Microalgae Growth: Research institutes and industries have shown increased interest in using microalgae to produce both low-value products like biofuels and high-value products like nutraceuticals. The anticipated depletion of fossil fuels and environmental concerns related to climate change are driving the search for renewable, clean, and sustainable energy sources. Microalgae appropriately meet this requirement. However, their small cell size and the diluted form in which they are grown pose various obstacles to economically producing them and efficiently converting them into biofuels. The selection of strains and their improvement, the effective use of sunlight for photosynthesis in ponds, the necessity of removing significant amounts of water to obtain concentrates, and improving the efficiency of conversion into biofuels are common challenges in the production and processing of microalgae.
However, the benefits of algae-derived renewable biofuels are significant. Importantly, when converting the raw biofuel into biogasoline, biodiesel, or aviation biofuel, they can directly replace their fossil fuel-derived equivalent with little or no modifications to the engines that are used. Furthermore, they also fulfill all the specifications of the fossil fuels they replace, and importantly, they demonstrate energy densities very similar to the fuels they are intended to substitute.
Since algae-based biofuels can be a direct replacement for their fossil fuel equivalents, there is great interest in producing algae that exhibit optimum characteristics for the manufacturing of biofuels. The following section discusses some of these important influencing parameters.

Influencing Parameters for Algae Growth:

Light:

Typically, light reactions and dark reactions form the basis of photosynthetic reactions. Microorganisms absorb and transform light energy into an energy carrier, releasing oxygen as a by-product during the light reaction phase. Oxygen can transmit its electrons to Photosystem I (PSI) and Photosystem II (PSII). The photo-oxidation of PSII components caused by intense light can lower algae productivity. Low light levels, however, are insufficient for the growth of microalgae. As a result, one of the crucial factors affecting microalgae growth is the optimal light intensity, with microalgae typically absorbing and using natural light with wavelengths between 400nm and 700nm.

Temperature:

A significant factor that influences algae growth is temperature. While excessively high temperatures prevent microalgae from respiring, high temperatures encourage the uptake and fixation of CO2 by microalgae. To avoid negative effects on the metabolism of microalgae cells and to reduce lipid accumulation, one should control the temperature at a level that promotes optimal microalgae growth. Many species of microalgae have varied optimal growth temperatures, most commonly falling between 20°C and 30°C. However, in large-scale applications suitable for biofuel production, the temperature must be maintained in the optimal range of 25°C to 30°C.

Carbon:

For photosynthesis, microalgae need inorganic carbon, which researchers can supply by enriching the ambient air with CO2 or providing it in the form of salts (bicarbonate). This process leads to the intensive cultivation of microalgae. It’s important to note that this carbon source needs to be solubilized for microalgae to utilize it for photosynthesis. Depending on the pH of the water, CO2 dissolves in various forms. Microalgae typically thrive in the media with a pH of 7. The dissolved carbon will therefore be in the form of CO2 and carbonate ions.

CO2 is one of the important factors influencing the photosynthesis of microalgae. The increase in CO2 is beneficial for the improvement of the photosynthetic efficiency of microalgae, thereby increasing their biomass yield.

Salinity, nutrients, and pH:

Different species of microalgae require varying salinities, nutrients, and pH levels. Microalgae need ideal salinity conditions to flourish healthily. One can add NaCl and Na2SO4 to the culture medium to increase salinity. However, high salinity typically limits microalgal growth. Various microalgae species require diverse levels of salinity. For example, marine microalgae may survive in environments with higher salinity than freshwater microalgae.

Several nutrients, such as C, O2, H2, N2, K, Mg, Ca, Fe, S, and P, are essential for microalgae growth. The four most crucial nutrients are C, O2, H2, and K, with C, O2, and H2 coming from water and air and N2, P, and K from the culture media. Microalgae growth primarily requires N2 and P as the main nutrient components. However, some rare varieties of microalgae also require other unique elements, such as Si, for diatoms.

In addition to controlling enzyme activity, phosphorus availability, ammonia toxicity, and inorganic carbon availability, the pH of the culture medium is crucial for microalgae growth. As the pH steadily rises, microalgae production increases and the culture medium transitions to being alkaline. When photosynthesis intensifies, OH ions build up over time, and microalgae easily absorbs CO2 from the atmosphere to produce biofuel. PH variations impact the permeability of microalgae cells and the shape of hydrated hydrogen ions in inorganic substances.

Mixing:

Mixing is crucial as it promotes maximum productivity and concentration. Increased environmental turbulence increases the exchange of nutrients and metabolites between cells and the culture medium. However, because algae are sensitive to induced shear, some mixing techniques, such as centrifugal pumps, are unsuitable. People often avoid subjecting microalgae to high turbulence mixing regimes because doing so increases the risk of causing cellular damage. Cell damage is quite likely in airlifts or bubble columns with significant turbulence.

Cultivation modes:

Microalgae that are in autotrophic mode use light and inorganic substances (CO2, water, and inorganic salts) for photosynthesis to create organic molecules. Microalgae that are in a heterotrophic mode need external carbon sources to develop in a dark environment. The biomass and lipid yield in the mixotrophic mode is found to be substantially higher than those in the autotrophic mode. Microalgae rapidly divide their cells when they are in a mixotrophic mode.

Microalgae Growth to Biofuels – Future Outlook:

Having a deeper understanding of the factors that impact the growth rate, biomass yield, and lipid accumulation is essential to maximize productivity and fully harness the benefits that microalgae can offer. The selection and improvement of high-yield species, the design and construction of low-cost, low-energy cultivation systems, the utilization of effective dewatering techniques, the adoption of non-chemical extraction techniques, and the implementation of wet conversion processes are necessary steps to enhance the sustainability of the microalgae industry.
Unique difficulties at numerous points along the production process chain continue to impede the large-scale production of microalgae growth for low-value products like biofuels. However, there is a lot of room for increased productivity at lower prices. The use of chemometric analysis and online measuring systems in the context of large-scale microalgae production in biorefineries will be a fascinating area of development in the coming years. As biofuel technology advances, the production of microalgae may potentially exhibit additional socioeconomic and environmental advantages that will support industry growth.