Review of Literature
2.6 Culturing of microalgae: mode of nutrition and reactor types
2.6.1 Mode of nutrition
Environmental parameters such as temperature and light intensity are also found to affect the lipid accumulation properties and composition of the strains. Increase in temperature increases the saturated fatty acid content in the total fraction and when there is decrease in temperature, the unsaturated fatty acid content increases in concentration (Hu et al., 2008). While Nannochloropsis salina showed increased lipid content with increase in temperature (Converti et al., 2009), no significant change in lipid content was observed in Chlorella sorokiniana when grown at various temperatures (Converti et al., 2009). Similar observations were found even when light intensity or photoperiod was changed. For instance, in Nannochlororpsis sp. lipid accumulation was found to be less influenced by light intensity (Simionato et al., 2011) while, in many other organisms such as Chlorella and Scenedemsus sp. lipid content were reported to increase with photoperiods (Liu et al., 2012). It is important to note that the effect of light conditions on lipid accumulation is not an individual phenomenon and it is often coupled with the impact of other critical parameters such as nutrient starvation. This confirms that a clear understanding on the effect of light intensity and photoperiod on lipid accumulation and its regulation is still lacking (Bruer et al., 2013).
Thus, these factors can be manipulated to enhance the growth and lipid content in the algal strains to the maximum.
uses sunlight as the energy source and CO2 as the carbon source for growth (Huang et al., 2010). Lipid productivity of the algal strains varies significantly from species to species (4 to 61 mg L-1 day-1) under photoautotrophic cultivation (Rodolfi et al., 2009; Lim et al., 2012).
Table 2.4 Different cultivation conditions with their respective carbon and energy sources utilized for algal growth showing the specific reactor systems required
Growth conditions
Carbon sources Energy sources Reactor types
Photoautotrophic Inorganic CO2 Light Photobioreactor
& open pond Heterotrophic Organic carbon Organic carbon Closed
bioreactor Mixotrophic Organic carbon &
inorganic CO2
Light & organic carbon
Photobioreactor
& open pond Photoheterotrophic Organic carbon Light & organic
carbon
Photobioreactor In general, nutrient limiting condition is provided to obtain enhanced accumulation of neutral lipid at the cost of growth under photoautotrophic condition. The major advantage of photoautotrophic cultivation mode is the utilization of inorganic CO2 for growth of microalgae which is usually fulfilled with the flue gas from industries in large scale operations (Mata et al., 2010). As the medium is not rich in organic nutrients very less contamination is expected under photoautotrophic mode which enables the use of open race way ponds and outdoor cultivation for algal growth (Mata et al., 2010). The major disadvantage with photoautotrophic systems is the light penetration and availability per cell under high cell densities. Under heterotrophic growth condition, the organism is grown under dark condition in the presence of organic carbon as the source of energy and carbon (Wang et al., 2012) which bypasses the requirement of a light source and associated light penetration problems. The lipid productivity varies from 0.7 to 1.8 g L-1 day-1 which is 100 times more than that obtained in photoautotrophic conditions (Chen et al., 2011). The major disadvantage of such cultivation mode is the cost of complex organic carbon source used
for the growth of microalgae. Many alternative cheaper carbon sources are being tested for maximizing the growth under heterotrophic cultivation condition, however much attention is still required in the field to have a sustainable bioprocess for biodiesel production.
Mixotrophic cultivation is another alternative that uses both the organic and inorganic carbon compounds as a source of carbon for growth along with light as the energy source (Table 2.4). Thus providing the opportunity for the strain to follow both heterotrophic and photoautotrophic mode of cultivation in which the growth is not strictly limited by light or carbon availability. The lipid productivity under mixotrophic condition varies from 0.01 to 11.8 g L-1 day-1 in different microalgal strains (Wang et al., 2014a). However, this was achieved with an additional organic carbon compound that can acts as carbon and energy source for algal growth. Recent technologies have addressed these feasibility issues by using waste waters rich in organic compounds thereby coupling mixotrophic biodiesel production with waste water treatment (Wang et al., 2014a).
Photoheterotrophic cultivation requires light as the chief energy source and organic carbon compounds as the source of carbon that is it requires both carbohydrate-sugars and light at the same time for maximal productivity (Chen et al., 2011). Irrespective of these cultivation conditions, simultaneous enhancement of lipid content along with uncompromised growth is necessary to attain maximum net lipid productivity. Therefore, net lipid productivity which considers both the lipid content and biomass productivity is used as the performance index for selection of the best productive strains and process for biodiesel production (Chen et al., 2011). Maximum net lipid productivity was reported for mixotrophic followed by heterotrophic and photoautotrophic cultivation conditions (Wang et al., 2014a). Various hybrid systems involving these cultivation conditions were also designed for optimal commercial scale biodiesel production processes. However, a complete realization of these technologies at commercial scale still remains unachieved.