absorbed by charophytes through theirrhizoids and photosynthetic thallus, and charophyte commu-nities can be a significant store of nitrogen in small waterbodies. Uptake by charophytes removes nutrients from thewater column that would otherwise be available for growthof other algae. The decline of charophytes following eu-trophication can be explained largely by decreases in waterclarity and competition with angiosperms.

ZygnematophyceaeThe Zygnematophyceae are among the most diverse greenalgae, with a variety of thallus types (filaments, unicells,colonies; Fig. 13), and approximately 4,000 described species.The group lacks flagella at all stages of the life cycle. Sex-ual reproduction, when present, involves conjugation orthe union of two haploid vegetative protoplasts (individualcells of filaments or unicells) to form a zygospore, whichundergoes meiosis to produce a new haploid thallus upongermination.

Zygnematophyceae contains some of the most beautifulmicroscopic organisms known (Fig. 15).TheZygnematophyceae is usually divided into two groups

Desmidiales and Zygnematales.Zygnematales are generally oblong, rod shaped, or cylin-

drical, and the smooth cell wall lacks pores; the primarywall is a homogeneous piece, lacking a median constriction.The family Zygnemataceae (14 genera, over 800 species)included filamentous algae.TheDesmidiales (41 genera, 3,500+ species) contains the

desmids, which are divided into four families, the Closte-riaceae, Gonatozygaceae, Peniaceae, and Desmidiaceae,the latter being the largest of the four families (36 genera,3,000 species, 12,000 subspecific taxa). Most are unicells.Each cell consists of two mirrorimage parts called semicellsthat are joined at a narrow midregion or isthmus wherethe nucleus is located. Chloroplasts and other nonnuclearcell contents are divided equally between semicells. Thestructure of semicells is often complex, with two, three, ormore planes of symmetry.

Chloroplasts

Chloroplast shapes range from asteroid (Cylindrocystis andZygnema) to laminate (Gonatozygon, Mesotaenium,Mougeo-tia) to ribbon-like (Spirogyra and Spirotaenia, Fig. 16). Anaxile, ridged chloroplast (stellate) is found in many desmids

7STREPTOPHYTA

https://www.carlsonstockart.com/photo/stonewort-chara-charophyte-green-algae-characeae/

Figure 13. Structural diversity in the Zygnematophyceae. (a)Spirogyra sp.; (b) Zygnema sp.; (c) Spirotaenia condensata; (d)Roya obtusa var. montana; (e) Netrium digitus; (f) Gonatozygonaculeatum; (g) Micrasterias rotata; (h) Euastrum evolutum var.glaziovii; (i) Xanthidium cristatum var. hipparquii. Structures: c– chloroplast, n – nuclear region at site of isthmus betweensemicells, p – pyrenoid, v – apical vacuole. Scale bar = 10 μm ineach micrograph

Figure 14. Micrasterias melitensis (left) and Euastrumapiculata (right) from the marvellous collection by Ernst Haeckel.

Figure 15. Desmidiales: Cosmarium (left) and Closterium(right). Cosmarium is a very species rich genus. It is a desmid,with two mirror-image like half cells, joint by an isthmus.Closterium has a characteristic half-lunar shape.

Figure 16. Sprirogyra:has one of the most conspicuous ribbonshaped chloroplasts, arranged in a spiral configuration in theperiphery of the cell.

includingNetrium, Closterium, andPenium. Species of Desmidi-aceae contain some of the largest and most elaborate chloro-plasts known among the green algae. Their chloroplasts areoften ridged, lobed, and highly dissected.Pigments include those typical of green algae and em-

bryophytes, i.e., the descendants of a common ancestor thatincludes all green algae and embryophytic plants: chloro-phylls a and b, β and γ-carotenes, and several xanthophylls.Chloroplasts usually contain one or more pyrenoids aroundwhich starch is stored.

Photomovement of chloroplasts

The laminate chloroplast of many taxa, e.g. Mougeotia andMesotaenium are able to moves within the cell. The chloro-plasts display maximum surface area or face toward low-intensity light. In high-intensity light, the chloroplast alignsitself with the edge profile toward the light. Presumably

8STREPTOPHYTA

thesemotions optimise photosynthetic performance andmin-imise damage to the photosynthetic apparatus.

Practical use

Members of the Zygnematophyta have not been exploitedfor economic use in any major way. A few species havebeen used in fish aquaculture. Some studies suggest thatgreen algae in general and Spirogyra in particular may beuseful for the detection and recovery of certain metals fromcontaminated waterways. Some conjugating green algae,including Spirogyra, Mougeotia, and the Desmidiales, areused as indicators of trophic status and water quality.

HabitatsMostly freshwater. These algae are common in ponds, ephemeralpools, marshes, and bogs, lakes, and streams. They readilycolonise artificial habitats, reservoirs, cattle tanks, roadsideditches, irrigation canals.They habit surfacemats, benthos aswell as plankton. Most

conjugating green algae are benthic or periphytic and growon surfaces or occasionally attached to substrates by meansof rhizoids or mucilage. Rhizoids that attach to substratemay be present in all of the filamentous Zygnematales (e.g.,Mougeotia, Spirogyra, and Zygnema).Many, but not all, are found in oligotrophic tomesotrophic

waters of moderate to low pH. The diversity of habitats oc-cupied spans a wide range and may be quite specific for indi-vidual species. Species show distinct preferences for certainhabitats characterised by water chemistry and productivity.This makes the group as a good indicators of habitat typesand water quality.

ReproductionAsexual reproduction is by fragmentation and cell division.A fundamental feature distinguishing theZygnematophyta

from other chlorophytes is sexual reproduction by conjugationinvolving the fusion of non-flagellate gametes (Fig. 17).Sexual cycles consist of:

1. conjugation (the physical joining of cells or filamentsand subsequent union of gametes to form a zygote)

2. formation of a thick-walled zygospore

3. a period of zygospore dormancy

4. and germination of the zygospore to produce vegetativecells.

Zygnematophyta display zygoticmeiosis—growing cellsare haploid, andmeiosis occurs in the zygote, the only diploidcell in the sexual cycle. Strains of speciesmay be homothallic

Figure 17. Sprirogyra:reproduction by conjugation.

— conjugation is intraclonal, or heterothallic — conju-gation is interclonal between plus and minus mating types.Optimal conditions for conjugation vary from species to

species. Filamentous Zygnematales often conjugate whenfilaments are transferred to nutrient-poor conditions.

ChlorophytaThe chlorophyte clade is composed of four algal classes (Fig.9):

Prasinophyceae — paraphyletic class of scaly naked uni-cellular flagellates, mostly marine.

Ulvophyceae — predominantly marine, but a number offreshwater species. All filamentous marine green algaeor larger green seaweeds belong here.

Chlorophyeae

Trebouxiophyceae

PrasinophyceaeHere belong primarily marine green flagellates with scalescomposed of acidic polysaccharides. Prasinophytes are re-garded as the modern representatives of the earliest greenalgae.Within the group, the flagellar number varies from one in

Pedinomonas to 16 in Pyramimonas cyrtoptera.Prasinophycean flagella typically emerge from an apical

depression or pit. The cell membrane of most forms is cov-ered with one or more layers of often extremely elaboratescales.Prasinophyceans generally also have a single plastid (though

it may be highly lobed), and usually possess at least onestarch-sheathed pyrenoid.The cells of most prasinophyceans are enclosed by one to

five layers of scales attached to the cell membrane, with thescales of each layer characteristic for the species.

9CHLOROPHYTA

Figure 18. Pyramimonas light microscopy (left) and SEM(right). Note the coverage of cell and flagella with scales.

Figure 19. EM image of cell scales in Pyramimonas (left). Notethat different types of scales are in many layers. Scheme ofPyramimonas flagellum, showing the coverage with flagellascales (right).

Pyramimonas

Pyramimonas is a flagellate unicell found in marine, brack-ish (incl. Baltic Sea), or freshwaters. Flagella, mostly 4, butcan be also 8 or 16, depending on species, emerge from adeep, narrow pit in the middle of four lobbed cell anterior.The cells are somewhat heart-shaped (cordate) (Fig. 19).There are several layers of body and flagellar scales (Fig.19).

Ostreococcus

Ostreococcus is a genus of unicellular coccoid or sphericallyshaped green algae. It includes prominent members of theglobal picoplankton community. Ostreococcus tauri has anaverage size of 0.8 μm in diameter and is the smallest eu-karyotic cell known (Fig. 20). The alga has a relativelylarge nucleus, a single chloroplast with a starch granule, amitochondrion, and a Golgi apparatus.Due to its small size, the genus was discovered as late as

in 1994.

UlvophyceaeMostlymulticellular thalloidmarine benthic green algae. Thelife cycle usually involves the alternation of a haploid thallus

Figure 20. Ostreococcus tauri. Left: The general organisationSource: [?] . Right: EM image, showing the the nucleus, themitochondria and the chloroplast. Source.

with a diploid thallus. Here belong the conspicuous coeno-cytic benthic algae of tropical marine waters.Six orders, first three are common algae in temperate wa-

ters, others are predominantly tropical:

Ulotrichales — uninculeate filamentous algae with a pari-etal chloroplast.

Ulvales —uninucleate cells with a parietal chloroplast; thal-lus is a hollow cylinder or a sheet, one or two cells thick.

Cladophorales — multinucleate filamentous algae with aparietal perforate or reticulate chloroplast.

Dasycladales — thallus has radial symmetry composed ofan erect axis bearing branches; thallus uninucleate butmultinucleate just before reproduction.

Caulerpales — coenocytic algae lacking cellulose in thewalls.

Siphonocladales — algae with segregative cell division.

UlotrichalesUlothrix (Fig. ) is found in quiet or running freshwater andoccasionally on wet rocks or soil. The thallus consists ofunbranched filaments of indefinite length that are attachedto the substratum by a special basal cell. All of the cellsexcept the basal one are capable of cell division and formingzoospores or gametes.Species with narrow cells form 1, 2, or 4 quadriflagellate

zoospores per cell, whereas those with broad cells form 2,4, 8, 16, or 32 zoospores per cell. The zoospores have aconspicuous eyespot and are liberated through a pore in theside of the parent wall. Zoospores that are not dischargedfrom the parent may secrete a wall and become thin-walledaplanospores. These later germinate to form a new filament.Gametes ofUlothrix are formed in the sameway as zoospores

but are biflagellate. The gametes are of the same size, withfusion occurring only between gametes from different fila-ments. The zygote remains for a while, settles, secretes athick wall, and undergoes a resting period during which it

10CHLOROPHYTA

https://genome.jgi.doe.gov/OstRCC809_1/OstRCC809_1.home.html

Figure 21. Ulothrix zonata. Filaments consist of many cells;rhizoid formed at one end. Cells 30–60 μm in diam., 15-30 μmlong; Source.

accumulates a large amount of storage material. The firstdivision of the zygote is meiotic, with the zygote forming 4to 16 zoospores or aplanospores.Inmany northern lakes,Ulothrix zonata grows abundantly

in early spring in shallow waters along shorelines. Ulothrixzonata is dominant until thewater temperature reaches 10°C,when it disappears owing to massive conversion of the thal-lus to zoospores.

Ulvales

Ulvales have a thallus that is either an expanded sheet one(Monostroma) or two cells (Ulva; Fig. 5.33) thick. Thethallus of Enteromorpha is a hollow cylinder.Ulva thallus is composed of two layers of cells, with each

cell having a large cup-shaped chloroplast toward the exte-rior of the cell (Fig. 5.33). The holdfast is formed by thecells of the thallus, sending down long slender filamentsthat coalesce to form the holdfast. The holdfast portion isperennial and proliferates new blades each spring.Cell division may occur anywhere in the thallus, but all

divisions are in a plane perpendicular to the thallus surface.Ulva (Fig. 5.33) has an isomorphic alternation of gener-

ations, with the gametophyte forming biflagellate gametesand the sporophyte producing quadriflagellate zoospores.Ulva is normally a marine genus although it can be found

in brackish waters, particularly in estuaries, and also in theBaltic Sea. It normally grows on rocks in the intertidal zone,Ulva is an opportunistic alga, capable of rapid colonisa-

tion and growth when conditions are favourable. This oc-curs because of a rapid growth rate and the ability to takeup and store nutrients available in pulsed supply. Becauseof this Ulva has proliferated in many eutrophied areas. Inenclosed and semienclosed waters Ulva comprises a largeproportion of drift plants, which may smother other benthiccommunities or be cast ashore where they decompose, caus-ing considerable aesthetic nuisance.

Ulva is commonly known as the sea lettuce or green laver,and has been eaten as a salad or used in soups.

Cladophorales

The filamentous genera in this order havemultinucleate cells,usually with a parietal or reticulate chloroplast. The fila-ments may be branched or unbranched.Cladophora (Fig. 5.35(b)) andChaetomorpha (Figs. 5.35(a),

5.36), each with an isomorphic alternation of generations,are common members.Cladophora. is found in freshwater and marine habitats. Itmay be the most ubiquitous macroalga in freshwaters world-wide. This filamentous alga can reach nuisance levels as aresult of eutrophication.Cladophora is predominantly benthic, and is often found

in the region of unidirectional flow or in periodic wave ac-tion. Cladophora is colonised by a wide variety of epiphytesbecause it offers a substrate that is anchored against flowdisturbance.

Dasycladales

Here belong tropical and subtropical marine algae, most ofthem calcified.Due to calcification they fossilise readily. TheDasycladales

has a paleontological record that extends back to the Precambrian–Cambrian boundary (ca. 570 million years ago). Of the 175known fossil genera, only 11 are extant. The Dasycladalesare in fact living fossils3

Acetabularia (Figs. 5.38, 5.40, 5.41), (mermaid’s wine-glass) is the best known. At maturity Acetabularia has anaked axis with a single gametangial disc at the apex.Acetabularia is a warm-water alga found in shallow pro-

tected lagoons and on the borders ofmangrove swamps, grow-ing on shells, coral fragments, and other algae. The thallusis calcified.

Caulerpales

Caulerpales contains the coenocytic or siphonaceous greenalgae. The non-septate thallus resembles a garden hose with-out any cross walls separating the usually large thallus, ex-cept during reproduction.Cellulose is usually not a wall component and is replaced

by a β-1,3 linked xylan or a β-1,4 linked mannan.Caulerpales are marine algae and occur as sea-weeds in

the warmer oceans.The coenocytic algae in the Dasycladales and Caulerpales

respond to injury by rapidly forming gel-like wound plugs,

3Living fossils are organisms that include extant clades that havesurvived for long intervals of geological time at low numerical diversityand exhibit primitive morphological characteristics that have undergonelittle evolutionary change.

11CHLOROPHYTA

http://protist.i.hosei.ac.jp/PDB/Images/Chlorophyta/Ulothrix/sp_7.html

thereby preventing loss of cytoplasm. The wound plugsare formed from extruded cytoplasm that forms the plugthrough interaction of carbohydrates and lectins (Ross et al.,2005). A new cell wall is formed under the gelatinous plug.

SiphonocladalesThese algae havemulticellular thalli, are wholly marine, andare usually tropical. The cells are multinucleate, with retic-ulate chloroplasts.

Acknowledgments

12CHLOROPHYTA

Introduction to green algaeMorphological diversityCalcifying chlorophytesOil algae chlorophytesBiotechnology

Cell structureCell wallPigmentationEyespot and phototaxisPhototaxis by the secretion of mucilage

ReproductionAsexual reproductionSexual reproduction

Where do green algae belong and classificationSubdivision of chlorophytes

StreptophytaCharophyceaeThallusReproductionHabtiats

ZygnematophyceaeChloroplastsPhotomovement of chloroplastsPractical use

HabitatsReproduction

ChlorophytaPrasinophyceaePyramimonasOstreococcus

UlvophyceaeUlotrichalesUlvalesCladophoralesDasycladalesCaulerpalesSiphonocladales

Acknowledgments