DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM CHLOROPHYTA (PASCHER 1914)

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Chlorophyta (klo-RA-fa-ta) is formed from two Greek roots that mean green (chloros -χλοερός); and plant (phyto -φυτό).  The reference is to the typical color of members of the phylum.

 

INTRODUCTION TO THE CHLOROPHYTA

This phylum includes most of the green algae, which may grow as colonies, unicells, filaments, and large seaweeds.  Indeed, their diversity rivals that of the Phaeophyta and the Rhodophyta.  They occur in almost all types of water and often are dominants in freshwater environments.  Their life histories are as varied as their forms.  In general, they exhibit the standard plant alternation of sporophyte-gametophyte generations (isomorphic to heteromorphic alternation).  Some have truncated their life histories such that they are haploid with zygotic meiosis (haplontic life history), or diploid with gametic meiosis (diplontic life history).  The diplontic taxa generally are among the siphonaceous pseudoparenchymatous seaweeds.  Within each type of sexual life history taxa vary from isogamy to anisogamy to oogamy.  Indeed, all three can be found in the genus Chlamydomonas. This suggests that oogamy (if it is derived) has evolved multiple times within the green algae.   In general, the sexual cycle serves to produce zygospores that form resting cysts.  Most of the reproduction is vegetative (mitosis, usually accompanied by fragmentation) or asexual (by the formation of zoospores, aplanospores, daughter colonies, etc.).

The green algae are of three types, each of which is represented in the systematic treatment of McCourt (1995) as a class.  The different classes are: Ulvophyceae, Chlorophyceae, and Trebouxiophyceae [see the relationships between the classes in Figure 1].  The monophyly of of each class was confirmed by analyses of Mishler et al. (1994), Krienitz et al. (2004), and Kapraun (2007).

 

 

FIGURE 1. MAJOR CLADES OF THE CHLOROPHYTA.  This cladogram shows the relationships between the three classes of the Chlorophyta according to McCourt (1995).  

 

 
ULVOPHYCEAE: THE BASAL GREEN ALGAE

Most taxa of the Ulvophyceae are marine, but some occur in abundance in freshwater habitats.  They can range from uninucleate to multinucleate filaments to siphonaceous forms to giant unicells.  The green seaweeds, most of which are diploid in the vegetative state, belong to this class. Basal bodies are cruciate and occur in a counter clockwise displacement. Members of this group can exhibit alternation of haploid and diploid generations or have a dominant diploid generation.  Rarely are they haplontic.  We represent them here by Ulva, Cladophora, Codium, and Acetabularia.

Ulva (Figure 2), known as sea lettuce, is a common member of the attached seaweed community attached to rocks and jetties in the turbulent wave zone of warm temperate marine environments.  They form a broad, flat thallus, usually two cells thick.  Despite the filmy appearance, Ulva is quite tough and survives well in zones of pounding  waves.  They exhibit an isomorphic alternation of generation (i.e. the haploid thalli look like the diploid thalli).  In the sexual cycle, gametophyte plants form gametangia in which biflagellate gametes are formed.  They are anisogamous; so, a larger gamete fuses with a smaller one to form a zygote that begins to develop into a diploid vegetative thallus, the sprorphyte.  Certain cells in the sporophyte form zoosporangia in which meiosis occurs and haploid quadriflagellate zoospores are formed and released to give rise to the gametophyte generation (see Figure 3).


FIGURE 2

 

FIGURE 3. LIFE CYCLE OF ULVA

The sexual life history of Ulva is isomorphic and isogamous.  The sporophyte (h) produces quadriflagellate  zoospores (j and j' ), which germinate to produce the gametophytes (a and a' ), which produce biflagellate cells (c and c' ) that may function as gametes (sexual reproduction) or zoospores (asexual reproduction).  There is no vegetative reproduction in this genus.

Van den Hoek et al. (1995)

 

 

Cladophora (Figure 4) is a branched filament that occurs in turbulent water, mainly freshwater.  The branches occur at the distal ends of of the cells, which have up to 50 nuclei and a large parietal net chloroplast.  They can grow quite profusely when conditions are right.  Cladophora glomerata "bloomed" in places like Lake Erie in response to phosphate enrichment.  Their abundance meant the "death" of Lake Erie until laws limiting the phosphate load brought about their control.  In central Pennsylvania, during warm periods of exceptional low flow, I have seen Cladophora overwhelm the periphyton community with strands that can grow to more than a half meter long.  Despite the appearance of slimy strands, Cladophora mats are rough to the touch because they do not produce mucilage but rather deposit lime in the cellulose strands of the wall.  The life history is very similar to that of Ulva.  Many, but not all, exhibit isomorphic alternation of generation with biflagellate isogametes and quadriflagellate zoospores (Figure 5).


FIGURE 4

 

FIGURE 5. LIFE CYCLE OF CLADOPHORA

The sexual cycle of Cladophora vagabunda (but not all Cladophora) is isogamous with an isomorphic alternation of generation.  The sporophyte releases asexual zoospores (i and i' ), which germinate to produce gametophytes (a and a' ).  The filament can also reproduce by fragmentation (vegetative reproduction).  

Van den Hoek et al. (1995)

 

 

Codium (similar to Caulerpa, Figure 6) can, according to the species, appear as small upright shrubs, spheres or flattened blades.  They all are formed from interwoven siphonaceous filaments (pseudoparenchymatous thallus) with the periphery textured by minute attenuate branches bearing gametangia and hairs.  The seaweed is diploid with meiosis occurring during gametogenesis.  The anisogametes are both motile and biflagellate, but they differ very much in size.  Species of Codium and Caulerpa have been implicated as noxious invasive taxa, and they threaten local marine coastal communities where they have been established (e.g. California, eastern US, Australia, and the Mediterranean Sea; see Figure 7).

 

 

FIGURE 7. LIFE CYCLE OF CODIUM

Codium has a life history that is similar to that of Caulerpa.  Both are diplontic with anisogamous sexual reproduction.  There is not asexual reproduction, though vegetative reproduction does occur. 

Van den Hoek et al. (1995)

 


FIGURE 6

Acetabularia is  a single attached giant cell that develops gametangial rays at the top.  The organisms are almost colorless; so, the overall appearance is that of a very delicate wine cup (thus, the common name, Venus' Wine Cup; see Figure 8).   I have seen these growing in Texas gulf coastal water on submerged rocks such that they made an almost continuous lawn.  The upright cell develops from a zygote that attaches and elongates.  The cell remains diploid, but the single nucleus becomes gigantic.  Then, gametangial rays begin to form at the top of the cell.  The nucleus undergoes meiosis and then divides repeatedly to form thousands of haploid nuclei that migrate to the gametangial rays.  There, they accrete cytoplasm and form haploid cysts (the attenuate gametophyte), which undergo more mitotic divisions to make about 20 haploid nuclei.  The rays, each with many cysts, release the cysts to the environment.  The cysts may take many weeks to mature and develop biflagellate gametes, which then leave the cysts through a lid-like operculum.  The zygote is formed by the fusion of the isogametes (see Figure 9).


FIGURE 8

 

FIGURE 9. LIFE CYCLE OF ACETABULARIA

Acetabularia has a sexual life history that is isogamous with a modified heteromorphic alternation of generation.  The zygote (l-b) germinates to make a uninucleate sporophyte.  The single cell begins to produce branches at its tip and then the diploid nucleus undergoes meiosis ultimately to produce hundreds of haploid nuclei which migrate to the gametangia at the tip of the cell.  The nuclei accrete cytoplasm about themselves and produce cysts within which gametes differentiate and are released. 

Van den Hoek et al. (1995)

 

 
CHLOROPHYCEAE

The Chlorophyceae includes taxa that are unicellular, filamentous or colonial.  There are two subgroups of this class known as the DO (directly opposed basal bodies) clade and the CW (clockwise arrangement of basal bodies) clade.  Flagella tend to be smooth (non-scaly), and their flagellar roots run in periphery of cell.  Generally, their life histories are haplontic.  We represent them here by Pediastrum, Hydrodictyon, Volvox, Chlamydomonas, and Oedogonium.

Colonial taxa are multicellular and quite distinctive in their appearance.  Pediastrum (Figure 10) and Hydrodictyon (Figure 11) form such distinctive colonies.  Hydrodictyon forms its colonial net bag of cells that join mostly in hexagons.  While in the colonial form the organism cannot form new cells for the colony.  Instead, the nuclei within any one cell accrete cytoplasm and differentiate into a biflagellate zoospore.  In the case of Hydrodictyon, this could be hundreds of zoospores, which do not leave the parent cell but swim to arrange themselves into a tiny bag of hexagonally-associated cells.  This autocolony then emerges from the parent cell in the asexual cycle.  At the induction of the sexual cycle, cells that resemble zoospores emerge from the parent cell and function as isogametes.  They fuse with gametes from a compatible mating type and form a zygote.  These have zygotic meiosis and, therefore, have a haplontic life history. This zygospore is the resting cyst that germinates when the environment again becomes amenable (Figure 12).  We have observed Hydrodictyon in abundance growing in relatively clean streams that have running water.


FIGURE 10

 

FIGURE 12. LIFE CYCLE OF HYDRODICTYON

Hydrodictyon, although quite different in form and growth habit from Pediastrum, is similar with regard to its life history.  The sexual life history is haplontic and isogamous (note d-l for gamete release and zygote formation).  The same cells that released gametes can release zoospores (d-p).  The meoitic spores (meiospores) function as the zoospore and produce q, a cell that undergoes autocolony formation (also illustrated d-i).  There is no vegetative reproduction in this organism.

Van den Hoek et al. (1995)

 


FIGURE 11

Volvox (Figure 13), in contrast to Hydrodictyon, is a motile colony of delicate green spheres of up to more than 1,000 cells, each with a cup-shaped chloroplast and a pair of flagella.  Typically, they occur in shallow ponds among vegetation, where turbulence cannot tear them apart.  In the asexual cycle, specialized cells begin to divide and form a hollow ball of cells, the requisite number for the mature colony.  Like Hydrodictyon, a mature colony of Volvox cannot increase in size by adding more cells.  The developing daughter colony then turns inside out because during the mitotic phase, the flagellar ends of the cells were directed inwards.  Fully formed, the daughter colonies roll around inside the cavity of the parent colony until the parent tears and released them.  The sexual cycle is oogamous.  Certain cells on the colony become enlarged and, therefore, functional eggs.  Others make very small elongate motile sperm that swim as a group until they encounter an egg.  The zygotes, which look like spiked balls, also remain on the inside of the parent colony until they are released.  Like Hydrodictyon, meiosis occurs inside the zygospore, which is the resting stage (see Figure 14).


FIGURE 13

 

FIGURE 14. LIFE CYCLE OF VOLVOX

The asexual cycle of Volvox is illustrated a-h with the formation of a coenobium or daughter colony.  The sexual cycle is oogamous and haplontic (i-n).  Sperm formation (i-k) is followed by the release of the sperm packet, which finds the stationary egg (l-m).  They fuse to form the zygote (n).

Van den Hoek et al. (1995)

 

 

Chlamydomonas (Figure 15) looks like a unicell of Volvox, and species of Chlamydomonas occur in almost all aquatic environments with low levels of turbulence.  They are small motile cells that divide within the old parent wall and emerge (as zoospores).  The individual cells can serve as functional gametes which fuse at the flagellar ends.  Zygotes are similar to those of Volvox (see Figure 16 for the life cycle of Chlamydomonas).  Chlamydomonas, when suddenly presented with an environmental difficulty, can withdraw the flagella and surround the nonmotile cell with a gelatinous layer (called a palmella stage), in which form phycologists generally refer to them as LGB (little green balls).  The Chlamydomonas form seems to have evolved multiple times according to molecular evidence.  Thus, the genus will be fragmented into multiple taxa in several different orders.  Dunaliella (Figure 17) is a unicell like Chlamydomonas, but, because it lives in highly saline environments, it requires almost no cell wall.


FIGURE 15

 

FIGURE 16. LIFE CYCLE OF CHLAMYDOMONAS

Chlamydomonas has a life history that includes an asexual cycle (h-k; most cells are produced this way) and a sexual cycle.  This one is isogamous and haplontic.

Van den Hoek et al. (1995)

 


FIGURE 17

Oedogonium (Figure 18) is an branched filament that occurs commonly in periphyton communities of freshwater environments.  Even though they live as simple filaments, there is a degree of specialization.  The bottom cell differentiates as a holdfast.  Certain cells in the filament can divide.  In this (and other filaments), the parent cell wall is conserved; however, in Oedogonium, the dividing cell causes the cell wall to separate as a cap at the apex and most of the wall goes to the non-dividing daughter cell.  As the cell divides over time, the caps stack at the apex in a distinctive way.  Certain cells in the filament can develop zoospores (asexual reproduction), which are relatively large cells with an antapical ring of paired flagella.  Sexual reproduction is quite distinctive in this group.  It is always oogamous, but the way in which the antheridia are formed can vary according to two types: macandrous and nannandrous antheridia.  In macandrous reproduction, the male filament makes smaller cells on an otherwise vegetative filament.  Within the small cells two sperm are formed, swim out and fertilize an enlarged oogonium through a port in its wall.  Nannandrous taxa have two stages in the formation of the antheridia.  First, they form the macandrous-like cells in which two small zoospores are formed.   The zoospores escape their cell walls and attach on the oogonial filament, either on the oogonium or on a cell joining it (this is species specific).  The small zoospore germinates to form a dwarf filament (called a dwarf male) with a holdfast, a vegetative cell, and a terminal antheridium.  The oogonium surrounds itself and the developed dwarf males with a mucilage sphere.  Fertilization occurs and the zygote becomes the resting spore.  Meiosis occurs within the zygote, which, upon germination, releases zoospores to begin the cycle (see Figure 19). 


FIGURE 18

 

FIGURE 19. LIFE CYCLE OF OEDOGONIUM

Oedogonium has an unusual sexual life history that is haplontic and oogamous.  Cells in the female filaments differentiate to form oogonia that bear a single egg.  The male filaments (in certain species) release androspores (ASP), specialized zoospores that seek out the oogonium and attach.  There, they germinate to produce a dwarf male filament which produces two sperm.  These fertilize the egg, making a zygote.  The zygote germinates by producing 2-4 zoospores which develop into the respective male and female filaments.  The filaments may reproduce asexually by producing zoospores or vegetatively by fragmentation.

Van den Hoek et al. (1995)

 

 
TREBOUXIOPHYCEAE

Trebouxiophyceae is a third group of green algae, and it lives primarily in the soil.  Its mitosis is distinctive in that centrioles position themselves at the sides of the spindle, a process called metacentric mitosis, which is considered to be a derived state from mitosis with polar centrioles.  Members of this class range from unicells to small filaments and sheets of cells.  Many of them occur as the phycobionts in lichens.  Sexual reproduction in the group is quite variable.  In motile cells (zoospores and motile gametes), the basal bodies occur in a counter clockwise displacement.


FIGURE 20

SYSTEMATICS OF THE CHLOROPHYTA

In this system, the Chlorophyta as a phylum is much abbreviated from systems like those of Margulis and Schwartz (1998).  Bold and Wynne (1985) present a very conservative classification scheme that is little changed from that of Smith (1950) and ignores the vast body of ultrastructural data that have accumulated over the past three decades (Pickett-Heaps and Marchant 1972; Pickett-Heaps 1975; and Mattox and Stewart 1984). Molecular evidence indicates that the phylum as indicated in this system is monophyletic with three large clades, each interpreted as a class  (Graham and Wilcox 2000; Van den Hoek et al. 1995).   The classes also correlate with some details of mitosis and cytokinesis (persistent telophase spindle as a phycoplast, occurrence and placement of centrioles, type of cytokinesis; van den Hoek et al. 1995).  A curious outcome of the molecular and ultrastructural work is that the morphology of the taxa is enormously variable.  For example, motile unicells, branched filaments, sheets of cells, and pseudoparenchymatous thalli seem to have evolved numerous times.  Some of them are so similar that they have been treated as sibling species in genera like  Chlamydomonas and Chlorella.  Similarly, oogamy seems to have evolved repeatedly as well.

 
FURTHER READING:

INTRODUCTION TO THE DOMAIN EUKARYA

 

LITERATURE CITED

Bold, H. C. and M. J. Wynne. 1985. Introduction to the Algae. 2nd Edition. Prentice-Hall, Inc. Englewood Cliffs. NJ. 

Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. 

Friedl, T. 1995. Inferring taxonomic positions and testing genus level assignments in coccoid green lichen algae: a phylogenetic analysis of 18S ribosomal RNA sequences from Dictyochloropsis reticulata and from members of the genus Myrmecia (Chlorophyta, Trebouxiophyceae Cl. Nov.). Journal of Phycology. 31:632-639. 

Graham, L. E., and L. W. Wilcox. 2000. Algae. Prentice Hall, Upper Saddle River, NJ.

Krienitz, L., E. H. Hegewald, D. Hepperle, V. A. R. Huss, T. Rohr, and M. Wolf. 2004. Phylogenetic relationship of Chlorella and Parachlorella gen. nov. (Chlorophyta, Trebouxiophyceae). Phycologia. 43: 529-542.

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company.  New York. 

Mattox, K. R. and K. D. Stewart. 1984. Classification of the green algae: a concept based on comparative cytology. In:  Irvine  , D. E. G. and D. M. John, eds. Systematics of the Green Algae. Academic Press. London. pp. 29-72.

McCourt, R. M., K. G. Karol, S. Kaplan, R. W. Hoshaw. 1995. Using rbcL sequences to test hypotheses of chloroplast and thallus evolution in conjugating green algae (Zygnematales, Charophyceae). Journal of Phycology. 31: 989-995.

Mishler, B. D., L. A. Lewis, M. A. Buchheim, K. S. Renzaglia, D. J. Garbary, C. F. Delwiche, F. W. Zechman, T. S. Kantz, and R. L. Chapman. 1994. Phylogenetic relationships of the "Green Algae" and "Bryophytes". Annals of the Missouri Botanical Garden. 81: 451-483.

Pascher A. 1914. Über Flagellaten und Algen. Berichte der Deutschen botanischen Gesellschaft. 32: 136-60.

Pickett-Heaps, J. D. 1975. Green Algae: Structure, Reproduction and Evolution in Selected Genera. Sinauer Associates, Inc. Sunderland , Massachusetts.

Pickett-Heaps, J. D. and H. J. Marchant. 1972. The phylogeny of the green algae: a new proposal. Cytobios. 6:255-264.

Smith, G. M. 1950. The fresh-water algae of the United States . McGraw-Hill Book Co. New York

Van den Hoek, C., D. G. Mann, and H. M. Jahns. 1995. Algae, An Introduction to Phycology. Cambridge University Press.  Cambridge.

 

By Jack R. Holt.  Last revised: 10/24/2016