Scenedesmus is agenus ofgreen algae, in the classChlorophyceae. They arecolonial and non-motile. They are one of the most common components ofphytoplankton in freshwater habitats worldwide.^[1]

Scenedesmus
Scenedesmus bijugus
Scientific classification
Clade:	Viridiplantae
Division:	Chlorophyta
Class:	Chlorophyceae
Order:	Sphaeropleales
Family:	Scenedesmaceae
Genus:	Scenedesmus Meyen, 1829
Type species
Scenedesmus obtusus Meyen, 1829
Species
Scenedesmus dimorphus Scenedesmus acuminatus 72 others (see text)

The starting point ofScenedesmus and related algae is in 1820, whenPierre Jean François Turpin observed these algae under a microscope. He classified them under thediatom genusAchnanthes; later authors moved them to different groups, until it was finally classified as agreen alga.^[1] The name comes from the Greek rootsskene, meaning "tent" or "awning", anddesmos, meaning "bond".^[2]

Currently, there are 74 taxonomically accepted species ofScenedesmus.^[3] Additionally, several subgenera have been identified, but vary according to the source. Hegewald denotesAcutodesmus,Desmodesmus, andScenedesmus as the three major categories.Acutodesmus is characterized as having acute cell poles, whileDesmodesmus andScenedesmus have obtuse/truncated cell poles (differentiated by the presence or absence of spines respectively). Fossil records dateScenedesmus from 70 to 100 million years ago withDesmodesmus suspected to be the youngest of these three groups.^[1]

Scenedesmus is one of the most common freshwater algae genera; however, the extremely diverse morphologies found within species make identification difficult.^[4] While most species are found across the world, certain species exist only in local populations such asS. intermedius andS. serratus which are found in New Zealand.^[4]

Scenedesmus can exist as unicells; they are also frequently found incoenobia of four or eight cells^[4] inside a parental mother wall. Various coenobial architectures have been described, including linear, costulatoid, irregular, alternating, or dactylococcoid patterns (Figure 1).^[4] 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.^[4] 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.^[4] 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 asDaphnia, that threaten to consume unicellular algae, the larger colonies provide significant security.^[4] 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.^[4][5]

The cells have other mechanisms of self-defense in addition to colonizing.Scenedesmus can be divided into two subgenera, the non-spinyScenedesmus and the spinyDesmodesmus. Although spineless, theScenedesmus subgenera cells have thick cells walls and mucilage, which may make them digestion-resistant. Some chemical compounds inScenedesmus 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.^[4] Cells defensively form these bristles whenkairomones are detected, an infochemical released byDaphnia thatScenedesmus has evolved to recognize as a warning signal.^[4][5]

During replication, the mother cell enlarges and becomesmultinucleate after multiple divisions. The cytoplasm then is cleaved intouninucleate daughter cells, usually developing as non-motileautospores. These daughter cells typically link up with other daughter cells to form a colony within the parental cell wall to be later released.^[6] The cells progress through a typical mitotic cycle similar to other members of Chlorophyceae, with the cytoplasm of the daughter cells becoming very dense.^[6] Eventually the mother cell wall breaks and releases the spores which adopt a normal cellular appearance.^[6] The cells at either end of the coenobium are different in morphology from those in the center.^[6] How the cells adhere to one another during development is still unclear, but it is known that a trilaminar sheath (TLS), composed ofalgaenan, is one of the first exterior structures to form, developing in patches before growing to connect into one continuous layer.^[6] The ornamented layer is the last component to develop.^[6]

The exterior ornamentation is highly variable within the genusScenedesmus. Staehelinet al. characterized two species in detail:S. pannonicus andS. longus.S. pannonicus assembles a tight-fitting "warty" layer compared to the loose "reticulate" layer found onS. longus.^[7] A shared feature between the two is a TLS found at the junction between neighboring cells that helps cement them together.^[7] An additional pectic layer observed on S. pannonicus forms a thick mesh of thin filaments originating from the warty layer.^[7] Another feature of the outer coenobial surface ofS. pannonicus is a combination of individual spikes (seemingly connected to the warts) and small spikelets that fuse to form combs that zigzag along the cell.^[7] An overview of these structures can be seen in Figure 2. The last major category of ornamentation is rosettes that are common to manyScenedesmus species.^[7] Rosettes are ring-shaped structures enclosing small mounds on the cell surface and are usually sitting upon a thicker layer of cell wall than the surrounding areas.^[7] No potential function for these structures has been suggested. WhileS. longus was not observed with the comb-like structures ofS. pannonicus, it did have two variations of spikelets forming between the TLS and reticulate layer to keep the two apart.^[7]

Scenedesmus obliquus is notable for the non-standard coding of itsmitochondrial DNA which may represent an intermediate form in the evolution of green algal mitochondrial DNA.^[8] This code is represented byNCBI translation table 22,Scenedesmus obliquus mitochondrial code.^[9]

AlthoughScenedesmus is capable of producing many kinds ofbiofuels such as biohydrogen, biodiesel, bioethanol and drop-in fuels, most extensive research has been done on the use ofScenedesmus for biodiesel production. Like all algae systems, the implementation of integrated biofuel production ofScenedesmus from the laboratory findings has challenges in large-scale production. Major challenges include nutrient supply and recycling, gas transfer and exchange, PAR (Photosynthetically Active Radiation) delivery, cultural integrity, environmental control, land and water availability, harvesting, and genetic and metabolic engineering^[10]

In 1942, Gaffron and Rubin may be credited with conducting an experiment that sparked H₂ production research in green algae usingScenedesmus obliquus.^[11] Algae produce H₂ gas under anaerobic conditions by providing hydrogenases with hydrogen ions derived from splitting of water molecules via photosynthesis.^[12] However, enzyme activity is transient due to inhibition from O₂ production via photosynthesis, a problem that continues to plague H₂ production.^[13]S. obliquus is traditionally known to utilize a nickel-iron hydrogenase, but usage of other iron hydrogenases in H₂ production is also reported.^[14] Hydrogenase enzyme activity inScenedesmus species is reported to be lower than that ofChlamydomonas reinhardtii.^[15] H₂ production independent of Photosystem II inScenedesmus has also been performed using redox equivalents of fermentative metabolism under dark anaerobic incubation.^[11][16] Research findings suggest that a sulfur-deprived environment triggers an imbalance in the photosynthesis and respiration relationship, resulting in net consumption of O₂, causing anaerobiosis, and switching to hydrogen production.^[13] Ultrasonication pretreatment has been effective in increasing fermentative bioenergy production fromScenedesmus oliquus YSW15.^[17] Biohydrogen production research usingScenedesmus is actively spurred by its applications towastewater treatment. (See subsequent section on waste management byScenedesmus).

Scenedesmus is known to have high biomass productivity among green algae, and has been actively researched for its use for biodiesel production. Its heterotrophic production of biomass and lipid in optimized conditions is reported to have higher efficiency than its autotrophic production.^{[18][19][20][21]} Optimization of biomass productivity as well as lipid content through varying concentration of supplemental nutrients has been done in numerous studies; currently,Scenedesmus lipid yield after optimization has reached ~60% dry cell weight, lower than some other algae.^[19][22] However,Scenedesmus is more efficient at capturing CO₂ than other algae.^[21] Like many algae species,Scenedesmus required nitrate-deficient condition to profoundly increase its lipid yield.^[23] A significant improvement (up to six-fold) of feedstock yields was achieved by adding varying concentrations of ethanol under a 12-hour photoperiod and in the dark.^[23] Under salinity stress via two-stage cultivation,Scenedesmus sp. BHU1 enhances both biomass productivity and lipid content.^[24] The most significant improvement in lipid production was obtained when stationary phase cultures were transferred to media deficient in nitrate for 7 days and phosphate for 3 days, respectively.^[19] Extraction of oils with methanol or ethanol from theScenedesmus remains a challenge and its lower lipid content adds to the cost of production.^[19] In a recent study,^[25]Scenedesmus abundans was isolated from Dal Lake, Kashmir and proved to be a suitable raw material for biodiesel production. The alga increased significantly in biomass and lipid content with the nitrogen concentration of 0.32g/L ofnitrogen. A two-step transesterification was found to be best suited fortransesterification, while Folch extraction was best for lipid extraction.^[25]

Scenedesmus, and other microalgae such asChlorella,Dunaliella,Chlamydomonas, andSpirulina, contain large amounts of carbohydrate (>50% of the dry weight), which make them attractive candidates for bioethanol production.^[26] In one study,^[27]Scenedesmus was used to yield high biomass productivity; its carbohydrate-rich biomass was then hydrolyzed with 2%sulfuric acid and underwent an SHF (Separate Hydrolysis and Fermentation) process to produce 8.55 g L⁻¹ of ethanol and a maximum yield of 0.213 g ethanol / g biomass within 4 hours of ethanol fermentation.

Isoprenoids are considered important metabolites that can be utilized as drop-in fuels, often as alkane chains.Scenedesmus conducts a pyruvate/glyeraldehyde 3-phosphate non-mevalonate pathway to synthesize isoprenoids. However, isoprenoid yields were too low (1.5~15 mg per 10 liter ofScenedesmus culture when cells reached 0.5-0.6 g L⁻¹) to be considered viable for future drop-in fuel production.^[28]

Scendesmus species are often able to grow in wastewater or other side streams and are thus regularly cultivated in such streams.^{[29][30][31][32]}In a study comparing the efficiency of ammonia and phosphorus removal from anagroindustrialwastewater byChlorella vulgaris andScenedesmus dimorphus,Scenedesmus exhibited better efficiency of removingammonia in cylindrical bioreactor while both algae removedphosphorus from the wastewater to the same extent.^[33]Algal Turf Scrubber (ATS) is one of many technologies that utilize algae for treating variety of wastes and industrially polluted waters.^[34] An algal turf scrubber in Florida, for example, removed phosphorus at a cost of$24 perkg whereasengineered wetland processes removed phosphorus at a cost of $77 per kg. While removing metallic wastes as well as organic substrates, growingScenedesmus biomass could be utilized for producing cattle feeds, organic fertilizers, paper, construction paper, and biodiesel.^[10]

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  Scenedesmus quadricauda
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  Scenedesmus brasiliensis
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