Heterokont algae are a monophyletic group with chloroplasts containing chloro­phyll a and c and the accessory pigment fucoxanthin which gives the group a golden-brown color. Marine, freshwater, and terrestrial heterokonts are known and range in the form of giant kelp (brown seaweeds), diatoms, Eustigmatophytes, and Chrysophytes. The later three groups have species that are high-lipid producers. Nannochloropsis (Eustigmatophytes) are primarily known from marine environ­ments but also occur in fresh and brackish water (Fawley and Fawley 2007). All of the species are small (diameter of about 2-3 pm) non-motile spheres with no distinct morphological features, and many are mixotrophic (Das et al. 2011). Nannochloropsis strains contain 30 % lipids under nutrient-replete growth condi­tions and over 60 % lipid content after nitrogen deprivation (Rodolfi et al. 2009; Huerlimann and de Nys 2010). The Chrysophyceae and Xanthophyceae are predominately freshwater organisms, although a substantial number of xantho — phytes are terrestrial.

Diatoms, widely studied as a feedstock for biodiesel production, are important members of planktonic and attached biofilm communities in both freshwater and marine environments (Round et al. 1990). Over 100,000 species are known which are estimated to contribute up to 45 % of total primary productivity in open oceans (Yool and Tyrrell 2003). Diatoms are distinguished by a unique silica cell wall composed of two separate valves and yellowish brown chloroplasts, surrounded by four membranes and containing the carotenoid pigment fucoxanthin as a photo­synthetic accessory pigment. Other xanthophylls are present as well as P-carotene and chlorophylls a and b. The main storage compounds are lipids (TAGs) and a P (1 ^ 3-linked carbohydrate chyrsolaminarin (Horner 2002). Several genera include species known for high lipid content including Nitzschia, Navicula, Amphiprora, Amphora, and Phaedodactylum (Griffiths and Harrison 2009). Diatoms lack flagella (except in sperm of some species), and their dense cell walls cause them to sink in the water column. Planktonic forms rely on turbulence to keep them in the photic zone, and many species regulate their buoyancy using intercellular lipids. Most diatoms are phototrophic, but a few groups are either obligate heterotrophs or are diurnally heterotrophic in the dark when supplied with a carbon source.

Silicon metabolism has relevance in the culture of diatoms as a feed source for biodiesel production. The silicon-laden cell wall is synthesized intercellularly by polymerizing silicic acid monomers (taken up by transporters from the media) in a specialized membranous compartment (Pickett-Heaps et al. 1990). Because silicate is a relatively expensive and an essential nutrient for diatoms, production costs can be raised significantly. However, silicon limitation prevents cell division and trig­gers rapid lipid biosynthesis which may allow for methods to control oil production in a two-stage production process. Nutrient limitation, including N and P that promote lipid hyper-accumulation in a variety of microalgae, has also been shown to promote lipid accumulation in diatoms (McGinnis et al. 1997). However, several studies suggest that Si deficiency stimulates lipid biosynthesis more rapidly and can result in up to 70 % (dry weight) lipid content (Adams et al. 2013).