Arthrospira platensis (commonly referred to as Spirulina , despite belonging to a morphologically distinct genus) is a filamentous cyanobacterium characterised by its beneficial nutritional content, left handed open helical form, and ability to grow under highly alkaline conditions. A. platensis is protein rich and has been utilised as a traditional foodstuff in multiple cultures, most notably in Central America and in the region surrounding Lake Chad. Recently A. platensis has experienced a resurgence as a health food and a source of the blue pigment phycocyanin for use as a food colourant.
Synechocystis sp. PCC6803 is a unicellular freshwater cyanobacterium commonly employed as a cyanobacterial model system. It is capable of both photo- and heterotrophic growth making it useful for studying photosynthetic processes, and has a well defined circadian clock. Synechocystis sp. PCC6803 is naturally competent allowing direct uptake of recombinant DNA, and has a well defined molecular toolbox including a sequenced genome. There has also been interest in the use of Synechocystis sp. PCC6803 for biofuel applications, both in itself and as a model for other third generation biofuel platforms.
Chlamydomonas reinhardtii is a motile, unicellular green microalga typically measuring around 10 µm in diameter. It is widely distributed, and is often isolated from soil and freshwater samples. C. reinhardtii has been used as a model organism for over 70 years for both basic and applied research, largely due to its ease of cultivation, rapid doubling time of 6-8 h, and established molecular toolbox (Harris, 2009). Noted areas of study include photosynthesis, phototaxis, cell wall biogenesis, cell cycle events, flagella assembly, mating processes, and nuclear/chloroplast interactions (Rochaix, 1995; Shimogawara et al., 1998; Merchant et al., 2007). Annotated sequences are available for the nuclear, chloroplast and mitochondrial genomes (Merchant et al., 2007, Scaife et al., 2015), and several extensive libraries of mutants have been generated. Transformation of all three genomes have been demonstrated, with nuclear and chloroplast manipulation becoming routine (Boynton et al. 1988; Kindle et al. 1989; Sodeinde and Kindle. 1993).
Recently C. reinhardtii has gained attention as a platform for commercial applications; these include recombinant protein expression (Mayfield et al., 2007; Rosales-Mendoza et al., 2012), biohydrogen production (Torzillo et al., 2015), and as a model testbed for biofuel technologies prior to shuttling into more industrially relevant, but less easy to manipulate, biofuel production strains.
Cultivation of C. reinhardtii is typically conducted mixotrophically on TAP (tris acetate phosphate) medium. Although suitable for lab-scale work, TAP medium is not appropriate for scale up due to its relatively high cost and susceptibility to contamination.
The genus Haematococcus is found globally, with reports of isolates from all continents with the exception of Antarctica, with hostile areas of isolation including the artic circle (Klochkova et al., 2013). H. pluvialis is of commercial interest due to its ability to produce copious amounts of astaxanthin, reaching up to 5 % dry weight in the encysted aplanospore state (Wayame et al., 2013). Astaxanthin is sold as a pigment for aquaculture and in animal feed, and is marketed as an antioxidant for the nutraceutical market. The H. pluvialis derived astaxanthin industry is commercially successful; however, several constraints are ever-present including issues of contamination and grazing, high extraction costs, high light requirements for encystment, and conversely, photo-bleaching (Shah et al., 2016).
Astaxanthin is produced under high light and nutrient deplete conditions (García-Malea et al., 2008). High temperature is rarely implemented to induce astaxanthin production, as it was reported to severely reduce biomass yield, and thus decrease astaxanthin productivity (Tjahjono et al. 1994). Currently the red stage of astaxanthin production is constrained by biomass production in the green stage, which requires strictly controlled culture conditions. Optimal reported temperatures for the vegetative growth of H. pluvialis are between 20 and 28°C (Wan et al., 2014), with temperatures in excess of 30°C shown to induce transition from the green vegetative stage to the red stage with the formation of aplanospores. Domínguez-Bocanegra et al., (2004) demonstrated optimal growth at an irradiance of 177 µmol photons/m²/s with higher density cultures achieved under continuous light.