Algae

Kingdom Protista is an artificial classification for eukaryotic organisms not classified as plants, fungi, or animals. It includes diverse groups like algae, fungus-like organisms, and single-celled heterotrophs. Algae are photosynthetic protists, lacking true leaves, stems, roots, or flowers, and contain various accessory pigments that determine their color and depth preference in water.

Chlorophyta, or green algae, are the closest relatives to modern land plants. They contain chlorophyll A and B, store sugars as starch, and have cellulose in their cell walls. They can be single-celled, filamentous, or colonial, found in fresh water, marine environments, and even on trees or animal hair. Many form symbiotic relationships, for example, with lichens or sea anemones.

Chromophyta is a diverse phylum, including golden brown algae (Chrysophyceae) and diatoms. Golden brown algae are mostly single-celled or colonial, possessing chlorophyll a, c, and fucoxanthin, giving them a brownish color. Diatoms are predominantly unicellular, rich in chlorophyll a and c, and notably have silica in their cell walls, contributing significantly to global oxygen production and forming 'sea foam' on beaches.

The true brown algae within Chromophyta include giant kelp (genus Fucus), known for their large size. These are exclusively multicellular marine organisms, featuring a body plan with a holdfast, stipe, and blade, often with gas-filled bladders for buoyancy. Kelp forests are vital marine ecosystems.

Rotophyta, or red algae, are distinguished by chlorophyll a, d, and phycobilins. Most species are considered seaweed, thriving in warmer and deeper waters than brown algae. They are typically filamentous or crustose and have complex life cycles. Economically, red algae are a source of agar.

Algae provide several significant products for human use. Diatoms yield oils for vitamin supplements and their silica-rich cell walls are harvested as diatomaceous earth for filtration, polishes, and paint. Algin, derived from brown algae (like giant kelp), is used as an additive in food products (ice cream, salad dressing) and industrial materials (paints, textiles). Agar, extracted from red algae, is crucial in microbiology for solidifying nutrient culture media.