Why do algae multiply in different ways?

Algae are a large group of simple plant organisms that, due to their diversity, reproduce in different ways, both sexual and asexual. Some species alternate between breeding methods in subsequent generations. Algae can exist as single-celled organisms called plankton, they can form colonial organisms such as algae, or they can combine with fungi to form lichens. Various species can live in fresh water, seawater, or wet rock.