Photobioreactors
3 Influence of Light on Microalgae
127
3.1 Photosynthetic Efficiency
Microalgae have an extremely high productivity compared with higher plants.
Some strains of high-temperature adapted green algae, e.g. Chlorella
pyrenoidosa, are able to double their cell number every 21 hours [15]. As
opposed to higher plants, photosynthetically inactive cells (such as roots or
trunks) are missing, and microalgae are a favoured subject of research for the
determination of complex photosynthetical processes, especially with regard to
quantum yield. Although the antenna complex of well-mixed microalgal cultures does not react to increased radiation intensity to the extent used by
phanerogames to convert efficiently high quantum yields to utilizable energy,
this is compensated largely by the mobility and the constant supply of carbon
dioxide, minerals and water [16].
Photosynthetic production of biomass is generally represented by the following equation:
otCO2 + ~H20 + 7NH3 q- minerals + nhu ~ ot[,CHON minerals] + 02
where nhu is the number of energy quanta of a certain frequency which is
required for the production of 1 mol 02. Thereby, a must not have the value 1,
but effects such as dark respiration or the energy required for metabolism maintenance will reduce the yield of fixed carbon with regard to
the oxygen quantity produced. In the classical model of photosynthesis, the
Z-scheme, 8 mol quanta absorbed are required for the evolution of 1 mol
02, at which approx. 3 mol ATP are produced, cyclic photophosphorylation included. They also are required for reducing t mol CO2 to glucose
in the dark reaction. The real requirement of quanta for photosynthesis,
however, is controversial and given by different authors as 6-16 quanta
[-17, 18].
The data of the calculated photosynthetic efficiency (PE), the quotient of the
energy accumulated in the biomass and the light energy absorbed, are correspondingly different. A theoretical maximum PE of approx. 29 % is calculated at
8 mol light quanta (X = 575 nm) per 1 mol fixed CO2. With regard to the solar
spectrum the conversion of the irradiated sunlight is corrected theoretically to
18.1% [-19]. This value, however, remains questionable under real cultivation
conditions [20], all the more since the real conversion rates do not exceed 10%
[,21]. After extensive examination, Oswald [22] calculated a maximum utilization of sunlight of approx. 5-6% for outdoor plants, which was also reached in
a continuously mixed culture in Richmond, California [7]. Algae, therefore, by
far exceed agricultural crops, such as C, maize with a PE of 1.3%, fast growing
tropical plants as sugar cane or bullrush millet with up to 4.3% [23] and are
even superior to technical equipment such as classical photocells or solar
collectors [-19, 24].
3 Influence of Light on Microalgae
127
3.1 Photosynthetic Efficiency
Microalgae have an extremely high productivity compared with higher plants.
Some strains of high-temperature adapted green algae, e.g. Chlorella
pyrenoidosa, are able to double their cell number every 21 hours [15]. As
opposed to higher plants, photosynthetically inactive cells (such as roots or
trunks) are missing, and microalgae are a favoured subject of research for the
determination of complex photosynthetical processes, especially with regard to
quantum yield. Although the antenna complex of well-mixed microalgal cultures does not react to increased radiation intensity to the extent used by
phanerogames to convert efficiently high quantum yields to utilizable energy,
this is compensated largely by the mobility and the constant supply of carbon
dioxide, minerals and water [16].
Photosynthetic production of biomass is generally represented by the following equation:
otCO2 + ~H20 + 7NH3 q- minerals + nhu ~ ot[,CHON minerals] + 02
where nhu is the number of energy quanta of a certain frequency which is
required for the production of 1 mol 02. Thereby, a must not have the value 1,
but effects such as dark respiration or the energy required for metabolism maintenance will reduce the yield of fixed carbon with regard to
the oxygen quantity produced. In the classical model of photosynthesis, the
Z-scheme, 8 mol quanta absorbed are required for the evolution of 1 mol
02, at which approx. 3 mol ATP are produced, cyclic photophosphorylation included. They also are required for reducing t mol CO2 to glucose
in the dark reaction. The real requirement of quanta for photosynthesis,
however, is controversial and given by different authors as 6-16 quanta
[-17, 18].
The data of the calculated photosynthetic efficiency (PE), the quotient of the
energy accumulated in the biomass and the light energy absorbed, are correspondingly different. A theoretical maximum PE of approx. 29 % is calculated at
8 mol light quanta (X = 575 nm) per 1 mol fixed CO2. With regard to the solar
spectrum the conversion of the irradiated sunlight is corrected theoretically to
18.1% [-19]. This value, however, remains questionable under real cultivation
conditions [20], all the more since the real conversion rates do not exceed 10%
[,21]. After extensive examination, Oswald [22] calculated a maximum utilization of sunlight of approx. 5-6% for outdoor plants, which was also reached in
a continuously mixed culture in Richmond, California [7]. Algae, therefore, by
far exceed agricultural crops, such as C, maize with a PE of 1.3%, fast growing
tropical plants as sugar cane or bullrush millet with up to 4.3% [23] and are
even superior to technical equipment such as classical photocells or solar
collectors [-19, 24].