Scenedesmus
| Scenedesmus | |
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| Scenedesmus bijunga | |
| Scientific classification | |
| Kingdom: | Plantae |
| Division: | Chlorophyta |
| Class: | Chlorophyceae |
| Order: | Chlorococcales |
| Family: | Scenedesmaceae |
| Subfamily: | Scenedesmoidea |
| Genus: |
Scenedesmus Meyen, 1829 |
| Type species | |
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Scenedesmus obtusus Meyen, 1829 |
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| Species | |
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Scenedesmus is a genus of green algae, in the class Chlorophyceae. They are colonial and non-motile.
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.
Scenedesmus is one of the most common freshwater algae genera; however, the extremely diverse morphologies found within species make identification difficult. 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.
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, 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.
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 are detected, an infochemical released by Daphnia that Scenedesmus has evolved to recognize as a warning signal.
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Wikipedia