Literature Review: Impact of Artificial Lights on the Growth of Microalgae in Photobioreactor

Light is one of the important parameters that controls the growth of algae cultivation. It directly controls the photosynthesis process carried out by algae.^[1] Illumination must be present in the system from 10 to 24 hours per day depending upon the algal species. For large-scale open pond system, sunlight is the primary source of ligh^[2]t. But the sunlight output greatly fluctuates with day-night cycles, different weather conditions and seasonal changes. Due to its unpredictable nature, artificial source of lighting is introduced to ensure smooth cultivation of algae.^[3]

Artificial lighting system provides uninterrupted supply of lighting irrespective of location and different weather conditions. The light intensity can also be controlled to prevent microalgae from entering photoinhibition (excess exposure to light that is required for photosynthesis) or stagnation (little exposure to light to sustain algae growth) region.^[4] So different types of light such as – incandescent lamp, fluorescent lamp, LEDs are being used in small scale pilot project or laboratory scale microalgae cultivation.^[5][6][7] However, LEDs shows an outstanding performance in many aspects that surpass the performance of incandescent lamp and fluorescent lights. LEDs can generate same illumination to incandescent or fluorescent bulbs while consuming fewer watts of energy. Also, the other advantages are longer lifespan, smaller size, low dissipation of energy, high photoelectric conversion efficiency etc. But what makes it more suitable for microalgae photobioreactors are its ability to emit desired range of wavelength and control the light intensity quite easily.

LEDs of different colors are widely available in market. For laboratory scale photobioreactors, various LED arrangements are designed based on the types of microorganisms and the purpose of microalgae cultivation. So, a literature review is done to have a better understanding of the effect of wavelengths and light intensities on microalgae photobioreactor.

1. Modeling of the influence of light quality on the growth of microalgae in a laboratory scale photo-bio-reactor irradiated by arrangements of blue and red LEDs^[8]

In 2014, Ignacio Niizawa, et. al designed a photobioreactor model interfaced with simulation algorithm to accurately predict the rate of absorption of photons in microalgal cultures irradiated with different arrangements of LED lights emitting in blue and red spectral regions.

Findings:

• Blue LED lights show better average photon absorption rates in the culture rather than red LEDs. However, dispersion around the average absorption value was also higher in blue LEDs which questions about the reliability and effectiveness of this radiant field.
• Although the absorption rate was lower than the blue LEDs, red LEDs show better result in production of biomass.

2.Luminescent photobioreactor design for improved algal growth and photosynthetic pigment production through spectral conversion of light^[9]

In 2013, Seyedeh Fatemeh Mohsenpour, et. al constructed luminescent acrylic photobioreactor to examine cultivation growth at various culture density.

Findings:

• Although red luminescent photobioreactor increased biomass production in both microalgae, this wavelength is not suitable for highly dense or deep cultures.
• No consistent pattern was observed for optimal light colors in pigment production, though green light tends to yield better results for only one species.

3.Use of Red and Blue Light-Emitting Diodes (LED) and Fluorescent Lamps to Grow Microalgae in a Photobioreactor^[10]

In 2012, Caner Koc,et. Al observed cell count, cell weight and cell size of a particular microalgae species by using different wavelengths of lights.

Findings:

• Red LED was the most efficient light in terms of cell concentration and weight.
• Blue LEDs showed best performance for maximum circular size of microalgae cells.

4.Maximizing biomass productivity and cell density of Chlorella vulgaris by using light-emitting diode-based photobioreactor^[11]

In 2012, Weiqi Fu, et. al reported the best growth rate condition for C. vulgaris by using adaptive laboratory evolution (ALE) technique.

Findings:

• Expensive 680 nm red LEDs were replaced by 660 nm of flashing LEDs at different duty cycle without hampering the growth density.
• ALE was achieved by maintaining consistent biomass concentrations at the beginning of each cycle through the removal of excess culture and its replacement with fresh medium.

5.Light emitting diodes (LEDs) applied to microalgal production^[12]

In 2014, Peter S.C. Schulze, et. al gathered information from various published sources to summarize the necessary light wavelengths for different microalgae species.

Findings:

• Color of the optimum light configuration depend on the predominant pigments within the microalgae species.
• Green algae exhibited their best growth performance when exposed to a combination of blue light, ranging from 10% to 30% with red light.

6.Application of light-emitting diodes (LEDs) in cultivation of phototrophic microalgae: current state and perspectives^[13]

In 2015, M. Glemser and his team conducted a review of both the internal and external illumination systems using artificial lights for photobioreactors.

Findings:

• Mainly focuses the effect of white lights (warm-white and cool-white) on biomass growth.
• Reviewed the effect of LEDs in industrial level photobioreactor.

7.Effects of light-emitting diodes (LEDs) on the accumulation of lipid content using a two-phase culture process with three microalgae^[14]

In 2016, Chae-Hun Ra, et. al conducted a comparative study to show the difference in lipid build up between three different microalgae group by using different wavelengths of LEDs. The experiment is conducted in two phases- phase I is for microalgae culture and phase II is for lipid extraction.

Findings:

• Blue LED showed the best performance for microalgae growth for all the three groups, which is followed by red LED and then fluorescent lamp.
• It was observed that green microalgae species exhibited poor growth under green LEDs, as the green light was not absorbed but rather reflected.
• Despite the poor performance of green LEDs in phase-I, they stand out in phase-II by producing the highest lipid contents among all the other LED configurations.

8.Effect of light quality supplied by light emitting diodes (LEDs) on growth and biochemical profiles of Nannochloropsis oculata and Tetraselmis chuii^[15]

In 2016, Peter S.C. Schulze, et. al tested two microalgae species by exposing to different non-tailored mono- or multichromatic LED light sources, and tailored light sources, such as- fluorescent light, di- or multichromatic LED mixes.

Findings:

• Precise light tailoring (i.e., use of light sources whose emission spectra closely match the absorption spectra of target microalgal species) increased photon absorption compared to other monochromatic and non-tailored light sources.
• Tailored di-chromatic LEDs (mixture of red and blue lights) outperformed all the other configurations in terms of microalgae growth.

9.Photosynthetic CO2 uptake by microalgae for biogas upgrading and simultaneously biogas slurry decontamination by using of microalgae photobioreactor under various light wavelengths, light intensities, and photoperiods^[16]

In 2016, Cheng Yan, et al. conducted a study at microalgae strain under varying wavelength, light intensity and photoperiods.

Findings:

• The mixed LED red: blue = 5:5 configuration consisted the optimal light wavelength for the microalgae photobioreactor.
• CO2 removal efficiency was also significantly higher in this fifty-fifty red and blue light configuration.

10.Optimization of spectral light quality for growth and product formation in different microalgae using a continuous photobioreactor^[17]

In 2016, Sascha Baer, et al. analyzed the effects of total 37 LED light configurations on three different microalgae species in terms of biomass growth.

Findings:

• RGB-LED strips are controlled to create different ratios of red, blue and green light (total 37 configuration).
• The ideal light conditions for the growth of various microalgae vary depending on the combination of red, blue, and green light. Nevertheless, red LED light consistently constitutes the highest proportion in every LED mixture.

11.Effect of LED Lights on the Growth of Microalgae^[18]

In 2018, Archana Pattanaik, et al. reviewed several case studies that uses artificial light for microalgae cultivation and summarized their results by finding some common traits of algae behaviour.

Findings:

• Growth rate was reported to be higher in red LED while blue light increased the accumulation of photosynthetic pigments.
• Some species grew best in mixed red-blue LED rather that monochromatic lights.

12.Light management technologies for increasing algal photobioreactor efficiency^[19]

In 2019, Emeka G. Nwoba, et al. reviewed many prototypes to raise photosynthetic efficiency of photobioreactor.

Findings:

• Different techniques like spectral conversion, shifting, filtration etc. made a better growth yield at laboratory scale, however using these innovations for large scale algae cultivation is not feasible.
• Suggested a hybrid PV-PBR system that use an insulated glazed photovoltaic panel to modify the solar spectrum to benefit microalgae with photosynthetically useful light while generating electricity from UV and IR wavelengths.

13.Effect of different wavelengths of LED light on the growth, chlorophyll, β-carotene content and proximate composition of Chlorella ellipsoidea^[20]

In 2021, Arpan Baidya and his team examined the growth and pigment content of Chlorella ellipsoidea by using various wavelength LED lights.

Findings:

• Highest culture growth and nutritional values were obtained by blue LED light exposure.
• Red LED showed the least performance even if it was compared with white and green LEDs.

Temperature[edit | edit source]

For outdoor open system photobioreactors, there is no control over temperature. But somehow, this fluctuation in temperature is adapted by microalgae and they can work over a wide range of temperatures. Microalgae can endure temperatures within the range of 15–30°C, with an optimal survival range of 20–25°C.^[21]

14.Outdoor Cultivation of Temperature-Tolerant Chlorella sorokiniana in a Column Photobioreactor Under Low Power-Input^[22]

In 2013, Quentin Bechet, et al. investigated the growth of C. sorokiniana with an optimal mixing in an outdoor condition.

Findings:

• Unregulated temperature did not adversely affect the culture's growth even the temperature had risen to 41°C.
• In comparison to other species, this experiment indicated that temperature-tolerant microalgae species would require less mixing, resulting in cost savings for mixing.

15.Response of Antarctic, temperate, and tropical microalgae to temperature stress^[23]

In 2013, Ming-Li Teoh, et al. examined temperature response variations among microalgae taxa from different regions (antarctic, temperature and tropical).

Findings:

• Chlorella spp. from the three regions exhibited consistent temperature stress responses, tolerating a wide range from 4 to 38 °C.
• However, the three Chlorella species showed varying responses in lipid content in different temperature.

16.Effects of Temperature and Other Operational Parameters on Chlorella vulgaris Mass Cultivation in a Simple and Low-Cost Column Photobioreactor^[24]

In 2015, Bio Sigui Bruno Bamba, et al. examined the impact of temperature, CO2 to air ratio and bubbling rate on biomass production in Chlorella vulgaris using a cost-effective 84-L column photobioreactor.

Findings:

• The growth rate significantly increased from 100 to 200 L h−1(735±5 to 1000±11 mg L−1); however, the growth was less pronounced from 200 L h−1 to 400 L h−1.
• Considering cost, the findings demonstrated elevated productivity and biomass accumulation at temperatures up to 30 °C with moderate (2%) to high (10%) CO2 levels.

17.Growth and photosynthesis of Chlorella strains from polar, temperate and tropical freshwater environments under temperature stress^[25]

In 2018, Kok-Keong and his team and explored the influence of increased temperatures on Chlorella strains derived from diverse latitudinal origins.

Findings:

• All four Chlorella strains exhibited growth and photosynthesis capacities at temperatures 1.5 to 6 times higher than their usual ambient conditions.
• Despite the brief ten-day exposure in this study, even under extreme growth-permissive temperatures, the cultures managed to undergo at least one cell division.
• The chlorella strains proved their eurythermal adaptivity through these experiments.

18.Pilot-Scale Cultivation of the Snow Alga Chloromonas typhlos in a Photobioreactor^[26]

In 2022, Floris Schoeters, et al. conducted a proof-of-concept investigation, cultivating C. typhlos in a photobioreactor situated within a minimally heated (frost-protection) greenhouse during colder intervals.

Findings:

• Effectively grown in 350 L working volume photobioreactor with a maximum growth rate of 1.082 g L−1.
• The growth period extended from the winter season into spring for these algae.

pH[edit | edit source]

pH requirements vary widely depending on the microalgae species. However, some trends are found out by doing some literature about various experiments conducted.

19.The Effects of pH on the Growth of Chlorella vulgaris and Its Interactions with Cadmium Toxicity^[27]

In 1991, Joseph W. Rachlin, et al. examined the effect of pH on the microalgae growth between the range of 3.0-9.0.

Findings:

• For 96-h exposure period, the medium pH level ranging from 3.0-7.0 remained constant throughout the experiment in contrast for pH level from 7.5-9.0.
• C. vulgaris exhibited optimal growth conditions in the range of 7.5 to 8.0.

20.Effect of pH on growth and biochemical responses of Dunaliella bardawil and Chlorella ellipsoidea^[28]

In 2010, Zeinab I. Khalil and his team observed the response of pH on the growth and biochemical responses on species mentioned.

Findings:

• C. ellipsoidea exhibited a significant increase in dry weight at a pH level of increased alkalinity, specifically at pH 10.
• It survived a wide range of pH level ranging from 4.0 to 11.0 indicating its use for outdoor cultivation.

21.Effect of pH on growth and lipid accumulation kinetics of the microalga Chlorella vulgaris grown heterotrophically under sulfur limitation^[29]

In 2016, Myrsini Sakarika, et al. conducted a various range of experiments to find the optimum pH value for biomass growth and lipid accumulation.

Findings:

• C. vulgaris had the capacity to thrive heterotrophically within a broad pH range of 5.0–8.0.
• pH 3.0, 4.0 and 11.0 considered extreme for this strain and pH 9.5 caused cell aggregation.
• The optimal pH was found to be 7.5 for both growth and lipid accumulation.

22.Effects of pH on cell growth, lipid production and CO2 addition of microalgae Chlorella sorokiniana^[30]

In 2017, Renhe Qiu, et al. measured cell growth and lipid content at different pH levels in flask cultivation.

Findings:

• pH level of 6.0 exhibited highest cell growth and lipid content without taking account of C02 intake.
• For optimized C02 supply in terms of cost effectivity, the pH level 8.0 shows the optimum result for algae growth and lipid production.

Agitation[edit | edit source]

Agitation can be done by in form of aeration system. CO2 can be supplied by aeration. Stirring is another common process to mix the culture. It can be done with impeller or rotating device in according to the design of photobioreactor. Stirring speed is important for steady-state microalgae growth. However, the speed can never be very high because it will damage the cell composition and collapse the culture. Impeller velocities of 0.2-0.5 ms-1 are ideal for mass cultivation but it should not exceed 1ms-1.^[31]One different approach is to use pump to circulate culture medium. For low scale PBR, spargers are used to mix medium by injecting CO2. However, spargers can be used in conjunction with any other agitator where the mixing is difficult or where there is a dead zone.

23.Flow Pattern, Mixing, Gas Hold-Up and Mass Transfer Coefficient of Triple-Impeller Configurations in Stirred Tank Bioreactors^[32]

In 2014, Minghui Xie, et al. compared the performance of four distinct triple-impeller configuration to find their compatibility.

Findings:

• Axial impellers combination offered more effective homogenization performance than the impellers with combined radial and axial flow.
• The radial impellers combination was identified as the least effective.

24.Design of Photobioreactors for Mass Cultivation of Photosynthetic Organisms^[33]

In 2017, Qingshan Huang, et al. elaborated the parameters and their influence on the successful operation of a photobioreactor.

Findings:

• Non agitated PBRs suffer from cell accumulation, mutual shading, exhaustion of carbon dioxide (CO2) etc which can lead to decrease in cell productivity, creating dead zone, photoinhibition, photooxidation.
• Agitation system increases the CAPex and OPex of a photobioreactor to a great extent.

25.Mixing and agitation in photobioreactors^[34]

In 2022, Paulo Cesar de Souza Kirnev, et al. addressed the importance of agitators in PBR systems, outlining the specific types required for each system.

Findings:

• Mechanical agitations i.e., impellers, paddle wheels are used for large scale microalgae cultivation whereas pumps are used for small or medium scale cultivation.
• In gas-driven systems, fluid movement is facilitated by gas bubbles generated through spargers or perforated tubes.

References[edit | edit source]