Light distribution and spectral composition within cultures of micro-algae: Quantitative modelling of the light field in photobioreactors

TLDR: The model is a general tool to predict light availability inside photosynthetic microorganism cultures and to optimize light supply, in respect to both intensity and spectral distribution, in technological applications.

Abstract: Light, being the fundamental energy source to sustain life on Earth, is the external factor with the strongest impact on photosynthetic microorganisms. Moreover, when considering biotechnological applications such as the production of energy carriers and commodities in photobioreactors, light supply within the reactor volume is one of the main limiting factors for an efficient system. Thus, the prediction of light availability and its spectral distribution is of fundamental importance for the productivity of photo-biological processes. The light field model here presented is able to predict the intensity and spectral distribution of light throughout the reactor volume based on the incident light and the spectral characteristics of the photosynthetic microorganism. It takes into account the scattering and absorption behaviour of the micro-algae, as well the adaptation of the biological system to different light intensities. Although in the form exposed here the model is optimized for photosynthetic microorganism cultures inside flat- type photobioreactors, the theoretical framework is easily extensible to other geometries. Our calculation scheme has been applied to model the light field inside Synechocystis sp. PCC 6803 wild-type and Olive antenna mutant cultures at different cell-density concentrations exposed to white, blue, green and red LED lamps, delivering results with reasonable accuracy, despite the data uncertainties. To achieve this, Synechocystis experimental attenuation profiles for different light sources were estimated by means of the Beer-Lambert law, whereby the corresponding downward irradiance attenuation coefficients Kd(λ) were obtained through inherent optical properties of each organism at any wavelength within the photosynthetically active radiation band. The input data for the algorithm are chlorophyll-specific absorption and scattering spectra at different mean acclimatisation irradiance values for a given organism, the depth of the photobioreactor, the cell-density and also the intensity and emission spectrum of the light source. In summary, the model is a general tool to predict light availability inside photosynthetic microorganism cultures and to optimize light supply, in respect to both intensity and spectral distribution, in technological applications. This knowledge is crucial for industrial-scale optimisation of light distribution within photobioreactors and is also a fundamental parameter for unravelling the nature of many photosynthetic processes.

Figures

Figure 1: Summarised modelling scheme to obtain PAR attenuation profiles and spectral ones.

Table 1: List of symbols and abbreviations.

Figure 2: Emission spectra in terms of relative power of the LED lamps used for the attenuation trials. Graph colours represent each LED characteristic colour (blue, green and red), whereas black curve corresponds to the white LED.

Figure 7: Left panel: Light attenuation profiles of Synechocystis WT (A) and Olive (B) strain cultures exposed to 2000 µmol photons ·m−2 · s−1 of white LED light at five different OD750 concentrations (0.1, 0.5, 1.0, 2.5 and 5.0) are depicted. Dots are the original source samples [38] and lines the simulation outcome. Darker colours correspond to denser suspensions. Right panel: Light attenuation profiles of Synechocystis WT (C) and Olive (D) strain cultures exposed to 1000 µmol photons ·m−2 · s−1 of four different LED lamps at an OD750 concentration of 1.0 are shown. Dots are the original source samples. Graph colours represent each LED characteristic colour (blue, green and red), whereas black curve corresponds to the white LED.

Figure 6: Modelled chlorophyll-specific downward attenuation function K?d(λ,Ed,i) for wild-type and Olive strain (green and brown colour, respectively) at the incident irradiance values of 40, 100 (dashed) and 170 (dotted) µmol photons ·m−2 · s−1.

Figure 11: Modelled mean downward attenuation coefficient within PAR rangeKd,PAR(Ed,acc) for WT cultures exposed to lamp intensities of 20 and 100 µmol photons ·m−2 ·s−1 in a 4-cm depth PBR at constant cell-densities up to 25,000 mg Chla · m−3 are depicted (right vertical axis). Additionally, resulting average irradiance Ed,acc is also plotted for such suspensions (dotted, left vertical axis). Green and blue curves stand for WT cultures grown at 20 and 100 µmol photons ·m−2 · s−1 incident radiation and similarly red and yellow curves represent Olive cultures cultivated at 20 and 100 µmol photons ·m−2 · s−1 incident radiation.

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http://doi.org/10.1016/j.algal.2017.01.004
http://hdl.handle.net/10251/101685
Elsevier

Light distribution and spectral composition within cultures of1
micro-algae: Quantitative modelling of the light field in photobioreactors2
David Fuente
1,*
, Joseph Keller
2
, J. Alberto Conejero
3
, Matthias Rögner
2
, Sascha Rexroth
2
, and Javier F.3
Urchueguía
1
4
1
Instituto de Aplicaciones de las Tecnologías de la Información y de las Comunicaciones Avanzadas,5
Universitat Politècnica de València, València, Spain6
2
Plant Biochemistry, Faculty of Biology and Biotechnology, Ruhr University Bochum, Germany7
3
Instituto Universitario de Matemática Pura y Aplicada, Universitat Politècnica de València, Valencia,8
Spain9
*
Corresponding author: dafueher@upv.es10
January 11, 201711
Abstract12
Light, being the fundamental energy source to sustain life on Earth, is the external factor with the strongest13
impact on photosynthetic microorganisms. Moreover, when considering biotechnological applications such as the14
production of energy carriers and commodities in photobioreactors, light supply within the reactor volume is one of15
the main limiting factors for an efficient system. Thus, the prediction of light availability and its spectral distribution16
is of fundamental importance for the productivity of photo-biological processes.17
The light field model here presented is able to predict the intensity and spectral distribution of light throughout the18
reactor volume based on the incident light and the spectral characteristics of the photosynthetic microorganism. It19
takes into account the scattering and absorption behaviour of the micro-algae, as well the adaptation of the biological20
system to different light intensities.21
Although in the form exposed here the model is optimized for photosynthetic microorganism cultures inside flat-22
type photobioreactors, the theoretical framework is easily extensible to other geometries. Our calculation scheme23
has been applied to model the light field inside Synechocystis sp. PCC 6803 wild-type and Olive antenna mutant24
cultures at different cell-density concentrations exposed to white, blue, green and red LED lamps, delivering results25
with reasonable accuracy, despite the data uncertainties. To achieve this, Synechocystis experimental attenuation26
profiles for different light sources were estimated by means of the Beer-Lambert law, whereby the corresponding27
1

downward irradiance attenuation coefficients K
d
(λ) were obtained through inherent optical properties of each28
organism at any wavelength within the photosynthetically active radiation band. The input data for the algorithm29
are chlorophyll-specific absorption and scattering spectra at different mean acclimatisation irradiance values for a30
given organism, the depth of the photobioreactor, the cell-density and also the intensity and emission spectrum of31
the light source.32
In summary, the model is a general tool to predict light availability inside photosynthetic microorganism cultures33
and to optimize light supply, in respect to both intensity and spectral distribution, in technological applications.34
This knowledge is crucial for industrial-scale optimisation of light distribution within photobioreactors and is also35
a fundamental parameter for unravelling the nature of many photosynthetic processes.36
Keywords: absorption, scattering, attenuation, inherent optical properties, modelling, Synechocystis37
1 Introduction38
1.1 Light research in aquatic ecosystems39
1.1.1 Introduction to Optics in Biology40
Photosynthesis is a very active research field in the life sciences due to the crucial importance of photosynthetic41
organisms as the fundamental source of all biomass in our planet. Particularly, much research has been done in42
understanding how light behaves inside different water bodies, such as inland, coastal and oceanic ecosystems.43
Concurrently, bio-optical researchers have developed several methodologies to estimate optical properties. In the44
year 1961 Preisendorfer defined the inherent (IOPs) and apparent optical properties (AOPs) of water bodies, founding45
optical oceanography [1]. Relating IOPs and AOPs have been an ongoing effort since then, and different authors46
have studied, experimentally as well as theoretically [2], the optical characteristics of water and cell suspensions as a47
function of water body features and metabolic variables such as the energy stored by algae upon light conditions [3].48
But oceanic optics is not the only field of interest in the study of light interaction with microorganisms. During49
the last 30 years, more interest has progressively been devoted to the development of closed photobioreactors (PBRs),50
aimed at the production of many substances of interest ranging from nutra- and pharmaceuticals, to bioenergetic51
compounds [4], [5]. As dense cultures are preferred to maximise production, light is normally the limiting factor to52
obtain a cost effective PBR operation. Although dense suspensions are a priori more appropriate for an efficient PBR53
utilisation [6], too concentrated cultures may increase operating costs [7] and completely deplete the system of light54
in most the external layers [8] as well. Therefore, optimisation of illumination conditions and cell density is required55
for improving overall photosynthesis performance and to minimise dark respiration and thus for achieving an optimal56
2

design of large-scale photobioreactors [9].57
From the point of view of light propagation, there are important differences between the conditions in open waters58
or inside a PBR aqueous phase. The use of artificial light sources in many PBR set-ups, unnatural light cycles,59
the geometry of the arrangement itself and its inherent limitation in culture depth, not present in most open waters,60
are just some of the differentiating factors. A crucial topic is the question of stratification. Whilst in open waters61
a given equilibrium stratification is established within the photic zone and substantial differences may be found in62
microorganism concentration and composition depending on depth, inside a PBR efforts are usually oriented towards63
obtaining a good mixing so that the photosynthetic cells can rapidly move towards the external and internal zones of64
the reactor. Accordingly, the culture inside the PBR volume is usually regarded as being homogeneous.65
Regarding the strategies to describe light distribution within water bodies, authors have either used algorithms that66
calculate the light field based on the radiative transfer equation describing light-matter interaction [10] or have applied67
stochastic methods such as Monte Carlo simulations [11, 12], which allow researchers to statistically follow the fate of68
individual photons within the medium. Relevant works based on this strategy have been published in the last decades.69
In this regard, in some cases the light field prediction is linked with experimental cell growth [13, 14] or coupled70
biomass production is modelled following a classical growth law such as Monod-type [15]. Several applications on71
different reactor shapes such as torus photobioreactors [16] or open ponds [17] can be found.72
In our approach we aim at creating a procedure in between the simple light models and exceedingly detailed73
simulations in order to get a holistic view of the interaction of light and biomass based on the IOPs of the cells of74
interest, which has not been described in literature and is novel to the field. To do so, we will derive a relationship75
connecting the light field profile within a PBR suspension knowing the cell density, lamp emission spectrum, culture76
depth, absorption and scattering coefficients of the culture acclimatised to different light intensities. Making some77
simplifying assumptions we arrive at an expression that can be easily solved and can even give rise to an analytic78
relationship between operating parameters of the culture and includes in an implicit manner photo-adaptation of79
the cells. Furthermore, we have tested our scheme using information from two sources, completed with our own80
experiments, on two different strains of Synechocystis sp. PCC 6803 (hereafter referred to as Synechocystis), the81
wild-type and the Olive mutant. The latter is a strain with truncated phycobilisome structure, where the phycobilisome82
core is present but the rods are absent [18].83
The model is able to predict the light attenuation caused by cultures in a considerable range of optical densities84
and light sources. Besides, the methodology proposed in this work follows a semi-mechanistic calculation procedure85
that can be generalised to other microorganisms and reactor geometries, whereas other published contributions are86
merely empiric fits or assume that absorption is the only factor for light attenuation. Moreover, this methodology is87
3

also capable of predicting spectral composition of light within the photic zone.88
In the following subsections we will explain the main features of our modelling approach and its assumptions:89
section 2 exposes the experimental information and underlines how our method can be used in practice combining90
existing information with novel experiments. Section 3 discusses the results and highlights some interpretations that91
can be obtained from these analyses. Section 4 contains the conclusions and further outlook of our work.92
1.1.2 Light spectrum influence in photosynthetic mechanisms93
As stated before, light spectral composition in a PBR is sometimes not just a given condition, but can be selected and94
optimised. For an optimal selection of the light source, it is not only important to consider lamps whose emission95
peaks overlap the cell absorption spectra, but also other factors such as scattering, quantum yield and excitation balance96
between both types of photosystems [19].97
Moreover, not only the light absorption capacity of the cells but also its efficiency in converting the captured photons98
into usable energy has to be taken into consideration. In this regard, the action spectrum represents the quantum yield99
of this efficiency upon light wavelength. It is important to note that the action spectra can vary depending on the100
pre-illumination conditions [20] or if supplementary light is applied. In the latter case, if cells are not exposed to some101
background light, the action spectrum can differ greatly from the absorptance spectrum in some wavelengths [21]. In102
other words, when using a monochromatic light source, the spectrum of the chosen lamp has to provide a balanced103
amount of quanta for both types of photosystems.104
While it is common practice to study how white light affects growth in photosynthetic microorganism cultures,105
including mechanistic approaches for the photo-adaptation phenomenon [22], less research has been performed on how106
other types of light sources impact photosynthesis rates and related mechanisms. Specifically in cyanobacteria, some107
contributions can be found regarding light colour effect on oxygen evolution [23], redox state of the plastoquinone pool108
[24], growth [25] in Synechocystis, biomass composition of Arthrospira platensis [26] or areal biomass productivity in109
Chlamydomonas reinhardtii [27]. In Zavrel et al. research [25] and Markou contribution [26], blue light led to lower110
growth than red in both species, whereas in [27] yellow light promoted the highest productivity. Available irradiance as111
a function of the remaining wavelengths can shed light on real photosynthesis rates as quanta are absorbed by pigments112
which have specific absorption spectra on one side while part of the light is scattered in a spectrally dependent way.113
Particularly in Synechocystis cultures, blue is the most scattered colour and red the least [28], though this phenomenon114
relies on the type of organism and the aquatic environment [29].115
Delving deeper in spectral composition of light publications, it must be noted that there are few experimental116
works which describe the wavelength dependent light distribution along the optical path-length. Measured spectra of117
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Frequently asked questions

Q1. What are the contributions in "Micro-algae: quantitative modelling of the light field in photobioreactors" ?

In this paper, a light field model is presented to predict the intensity and spectral distribution of light throughout the reactor volume based on the incident light and the spectral characteristics of the photosynthetic microorganism.