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Yellow-brown Algae, Brown Algae, and Diatoms

These algae are distinguished from other algae and higher plants by the type of chlorophyll they use. While most algae and plants use chlorophyll a and b, these algae use chlorophyll a and c, but not b. Most are unicellular or colonial, and they usually reproduce asexually. Yellow-brown algae are mostly freshwater dwellers, while diatoms live in both fresh- and saltwater. Brown algae are almost exclusively saltwater dwellers.

Diatoms are somewhat distinct from other algae in this group. Their cell walls are box-like, with a top and bottom that are fitted together. The cell walls have a high silica content, giving them a glassy appearance. The shells of dead diatoms are used in polishing products and detergents. What makes them truly different from other primitive plant-like organisms is that their non- reproductive cells are normally diploid rather than haploid.

All brown algae are all multicellular. In addition, they are the largest of the algae that possess chlorophyll c, growing to lengths of 45 meters or more. The thallus may be flat or three dimensional in structure, but none possess the complex internal tissues of higher plants.

Unlike green and red algae, brown algae the life cycle of brown algae includes an alternation of generations.

Figure %: Life cycle of the Brown Algae
This term describes a reproductive strategy that involves a succession of haploid and diploid phases. Spores produce a multicellular haploid thallus. The thallus produces isogamus gametes. Fertilization occurs when two gametes meet and a diploid zygote is formed. The zygote then gives rise to a multicellular diploid structure, which in some cases is indistinguishable from the haploid structure. The diploid thallus produces haploid spores through meiosis.

Euglenoids

The Euglenoids are the least algae-like of the algae. They are unicellular and motile, and they lack a key plant-like structure: the cell wall. For these reasons, they are often categorized as protests. Most euglenoids are photosynthetic, but some lack chlorophyll and are heterotrophic (requiring complex organic compounds of nitrogen and carbon for metabolic synthesis).

The structure of a typical euglenoid, Euglena is pictured below.

Figure %: Structure of Euglena
The euglena has several organelles typical of eukaryotes. The chloroplasts of the Euglena are surrounded by three membranes, indicating that they are the result of endosymbiosis of a eukaryote, most likely a green algal cell. Euglena also have a light sensitive stigma which allows them to move toward light sources for better photosynthesis. The two flagella found in the anterior invagination are not the same as the flagella found in prokaryotes. gives a comparison of the structures of eukaryotic and prokaryotic flagella. The flagella of Euglena are rooted in the cell membrane, and thus cannot rotate like those of prokaryotes. Instead, they beat back and forth in a whip-like motion.

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