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Red algae These groups have chloroplasts containing chlorophylls a and c, and phycobilins. The shape varies from plant to plant; they may be of discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon shaped. They have one or more pyrenoids to preserve protein and starch. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with red algae suggest a relationship there.
In , Samuel Gottlieb Gmelin — published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves. Harvey — and Lamouroux  were the first to divide macroscopic algae into four divisions based on their pigmentation.
This is the first use of a biochemical criterion in plant systematics. Unlike macroalgae , which were clearly viewed as plants, microalgae were frequently considered animals because they are often motile.
Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae: cyanophytes , rhodophytes , chrysophytes , xanthophytes , bacillariophytes , phaeophytes , pyrrhophytes cryptophytes and dinophytes , euglenophytes , and chlorophytes. Later, many new groups were discovered e. With the abandonment of plant-animal dichotomous classification, most groups of algae sometimes all were included in Protista , later also abandoned in favour of Eukaryota.
However, as a legacy of the older plant life scheme, some groups that were also treated as protozoans in the past still have duplicated classifications see ambiregnal protists. Some parasitic algae e. In other cases, some groups were originally characterized as parasitic algae e. Furthermore, groups like the apicomplexans are also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.
Relationship to land plants[ edit ] The first land plants probably evolved from shallow freshwater charophyte algae much like Chara almost million years ago. These probably had an isomorphic alternation of generations and were probably filamentous.
Fossils of isolated land plant spores suggest land plants may have been around as long as million years ago. The only groups to exhibit three-dimensional multicellular thalli are the reds and browns , and some chlorophytes.
Some of the more common organizational levels, more than one of which may occur in the lifecycle of a species, are Colonial : small, regular groups of motile cells Capsoid: individual non-motile cells embedded in mucilage Coccoid: individual non-motile cells with cell walls Palmelloid: nonmotile cells embedded in mucilage Filamentous: a string of nonmotile cells connected together, sometimes branching Parenchymatous: cells forming a thallus with partial differentiation of tissues In three lines, even higher levels of organization have been reached, with full tissue differentiation.
The innovation that defines these nonalgal plants is the presence of female reproductive organs with protective cell layers that protect the zygote and developing embryo. Hence, the land plants are referred to as the Embryophytes. Physiology[ edit ] Many algae, particularly members of the Characeae ,  have served as model experimental organisms to understand the mechanisms of the water permeability of membranes, osmoregulation , turgor regulation , salt tolerance , cytoplasmic streaming , and the generation of action potentials.
Phytohormones are found not only in higher plants, but in algae, too. In these symbioses, the algae supply photosynthates organic substances to the host organism providing protection to the algal cells. The host organism derives some or all of its energy requirements from the algae. Examples are: Main article: Lichen Rock lichens in Ireland Lichens are defined by the International Association for Lichenology to be "an association of a fungus and a photosynthetic symbiont resulting in a stable vegetative body having a specific structure.
In nature they do not occur separate from lichens. It is unknown when they began to associate. A photobiont may be associated with many different mycobionts or may live independently; accordingly, lichens are named and classified as fungal species. The photobiont possibly triggers otherwise latent genes in the mycobiont. Lichen thus share some of the habitat and often similar appearance with specialized species of algae aerophytes growing on exposed surfaces such as tree trunks and rocks and sometimes discoloring them.
Main articles: Coral , Coral reef , and Symbiodinium Floridian coral reef Coral reefs are accumulated from the calcareous exoskeletons of marine invertebrates of the order Scleractinia stony corals.
These animals metabolize sugar and oxygen to obtain energy for their cell-building processes, including secretion of the exoskeleton, with water and carbon dioxide as byproducts. Dinoflagellates algal protists are often endosymbionts in the cells of the coral-forming marine invertebrates, where they accelerate host-cell metabolism by generating sugar and oxygen immediately available through photosynthesis using incident light and the carbon dioxide produced by the host.
Reef-building stony corals hermatypic corals require endosymbiotic algae from the genus Symbiodinium to be in a healthy condition. Main article: Sea sponge Endosymbiontic green algae live close to the surface of some sponges, for example, breadcrumb sponges Halichondria panicea.
Asexual reproduction permits efficient population increases, but less variation is possible. Commonly, in sexual reproduction of unicellular and colonial algae, two specialized, sexually compatible, haploid gametes make physical contact and fuse to form a zygote.
To ensure a successful mating, the development and release of gametes is highly synchronized and regulated; pheromones may play a key role in these processes. Another checklist reports only about 5, species. Regarding the difference of about 15, species, the text concludes: "It will require many detailed field surveys before it is possible to provide a reliable estimate of the total number of species Most of these are listed in List of seaweeds of South Africa.
These exclude phytoplankton and crustose corallines. Most estimates also omit microscopic algae, such as phytoplankton. The most recent estimate suggests 72, algal species worldwide. This dispersal can be accomplished by air, water, or other organisms.
Due to this, spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms. The spores of freshwater algae are dispersed mainly by running water and wind, as well as by living carriers.
Ocean water presents many vastly different habitats based on temperature and nutrient availability, resulting in phytogeographic zones, regions, and provinces. It is, therefore, possible to identify species occurring by locality, such as "Pacific algae" or "North Sea algae". When they occur out of their localities, hypothesizing a transport mechanism is usually possible, such as the hulls of ships.
For example, Ulva reticulata and U. Mapping is possible for select species only: "there are many valid examples of confined distribution patterns. Microscopic forms that live suspended in the water column phytoplankton provide the food base for most marine food chains.
In very high densities algal blooms , these algae may discolor the water and outcompete, poison, or asphyxiate other life forms. Algae can be used as indicator organisms to monitor pollution in various aquatic systems. Due to this, the species composition of algal populations may shift in the presence of chemical pollutants.
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