Industrial Microalgae Cultivation Artificial Lighting of High-Density Flat-Panel Airlift Photobioreactors In this short-communication on artificial lighting, I will give you insights into our activities and findings. regarding the lighting of systems for the indoor production of photosynthetic microorganisms.

1. Industrial Microalgae Cultivation
Artificial Lighting of High-Density
Flat-Panel Airlift Photobioreactors
Peter Bergmann
CSO, Subitec GmbH

2. c SEPTEMBER, 2017
SUBITEC GMBH, JULIUS-HÖLDER-STR. 36, 70597 STUTTGART, GERMANY
WWW.SUBITEC.COM/EN/
WWW.LINKEDIN.COM/COMPANY/SUBITEC-GMBH
INFO@SUBITEC.COM

3. Contents
1 Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Author’s Foreword 4
2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Background 5
2.2 Objective 5
3 Experimental Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Organisms and Light Supply 6
3.2 Statistical Treatment 7
4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. 1. Preface
1.1 Author’s Foreword
Dear customers, dear partners, dear colleagues and friends,
time has passed since I published our first short-communication on PBR operating strategies in
which we disclosed that, by smart process operation, microalgal production costs on industrial scale
can be significantly reduced. I was impressed by the great interest aroused by the release and it is
about time to thank all the readers for their appreciated time and comments.
In the meantime, I’ve been constantly confronted with questions regarding the lighting of systems
for the indoor production of photosynthetic microorganisms. Well, "give light, and the darkness will
disappear of itself" (Desiderius Erasmus) would be a straightforward strategy, but things usually turn
out to be more complicated than is initially thought.
In this short-communication on artificial lighting, I will give you insights into our activities and
findings. As always, please feel free to give feedback of any kind: questions, amendments, remarks –
no matter if positive or negative. I want to encourage communication and discussion on the topic as
this is the best way to drive progress.
Yours sincerely,
Peter Bergmann
P.BERGMANN@SUBITEC.COM

5. 2. Introduction
2.1 Background
Microalgae production costs are still the main limiting factor to penetrating mass markets, such as
food and feed. Therefore, economic viability is currently only given in high value (e.g. astaxanthin)
or speciality products such as microalgae as live feed for application in aquaculture. This tendency
is also confirmed by the inquiries we receive for microalgae production systems. In the past, many
inquiries were directed towards large-scale outdoor production for biofuel and feed production.
Today, most inquiries are for high-value applications. These are characterized by decisive customer
requirements, one of the major one being product quality (e.g. composition, concentration of target
molecules). A further eminent need is represented by a continuous and stable production in order
to fulfil contractual supply and/or delivery obligations and to serve process interdependencies (e.g.
in the case of utilizing microalgae as live feed). In any case, the value creation of the markets in
question allows for the indoor production of the biological resources utilizing artificial lighting under
highly controlled conditions.
2.2 Objective
Even with smaller indoor microalgae production systems, the area that needs to be illuminated
easily sums up to thousands of square meters. Therefore, it is of utmost importance for engineering
companies like Subitec to install the most efficient lighting with respect to microalgal growth. Hereby,
light quality has an impact not only on microalgal growth kinetics, but also on metabolites 1 and even
biofilm formation 2. As data on high-density microalgae cultures is scarce, Subitec has conducted
much research on its way from empiricism to empirical evidence.
1Schulze, Peter SC, et al. "Light emitting diodes (LEDs) applied to microalgal production." Trends in biotechnology
32.8 (2014): 422-430.
2Han, Pei-pei, et al. "Effects of light wavelengths on extracellular and capsular polysaccharide production by Nostoc
flagelliforme." Carbohydrate polymers 105 (2014): 145-151.

6. 3. Experimental Design
3.1 Organisms and Light Supply
The present short-communication depicts the results obtained for cultivations performed in 6 L FPA-
PBRs utilizing the green alga Chlorella sorokiniana SAG 211-8k and the diatom Phaeodactylum
tricornutum UTEX 640. Therefore, modified DSN and M&M were used, respectively. Light
was supplied 24/7 by commercially available light sources including high pressure sodium vapor
(HPS) discharge lamps and multiple light-emitting diodes (LED). Subitec also offers one LED with
adjustable spectrum for its 6 L FPA-PBR, specifically developed for experiments on the topic in
question (see Figure 3.1). Light was set to identical photon-flux densities (PFDs) of 370 µmol m-2 s-1
measured with a spectrometer. Applied light qualities are depicted in Figure 3.2. Hereby, main
differences manifested in different red : blue ratios and the emittance in the so called green gap.
Figure 3.1: Subitec’s LED with adjustable spectrum for experiments on light quality at 6 L scale

7. 3.2 Statistical Treatment 7
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Figure 3.2: Light qualities applied during experiments at 6 L scale
3.2 Statistical Treatment
Experiments were performed in triplicates at minimum. In order to cope with fluctuations in growth
deriving from the biological system as well as technical limits (e.g. slight fluctuations of temperature
and CO2 supply) data was evaluated by overlaying the available growth data for given experiments.
This procedure also allowed for the presentation of an otherwise vast data pool gained in multiple
photobioreactors resulting from randomized experiments. Growth data was fitted to result in a single
representative growth curve for each experiment. The fitted growth curves were then differentiated
(first derivative) to result in curves of volumetric productivity (Pvol.) over the course of the dry weight
(DW).

8. 4. Results and Discussion
As can be seen in Figure 4.1, growth of both organisms was significantly influenced by the applied
light quality with the influence being more prevalent for C. sorokiniana.
Here, light sources characterized by the lack of emittance in the green gap performed significantly
worse than others. This not only addressed productivity and final biomass yield but also process
stability with respect to reproducibility and biofilm formation. Light sources which predominant
emission overlapped with the absorption peaks of chlorophylls (around 450 nm and 680 nm) per-
formed worst. This is, amongst other reasons, due to the fact that blue and red light is efficiently
absorbed within the first millimeters of bioactive volume, not allowing the majority of the cultivation
volume to photosynthesize. On the contrary, green and yellow photons of e.g. HPS lighting penetrate
deeper into the culture and, in contradiction to a popular opinion, are as efficiently utilized for
assimilatory photochemistry than others, thereby increasing productivity and biomass yield.
For P. triconutum, similar findings apply albeit in alleviated terms. This is due to the fact that
this diatom is rich in carotinoids, reducing effects triggered by the the green gap.
Generally speaking and despite the general consensus that red and blue light is beneficial for
microalgal proliferation due to the congruency with the absorption maxima of chlorophyll, studies
reveal that this statement is only partially true for dense cultures. Besides the wavelength-dependent
specific absorption characteristics of photosynthetic and accessory pigments, physical aspects of
e.g. light penetration depth come into effect as well. The data reveal that a congruency of applied
light quality and chlorophyll specific absorption characteristics predominantly (but not exclusively)
shows negative effects on algal growth with respect to biofilm formation, biomass productivity and
biomass concentration.
The data also reveals that process development and optimization towards the indoor production
of added-value products requires a close look on the applied light source, a theme that Subitec has
intensive knowledge in and applies this information to help our customers improve their cultivation
processes.

9. 9
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(a) C. sorokiniana
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(b) P. tricornutum
Figure 4.1: Growth kinetics of C. sorokiniana and P. tricornutum cultivated utilizing different light
qualities.

10. 5. Conclusion
! Light quality has a significant influence on (i) (maximum) productivity; (ii) final biomass
concentration and (iii) process stability
! Despite “bad” spectral properties, good performance of HPS lamps along with strain compre-
hensive applicability
! No influence of the red : blue ratio was observed, rather a significant influence of the emittance
in the green gap is presumed
! Strain- and product-specific differences necessitate the inclusion of light quality in process
development and optimization strategies
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any use or operation of any methods, products, instructions or ideas contained in the material herein.