Laminariales

Laminariales is an order of large brown algae in the class Phaeophyceae, commonly known as kelp. Members form some of the tallest and fastest-growing seaweeds in temperate and subpolar marine environments and play a key role in coastal ecosystems. The order includes several families, such as Laminariaceae, Lessoniaceae, and Alariaceae, with prominent genera including Laminaria, Macrocystis, and Nereocystis.

The kelp thallus typically consists of a holdfast that anchors the plant to the substrate, a hollow stipe, and broad blades that capture light. Many species bear gas-filled bladders, or pneumatocysts, along the blades or near their base to maintain buoyancy and position the fronds toward the surface.

Kelp inhabit cold to temperate coastal waters worldwide, especially along rocky shores. They form dense underwater forests known as kelp forests, usually in shallow depths where light and nutrients are adequate. Growth is rapid under favorable nutrient conditions and temperature regimes.

Laminariales display alternation of generations. The conspicuous diploid sporophyte is the kelp plant, while microscopic haploid male and female gametophytes produce gametes in conceptacles. Fertilization yields a zygote that grows into a new sporophyte. Spores released from sporangia develop into male or female gametophytes, completing the cycle. This heteromorphic alternation means the large adult is the sporophyte.

Kelp forests provide habitat and food for a wide range of marine organisms and help protect shorelines from erosion. Humans harvest kelp for food (such as kombu), and for industrial products, notably alginates used as thickeners and stabilizers in foods and cosmetics. They are sensitive to temperature changes and nutrient availability; rising sea temperatures and pollution threaten kelp beds in some regions.