Scenedesmus

''Scenedesmus'' is a genus of green algae, in the class Chlorophyceae. They are colonial and non-motile. They are one of the most common components of phytoplankton in freshwater habitats worldwide.

Taxonomy

The starting point of ''Scenedesmus'' and related algae is in 1820, when Pierre Jean François Turpin observed these algae under a microscope. He classified them under the diatom genus '' Achnanthes''; later authors moved them to different groups, until it was finally classified as a green alga. The name comes from the Greek roots ''skene'', meaning "tent" or "awning", and ''desmos'', meaning "bond".

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.Hegewald, Eberhard H. "Taxonomy and Phylogeny of ''Scenedesmus''." The Korean Journal of Phycology 12.4 (1997): 235-46.

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'', 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 eight-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 μm 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.

Reproduction and colony formation

During replication, the mother cell enlarges and becomes multinucleate after multiple divisions. The cytoplasm then is cleaved into uninucleate