Scenedesmus
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''Scenedesmus'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...
, in the class
Chlorophyceae The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. They are usually green due to the dominance of pigments chlorophyll a and chlorophyll b. The chloroplast may be discoid, p ...
. They are
colonial Colonial or The Colonial may refer to: * Colonial, of, relating to, or characteristic of a colony or colony (biology) Architecture * American colonial architecture * French Colonial * Spanish Colonial architecture Automobiles * Colonial (1920 au ...
and non-motile.


Taxonomy

Currently, there are 74 taxonomically accepted species of ''Scenedesmus''. Additionally, several subgenera have been identified, but vary according to the source. Hegewald denotes ''Acutodesmus'', '' Desmodesmus'', and ''Scenedesmus'' as the three major categories. ''Acutodesmus'' is characterized as having acute cell poles, while ''Desmodesmus'' and ''Scenedesmus'' have obtuse/truncated cell poles (differentiated by the presence or absence of spines respectively). Fossil records date ''Scenedesmus'' from 70 to 100 million years ago with ''Desmodesmus'' suspected to be the youngest of these three groups.


Basic biology

''Scenedesmus'' is one of the most common freshwater algae genera; however, the extremely diverse morphologies found within species make identification difficult.Lürling, Miquel. The Smell of Water: Grazer-Induced Colony Formation in Scenedesmus. Thesis. Agricultural University of Wageningen, 1999. While most species are found across the world, certain species exist only in local populations such as ''S. intermedius'' and ''S. serratus'' which are found in New Zealand.


Coenobia and cell growth

''Scenedesmus'' can exist as unicells; they are also frequently found in coenobia of four or eight cells inside a parental mother wall. Various coenobial architectures have been described, including linear, costulatoid, irregular, alternating, or dactylococcoid patterns (Figure 1). The formation of coenobia is dependent on a number of factors. A higher proportion of unicellular organisms was found at high light intensities and high temperatures, suggesting that at higher growth rates the organisms prefer to be non-colonized. Successful growth and division for algae relies on a balance between maintaining buoyancy in the euphotic zone (containing ideal light and nutritional conditions) and avoidance of grazing predators. Larger colonies have a smaller surface-to-volume ratio, which limits nutrient uptake and light harvesting, and the large mass promotes sinking. However, in the presence of grazers, such as ''
Daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembl ...
'', that threaten to consume unicellular algae, the larger colonies provide significant security. This threat can be so significant that the cells will coalesce into these 8-cell colonies even in severely limiting growth conditions in order to reduce grazing vulnerability or while in nutrient-deplete conditions.


Defense mechanisms

The cells have other mechanisms of self-defense in addition to colonizing. ''Scenedesmus'' can be divided into two subgenera, the non-spiny ''Scenedesmus'' and the spiny ''Desmodesmus''. Although spineless, the ''Scenedesmus'' subgenera cells have thick cells walls and mucilage, which may make them digestion-resistant. Some chemical compounds in ''Scenedesmus'' could even be toxic to certain organisms upon consumption. Bristles of up to 100 um may form a net in both spiny and non-spiny varieties to discourage predation even further. Cells defensively form these bristles when
kairomones A kairomone (a coinage using the Greek καιρός ''opportune moment'', paralleling pheromone"kairomone, n.". OED Online. September 2012. Oxford University Press. http://www.oed.com/view/Entry/241005?redirectedFrom=kairomone (accessed 3 Octobe ...
are detected, an infochemical released by ''
Daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembl ...
'' that ''Scenedesmus'' has evolved to recognize as a warning signal.


Reproduction and colony formation

During replication, the mother cell enlarges and becomes
multinucleate Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordinat ...
after multiple divisions. The cytoplasm then is cleaved into
uninucleate {{Short pages monitor