6.5.1 Cyanobacteria

Divisions of prokaryotic algae, the Cyanophyta and Prochlorophyta, include the cyanobacteria (blue-green algae) and the Prochlorophyta, which use a unique form of chlorophyll, divinyl-chlorophyll, lack red and blue phycobilin pigments, and have stacked thylakoids. These organisms are oxygen-evolving photosynthetic bacteria and have cell wall structures similar to gram-negative bacteria which include a cell membrane, a layer of peptidoglycan, and an outer membrane. Of all the microorganisms described in this chapter, the cyanobacteria pose the least difficulty in terms of cell lysis for lipid extraction. Although cyanobacteria including Spirulina and Nostoc have been used for food for centuries, they tend to have high protein concentrations but low levels of lipids (Becker 1994). However, the cyanobacteria may provide some significant advantages in biofuel production including bacterial cell walls that are easily lysed for lipid extraction and the ability to grow in extreme conditions allowing species control in outdoor ponds, and many have a filamentous morphology that enables facile harvest.

The genetics of cyanobacteria are now well developed (Park et al. 2013), and many strains are easily transformed. Another argument for the use of cyanobacteria is the production of compounds important to the chemical and nutraceutical industries whose current market value is far greater than generic algal oil. For example, Arthrospira platensis produces significant quantities of phycocyanin and gama linolenic acid (C18:3, ю6, GLA) (Colla et al. 2004).

Various groups have engineered cyanobacteria to overexpress either native alkane biosynthetic genes or regulatory genes (Rosgaard et al. 2012; Wang et al.

2013) and have determined compatibility issues between the host and high levels of alkanes synthesized through engineered pathways.