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Photoautotrophic microorganisms offer tremendous potential for sustainable lipid production, carbon dioxide storage, and green chemistry such as biofuel production from microalgae beds. One of the major challenges facing microalgae farming and other photochemical reactors is light transmission efficiency. Achieving a balanced budget at scale requires specialized photon management at all levels of the reactor, from light harvesting and efficient injection and distribution within the reactor to the design of optical antennas and molecular energy transfer pathways. This paper discusses a biomimetic approach to light dilution that enables uniform illumination of large reactor volumes with high light density. We demonstrate that incorporation of side-emitting optical fibers into the reactor increases the fraction of illuminated volume by more than two orders of magnitude, even at cell densities as low as approximately 5 x 10⁴ mL⁻¹. Using the green alga Haematococcus pluvialis as a model system, we demonstrate that proliferation can be enhanced by up to 93%. Beyond microalgae, the versatile nature of side-emitting optical fibers allows for the introduction and dilution of light with tailored spectral and temporal properties into virtually any reactor.
The aquatic biosphere accounts for approximately half of global carbon sequestration1, where photosynthesis is used to store solar energy and carbon dioxide in complex compounds. The use of microbial cultures, such as microalgae-based bioreactors, offers significant opportunities for sustainable energy conversion, carbon dioxide storage, and fine chemical production2,3. For example, significant advances in genetic engineering, lipid harvesting, and purification suggest large-scale production of biodiesel from microalgae within the next decade4. Even in direct comparison with photovoltaic technologies, which offer higher conversion efficiencies5, microbial photosynthesis offers the widest versatility and remains a clear advantage in producing high-quality products using green chemistry6,7,8,9,10. Thus, although various technical challenges appear to have delayed the initial roadmap, at least in the biofuels area, the prospects for microalgae technology remain intact11.
The fundamental challenge facing aquatic photosynthesis, and indeed all photochemical reactions, is the efficient delivery of light to the photochemical apparatus. This starts with the light source (and, in the case of sunlight, its location)12 and encompasses all subsequent optical interfaces down to the design of intracellular antennas (e.g.13). Realistic estimates of the theoretical productivity of microalgae reactors predict an annual crude oil production of approximately 35 lm².14 In the real world, best-case scenarios achieve only one tenth of this value. In addition to the intrinsic efficiency of the photosynthetic apparatus, the main factors contributing to this variability are related to the lighting efficiency: spectral losses due to the spectral mismatch between the incident radiation and the response curve of the photosynthetic dyes15; transmission losses due to reflective surfaces, intermediate layers, or containers16; and photon utilization losses due to quenching effects such as photoinhibition, heat generation or mismatch between photon energy and dye band gap14,17. In this context, reactor layout and implementation remain critical factors for economic success18: Improving the productivity of microalgae cultivation systems requires specialized optical engineering and photon management at the reactor level19,20,21,22. The key question is: how can light with suitable spectral and temporal characteristics be delivered to the darkest regions of the reactor containment in the most efficient and uniform manner? This general theme covers many similar problems that arise in different applications or at different scales (e.g. soil illumination23 or artificial microbial plots24).
Microalgae cultivation requires light regimes with spectrally adapted intensities and complex time cycles.16 Starting from a minimum threshold of light intensity, the performance of the photosynthetic apparatus typically increases with increasing irradiance until photoinhibition occurs with an empirical threshold of photon flux density (PPD, integrated over the spectral range 400–700 nm) of 200–400 μmol m−2 s−1.17 Depending on the algae species, light densities in the range of 50–200 μmol m−2 s−1 are commonly used.25 These are applied to adapted light–dark cycles facilitated by stirring.26,27 The reactors themselves are implemented as open ponds, kilometers of glass or plastic pipes, or flat plate devices.28
Providing light to deeper areas of microalgae reactors requires a compromise between the bulk density of microorganisms and the efficiency of illumination: the penetration depth of incident light into the reactor is reduced by scattering and absorption. Consequently, for a given local light intensity, microbial population density, pigment concentration, and reactor geometry, all these factors impose limitations. For example, in a reactor containing the green microalga Haematococcus pluvialis, the relative transmittance of the algae suspension decreases exponentially with increasing algae population density (Fig. 1a). Even at concentrations close to the technically acceptable lower limit of reactor throughput, the light penetration depth quickly drops below 10 mm (Fig. 1b). In modern reactor technologies, the loading depth varies from a few centimeters (flat and tubular reactors) to ~40 cm (open pond reactors). 29 Light dilution can result in highly uneven light distribution, resulting in highly oversaturated outer regions of the reactor and complete darkness inside. Special liquid mixing and agitation strategies can only partially solve this problem. Fortunately, studies have shown that the productivity of microalgae reactors can be increased by approximately four times through optimal volumetric light dilution. 30
Light attenuation and light penetration into a microalgae layer. (a) Spectral distribution of direct molar absorbance in the visible region of the spectrum for a reactor filled with Haematococcus pluvialis. The arrow indicates the absorption of chlorophyll a at about 680 nm. The inset shows examples of the change in wavelength of the absorption coefficient for different algae concentrations (the labels indicate five selected concentrations, in units of 10⁴ mL⁻¹). (b) Relationship between light penetration depth and algae population density. (c) Representative photographs of different concentrations and days of cultivation are shown (the width of the vessel shown is 35 mm). (d) Spectral photon fluence for solar irradiation at an air mass (AM) of 1.5 (irradiance of 1000 W/m²), calculated according to the standard tables of ASTM G173-03. The highlighted area corresponds to the photosynthetic range (PSR). (e) Photon spectral flux density inside the reactor at 100% solar energy injection (AM 1.5, taken from (d)). (f) Total photon flux in the spectral range 400–700 nm (PSR) as a function of reactor depth and algae concentration. The dotted line in (f) denotes the empirical threshold between typical reactor operation and supersaturation.
The above-mentioned issues are applicable to almost all photochemical machines. Alternative illumination schemes that can provide volumetrically uniform light delivery throughout the reactor containment have attracted considerable attention. 31 Immersion of the light source inside the reactor increases the individual illumination proximity compared to illumination from external surfaces, thereby significantly overcoming the limitations associated with light attenuation. In addition, such systems allow decoupling of light generation or collection from the algae culture by using fiber optic waveguides. 32, 33, 34 In addition to significantly reducing thermal loads, this provides a comprehensive set of tools for spectral engineering, including intensity modulation, time cycling, and spectral conversion. Consequently, various designs have been tested to achieve volumetric light dilution, such as light guides with diffuse surfaces such as plates, tubes, rods, or optical fibers. 35 , 36 , 37 , or optical fibers, 38 , 39 , 40 , 41 , 42 . The latter are particularly promising for several reasons: first, they provide maximum flexibility in the geometric construction of light threads with adaptive illumination density and microfluidic properties. Second, by using a lattice consisting of several light-emitting fibers, very large effective illumination areas can be achieved at the fiber-fluid interface.
We can learn lessons from nature regarding volumetric light dilution by observing the opposite problem faced by systems that efficiently harvest light in low-light conditions. A striking example is provided by silica sponges, such as the archetypal hexapod species Euplectella aspergillum.43,44 E. aspergillum is composed of basal spicules that form its skeleton (Figure 2a). The structure of these biosilica fibers spans multiple levels, down to the nanoscale silica spheres arranged in a pattern that allows for the transmission of multimode light.38 Interestingly, at higher levels, the refractive index of the spicules decreases from the core to the surface. This reduces the reflectivity of the surface in an aqueous environment, thereby improving light harvesting.16 Furthermore, lens-like structures at the ends of the fibers and individual spines on their surface have been identified as illumination points, further enhancing the light harvesting efficiency (see also Figure 2b). Interestingly, this design not only collects light but also provides volumetric illumination. This occurs when light is injected from the fiber tip, distributed throughout the sponge, and re-emitted from the surface of the spike. Figures 2b–e illustrate this using Fourier transform microscopy to quantitatively display the angular distribution of light emission. The light guide tip has a diameter of approximately 50 μm (Figure 2c) and an average refractive index of approximately 1.4338. In water with n = 1.33, this corresponds to a numerical aperture (NA) of approximately 0.53, similar to that of typical PMMA optical fibers (NA ~ 0.5). Light emission from the spicule occurs primarily due to scattering from the spike (Figure 2b) or internal defects (Figure 2c). In addition, due to the surface refractive index gradient of 38, a pronounced attenuation field is formed around most of the spicule surface.
Light transmission and lateral emission in the deep-sea sponge E. aspergillum. (a) General view of the pore (top, scale bar 1 cm) and basal (bottom, scale bar 1 mm) regions. (b) Detailed view of the spicule with distinct spiny protrusions (top) and green light emission from the spicule region (bottom). (c) Detailed view of a homogeneous region of E. aspergillum spicule fibers (top) and green light emission concentrated at a specific internal scattering center (bottom). (d) Schematic diagram of the angular distribution of light emission and (e) the pole figure of light emission recorded by Fourier microscopy (objective numerical aperture = 0.8) on the E. aspergillum spicule shown in panel (c). The lower images in (b) and (c) are superimposed.
The angular scattering characteristics of light from Aspergillus spicules and side-emitting optical fibers were obtained using Fourier transform microscopy. As shown in Figure 2e, the Aspergillus spicule fiber exhibits strong scattering at small angles (θ < 40°) to the light propagation direction. This indicates that the refractive index fluctuations causing the observed light emission have a spatial frequency higher than the frequency of the scattered light. 46 The absence of backscattering suggests that Rayleigh scattering (caused by fluctuations at frequencies higher than the frequency of visible light) plays only a minor role in the scattering process.
Lateral emitting optical fibers mimic the luminescent behavior of Aspergillus spicules. Through post-processing (or modification of the fiber drawing process or the preform itself), their luminescent behavior can be tailored to specific spectral, lateral, and angular properties. Figure 3 shows several examples of this high versatility, including: initially cloudy bare plastic fiber (Figure 3b), chemically treated plastic fiber (Figure 3c), surface roughening of plastic fiber (Figure 3d), laser writing of gratings on silica glass fiber (Figure 3e), and coatings containing photoconverting dyes (Figure 3f15). All of these approaches create light-scattering refractive index fluctuations within or on the fiber surface, facilitating light extraction from the guiding core. In principle, any of these options could be used in the form of centimeter- to meter-long fiber bundles for immersion illumination in aqueous media (Figure 3g). Practical considerations for each case include the ability to sterilize/autoclave the fiber prior to experimentation and the preservation of scattering effects when immersed in liquid.
Side-emitting optical fibers for volumetric illumination of aqueous suspensions. (a) Experimental setup for light injection and emission characterization. (b–f) Examples of luminescent fiber types: uncoated PMMA fiber with intrinsic turbidity (b), PMMA fiber etched in acetone (c), PMMA fiber scratched with sandpaper (d), quartz fiber with a laser-etched grating (e), and PMMA fiber with a CaSrS:Eu2+/epoxy resin coating (f). Photographs (b–f) were taken in air. In (g), the PMMA fiber was immersed in water. The diameter of the plastic fiber in photographs (b–g) is d = 750 μm. The diameter of the quartz fiber in photographs (e) is d = 400 μm. For the photographs, light was introduced using a 100 mW green continuous-wave laser. Panels (h–j) show the emission characteristics of selected PMMA fibers in air. (h) The spectral dependence of light attenuation over a fiber section approximately 50 cm long is shown using a machined plastic optical fiber (as indicated). (i) The corresponding normalized side emission intensity is shown. (e) The emission pole diagram of light from the plastic optical fiber shown in (g) was obtained using a Fourier transform microscope with an objective numerical aperture of 0.8.
At a constant scattering function, the fraction of light directed into the core decreases exponentially (Fig. 3h). At the same time, the intensity of light emitted from the fiber surface decreases along its length (Fig. 3i). Beyond the scope of this report, achieving uniform lateral emission from optical fibers remains a challenging task, and various solutions have been proposed.47,48
For simplicity, the following description uses plastic optical fibers similar to those shown in Figures 3d and 3h. Their light attenuation varies from ~0.5 dB/cm to 3.5 dB/cm depending on the surface roughness. In the demonstration case, the length of the light-scattering section of the fiber is 10 cm. To attenuate 99% of the incident light due to scattering (relative to the emission in air), we aim for a level of ~2 dB/cm (Figure 3h; see Materials and Methods for more details). The angular emission characteristics of the reference plastic fiber are very similar to those observed in E. aspergillum spicules (Figure 3j). Pronounced small-angle emission and the absence of backscattering confirm that the refractive index fluctuations occur at specific frequencies above the frequency of the scattered light. Compared to the angular scattering pattern of E. aspergillum, which also shows a high-angle scattering component, the Fourier map of the technical fiber is relatively uniform. The high-angle reflections observed in E. aspergillum are associated with specific frequencies, similar to the scattered light caused by finer structures and/or sharper transitions in the refractive index (e.g., cracks, pores, or similar defects).
Let us now consider the theoretical limit of local illumination improvement using the above approach. For this purpose, we compared a traditional setup in which the reactor containment is illuminated from above (Figure 4a) with a setup in which a lateral optical fiber is immersed in the reactor cavity (Figure 4b). The illumination volume VI is determined by the illuminated surface area A and the light penetration depth into the algae solution Lp: {V}_{I}=A{L}_{p}\. Based on the Beer-Lambert law, we define the penetration depth as the maximum distance at which the light transmittance decreases to 1/e (for radiation perpendicular to the fiber axis).
The performance of a fiber-enhanced algae reactor was analyzed by comparing the illuminated volume of an open reactor (without optical fiber) illuminated from above and a similar reactor containing optical fibers with side emission. The top sketch shows the two types of reactors ((a) open reactor, (b) fiber-enhanced reactor), where gray represents the reactor volume, green the light penetration depth into the algae solution (and hence the illuminated volume) and red the optical fibers. (c) Plot of the gain (per unit length) as a function of algae concentration and wavelength (the \({K}_{n}^{\max }\) value is represented by the top color bar) using the maximum number of optical fibers (N = Nmax, Rf = 500 µm). The vertical green dotted line represents the chlorophyll absorption band at ~680 nm. (d) Example of the dependence of the gain on the algae concentration and the number of fibers in the reactor (the upper color band corresponds to the Kn value) at the chlorophyll absorption wavelength (represented by the vertical green line in (c)) calculated for a given reactor configuration. The gray area corresponds to the unrealistic case when N > Nmax.
\({L}_{p}={(\varepsilon c)}^{-1}\), where the absorbance of a particular suspension is ε and the concentration of microalgae is c. For simplicity, the microalgae are considered as uniform absorbers and the light is collimated. ε is the sum of the contributions of absorption, scattering, shadowing effects, etc. Light dilution is achieved by immersing several optical fibers into the sample volume, each representing a separate beam of light. Obviously, the efficiency of light dilution depends on the particular configuration of the fiber array, such as a simple square, a denser hexagonal, or even a three-dimensional geometry such as a spiral or a pillar. For the simple case of a uniform square array of N optical fibers with immersion length Lf and radius Rf, then
where the int function is needed to obtain discrete values of Nmax to achieve the step behavior shown in Figure 2d. The value of Nmax represents the limit at which the illumination volumes around each fiber begin to overlap. The constraints of the model are \(N\le {N}_{\max }\) and \({L}_{f}\le L\). The degree to which the illumination volume can be increased using embedded fibers can be described by the gain \({K}_{n}=\pi N{L}_{f}[{({R}_{f}+{L}_{p})}^{2}-{R}_{f}^{2}]/(A{L}_{p})\). The maximum gain per unit length (for the maximum possible number of fibers and Lf = L) is given by:
If the radius of the optical fiber used is significantly less than the light penetration depth, Rf ≪ Lp, then the maximum gain per unit length can be simplified to
The latter condition is fulfilled by using optical fibers with a typical radius of Rf < 500 μm and Lp > 5 mm (Fig. 1). Consequently, higher gains can be achieved in suspensions with higher algal population densities, making fiber-based approaches particularly suitable for illuminating optically dense suspensions. Overall, this analysis shows that even for N < Nmax (which may be due to rheological and microfluidic factors and is beyond the scope of this paper), an increase in illumination volume by one to two orders of magnitude can easily be achieved. As will be shown below, in real culture experiments, the microbial density is not constant but increases with incubation time. Consequently, Lp decreases and fiber-optic illumination becomes increasingly more efficient compared to traditional lighting.
It is important to note that this evaluation method provides a simple metric for determining the relative gain in a volume receiving at least 1/e of the original light intensity injected into the fiber-suspension contact region. It does not account for local intensity variations even within this illuminated volume. Since the sensitivity of the relevant photochemical mechanism may be intensity dependent, understanding this sensitivity function is necessary to quantitatively infer the actual gain of the system performance.
At an initial microalgae cell density of ~0.7 x 104 mL−1, the light penetration depth into the reactor is about 50 mm (Fig. 1b). Consequently, the difference in illuminated volume between the traditional top-illuminated (CL) configuration and the fiber-illuminated (FL) configuration is minimal: for the L = 95 mm reactor used in this demonstration experiment, top-illuminated illumination achieves I/I0 > 1/e in more than 50% of the total reactor volume. The rest of the reactor volume remains illuminated with an intensity 37% below the incident irradiance, decreasing to about 15% towards the bottom of the reactor. The practical threshold at which fiber-illuminated illumination becomes effective is observed at an algae cell density of ~5 x 104 mL−1 (Lp < 5 mm, Vl/VtotCL ~ 5%). At this value, the fraction of the volume illuminated by the CL configuration becomes so small that switching to the FL configuration appears advantageous. This is confirmed by the experimental data presented in Figures 5d–g. During 25 days of cultivation, the algal cell number, chlorophyll a concentration and photosynthetic activity increased in both the CL and FL configurations compared to the dark control. Significant effects of the lighting method began to appear after about five days of cultivation, when the microbial density reached a threshold value of approximately 5 104 mL−1. From this point on, the continued increase in microalgal density (accompanied by a rapid decrease in Lp) led to a significant effect of volumetric illumination in the FL configuration, resulting in a 93% increase in the absolute algal number density (after 22 days) compared to CL illumination. At the same time, the photoluminescence intensity of chloroplasts increased by approximately 76%, and DCMU-Fm by 87%.
Performance of the Haematococcus pluvialis microalgae reactor under different lighting conditions. Panels (ac) show different reactor configurations: no lighting (dark, (a)), conventional overhead lighting above the algae bed (CL, (b)), and side-emitting fiber optic cable lighting (FL, (c)). Panels (dg) show the relationship between reactor performance and the cultivation process by monitoring the algae population density (d), absolute chlorophyll a photoluminescence intensity (e), reactor optical density (f), and chlorophyll a photoluminescence intensity after DCMU treatment (g).
Efficient light transmission is one of the main obstacles for large-scale development of microalgae-based bioreactors. This is because the increased density of pigments and cells leads to strong attenuation of light, reducing the penetration depth of light into the bioreactor layer to a few millimeters. Consequently, conventional illumination from an external light source can lead to strong supersaturation or even photoinhibition of the bioreactor surface, while the interior remains dark. Lateral-emitting optical fibers offer a biomimetic approach to light dilution and uniform illumination of large bioreactor volumes with high optical density. In such fibers, lateral light emission is achieved by introducing a longitudinal scattering feature. Fiber designs range from intrinsically opaque materials and easily created surface roughness by grinding, scratching or chemical treatment to methods such as laser writing of scattering centers or deposition of functional coatings. The temporal, spectral and spatial properties of the light emission can be widely modified. This facilitates adaptation to a specific microbial environment, for example by providing spectrally selective lighting or light/dark cycles. Although challenges remain with side-scatter uniformity, cleaning and disinfection of fibres, and algae growth and fouling on the surface, solutions are expected to emerge through the introduction of secondary surface modifications such as side-scatter attenuation features and repellent layers.
Performance analysis shows that the fraction of the reactor volume illuminated can be easily increased by more than two orders of magnitude when using spatially diluted illumination using side-emitting optical fibers compared to conventional illumination. This increase is achieved even at relatively low cell densities, such as approximately 5 x 10^4 mL^-1 for a reactor colonized with the green alga Haematococcus pluvialis. Using H. pluvialis as a model system, we demonstrated an increase in the proliferation rate of up to 93% and in chlorophyll a activity by approximately three-thirds.
Characterization of the fibers and spicules involved various microscopic techniques including surface roughness studies using widefield confocal microscopy (Zeiss Smartproof 5, Germany), surface roughness studies using digital image processing (Keyence VHX 6000, Japan) and polarization studies using a polarizing microscope (Zeiss JENAPOL, Germany). Fourier transform microscopy was used to analyze the angular lateral radiation of Aspergillus spicules and light-guide fibers by introducing a Bertrand lens into the optical path of the polarizing microscope. Fourier transform microscopy exploits the property of a lens to decompose the radiation field in the front focal plane into angular components in the back focal plane, known as the Fourier transform of the lens. The result of the measurement is the pole figure shown in Figures 2e and 3j, which can be interpreted as the hemisphere in Figure 2d when viewed from above: each ray emanating from the center of the sphere, coinciding with a feature on the fiber surface at the focus of the lens, passes through the sphere at a specific location corresponding to the angles ϕ and θ. The specific intensity of this direction is represented by the color scale of the data plots. The maximum angle θ that can be measured with this setup is determined by the numerical aperture (NA) of the lens = sin θ (in air). Figure 3a shows a schematic of the complete setup for light injection and emission characterization, including Fourier microscopy and intensity analysis. In these experiments, a continuous-wave laser source with variable frequency (100 mW) was used as a light source. Quantitative analysis of the radiant intensity was performed using a motorized scanning stage, shown in Figure 3a, equipped with a custom-made integrating sphere (PTFE) and a spectrometer (Maya2000 Pro). The shortening method was used to measure the optical loss inside the fiber, examining the variation of the transmitted intensity as a function of the fiber length.
The microalga Haematococcus pluvialis (strain 192.80, acquired in 2014 from the SAG algae culture collection, University of Göttingen, Germany) was used as a model species to demonstrate the reactor performance. Haematococcus pluvialis is a green freshwater microalga. In the non-motile state, it produces the antioxidant carotenoid dye astaxanthin, which has a wide range of applications in nutrition, cosmetics, and healthcare.49,50 Due to its relatively high stability, this technological importance makes it a widely used model in laboratory studies, including reactor design and performance analysis (e.g.,19). Stock cultures were grown in Bold’s basal medium with vitamins (BBM-V), starting from 100 ml of suspension to ensure sufficient material for reactor testing. BBM-V was produced according to established protocols.51,52 Cultivation was carried out in 600 mL clear polycarbonate bottles for 20 days (the suspension volume doubled after 15 days) at 18 °C under simulated diurnal conditions of 14 h of light (10-28 μE) and 10 h of darkness. Gravimetric analysis showed that the dry algal biomass at this stage was (1.31 ± 0.01) g/kg of suspension. After the initial incubation, 1 L of BBM-V was added to 200 mL of the algal suspension. 360 mL of this mixture was then distributed among three test reactors, each designed for dark, closed, and closed light (FL) configurations. To quantify the effects of different lighting configurations, 500 μl of suspension were removed after designated incubation intervals (typically 1 day) to determine changes in microbial population density and photosynthetic activity over time. Unless otherwise stated, samples were fixed with 15 μl of Lugol’s iodine. Microalgal density was determined directly by cell counting and indirectly by measuring the optical density of the samples. Cell counts were performed in a Fuchs-Rosenthal counting chamber with a depth of 0.2 mm, a grid size of 0.25 mm, and a total volume of 3.2–6.4 μl (256–512 wells). In all experiments, the absolute number of algae counted ranged from 40 to 650. The initial algal cell density in the suspension determined in this way was approximately 0.692 x 104 ml−1.
Optical extinction was recorded in the spectral range of 300–900 nm using a UV–vis spectrophotometer (Shimadzu UV-3101) with suspension samples in 9.98 mm × 9.98 mm quartz cuvettes. The optical extinction values used in Figure 5 were extracted from the absorbance at 678 nm (path length 9.98 mm). Photosynthetic activity was subsequently estimated based on chlorophyll a photoluminescence. Fluorescence spectroscopy was performed according to the protocol of Maxwell and Johnson53 (Mithras Lb 940, excitation was achieved with a 75 W halogen lamp). An excitation wavelength of 430 nm was chosen to match the maximum excitation band of chlorophyll a in H. pluvialis19. Emission intensities were recorded at 460 nm. Samples were prepared in 96-well plates with a volume of 0.2 ml per sample. Measurements were performed immediately after sampling, after 0.5 h of storage in the dark and after the addition of 15 μl of the algicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). DCMU inhibits PSII, providing the maximum level of Fm fluorescence.
Unless otherwise stated, all equipment and tools used for the preparation and processing of algal suspensions were sterilized in an autoclave at 120°C for 1 hour prior to use.
The test reactor has a cross-sectional area of 35 mm × 108 mm (A = 3780 mm²) and a rectangular shape with a height of 130 mm. The filling volume is 360 ml, which corresponds to a reactor depth L of approximately 95 mm. Figures 5a–c schematically show three reactors operating in parallel.
To illuminate the reactor in the demonstration experiments, plastic optical fiber (PMMA) sheets with a diameter of 750 μm and a length of 35 cm were used. Uniform surface roughness was created on the last ~10 cm of the fiber surface by lightly sanding with 240-grit silicon carbide sandpaper (see Figure 3d). The roughened fibers were assembled into 400 fiber bundles of 30–45 cm in length for installation in the reactor setup, as shown in Figure 5c. One end (non-rough side) of the fiber bundle was fixed with 2 cm of heat-shrinkable tubing and polished. Light was injected into the fiber bundle from the fixed side using a 15-W RGB LED generator (Stiers, Germany). The total injected photon flux from all elements was ~20 μmol m−2 s−1. In the CL configuration, similar fiber bundles were placed on top of the microalgae suspension without immersion; in the FL configuration, the fiber bundles were arranged so that the rough part was immersed in the liquid. The fiber bundles were sterilized before use in the algae suspension. The cultivation experiment lasted 25 days, three reactors were operated simultaneously and repeated once.
The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.
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Post time: Sep-05-2025