Algae

Algae are chlorophyll-containing eukaryotic organisms which may be either unicellular or colonial. The colonial forms are usually organized as long filaments, either straight or branched, but in some cases as blades resembling leaves. However, there is little differentiation among cells. The gold-brown, brown, and red algae contain special pigments in addition to the chlorophylls.
The euglenids (Euglenophyta) and dinoflagellates (Pyrrophyta), discussed in the protozoa section, can equally well be regarded as algae. The bright green Chlorophyta, unicellular or filamentous algae, are definitely plants, however. Of biochemical interest is Chlamydomonas, a rather animal-like creature with two flagella and a carotenoid-containing eyespot or stigma. Chlamydomonas contains a single chloroplast. The“pyrenoid”, a center for the synthesis of starch, lies, along with the eyespot, within the chloroplast. The
organism is haploid with “plus” and “minus” strains and motile gametes. Zygotes immediately undergo meiosis to form haploid spores. With a well-established genetic map, Chlamydomonas is another important organism for studies of biochemical genetics. The filamentous Ulothrix shows its relationship to the animals through formation of asexual spores with four flagella and biflagellate gametes. Only the zygote is diploid. On the other hand, the incomparably beautiful Spirogyra has no motile cells. The ameboid male gamete flows through a tube formed between the two mating cells, a behavior suggesting a relationship to higher green plants.

Two frequently studied fungi. Top (including ascus): the yeast Saccharomycescerevisiae. Below: Neurospora crassa showing various stages. After J. Webster.

Some unicellular algae grow to a remarkable size. One of these is Acetabularia, which lives in the warm waters of the Mediterranean and other tropical seas. The cell contains a single nucleus which lies in the base or rhizoid portion. In the mature alga, whose life cycle in the laboratory is 6 months, a cap of characteristic form develops. When cap development is complete, the nucleus divides into about 104 secondary nuclei which migrate up the stalk and out into the rays of the cap where they form cysts. After the cap decays and the cysts are released, meiosis occurs and the flagellated gametes fuse in pairs to form zygotes which again grow into diploid algae. Because of its large size and the location of the nucleus in the base, the cells can be cut and grafted. Nuclei can be removed or transplanted and growth and development can be studied in the presence or absence of a nucleus. The green algae Volvox live in wheel-like colonies of up to several thousand cells and are useful for biochemical studies of differentiation. Look through the microscope at almost any sample of algae from a pond or aquarium and you will see little boatlike diatoms slowly gliding through the water. The most prominent members of the division Chrysophyta, diatoms are characterized by their external “shells” of silicon dioxide. Large and ancient deposits of diatomaceous earth contain these durable silica skeletons which are finely marked, often with beautiful patterns. The slow motion of diatoms is accomplished by streaming of protoplasm through a groove on the surface of the cell. Diatoms are an important part of marine plankton, and it is estimated that three-fourths of the organic material of the world is
produced by diatoms and dinoflagellates. Like the brown algae, Chrysophyta contain the pigment fucoxanthin. Other groups of algae are the brown and red marine algae or seaweed. The former (Phaeophyta) include the giant kelps from which the polysaccharide algin is obtained. The Rhodophyta are delicately branched plants containing the red pigment phycoerythrin. The polysaccharides, agar and carrageenin, a popular additive to chocolate drinks and other foods, come from red algae. Symbiotic associations of fungi with either true
algae or with cyanobacteria are known as lichens. Over 15,000 varieties of lichens grow on rocks and in other dry and often cold places. While the algae appear to benefit little from the association, the fungi penetrate the algae cells and derive nutrients from them. Although either of the two partners in a lichen can be cultured separately, the combination of the two
is capable of producing special pigments and phenolic substances known as depsides which are not formed by either partner alone.

A few species of algae.