7.29: Overview of Algae

The kingdom Archaeplastida encompasses red and green algae, along with land plants. Unlike other protists with chloroplasts that arose through secondary endosymbiosis, only red and green algae originated from primary endosymbiotic events. This diverse group of eukaryotic organisms contains chlorophyll and performs oxygenic photosynthesis.

Algae exist in various forms, from large brown kelp in coastal waters to green scum in puddles and stains on rocks or soil. Some species are responsible for food poisoning. They can exist as unicellular organisms, filamentous chains, or thalli. Though historically classified as plants, algae lack embryos and the structural complexity of true plants. Instead, they are defined as photoautotrophs that do not possess roots or stems. Their classification remains dynamic as DNA and structural analyses refine groupings within super clades.

Predominantly aquatic, algae thrive in diverse environments, including soil, tree surfaces, and even unusual habitats like the fur of South American sloths and polar bears, provided sufficient moisture is present. They rely on water for physical support, reproduction, and nutrient diffusion. While generally found in cool temperate waters, brown algae such as Sargassum form large floating mats in the subtropical Sargasso Sea, and some species thrive in Antarctic waters.

Algae are simple eukaryotic photoautotrophs that lack the tissues found in plants, such as roots, stems, and leaves. Identifying unicellular and filamentous algae requires microscopic examination. Most algae inhabit oceanic environments, with their distribution influenced by nutrient availability, light wavelengths, and suitable growth surfaces.

The body of a multicellular alga is called a thallus. In larger multicellular algae, commonly known as seaweeds, the thallus consists of branched holdfasts for anchoring, stemlike stipes (often hollow), and leaflike blades. Unlike vascular plants, algae lack conductive tissue (xylem and phloem) and absorb nutrients directly from water. The stipe is not lignified and does not provide structural support; instead, the surrounding water supports the thallus. Some algae are buoyed by gas-filled structures called pneumatocysts.

Algae reproduce both asexually and sexually. Multicellular algae with thalli or filamentous forms can reproduce by fragmentation, where each fragment develops into a new organism. Unicellular algae divide through mitosis, followed by cytokinesis, producing two genetically identical cells. Sexual reproduction also occurs, sometimes after multiple asexual generations. Some species exhibit alternation of generations, where offspring from sexual reproduction undergo asexual reproduction, followed by a generation that reproduces sexually.

Algae encompass multiple phyla, classified based on rRNA sequences, structures, pigments, and other characteristics. Most algae are photosynthetic, with chlorophyll a and accessory pigments responsible for their distinctive colors. However, some groups, such as oomycotes (fungal-like algae), are chemoheterotrophs. Photosynthetic algae predominantly inhabit the photic zone of aquatic environments, where sufficient light is available for photosynthesis.