Scenedesmus

Scenedesmus can exist as unicells; they are also frequently found in coenobia of four or eight cells[3] inside a parental mother wall. Various coenobial architectures have been described, including linear, costulatoid, irregular, alternating, or dactylococcoid patterns (Figure 1).[3] 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.[3] 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.[3] 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.[3] 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.[3][4]

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.[3] Cells defensively form these bristles when kairomones are detected, an infochemical released by Daphnia that Scenedesmus has evolved to recognize as a warning signal.[3][4]

During replication, the mother cell enlarges and becomes multinucleate after multiple divisions. The cytoplasm then is cleaved into uninucleate daughter cells, usually developing as non-motile autospores. These daughter cells typically link up with other daughter cells to form a colony within the parental cell wall to be later released.[5] The cells progress through a typical mitotic cycle similar to other members of Chlorophyceae, with the cytoplasm of the daughter cells becoming very dense.[5] Eventually the mother cell wall breaks and releases the spores which adopt a normal cellular appearance.[5] The cells at either end of the coenobium are different in morphology from those in the center.[5] How the cells adhere to one another during development is still unclear, but it is known that a trilaminar sheath (TLS), composed of algaenan, is one of the first exterior structures to form, developing in patches before growing to connect into one continuous layer.[5] The ornamented layer is the last component to develop.[5]

In 1942, Gaffron and Rubin may be credited with conducting an experiment that sparked H₂ production research in green algae using Scenedesmus obliquus.[10] Algae produce H₂ gas under anaerobic conditions by providing hydrogenases with hydrogen ions derived from splitting of water molecules via photosynthesis.[11] However, enzyme activity is transient due to inhibition from O₂ production via photosynthesis, a problem that continues to plague H₂ production.[12] S. obliquus is traditionally known to utilize a nickel-iron hydrogenase, but usage of other iron hydrogenases in H₂ production is also reported.[13] Hydrogenase enzyme activity in Scenedesmus species is reported to be lower than that of Chlamydomonas reinhardtii.[14] H₂ production independent of Photosystem II in Scenedesmus has also been performed using redox equivalents of fermentative metabolism under dark anaerobic incubation.[10][15] 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.[12] Ultrasonication pretreatment has been effective in increasing fermentative bioenergy production from Scenedesmus oliquus YSW15.[16] Bio-hydrogen production research using Scenedesmus is actively spurred by its applications to wastewater treatment. (See subsequent section on waste management by Scenedesmus).

Scenedesmus is known to have high biomass productivity among green algae, and has been actively researched for its use for bio-diesel production. Its heterotrophic production of biomass and lipid in optimized conditions is reported to have higher efficiency than its autotrophic production.[17][18][19][20] 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.[18][21] However, Scenedesmus is more efficient at capturing CO₂ than other algae.[20] Like many algae species, Scenedesmus required nitrate-deficient condition to profoundly increase its lipid yield.[22] A significant improvement (up to six-fold) of feedstock yields was achieved by adding varying concentrations of ethanol under a 12-hr photoperiod and in the dark.[22] 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.[18] Extraction of oils with methanol or ethanol from the Scenedesmus remains a challenge and its lower lipid content adds to the cost of production.[18] In a recent study,[23] 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 of nitrogen. A two-step transesterification was found to be best suited for transesterification, while Folch extraction was best for lipid extraction.[23]

Scenedesmus, and other microalgae such as Chlorella, Dunaliella, Chlamydomonas, and Spirulina, contain large amounts of carbohydrate (>50% of the dry weight), which make them attractive candidates for bio-ethanol production.[24] In one study,[25] 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 of Scenedesmus culture when cells reached 0.5-0.6 g L⁻¹) to be considered viable for future drop-in fuel production.[26]