Red algae in the context of "Glaucophyte"

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.

Plants are the eukaryotes that comprise the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars from carbon dioxide and water, using the green pigment chlorophyll. Exceptions are parasitic plants that have lost the genes for chlorophyll and photosynthesis, and obtain their energy from other plants or fungi. Most plants are multicellular, except for some green algae.

Historically, as in Aristotle's biology, the plant kingdom encompassed all living things that were not animals, and included algae and fungi. Definitions have narrowed since then; current definitions exclude fungi and some of the algae. By the definition used in this article, plants form the clade Viridiplantae (green plants), which consists of the green algae and the embryophytes or land plants (hornworts, liverworts, mosses, lycophytes, ferns, conifers and other gymnosperms, and flowering plants). A definition based on genomes includes the Viridiplantae, along with the red algae and the glaucophytes, in the clade Archaeplastida.

Viridiplantae (lit. 'green plants'; kingdom Plantae sensu stricto) is a clade of around 450,000–500,000 species of eukaryotic organisms, most of which obtain their energy by photosynthesis. The green plants are chloroplast-bearing autotrophs that play important primary production roles in both terrestrial and aquatic ecosystems. They include green algae, which are primarily aquatic, and the land plants (embryophytes, Plantae sensu strictissimo), which emerged within freshwater green algae. Green algae traditionally excludes the land plants, rendering them a paraphyletic group, however it is cladistically accurate to think of land plants as a special clade of green algae that evolved to thrive on dry land. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them.

Viridiplantae species all have cells with cellulose in their cell walls, and primary chloroplasts derived from endosymbiosis with cyanobacteria that contain chlorophylls a and b and lack phycobilins. Corroborating this, a basal phagotroph Archaeplastida group has been found in the Rhodelphidia. In some classification systems, the group has been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional plant kingdom of embryophytes to include the green algae. Adl et al., who produced a classification for all eukaryotes in 2005, introduced the name Chloroplastida for this group, reflecting the group having primary chloroplasts. They rejected the name Viridiplantae on the grounds that some of the species are not plants as understood traditionally. Together with Rhodophyta, glaucophytes and other basal groups, Viridiplantae belong to a larger clade called Archaeplastida which in itself is sometimes described as Plantae sensu lato.

The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats of unicellular archaeplastids evolved through endosymbiosis, through multicellular marine and freshwater green algae, to spore-bearing terrestrial bryophytes, lycopods and ferns, and eventually to the complex seed-bearing gymnosperms and angiosperms (flowering plants) of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones; for example, the ascendance of flowering plants over gymnosperms in terrestrial environments.

There is evidence that cyanobacteria and multicellular thalloid eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian, around 850 million years ago.

The Archaeplastida (or kingdom Plantae sensu lato "in a broad sense"; pronounced /ɑːrkɪˈplæstɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It also includes the non-photosynthetic lineage Rhodelphidia, a predatorial (eukaryotrophic) flagellate that is sister to the Rhodophyta, and probably the microscopic picozoans. The Archaeplastida have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly through a single endosymbiosis event by phagocytosis of a cyanobacterium. All other groups which have chloroplasts, besides the amoeboid genus Paulinella, have chloroplasts surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae. Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events.

The cells of the Archaeplastida typically lack centrioles and have mitochondria with flat cristae. They usually have a cell wall that contains cellulose, and food is stored in the form of starch. However, these characteristics are also shared with other eukaryotes. The main evidence that the Archaeplastida form a monophyletic group comes from genetic studies, which indicate their plastids probably had a single origin. This evidence is disputed. Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis. Photosynthetic organisms with plastids of different origin (such as brown algae) do not belong to the Archaeplastida.

Coralline algae are red algae in the order Corallinales, characterized by a thallus containing calcareous deposits within its cell walls, giving it hardness. The colors of these algae are typically some hue of pink, or another shade of red, but some species can be purple, yellow, blue, white, or gray-green. Typically, these algae grow in a crustose manner (encrusting rocks and other hardscape); in the intertidal zone of rocky shorelines, and within coral reefs, these algae appear as an abundance of colorful patches on rock surfaces. Unattached specimens (maerl, rhodoliths) may form relatively smooth compact balls, or forming warty to fruticose thalli.

The red algae belong to the division Rhodophyta, within which the coralline algae form the order Corallinales. There are over 1600 described species of nongeniculate coralline algae. The corallines are presently grouped into two families on the basis of their reproductive structures. Most are marine, though one species lives in freshwater; Pneophyllum cetinaensis.

Chromista is a proposed but controversial biological kingdom, refined from the Chromalveolata, consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all eukaryotes whose plastids contain chlorophyll c and are surrounded by four membranes. If the ancestor already possessed chloroplasts derived by endosymbiosis from red algae, all non-photosynthetic Chromista have secondarily lost the ability to photosynthesise. Its members might have arisen independently as separate evolutionary groups from the last eukaryotic common ancestor.

Chromista as a taxon was created by the British biologist Thomas Cavalier-Smith in 1981 to distinguish the stramenopiles, haptophytes, and cryptophytes. According to Cavalier-Smith, the kingdom originally consisted mostly of photosynthetic eukaryotes (algae), but he later brought many heterotrophs (protozoa) into the proposed group. As of 2022, the kingdom was nearly as diverse as the kingdoms Plantae and Animalia, consisting of nine phyla. Notable members include marine algae, potato blight, dinoflagellates, Paramecium, the brain parasite Toxoplasma, and the malarial parasite Plasmodium.