Algae are very simple, chlorophyll-containing organisms in a family that includes more than 20,000 different known species. In cosmetics, algae act as thickening agents, water-binding agents, and antioxidants.
Some algae are also potential skin sensitizers. For example, the phycocyanin present in blue-green algae has been suspected of sensitization on the basis of patch tests.
Other forms of algae, such as Irish moss and carrageenan, contain proteins, vitamin A, sugar, starch, vitamin B1, iron, sodium, phosphorus, magnesium, copper, and calcium. Most of these are beneficial for skin, as emollients, soothing agents, or antioxidants.
New research shows forms of brown algae show promise for improving the appearance of aging skin. As a plant ingredient, all types of algae are prone to breaking down with routine exposure to air and light. Because of this, avoid anti-aging products with algae that are packaged in jars.
References for this information:
Bioresource Technology, May 2015, pages 355-362
Marine Pharmacognosy: Trends and Applications, 2013
Marine Drugs, January 2013, pages 146-164