Stonewort in the context of Chara (alga)

Algae (/ˈældʒiː/ AL-jee, UK also /ˈælɡiː/ AL-ghee; sg.: alga /ˈælɡə/ AL-gə) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not land plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, such as cyanobacteria, Chlorella, and diatoms, to multicellular macroalgae such as kelp or brown algae which may grow up to 50 metres (160 ft) in length. Most algae are aquatic organisms and lack many of the distinct cell and tissue types, such as stomata, xylem, and phloem that are found in land plants. The largest and most complex marine algae are called seaweeds. In contrast, the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and stoneworts. Algae that are carried passively by water are plankton, specifically phytoplankton.

Algae constitute a polyphyletic group because they do not include a common ancestor, and although eukaryotic algae with chlorophyll-bearing plastids seem to have a single origin (from symbiogenesis with cyanobacteria), they were acquired in different ways. Green algae are a prominent example of algae that have primary chloroplasts derived from endosymbiont cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from endosymbiotic red algae, which they acquired via phagocytosis. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction via spores.

Charophyta (UK: /kəˈrɒfɪtə, ˌkærəˈfaɪtə/) is a paraphyletic group of freshwater green algae, called charophytes (/ˈkærəˌfaɪts/), sometimes treated as a division, yet also as a superdivision. The terrestrial plants, the Embryophyta emerged deep within Charophyta, possibly from terrestrial unicellular charophytes, with the class Zygnematophyceae as a sister group.

With the Embryophyta now cladistically placed in the Charophyta, it is a synonym of Streptophyta. The sister group of the charophytes are the Chlorophyta. In some charophyte groups, such as the Zygnematophyceae or conjugating green algae, flagella are absent and sexual reproduction does not involve free-swimming flagellate sperm. Flagellate sperm, however, are found in stoneworts (Charales) and Coleochaetales, orders of parenchymatous charophytes that are the closest relatives of the land plants, where flagellate sperm are also present in all except the conifers and flowering plants. Fossil stoneworts of early Devonian age that are similar to those of the present day have been described from the Rhynie chert of Scotland. Somewhat different charophytes have also been collected from the Late Devonian (Famennian) Waterloo Farm lagerstätte of South Africa. These include two species each of Octochara and Hexachara, which are the oldest fossils of Charophyte axes bearing in situ oogonia.