Microalgae can also function in mixotrophic nutrition mode by combining both the auto­trophic and the heterotrophic mechanisms. It facilitates fixing atmospheric CO2 as well as con­suming the organic molecules and micronutrients from the growing environment (Figure 8.8). Microalgae can assimilate available organic compounds as well as atmospheric CO2 as a car­bon source in mixotrophic mode. The CO2 released by microalgae via respiration will again be trapped and reused in mixotrophic nutritional mode. It differs from photoheterotrophic nutrition mode in terms of CO2 utilization. The mixotrophs have the ability to utilize organic carbon; therefore, light energy is not a limiting factor for biomass growth (Chang et al., 2011). The acetyl-CoA pool will be maintained from both carbon sources—that is, by the CO2 fixation (Calvin cycle) and intake from outside the cell, which can further make malonyl-CoA. The photosynthetic metabolism utilizes light and CO2 for growth and organic photosynthate pro­duction, whereas respiration uses the organic photosynthates produced during photosynthe­sis. If an external carbon source is available in the system, there is a less loss of photosynthate during respiration, and the algae utilize the available excess photosynthates for biomass development. Mixotrophic cultures show reduced photoinhibition and improved growth rates over autotrophic and heterotrophic cultures (Chojnacka and Noworyta, 2004).

8.3 NUTRITIONAL MODE OF MICROALGAE

Mixotrophic Nutrition

Lipid

Glucose

FIGURE 8.8 Mixotrophic mode of nutrition in algal cells towards CO2 fixation and glucose assimilation for lipid biosynthesis

Algae have the flexibility to switch their nutritional mode based on substrate availability and light condition. If simpler carbohydrates are present in the system, algae shift towards heterotrophic nutrition from autotrophic mode to save energy. Scenedesmus obliquus readily adapted to heterotrophic growth in dark conditions utilizing glucose (Abeliovich and Weisman, 1978). Heterotrophic cells differed significantly from photoautotrophic cells with respect to several physiological properties such as the rate of photoassimilation of CO2 and the rate of incorporation of carbon and chlorophyll a concentration. Algal cells in an oxidation pond shared features common to both photoautotrophic and heterotrophic cells (Abeliovich and Weisman, 1978), associating with the mixotrophic mode of operation. Bacteria seem to play a minor role in biological oxygen demand reduction in high-rate oxidation ponds, and their role is probably confined to degradation of biopolymers, thus producing substrates for algal consumption.

The advantages of mixotrophic nutrition are its independence in terms of both photosyn­thesis and growth substrates (Kong et al., 2012). The mixotrophic growth regime is a variant of the heterotrophic growth regime, where CO2 and organic carbon are simultaneously assim­ilated and both respiratory and photosynthetic metabolism operates concurrently (Kaplan et al., 1986; Lee, 2004; Perez-Garcia et al., 2011). Mixotrophism is often observed in ecological water bodies, where the homeostatic structure and function of living systems are supported by chemical, physical, and organic activity in biota that balance the ecological status. Water ecosystems generally consist of nutrients and organic carbon as integral parts (Venkata Mohan et al., 2009), where microalgae, along with other living components, function together symbiotically. Some microalgal species are not truly mixotrophs but have the ability to switch between phototrophic and heterotrophic metabolisms, depending on environmental condi­tions (Kaplan et al., 1986). Microalgae-accumulating lipids are generally grown in natural

water bodies; therefore, ecological water bodies embedded with diverse microalgae species can be considered as potential reservoirs for harnessing biodiesel. In this regard, an attempt was made to explore the ability and potential of mixed microalgae cultures derived from dif­ferent water bodies in extracting lipids, which can be further transesterified to biodiesel. The study also focused on the economic mode of lipid production from the treatment of domestic sewage. The growth of algae was shown to be highest under mixotrophic conditions, with higher biomass productivity under photoautotrophic conditions (Bhatnagar et al., 2010; 2011). Mixotrophic cultivation was shown to be a good strategy to obtain a large biomass and high growth rates (Ogawa and Aiba, 1981; Lee and Lee, 2002), with the additional benefit of producing photosynthetic metabolites (Chen, 1996; Perez-Garcia et al., 2011). Solazyme, a renewable oil company in the United States, has developed an integrated algal cultivation process by dark heterotrophic mechanisms, giving carbon sources externally. The company is using various forms of waste material as feedstock for the cultivation of algae in fermenters and harnessing as much as 75% of oil on the basis of dry cell weight. The company is antic­ipating in selling algal oil to commercial refineries by the end of 2013.