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).

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).

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).

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).

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.

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).

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 17 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 16) is a unicell like Chlamydomonas, but, because it lives in highly saline environments, it requires almost no cell wall.

Oedogonium (Figure 18) is a 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).

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.