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Algae (UK: / ˈ æ l ɡ iː / AL-ghee, US: / ˈ æ l dʒ iː / AL-jee; [3] sg.: alga / ˈ æ l ɡ ə / AL-gə) is an informal term for any organisms of a large and diverse group of photosynthetic eukaryotes, which include species from multiple distinct clades.
Algae lack the various structures that characterize plants (which evolved from freshwater green algae), such as the phyllids (leaf-like structures) and rhizoids of bryophytes (non-vascular plants), and the roots, leaves and other xylemic/phloemic organs found in tracheophytes (vascular plants). Most algae are autotrophic, although some are ...
Green algae and plants possess two forms of this pigment: chlorophyll a and chlorophyll b. Kelps, diatoms, and other photosynthetic heterokonts contain chlorophyll c instead of b, while red algae possess only chlorophyll a. All chlorophylls serve as the primary means plants use to intercept light in order to fuel photosynthesis.
Recent work suggests that non-photosynthetic bacterial microbiomes associated with lichens may also have functional significance to lichens. [10] Most mycobiont partners derive from the ascomycetes, and the largest class of lichenized fungi is Lecanoromycetes. [11] The vast majority of lichens derive photobionts from Chlorophyta (green algae). [9]
The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis, at first by cyanobacteria and later by photosynthetic eukaryotes, radically changed the composition of the early Earth's anoxic atmosphere, which as a result is now 21% oxygen.
In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This oxygenic photosynthesis is by far the most common type of photosynthesis used by living organisms. Some shade-loving plants (sciophytes) produce such low levels of oxygen during photosynthesis that they use all of it themselves instead of releasing it to the atmosphere. [12]
Diatoms are photosynthetic unicellular algae populating the oceans and other waters around the globe. They form a (disputed) phylum containing about 100,000 recognised species. They form a (disputed) phylum containing about 100,000 recognised species.
Chloroplastida is the term chosen by Adl et al. for the group made up of the green algae and land plants (embryophytes). Except where lost secondarily, all have chloroplasts without a peptidoglycan layer and lack phycobiliproteins. The chlorophyte Stigeoclonium. Chlorophyta Pascher, 1914, emend. Lewis & McCourt, 2004 – green algae (part)