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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.
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria (formerly called blue-green algae). The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated ...
Microalgae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen [2] and use the greenhouse gas carbon dioxide to grow photoautotrophically. "Marine photosynthesis is dominated by microalgae, which together with cyanobacteria, are collectively called phytoplankton."
In general, photosynthesis in cyanobacteria uses water as an electron donor and produces oxygen as a byproduct, though some may also use hydrogen sulfide [79] a process which occurs among other photosynthetic bacteria such as the purple sulfur bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle. [80]
Heterotrophic organisms have to take in all the organic substances they need to survive. All animals , certain types of fungi , and non-photosynthesizing plants are heterotrophic . In contrast, green plants , red algae , brown algae , and cyanobacteria are all autotrophs , which use photosynthesis to produce their own food from sunlight.
Most brown algae live in marine environments, where they play an important role both as food and as a potential habitat. For instance, Macrocystis , a kelp of the order Laminariales , may reach 60 m (200 ft) in length and forms prominent underwater kelp forests that contain a high level of biodiversity. [ 4 ]
Most organisms forage, hunt, or use photosynthesis to get food, but around 50 million years ago — long before humans were around — ants began cultivating and growing their own food.
In a reversal of the pattern on land, in the oceans, almost all photosynthesis is performed by algae and cyanobacteria, with a small fraction contributed by vascular plants and other groups. Algae encompass a diverse range of organisms, ranging from single floating cells to attached seaweeds. They include photoautotrophs from a variety of groups.