As photosynthetic organisms, microalgae require light and CO2, appropriate temperature
(generally 15–35°C) and pH (1.5–10, but generally 6–7.5), and a supply
of nutrients (Havlik et al. 2013). Some algae may also consume organic carbon
when it is available.
Light and CO2 are the two most important growth factors for phototrophic microalgae
(Havlik et al. 2013). Light provides the energy for microalgal biomass
production, whereas CO2 provides the carbon for that biomass. Provision of light is
largely dependent on the reactor design, which may be an open system, such as a
pond, or a closed photobioreactor (PBR). In either case, light may be obtained
directly from sunlight, but PBRs may also be fed with artificial light. CO2 is generally
provided by sparging air or CO2 enriched gas to the production system, but may
also be added as bicarbonate solution.
Nitrogen and phosphorous, plus other nutrients are essential for growth, but
their depletion can result in accumulation of storage molecules such as lipids,
starch, or pigments. When these are the desired end-product (as in lipids for biodiesel),
nutrient levels must be carefully monitored.
When microalgae utilise organic carbon sources for growth, light is no longer
limiting and higher biomass concentrations and higher growth rates can be
achieved than by phototrophic growth. Growth on organic carbon sources may be
completely heterotrophic, but with some species may also be mixotrophic if light is
supplied. Biomass production under mixotrophic conditions is generally similar to
that in heterotrophic conditions, or slightly higher (Wang et al. 2013). Mixotrophy has
been shown to increase the lipid content and yield of lipid on organic substrate in
some species (Wan et al. 2011, Wang et al. 2013). Indeed cultivation processes
based on both phototrophic, heterotrophic, and mixotrophic growth are being commercialized
(Eckerberry 2012).