The morphology, pigments, metabolic capabilities, and cell wall structure of the eukaryotic microalgae are quite diverse because they represent a multitude of phylogenetically distinct groups of organisms. Recent molecular evidence suggests that the “algae” fit into very different evolutionary lineages including those related to plants, fungi, or animals (Lucentinii 2005) via one or more serial endosymbiotic events (Moestrup 2001a, b). Three classes of primary interest for biofuels pro­duction include the golden-brown algae (Chrysophycea), prymnesiophytes (Prymnesiophyceae), and the eustigmatophytes (Eustigmatophyceae) (Sheehan et al. 1987) and for bioremediation, the Chlorophycea.

6.5.2.1 Chlorophyta

The green algae are a large group estimated to have over 13,000 species, (Guiry

2012) from which the higher plants emerged. Green algae, in common with land plants, have chloroplasts that contain chlorophyll a and b, as well as the accessory pigments P-carotene and xanthophylls, and store carbon as starch and lipids. Their distribution is ubiquitous with marine, freshwater, and terrestrial species and groups adapted to extremes including deserts, arctic zones, hypersaline waters, and deep — sea thermal vents (Lewis and Lewis 2005; De Wever et al. 2009). Several strains are known to have rapid growth rates and high lipid content and have been a focus of biofuel research. Chorella spp., used since the 1950s as a food supplement, have short doubling times and can be cultured to produce between 30 and 55 % lipid content (Becker 1994; Miaoa and Wu 2006). Species of Scenedesmus, Ettlia, Nannochloris, and Monoraphidium grow rapidly (doubling times from 7 to 12 h under nutrient-replete conditions) and have lipid contents ranging from 30 to over 60 % (Griffiths and Harrison 2009). A number of genera in the order of Chloro — coccales, including Actinastrum, Scenedesmus, Chlorella, Closerium, and Gol — enkinia, tend to dominate algae communities in eutrophic waters (Rawson 1956) and algae wastewater ponds (Martinez et al. 2000; Benemann and Oswald 1996). Many are either heterotrophic or mixotrophic.