7.32: Other Algae

The group Stramenopiles include some phototrophic microorganisms. Members of this group possess flagella covered in numerous short, hairlike extensions, a feature that inspired the group's name, derived from the Latin words for "straw" and "hair." Some of the main categories of Stramenopiles include diatoms, golden algae, and brown algae.

Diatoms are unicellular, photosynthetic eukaryotes, with over 200 known genera. They play a key role in the planktonic communities of both marine and freshwater environments. A defining characteristic of diatoms is their silica-based cell wall, which is reinforced with proteins and polysaccharides. This protective structure, known as a frustule, helps deter predators and remains intact long after the cell itself has decomposed. Diatom frustules come in a variety of intricate shapes and often display symmetrical patterns. Some species exhibit pinnate symmetry, while others have radial symmetry. Due to their silica composition, these structures can persist for millions of years in sediment layers. As a result, diatom fossils are among the most well-preserved examples of ancient unicellular eukaryotes, with evidence indicating their emergence around 200 million years ago.

Golden algae and brown algae represent additional major branches of the Stramenopiles. Golden algae, or chrysophytes, are primarily found in marine and freshwater environments, where they function as phototrophs. Some species can also consume organic material through phagocytosis or by absorbing dissolved compounds. While most golden algae are unicellular and motile, some, such as Dinobryon, form colonies. Their golden-brown color comes from the presence of the carotenoid pigment fucoxanthin. Instead of the chlorophyll a found in many other photosynthetic organisms, golden algae primarily use chlorophyll c and lack the phycobiliproteins characteristic of red algae. A well-studied example of this group is Ochromonas, a unicellular species with one or two chloroplasts per cell.

Brown algae, in contrast, are exclusively marine and entirely multicellular, with no known unicellular representatives. Many brown algae are macroscopic, with some species reaching impressive sizes. Giant kelp, such as Macrocystis, can grow up to 50 meters in length, while Fucus, a common seaweed in intertidal zones, can grow up to 2 meters. The brown or greenish-brown color of these algae depends on the concentration of fucoxanthin in their cells. Brown algae are among the most abundant marine seaweeds, and in certain coastal regions, their rapid growth can create issues when large amounts wash ashore and decompose, causing unpleasant odors.