On-line control of light intensity in a microalgal bioreactor using a novel automatic system

doi:10.1016/j.enzmictec.2007.12.002

Enzyme and Microbial Technology

Volume 42, Issue 7, 5 June 2008, Pages 554-559

https://doi.org/10.1016/j.enzmictec.2007.12.002 Get rights and content

Abstract

The influence of light intensity upon biomass and fatty acid productivity by the microalga Pavlova lutheri was experimentally studied using a novel device. This device was designed to automatically adjust light intensity in a photobioreactor: it takes on-line measurements of biomass concentration, and was successfully tested to implement a feedback control of light based on the growth rate variation. Using said device, batch and semicontinuous cultures of P. lutheri were maintained at maximum growth rates and biomass productivities – hence avoiding photoinhibition, and consequent waste of radiant energy. Several cultures were run with said device, and their performances were compared with those of control cultures submitted to constant light intensity; the biomass levels attained, as well as the yields of eicosapentaenoic and docosahexaenoic acids were calculated – and were consistently higher than those of their uncontrolled counterpart.

Introduction

Microalgae are currently cultivated to produce a vast number of high added-value products, e.g. pigments and polyunsaturated fatty acids [1]. Production on the industrial level is usually performed in open ponds or raceways; however, they often lead to low biomass productivity, so they are restricted to only a few species [2]. Several closed systems – usually of the tubular or flat-panel types, have been developed [3]; however, they present difficulties for effective control, require a large area of land and are expensive to operate. Therefore, compact and sterilizable photobioreactors are urged [4].

One of the major parameters that affect microalga growth is light [2]; hence, a light control system is desirable for closed photobioreactors. It is well known that, in a batch culture run under constant light intensity provided externally, the amount of light actually available to cells is affected by mutual shading [5], [6]; this affects negatively both their growth rate and biochemical composition. On the other hand, excess light can cause photoinhibition, thus wasting energy and promoting cell death. Therefore, assessment of the light available for photosynthesis throughout culture time is an important step toward accurate and continuous control of light intensity.

A model that describes light-limited growth of microalgae in steady-state, continuous cultures was proposed by Evers [7] – and later applied by Grima et al. [6]; this model takes into account the average light intensity inside a photobioreactor, as a function of incident light and biomass concentration. Use of this type of models allows calculation of the average light intensity associated with the maximum growth rate, hence avoiding photoinhibition; this feature is of major importance in what concerns light control. In this work, such a model was (for the first time) successfully applied to semicontinuous cultures under quasi-steady-state conditions.

Recall that Pavlova lutheri is widely employed to feed fish, bivalves and crustaceans, owing to its high content of polyunsaturated fatty acids [8], [9], [10] – mainly eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, which are claimed to be beneficial for human health [11], [12]. Our research effort encompassed growth of P. lutheri in a closed photobioreactor, operated either batch- or semicontinuous-wise. An automatic feedback system for control of light intensity – that uses information pertaining to growth rate, was thus designed and tested. The production rates of both EPA and DHA by that microalga were assayed under various conditions of light and growth rate, so as to elucidate the features of said controlled process in a model system.

Section snippets

Culture conditions

A genetically improved strain (II#2) of P. lutheri [13] was employed, using artificial sea water, ASW [14], as cultivation medium. Cultures of 1.5 L were performed in a 2-L bioreactor (Braun, Germany), with height and internal diameter of 240 and 130 mm, respectively – under both batch and semicontinuous modes. The temperature was maintained at 20 ± 0.5 °C via a refrigeration jacket, the stirring rate was set to 50 rpm, and the pH was kept at 8.0 ± 0.2 by addition of 1 M NaOH or HCl, as appropriate. All

Results

To study the influence of I_av on the growth rate of P. lutheri, several QSS stages – characterized by as many values of I_av, were achieved, so as to correspond to distinct biomass concentrations and incident lights. Two biomass concentrations were accordingly chosen, coupled with a wide range of I_av – in attempts to reduce mutual correlation of the data. The biomass concentrations in the various QSS stages are depicted in Fig. 2, in terms of cell number and ash-free dry wheight.

The model by

Discussion

Inspection of the results depicted in Fig. 2, Fig. 3, coupled with Eq. (2), one concludes that it is possible to anticipate when cultures will be light-limited or not; in order to attain growth rates close to the maximum (hence avoiding excess I₀, that would lead to both photoinhibition and energy waste), it was assumed that I_av should lie in the vicinity of 0.65 W m⁻².

The first datum in Fig. 3 was obtained when I₀ was set equal to zero (i.e. no light was provided to the reactor) – and thus

Conclusions

Microalgae are currently used as polyunsaturated fatty acid-rich biomass in the aquaculture industry, so systems that automatically monitor and control algal culture parameters are urged. Since light is a crucial parameter in microalgal growth and metabolite production, its control is required if the highest production yields are sought. In this research effort, a simple system aimed at controlling light intensity in a microalgal bioreactor was designed and tested; said system permitted maximum

References (18)

A.S. Mirón et al.
Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae
J Biotechnol
(1999)
J.C. Ogbonna et al.
An integrated solar and artificial light system for internal illumination of photobioreactors
J Biotechnol
(1999)
G. Lepage et al.
Improved recovery of fatty acid through direct transesterification without prior extraction or purification
J Lipid Res
(1984)
P.A. Thompson et al.
Nutritional value diets that vary in fatty acid composition for larval Pacific oyster (Crassostres gigas)
Aquaculture
(1996)
J.K. Volkman et al.
Fatty-acids from microalgae of the genus Pavlova
Phytochemistry
(1991)
A. Reis et al.
Eicosapentaenoic acid-rich biomass production by the microalga Phaeodactylum tricornutum in a continuous-flow reactor
Biores Technol
(1996)
H. Tatsuzawa et al.
Changes in lipid and fatty acid composition of Pavlova lutheri
Phytochemistry
(1995)
K. Yamaguchi
Recent advances in microalgal bioscience in Japan, with special reference to utilization of biomass and metabolites: a review
J Appl Phycol
(1996)
M.A. Borowitzka
Closed algal photobioreactors: design considerations for large-scale systems
J Mar Biotech
(1996)

There are more references available in the full text version of this article.

Cited by (16)

Light map optimization via direct chlorophyll fluorescence imaging in algal photobioreactors
2023, Algal Research
Microalgae are photosynthetic organisms that can be grown in photobioreactors (PBRs). The light placement within the PBR is crucial to optimize growth. Currently, the light path is often estimated from various mathematical models, which may result in less efficient placements of lights. Here we describe a novel fluorescence imaging technique to map the light within a bioreactor. These light maps describe the photosynthetic photon flux density (PPFR) inside the PBR, resulting in a precise light pattern of the LEDs and potential light reflections of the reactor walls. Furthermore, when compared with conventional imaging measurements that uses visible light, the fluorescence imaging technique has the advantage that only fluorescence emissions of microalgae are recorded. Regions of the PBR that show fluorescence emission can be assumed to have photosynthetic activity with sufficient irradiance of the specific photosynthetic active radiation (PAR) wavelengths. This method is adaptable to complex photobioreactor geometries and could be used to identify light-saturated areas. Two microalgae species and three colours of LEDs (red, blue, and white) were analysed in regards to light penetration and light distribution at different cell concentrations.
Scale-up of photo-bioreactors for microalgae cultivation by π-theorem
2020, Biochemical Engineering Journal
Robust microalgae cultivation at industrial scale needs complex scale-up of photobioreactors since the same yields obtained at lab-scale are hardly reached during pilot or industrial production. In this paper we propose a procedure based on Buckingham π-theorem to perform the scale-up of photobioreactors used to cultivate Chlorella Vulgaris fed by CO₂ and wastewater rich in glycerol. This theoretical approach is usually overlooked in favor of the synergy of modeling tailored on the process and piloting, but it grants excellent generalizability. An experimental campaign at three levels was designed and carried out to evaluate the characteristic dimensionless numbers individuated by the theoretical formulation. Since scale-up regards both geometrical dimensions and type of reactor, passing from lab-scale stirred tanks (STRs) to pilot scale tubular and airlift, particular attention was devoted to defining characteristic lengths inside the dimensionless numbers. Moreover, since scale-up also regards the operating mode, scaling from discontinuous to semi-continuous to continuous, some interesting dimensionless numbers arise other than $R e$ , $S h$ , ${D a}_{I I}$ . They are mainly related to the type of biological process and its operating mode and are the ratios O₂/CO₂ and T/T_opt, the ratio between the incident light intensity and the saturation constant, the absorbance, the ratio between the final and the initial concentration c/c₀, the ratio between the maximum increase in cell population and its initial concentration, the ratio between the estimated specific kinetic constant and a variable representing the characteristic time of mixing inside the chosen reactor.
Preliminary outdoor tests confirmed the operability of the scaled-up airlift reactors reaching c/c₀ = 5.3–7.5, with c = 1.15 g L⁻¹ at extraction intervals of 5 days. They were operated under optimal light conditions of incident light greater than Chlorella v. saturation constant and absorbance = 40 and at incipient churn flow (Re = 10000–11000) with dimension of fluegas bubbles around 5 mm, apt to guarantee Sh = 1500–2500; and with a calculated Da_II around 25.
Continuous cultivation of photosynthetic microorganisms: Approaches, applications and future trends
2015, Biotechnology Advances
Citation Excerpt :
The pH control through the injection of CO2 is the most common method, mainly because it is also a source of inorganic carbon. Generally, biomass feedback controls are applied on batch or semi-continuous cultivation systems (Meireles et al., 2008; Sandnes et al., 2006), while feedforward controls (that use biomass growing models) are proposed for continuous systems (Baquerisse et al., 1999; Becerra-Celis et al., 2008; Quinn et al., 2011). The main strategies for control of biomass concentration developed are feedback control based in spectrophotometer techniques (Meireles et al., 2008; Sandnes et al., 2006), feedforward control based in the inflow rate (Abdollahi and Dubljevic, 2012; Becerra-Celis et al., 2008) or a joint control (feedback and feedforward) using CO2 gas flow rate accordingly with the light intensity regime (Buehner et al., 2009).
The possibility of using photosynthetic microorganisms, such as cyanobacteria and microalgae, for converting light and carbon dioxide into valuable biochemical products has raised the need for new cost-efficient processes ensuring a constant product quality. Food, feed, biofuels, cosmetics and pharmaceutics are among the sectors that can profit from the application of photosynthetic microorganisms.
Biomass growth in a photobioreactor is a complex process influenced by multiple parameters, such as photosynthetic light capture and attenuation, nutrient uptake, photobioreactor hydrodynamics and gas–liquid mass transfer.
In order to optimize productivity while keeping a standard product quality, a permanent control of the main cultivation parameters is necessary, where the continuous cultivation has shown to be the best option. However it is of utmost importance to recognize the singularity of continuous cultivation of cyanobacteria and microalgae due to their dependence on light availability and intensity.
In this sense, this review provides comprehensive information on recent breakthroughs and possible future trends regarding technological and process improvements in continuous cultivation systems of microalgae and cyanobacteria, that will directly affect cost-effectiveness and product quality standardization. An overview of the various applications, techniques and equipment (with special emphasis on photobioreactors) in continuous cultivation of microalgae and cyanobacteria are presented. Additionally, mathematical modeling, feasibility, economics as well as the applicability of continuous cultivation into large-scale operation, are discussed.
The effects of illumination factors on the growth and HCO<inf>3</inf><sup>-</sup> fixation of microalgae in an experiment culture system
2014, Energy
Citation Excerpt :
For the microalgae cultivation, besides carbon source, light condition is the important and basic energy source for microalgae [13] that affects microalgal photosynthesis kinetics. The light source [14], light–dark cycle [15], illumination interval [16], light intensity [17] and illumination area [18] have effect on the growth and metabolism of microalgae. In Cai ZhuoPing's study, the microalgae were cultivated in a plant growth chamber under the common background conditions of 23 °C with a 12 h light/12 h dark cycle provided by cool-white fluorescent light [19].
Illumination factors can affect growth of microalgae and HCO₃⁻ fixation. In order to optimize microalgae growth in culture system and HCO₃⁻ fixation from wastewater, the effects of light source, light interval on the growth of microalgae, Chlorella sp. and Scenedesmus obliquus sp., were studied in experiment culture system. The effects of light intensity and available illumination area on HCO₃⁻ fixation of microalgae were also considered. Microalgae were grown in a constant light incubator-shaking table experiment system (CLIST system) to assess biomass, pH, DO, and HCO₃⁻ concentration at 30 °C cultivation temperature. The light interval evaluated were 15 min, 1 h, 4 h, 8 h and 12 h, respectively. HCO₃⁻ fixation of microalgae was investigated at five different light intensities (1000, 2000, 3700, 6800 and 8200 lux) and four available illumination areas (45, 80, 120, 200 cm²) in CLIST system. Under the light interval of 12:12, light intensity of 6800 lux and available illumination area of 200 cm², the removal rate of HCO₃⁻ in the water was found to be 65.7%, with a maximum biomass concentration above 1400 mg/L. In this study, 0.953 g HCO₃⁻ in per litre of wastewater would be captured by microalgae, in other word, producing 1.00 g of microalgae biomass fixed 1.10 g of HCO₃⁻.
Fatty acid composition of several wild microalgae and cyanobacteria, with a focus on eicosapentaenoic, docosahexaenoic and α-linolenic acids for eventual dietary uses
2011, Food Research International
Citation Excerpt :
Recall that those microorganisms are the most basic food source in aquaculture, so they are currently used in hatcheries to feed crustaceans and bivalves, or to feed rotifers, copepods and brine shrimps — which will eventually be used to feed fish intended for human consumption. Although the fatty acid overall composition of a few microalgae and cyanobacteria is reasonably well documented, several native strains are still lacking trustworthy data possessing sufficient detail (Chuecas & Riley, 1969; Dunstan, Volkman, Barrett, & Garland, 1993) — with a few notable, yet topical exceptions (e.g. Carvalho & Malcata, 2000; Meireles et al., 2007). Our work has attempted to fill some of the aforementioned gaps of knowledge, via systematic description of the fatty acid profile — with a particular focus on total ALA, EPA and DHA contents, of 30 autochthonous strains of cyanobacteria and microalgae.
A total of 13 species of microalgae and 14 strains of cyanobacteria, collected directly in the Portuguese coast and lagoons, were characterized for their fatty acid contents, focusing on two with a market potential — i.e. eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA); and another already with alternative (yet somehow more expensive) natural sources — i.e. α-linolenic (ALA) acid. The purpose of this work was their eventual inclusion as additives in food or feed. ALA was the most abundant PUFA in Nannochloropsis sp. (0.616 ± 0.081 mg_FA.L_culture^− 1.d^− 1), and EPA in Phaeodactylum tricornutum (0.148 ± 0.013 mg_FA.L_culture^− 1.d^− 1); Pavlova lutheri was particularly rich in EPA (0.290 ± 0.005 mg_FA.L_culture^− 1.d^− 1) and DHA (0.140 ± 0.037 mg_FA.L_culture^− 1.d^− 1). Despite several previous reports on similar topics and encompassing some of our microalgal species, the wild nature of our strains accounts for the novelty of this work — in addition to the characterization of a few wild cyanobacteria. Eustigmatophyceae class was the best producer of ALA, while Prymnesiophyceae was the best for EPA and ALA. Nodularia harveyana exhibited the highest ALA level (0.611 ± 0.022 mg_FA.L_culture^− 1.d^− 1) and Gloeothece sp. was highest in EPA (0.030 ± 0.004 mg_FA.L_culture^− 1.d^− 1).
Development of a flow-through microscopic multitesting system for parallel monitoring of cell samples in biotechnological cultivation processes
2010, Journal of Biotechnology
The automated monitoring of cell variables in cultivation processes is a key technology in modern bioscience. In this article an innovative analytical tool for monitoring of cell densities in shaking-flask cultivations – consisting of a flow-through microscope with an automated image analysis software integrated in a FIA sampling system – is presented. This atline multitesting system was optimized by varying the height of the microscopes sampling zone. A calibration of the system was performed by correlating the FIA result peaks to known concentrations of Baker's yeast. It was successfully applied in cell density monitoring of cultivation processes of Saccharomyces cerevisiae with a good correlation with offline methods, and further used to monitor 3 parallel cultivations. This methodology was successfully transferred to Bacillus megaterium cultures and applied to measure the cell densities of parallel cultivation setups of B. megaterium and S. cerevisiae.

View all citing articles on Scopus

View full text

Article preview

Enzyme and Microbial Technology

Abstract

Introduction

Section snippets

Culture conditions

Results

Discussion

Conclusions

References (18)

Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae

J Biotechnol

An integrated solar and artificial light system for internal illumination of photobioreactors

J Biotechnol

Improved recovery of fatty acid through direct transesterification without prior extraction or purification

J Lipid Res

Nutritional value diets that vary in fatty acid composition for larval Pacific oyster (Crassostres gigas)

Aquaculture

Fatty-acids from microalgae of the genus Pavlova

Phytochemistry

Eicosapentaenoic acid-rich biomass production by the microalga Phaeodactylum tricornutum in a continuous-flow reactor

Biores Technol

Changes in lipid and fatty acid composition of Pavlova lutheri

Phytochemistry

Recent advances in microalgal bioscience in Japan, with special reference to utilization of biomass and metabolites: a review

J Appl Phycol

Closed algal photobioreactors: design considerations for large-scale systems

J Mar Biotech

Cited by (16)

Light map optimization via direct chlorophyll fluorescence imaging in algal photobioreactors

Scale-up of photo-bioreactors for microalgae cultivation by π-theorem

Continuous cultivation of photosynthetic microorganisms: Approaches, applications and future trends

The effects of illumination factors on the growth and HCO<inf>3</inf><sup>-</sup> fixation of microalgae in an experiment culture system

Fatty acid composition of several wild microalgae and cyanobacteria, with a focus on eicosapentaenoic, docosahexaenoic and α-linolenic acids for eventual dietary uses

Development of a flow-through microscopic multitesting system for parallel monitoring of cell samples in biotechnological cultivation processes