Photobioreactors
131
before remains controversial. According to the literature published, a growth
promoting effect can neither be supposed nor excluded.
Besides physics of illumination, the biochemical adaptation to the prevailing
light conditions also plays an important role. Beale and Appleman [54] and
Herron and Mauzerol [55] observed a significantly increased chlorophyll production of Chlorella and a mixotrophic culture of yellow Chlorella mutants,
respectively, at low light intensity, so that the statistical probability of an
absorption of the quanta striking the cell surface is confirmed. Sukenik et al.
[56] have adapted the green alga Dunaliella salina to extremely intensive light.
Contrary to "normal cells" an inhibition of photosynthesis could not be detected
even in diluted culture at 2200 gE m- 2 s- 1 PAR. Ben-Amotz [57] established
an increase of the t-carotene content of the same alga at 8-fold at similar light
intensity (control 50 gEm -2 s-1), the carotene/chlorophyll ratio having increased by a factor of 35. Falkowski [58] has investigated the pigment production or pigment degradation during transition transformation and has calculated
the first-order kinetics for chlorophyll a. The adaptation of pigment development
and growth of Dunaliella salina from 45 gE m- 2 s- 1 to 500 gE m- 2 s- 1 (scalar
measured values) and vice versa was finished after 80-100 h.
3.5 Utilization of Light-Source Energy
In general, photobioreactors can be supplied with light energy by natural or
artificial illumination. The utilization of solar energy is by far the most important source for the production of algal biomass. Beside open pond technology,
sunlight can also be applied in semi-closed systems [59] or, with appropriate
technical expense, in completely closed plants [60]. Nevertheless, a number of
disadvantages are inherent to this cost-effective energy, e.g. sunlight is not
available at any time (day/night rhythm), its quality and quantity is subject to
significant fluctuations determined by geography, weather and season, and thus
exactly reproducible light conditions are not feasable. Furthermore, the solar
emission spectrum comprises the range 120 nm to 100 gm, i.e. more than 50% of
the radiation energy cannot be utilized photosynthetically. Therefore, artificial
light sources are used almost exclusively for the defined production of microalgae on a laboratory scale.
Fluorescent lamps, a cost-effective means, are used most frequently for the
cultivation of phototrophic organisms. The bands emitted from the mercury
vapour can be converted to continuous radiation by modifying the composition
of the fluorescent material, which allows for an optimum adaptation to the
effective spectrum of photosynthesis. Efficiency and applicability of the fluorescent spectrum are easily determined by appropriate parameterization [37].
The proportion of disadvantageous IR and UV-radiation is very small. A surface quantum flux density of 300-500 gE m- 2 s - 1 allows for a very close contact
with the suspension of organisms so that the relatively high efficiency of 15%
(compared with 5% for tungsten lamps) can be utilized quantitatively.
131
before remains controversial. According to the literature published, a growth
promoting effect can neither be supposed nor excluded.
Besides physics of illumination, the biochemical adaptation to the prevailing
light conditions also plays an important role. Beale and Appleman [54] and
Herron and Mauzerol [55] observed a significantly increased chlorophyll production of Chlorella and a mixotrophic culture of yellow Chlorella mutants,
respectively, at low light intensity, so that the statistical probability of an
absorption of the quanta striking the cell surface is confirmed. Sukenik et al.
[56] have adapted the green alga Dunaliella salina to extremely intensive light.
Contrary to "normal cells" an inhibition of photosynthesis could not be detected
even in diluted culture at 2200 gE m- 2 s- 1 PAR. Ben-Amotz [57] established
an increase of the t-carotene content of the same alga at 8-fold at similar light
intensity (control 50 gEm -2 s-1), the carotene/chlorophyll ratio having increased by a factor of 35. Falkowski [58] has investigated the pigment production or pigment degradation during transition transformation and has calculated
the first-order kinetics for chlorophyll a. The adaptation of pigment development
and growth of Dunaliella salina from 45 gE m- 2 s- 1 to 500 gE m- 2 s- 1 (scalar
measured values) and vice versa was finished after 80-100 h.
3.5 Utilization of Light-Source Energy
In general, photobioreactors can be supplied with light energy by natural or
artificial illumination. The utilization of solar energy is by far the most important source for the production of algal biomass. Beside open pond technology,
sunlight can also be applied in semi-closed systems [59] or, with appropriate
technical expense, in completely closed plants [60]. Nevertheless, a number of
disadvantages are inherent to this cost-effective energy, e.g. sunlight is not
available at any time (day/night rhythm), its quality and quantity is subject to
significant fluctuations determined by geography, weather and season, and thus
exactly reproducible light conditions are not feasable. Furthermore, the solar
emission spectrum comprises the range 120 nm to 100 gm, i.e. more than 50% of
the radiation energy cannot be utilized photosynthetically. Therefore, artificial
light sources are used almost exclusively for the defined production of microalgae on a laboratory scale.
Fluorescent lamps, a cost-effective means, are used most frequently for the
cultivation of phototrophic organisms. The bands emitted from the mercury
vapour can be converted to continuous radiation by modifying the composition
of the fluorescent material, which allows for an optimum adaptation to the
effective spectrum of photosynthesis. Efficiency and applicability of the fluorescent spectrum are easily determined by appropriate parameterization [37].
The proportion of disadvantageous IR and UV-radiation is very small. A surface quantum flux density of 300-500 gE m- 2 s - 1 allows for a very close contact
with the suspension of organisms so that the relatively high efficiency of 15%
(compared with 5% for tungsten lamps) can be utilized quantitatively.