Phaeophyceae (Brown Algae): Brown Algae are a large group of multicellular algae, which comprises the class Phaeophyceae and it includes many cold-water environment seaweeds that are found in the northern hemisphere. Most of the species belonging to the Brown algae group is found in the marine environment where they play two important roles, which are following:

• The first role brown algae perform in the marine environment is they act as an important source of food, and thus they maintain the food cycle and food web there.
• And, the second role which they perform there is that they act as potential habitat in the marine environment.

Brown algae also possess some unique characteristics in terms of their growth and development, as many seaweeds that belongs to the brown algae group are the largest and fastest-growing seaweeds. For example, look at the Macrocystis member of this group. The stipes of Macrocystis can grow up to 6 cm (which is equal to 2.5 inches) in a single day, and their fronds may grow as much as 50 cm (which is equal to 20 inches) per day.

They are multicellular algae. They are filamentous and branched. The plant body is thallus, i.e. they lack true roots, stem and leaves.

• They have a root-like structure called a holdfast, which anchors them to their substrate. The structure of holdfast varies in different species. They do not take part in nutrient or water uptake like roots in plants. They prevent algae from being flown away by the water current.
• There is a small stalk present, which is more like a stem. It is called a stipe. Fucus shows three distinct layers in the stipe region, namely pith (central region), cortex and outer epidermis. Stipe may be rigid or elastic. The centre of the stipe is hollow and helps in keeping the algae buoyant.
• They have a flattened structure called the lamina, blade or frond, which resembles leaves. The surface of the lamina may be smooth or wrinkled. Sometimes it is coated with slime to prevent attachment of epiphytes.
• Some of the brown algae contain special gas-filled bladders called pneumatocysts. They are mostly present near the lamina. It provides buoyancy to algae and helps to keep the part of the algae near the surface for photosynthesis.
• The cell wall is made up of two layers, outer gummy, made up of algin and inner cellulose, which provides strength.
• They are autotrophic and contain chlorophyll ‘a’ and ‘c’. They also contain carotenoids and xanthophyll.
• Chloroplast usually does not contain pyrenoids but some species have pyrenoids in them.
• Fucoxanthin (C₄₂H₅₈O₆) is the main xanthophyll pigment and is responsible for their characteristic colour.
• Food is stored in the form of complex carbohydrates, viz. Laminarin or mannitol.
• Laminarin is a polysaccharide of glucose. It is a linear polymer with β(1→3) and β(1→6) (branching) linkages.
• Mannitol is a sugar alcohol and other than being carbon storage it also acts as an osmoprotectant, antioxidant. It protects them in their harsh habitat.

Reproduction in Phaeophyceae (Brown Algae)

They reproduce by both asexual and sexual means.

• Vegetative reproduction is by fragmentation.
• Asexual reproduction is by the formation of motile zoospores. They are pear-shaped and have two unequal flagella.
• Sexual reproduction is by the formation of motile gametes, which are also biflagellate with two laterally attached flagella.
• Sexual reproduction may be oogamous, isogamous or anisogamous.
• Gametes unite in water or within oogonium.

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By Team Learning Mantras