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
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 siphonaceouspseudoparenchymatous
seaweeds. Within each type of sexual life history taxa vary
from isogamy to
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,
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
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).
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.
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).
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).
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.
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).
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.
den Hoek et al. (1995)
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
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.
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.
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).
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
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.
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).
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.
den Hoek et al. (1995)
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.
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.
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