A tricky
question!? Yes indeed, and after the experience we made can this only be soled
with practical experience.

The light
used for the cultivation of phototrophic micro-organisms has to be in the
spectral range of ~, 400 to 700 nm to activate the photosynthesis pigments
[Heath, in 1972; Lawlor, in 1992]. An optimum irradiation strength (number of photons
per surface needed and time [μE m-2 s-1], the photo synthetic photon flux density (PPFD) is different from specie to
specie and can be derived from the light saturation curve.

Below
minimum irradiation strength a photosynthetic driven micro-organism will use
more energy for his (preservation) metabolism than it could get from the
irradiated light (measurable photosynthesis rate and total oxygen balance
negative).

The minimum
necessary irradiation strength for a measurable positive photosynthesis rate
will show in the light saturation curve and be market as light compensation
point [Goldmann, in 1979; Kohl and Nicklisch, in 1988]. A growing PPFD rate will result in the production
of oxygen until it reaches a constant maximum level of oxygen saturation. In
the linear area of the light saturation curve every photon will be
photo-chemical absorbed by photosynthesis pigments thus enabling the
photosynthesis process. The photon exploitation at this level is at its maximum
(maximum quantum yield); however, the growth rate of photosynthesis-active
organisms is limited due to low a light intensity. We are now in the transition
area of the light saturation curve when the photosynthesis rate is at its
highest level but the amount of photons used for the photosynthesis are at a
minimum. The light saturation is based on the limited reaction rate of
plastoquinone or the oxidation of NADPH (Nicotinamide adenine dinucleotide
phosphate) molecules in the Calvin cycle [Matthijs et al., 1996]. A part of the
electrons animated in the PS II (photo synthesis system II/Calvin curve) can
not be used, but this energy is being converted into warmth and fluorescence. Raising
the light intensity may now result in a photo inhibition (reducing the photosynthesis
rate) and may destroy photosynthesis receptors, light-absorbent pigments and
the thylakoid membran [Hopkin

and H?ner,
in 2004].

The density
of cells during cultivation will lead to an increasing shading effect which
will have to be compensated by an adjustment of light entry. As a result this limitation
of light during the growths will not leave enough protons available for an
optimal supply. Contradictory too much light entry in the beginning of cell
cultivation would cause cell damage (photo inhibition). Another parameter which
influences the light entry is the upper surfaces /

Volume
(A/V) - relation of the photo bioreactor.

A
homogeneous irradiation of all cells combined with an ideal A/V-relation, this
in connection with a sufficient mixing would prevent a continuous change of
light conditions from the unlit centre of the reactor to the illuminated
surface.

If light is so vitally important for algae, - as much as too much light is
damaging, - just how much light do we need for algae breeding in a PBR system?



And what can we learn from all this?

They need light, off course, but are shy of direct solar
radiation. How can we measure how much light? We use a patented Model-Reactor
(Pat.Nr. 202007013 401.1) with an LED light source to find out the optimum
growth level of algae species. Here we can simulate the light conditions of Germany, Spain, Australia or even India. Also different
nutrition requirements can be tested. It has shown for us that here in Central Europe only 10% of
solar radiation is necessary to achieve an optimum growth rate. Thus algae
cells are converting 5% of the given light into bio energy (something that i.e.
the sugar cane can only do with 1% effectiveness). We are able to boost the
productivity of algae up to 100 gram per m? per day with this method of finding
the right specie. But we even learned more. In a compact algae culture which is
mixed up by streaming water, some algae will come into the (light) surface,
some will return into the dark. From the view of an algae cell it might appear
as if the light is flickering. This Disco-light effect was researched as well.
Surprisingly the algae liked it very much! They are able to store the captured
light and will use it later, in the dark, for their growth.

So how could you measure your light intensity compared to what you have as
equipment (type of reactor), and to find out with the help of a light saturation
curve just how much light you may need (that you will be able to find out just
how much photons you may need per cell I may have the freedom not to believe?..
this was a joke, wasn?t it?) for optimum growth.

You would need a good photo meter (Quantum Sensor to measure photo
synthetically active radiation (PAR ? LI-COR Biosiences) and at least 6 til 12 month time to go through all possible light conditions
with your PBR. Good Luck!

You may see the ?Specification ?bio reactor? which you will find on my
profile. You may notice that we are working with a continuous harvesting system
that gives an excellent control over the cell density in our reactor model. Thus
we can keep productivity high do not have to stress with light limitation
factors through shading. And we work with greenhouses, - not only because
of  our northern Europe climate! ? no,
we even shade our cultures up here in the north.. Andreas-Algae Nova