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Biorefinery of microalgae biomass cultivated in wastewaters 153

inorganic carbon as from the last, microalgae cells have to perform an extra step
(photosynthesis) to produce α-KG. In addition, glucose was found to be more
energy efficient than citrate in the formation of α-KG. Glucose is converted to
α-KG through glycolysis and Krebs cycle which results in the production of 2 mol
of ATP and 6 mol of nicotinamide adenine dinucleotide (NADH). In contrast, the
transformation of citrate into α-KG is done through Krebs cycle with the generation
of only 1 mol of NADH.
Adding glucose to the culture medium is beneficial since it increases the assimi-
lation of NH 3 -N and cells viability. The resulting α-KG is then used to assimilate
NH 3 -N entering the cells before it threatens cell metabolism (Lu et al., 2018).
Unfortunately, the addition of glucose to wastewaters with high concentrations of
NH 3 is not economically viable. However, the addition of wastewaters with high
organic carbon contents could be used as a viable alternative.

7.2.2 Turbid liquid wastes and light penetration

The use of turbid wastewaters in algae culture impairs light penetration, thereby
limiting the growth of the biomass. Under light-limiting conditions, alternative
sources of energy and carbon are required so that algae maintain their metabolic
functions. Turbid liquid wastes, such as slurries, coming from anaerobic digesters
and landfill leachates (LLs) are usually rich in organic matter that can be used by
algae as sources of energy and carbon. Microalgae cells can then shift their metabo-
lism toward heterotrophy to meet their energy needs.
To achieve their maximum growth rate, microalgae require a specific light inten-
sity, known as the saturation level. When the light intensity is above the saturation
level, the growth is inhibited. If it is below the saturation level, the light becomes a
limiting factor for optimal growth. Under the saturation level, the increase of light
intensity shortens the duration of the steps leading to RNA and protein synthesis
and it increases their number. In contrast the processes in the DNA replication-
division sequence are independent of the supply of external energy to the cell
(Zachleder et al., 2016).
The overall reaction that takes place in photosynthesis can be written as follows:

Light
nCO 2 1 nH 2 O 2 CH 2 Oð Þ 1 nO 2 (7.4)
n
Carbohydrate
In the photosynthesis process, one O 2 molecule evolves per 2500 molecules of
chlorophyll under maximal efficiency of light-energy utilization. Moreover, eight
quanta of light (photons) are required to reduce one CO 2 molecule to carbohydrate
and liberate one O 2 molecule. Then, one quantum event is mediated by around 300
chlorophyll molecules (called photosynthetic unit of chlorophyll) (Giese, 1979).
Light is made of photons that can be used by microalgae to generate energy. A
photon is a discrete unit of quantum flux in the photosynthetically active radiation,
which is the region of light used by algae. A photon flux is commonly measured