Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Setting the Scene
Kingdom Protista
Algae

Algae represent a highly diverse group of chlorophyll-containing eukaryotes that occur in both unicellular and colonial forms. Colonial algae typically form long filaments, either straight or branched, and sometimes develop sheet-like structures resembling leaves. However, cellular differentiation is practically absent. Golden-brown, brown, and red algae contain a range of specific pigments In addition to chlorophyll.

Euglenoids (Euglenophyta) and dinoflagellates (Pyrrophyta), mentioned in the section on Protozoa, can be classified as algae with equal justification. However, bright green algae (Chlorophyta), whether unicellular or filamentous, are undoubtedly plants. On the other hand, Chlamydomonas, a biochemically popular Organism featuring two flagella and a carotenoid-containing eyespot (stigma), more closely resembles an animal Cell (Fig. 1-9). Chlamydomonas contains a single chloroplast, within which, alongside the eyespot, lies a pyrenoid—the center for starch synthesis. The haploid Cells of Chlamydomonas exist in two mating types, designated as "plus" and "minus". Zygotes formed by the fusion of motile Gametes immediately undergo meiotic division to yield haploid spores. The Chlamydomonas chromosome has been mapped in considerable detail, making this organism a frequent choice for biochemical genetic research.

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FIG. 1-9. Selected species of algae.

Ulothrix, a filamentous alga, produces vegetative spores with four flagella and gametes with two flagella, thereby bearing a resemblance to animal cells; only the zygote is diploid. In contrast, the Cells of the remarkably beautiful Spirogyra (Fig. 1-9) are non-motile, and its amoeboid male gamete travels through a conjugation tube connecting two paired cells. This reproductive feature points to an evolutionary link between Spirogyra and higher green plants.

Certain unicellular algae attain considerable size. A prime example is Acetabularia (Fig. 1-9), which inhabits the warm waters of the Mediterranean and other tropical seas. The Cell of this alga contains a single Nucleus located at its base (the rhizoid). In the mature alga, whose life cycle ranges from 6 months (in laboratory conditions) to 1 year (in nature), a characteristic cap-like outgrowth develops. Upon completion of this Structure's development, The Nucleus divides into approximately 10⁴ secondary nuclei that migrate up the stalk and into the radial rays of the cap, where cysts are eventually formed. The cap then decays, releasing the cysts; Meiosis takes place within them, and the resulting flagellated gametes fuse in pairs to form a zygote, from which a new diploid alga develops.

Owing to its large size and the basal localization of the nucleus, Acetabularia has proven to be a convenient model organism for morphogenesis research [38, 39]. Growing algae were sectioned into distinct parts—rhizoidal, basal, and apical—after which the nucleus was removed and transplanted from one part to another; severed parts from different plants were fused to produce multi-nucleated algae; and the apical portion of one species was joined to the rhizoidal portion of another, and so on. Experiments of this kind demonstrated that cap Morphology (shape) is governed by Genetic information originating from the nucleus. Nevertheless, Protein Synthesis AND cell growth proceed even in the absence of the nucleus. This implies that information-bearing molecules (presumably mRNA) migrate from the nucleus to the growth zone at a very early stage of algal development. The continuous presence of the nucleus is not required for Protein synthesis in the remainder of the cell.

Examining any sample of pond or aquarium Water under a Microscope will invariably reveal tiny diatoms gliding slowly through the water like little boats. Diatoms, belonging to the Chrysophyta, are widely known for possessing an outer shell composed of silica. These intricate, often strikingly beautiful siliceous skeletons (Fig. 1-9) are exceptionally durable and form vast ancient deposits of diatomaceous earth. Diatoms move very slowly through a highly unusual mechanism—by the streaming of protoplasm along a groove on the cell surface. Diatoms constitute a major component of marine plankton. It is estimated that three-quarters of the world's organic matter is produced by diatoms and dinoflagellates. Like brown algae, chrysophytes contain the pigment fucoxanthin.

There are two additional major algal groups: the brown algae (Phaeophyta) and the red algae (Rhodophyta). The former include giant kelp, which serves as the source of the polysaccharide alginate. The latter are represented by plants with numerous delicate branches containing the red pigment phycoerythrin. The Polysaccharides Agar and carrageenan, frequently added to chocolate drinks and other food products, are extracted precisely from these red algae.