Bryopsis, often referred to as hair algae, is a genus of marine green algae in the family Bryopsidaceae. Species in the genus are macroscopic, siphonous marine green algae that are made up of units of single tubular filaments. They can form dense tufts up to 40 cm in height. Each cell is made of up an erect thallus that is often branched into pinnules. Approximately 60 species have been identified in this genus since its initial discovery in 1809. The ecological success of Bryopsis has also been attributed to its associations with endophytic bacteria that reside in the cytoplasm of their cells.

Species in this genus are known to be pests in aquariums and associated with green tides due to macroalgal blooms. However, Bryopsis also contains unique chemical properties, most notably, Kahalalide F (KF), a depsipeptide that is being studied for its antitumor properties in human cancer cells and also has important ecological significance in protecting the algae against herbivory. The removal of algal blooms for the extraction of KF may be a pragmatic approach to eradicating Bryopsis from green tides and aiding the economic burden of producing KF experimentally for clinical trials.

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