Appl Microbiol Biotechnol (2001) 57:287–293

DOI 10.1007/s002530100702

MINI-REVIEW

O. Pulz

Photobioreactors: production systems for phototrophic microorganisms

Received: 18 January 2001 / Received revision: 17 April 2001 / Accepted: 18 April 2001 / Published online: 22 August 2001
© Springer-Verlag 2001

Abstract Microalgae have a large biotechnological These systems have to be evaluated in their various
potential for producing valuable substances for the feed, configuration concepts regarding their potential produc-
food, cosmetics and pharmacy industries as well as for tivity and economic feasibility. The most important and
biotechnological processes. The design of the technical most obvious differences in microalgal production
and technological basis for photobioreactors is the most systems are the exposure of the microalgal culture to the
important issue for economic success in the field of environment.
phototrophic biotechnology. For future applications, Open systems can be divided into natural waters
open pond systems for large-scale production seem to (lakes, lagoons, ponds) and artificial ponds or containers,
have a lower innovative potential than closed systems. erected in very different ways. Regarding the technical
For high-value products in particular, closed systems of complexity, open systems such as the widespread raceway
photobioreactors seem to be the more promising field for ponds may vary considerably, but they are still much
technical developments despite very different approaches simpler than more recent closed systems for the cultivation
in design. of microalgae. Most of these closed systems consist of
tubular photobioreactors with tubes of various shapes,
sizes, and length as well as the transparent materials
Introduction used. In all cases the biotechnological solutions for
optimum growth, with the main factors being light and
Both macro- and microalgae have an important role to turbulence, are key issues for success (Tredici 1999).
play in the current world economy with an approximate
turnover of US$ 5 billion per year. Microalgae are
unique and valuable microorganisms in various respects Photobioreactor design
because they are the initial biological CO2/O2 exchangers
on this planet, the most important primary producer of Microalgae are found growing within nearly every biotope
biomass, and one of the most variable ecological groups because of their ecological diversity and their physiologi-
of organisms. For microalgae in particular, recent bio- cal adaptability. This diversity explains why almost
technological and technical advances call for a reappraisal everybody who is active in microalgal cultivation has
of their possible future contribution to developments in their own technical solutions in mind for mass cultivation.
such areas as food, cosmetics, pharmaceuticals, feed, Within this multitude of technical solutions one can
agriculture, aquaculture and the environment (Soeder basically distinguish between open ponds or reactors,
1986; Borowitzka and Borowitzka 1988; Borowitzka which are open to air, and closed systems (Table 1). In
1992; Sirenko and Pulz 2000; Tsoglin and Gabel 2000). all cases the main efforts have to be made to introduce
Therefore, the biotechnological basis for the most efficient light energy into the dispersion of microalgae (Ogbonna
production of microalgal biomass is a key issue for the and Tanaka 2000; Pulz and Scheibenbogen 1998).
future impact of these organisms.
Technical systems for the production of phototrophic
microorganisms are termed photobioreactors (PBR). Open systems
O. Pulz (✉) Open ponds resemble most closely the natural milieu of
IGV Institute for Cereal Processing, Arthur-Scheunert-Allee 40/41,
14558 Bergholz-Rehbrücke, Germany microalgae. Despite a certain variability in shape, the
e-mail: pulz@igv-gmbh.de most common technical design for open pond systems
Tel.: +49-33200-89151, Fax: +49-33200-89220 are raceway cultivators driven by paddle wheels and
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Fig. 1 Typical open pond
production site using a raceway
arrangement

Table 1 Systematics of
cultivation equipment Basic type Technical variables

1. Open system
Cuvette, container, stirred vessel Material (glass, plastics)
Natural water Turbulence development (pumping, stirring)
Raceway pond Flow direction (horizontal/vertical)
Inclined surface device Surface to volume-ratio
2. Closed system
Plastic sleeves O2 removal > CO2 input
Fermenter-like tank Type and duration of illumination
Tubular PBR Temperature control
Laminar PBR Sterilization

usually operating at water depths of 15–20 cm (Fig. 1). Closed systems

At these water depths, biomass concentrations of up to
1,000 mg/l and productivities of 60–100 mg/(l day–1), Closed PBRs are characterized by the regulation and
i.e., 10–25 g/(m2 day–1), is possible. Similar in design control of nearly all the biotechnologically important
are the circular ponds which are common in Asia and the parameters as well as by the following fundamental
Ukraine (Becker 1994). benefits (Pulz 1992): a reduced contamination risk,
Significant evaporative losses, the diffusion of CO2 to no CO2 losses, reproducible cultivation conditions,
the atmosphere. as well as the permanent threat of controllable hydrodynamics and temperature, and flexible
contamination and pollution, are the major drawbacks of technical design.
open pond systems. Also, the large area required must The scale-up of simple closed container-based
not be underestimated. The main disadvantage of this systems (tanks, hanging plastic bags) as a first generation
principle in terms of productivity seems to be the light of closed PBRs was soon faced with serious limitations
limitation in the high layer thickness. Technically it is because at a volume of 50–100 l it is no longer possible
possible to enhance light supply by reducing the layer to effectively introduce the light energy required for
thickness to a few centimeters or even millimeters, using successful biomass development. Several technical
thin layer inclined types of culture systems. Another approaches to underwater lighting, for instance with
problem of open systems is the maintenance of the submersed lamps or light diffusing optical fibers on the
desired microalgal population, which is possible only for one hand, or with pillar-shaped photobioreactors on the
extremophilic species and even there some contamina- other hand, have been tried, but have not been successful
tion risks remain. in application (Gerbsch et al. 2000; Semenenko et al.
Until recently, open systems were the most important 1992). However, this principle seems to be of future
design principle for microalgal production (Richmond relevance only for the aquaculture of certain selected
1990). However, the preparation of high-value products species.
from microalgae for applications in pharmacy and Closed photobioreactors (Fig. 2) are currently tested for
cosmetics appears to be feasible only on the basis of closed microalgal mass cultures in the following configurations:
photobioreactors with the ability to reproduce production (1) tubular systems (glass, plastic, bags), (2) flattened,
conditions and to be GMP-relevant (GMP: good manu- plate-type systems, and (3) ultrathin immobilized configu-
facturing practice following ISO and EC guidelines). rations. Vertical arrangements of horizontal running
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tubes or plates seem to be preferred for reasons of light

distribution and appropriate flow.
Since about the 1990s, parameters such as species-
efficient light incidence into the photobioreactor lumen,
light path, layer thickness, turbulence and O2 release
from the total system volume have gained in importance
(Table 2). Closed or almost closed systems based on very
different design concepts have already been implemented
and tested up to pilot scale. The latest developments
seem to be directed toward photobioreactors of a tubular
configuration or of the compact-plate type as well as
combinations of these main design principles in order to
distribute the light over an enlarged surface area (Tredici
and Materassi 1992; Gabel and Tsoglin 2000).
Assuming that light for photosynthesis should be
continuously available to the receptor in the microalgal
cell, a lamination or other enlargement of the reactor
surface directed toward the light source seems to be the
prime aim. For microalgae this idea may include an
appropriate lowering of net light energy supply for the
suspension because of the significantly lower level of
light saturation needed for these organisms. The basic
principle of the laminar concept for thin layer plate or
thin diameter tube photobioreactors mimics the leaves of
higher plants. For instance a 100-year-old,10 m high
lime tree shading an area of 100 m2 has a leaf surface
area of more than 2,500 m2. Expressed as a surface:
volume ratio this amounts to a value of 2.5 m2/m3.
On the basis of these considerations, the trend toward
developing closed photobioreactors as already described
Fig. 2 Closed plate type photobioreactor fed with high CO2 levels
from a lime production plant in central Germany is paralleled by conceptions of the use of relatively thin
light-exposed reactor lumina. The tube diameter in tubular
photobioreactors is reduced significantly and laminar,

Table 2 Advantages and disadvantages of open and closed algal cultivation plants

Parameter Open ponds (raceway ponds) Closed systems (PBR systems)

Contamination risk Extremely high Low

Space required High Low
Water losses Extremely high Almost none
CO2-losses High Almost none
Biomass quality Not susceptible Susceptible
Variability as to cultivatable Not given, cultivation possibilities High, nearly all microalgal varieties
species are restricted to a few algal varieties may be cultivated
Flexibility of production Change of production between Change of production without
the possible varieties nearly impossible any problems
Reproducibility of production Not given, dependent on exterior conditions Possible within certain tolerances
parameters
Process control Not given Given
Standardization Not possible Possible
Weather dependence Absolute, production impossible during rain Insignificant, because closed configurations
allow production also during bad weather
Period until net production is reached Long, approx. 6–8 weeks Relatively short, approx. 2–4 weeks
after start or interruptions
Biomass concentration during production Low, approx. 0.1–0.2 g/l High, approx. 2–8 g/l
Efficiency of treatment processes Low, time-consuming, large volume flows due High, short-time,
to low concentrations relatively small volume flows
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Table 3 Basic values of various cultivation plants (data obtained from IGV: German cultivation sites at natural illumination)

Unit Raceway Incl. Surface type Tubular Plate

Open pond, high Open pond, low
layer thickness layer thickness Semi-closed Semi-closed
tubular system plate system

Illuminated surface m2 500 200 600 500

Total volume m3 75 5 7 6
Space required m2 550 250 110 100
Layer thickness cm 10–30 0.5–1 4 3
Flow rate cm s–1 30–55 30–45 50–60 120
Biomass conc.(DW) mg l–1 300–500 3,000–6,500 5,000–8,000 5,000–8,000
Productivity (DW) g l–1 d–1 0.05–0.1 0.8–1.0 0.8–1.2 0.8–1.3

plate-type configurations are strongly favored. The tubular proportional to light intensity, until at high illumination
or pipe design principle is the most important basis of intensities damage to the photosynthetic receptor system
completely or partially closed cultivators in plastic ducts occurs within a few minutes (photoinhibition). In most
and especially in glass or plastic tubes. The development microalgae, photosynthesis is saturated at about 30% of the
of closed photobioreactors, which had intensified by the total terrestrial solar radiation, i.e. 1,700–2,000 µE/(m2 s).
end of the 1980s, seems to be of significant future Some picoplankton species grow with optimal rates at
importance. Compared with laminar, plate-type systems, 50 µE/(m2 s) and are photoinhibited at 130 µE/(m2 s).
the tubular systems seem to have identical configuration Phanerophytes, like most agricultural crops with light
potentials, especially in cases of vertical packing of limitation values of 900 µE/(m2 s), are clearly adapted to
horizontally oriented tubes (Broneske et al. 2000; Molina higher PFD than microalgae are.
Grima et al. 2000; Table 3). Stirred fermenters illuminated with various submersed
luminous elements or light pipes facilitate an average
productivity in the range of 100–1,000 mg DW/(1 day).
Biotechnological problems and preconditions This appears to be the upper limit at a surface :volume
for PBR design ratio of 2–8 m2/m3 typical for this illumination design.
Laboratory bioreactors based on this principle are very
The normal, i.e., natural life conditions of microalgae well suited to physiological and autecological studies as
which are subject to biotechnological research, are as well as for the establishment and testing of miniature
follows: maximum cell densities of 103 cells/ml, average ecosystems, but they cannot be used for scaling up. In
distance between cells of 1,350 µm or 250 times the cell tubular or plate-type photobioreactors, surface:volume
diameter, vertical or horizontal displacements of 5×10–3 ratios of 20–80 m2/m3 and light incidence values (PAR)
to 3×10–5 m/s, photon flux density (PFD) usually well up to 1,150 µE/(m2 s) are achieved. At a layer thickness
within the light limited area, light supply subject to of up to 5 mm, a productivity of 2–5 g DW/(l day) can
daytime rhythm, CO2 and nutrient conditions, generally be achieved (Chini Zitelli et al. 2000).
far from optimal, and prolonged stability of pH value, Despite growing interest in recent years, there are
ion concentrations and temperature. only a few references in the literature regarding the
In contrast, for cultivation systems, we have to apply short-term processes of photoadaptation, on light inhibition
very different conditions, namely: cell densities up to 108 or saturation effects in closed photobioreactors. Photo-
cells/ml, average distance between cells reduced to adaptation requires at least 10–40 min, which can explain
60 µm or 10 times the cell diameter, spatial displacements the discrepancy between the productivity of open-air
ranging from 0.3 to 1.2 m/s, turbulence-conditioned PFD algal cultures and their light optimum. Photoinhibitory
variations with frequencies of 0.1–1,000 s superseding processes, too, are time-dependent; however, in this case
the daytime rhythm, generally optimum or surplus nutrient irreversible destruction is supposed to occur even after
and CO2 supply, pH values and temperatures undergoing only a few minutes of light stress, exceeding 50% damage
completely nonphysiological variations, and nearly after 10–20 min.
continuous mechanical stress on the cell walls and the
cells themselves (Tredici 1999).
CO2/O2 balance

Light energy For a high photosynthesis rate, the CO2/O2-balance has

to be adjusted in a way that the prime carboxylating
Light as the energy source for photoautotrophic life is enzyme, Rubisco, furnishes CO2 for the Calvin cycle but
the principal limiting factor in photobiotechnology (Kirk does not use O2 for photorespiration. Hence, in algal
1994). At illumination intensities above the light cultures of high cell densities, sufficient CO2 must be
compensation point the rate of photosynthesis is directly available, while evolved O2 has to be removed before
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Temperature

Temperature influences respiration and photorespiration

more strongly than photosynthesis. When CO2 or light is
limiting for photosynthesis, the influence of temperature
is insignificant. With an increase in temperature, respira-
tion will rise significantly, but the flux through the
Calvin cycle increases only marginally. Thus, the net
efficiency of photosynthesis declines at high temperatures.
This effect can worsen in suspension cultures by the
difference in decrease of CO2 and O2 solubility at elevated
temperatures.

Salinity, nutrients, and pH-value

A sufficient nutrient supply for microalgae is a precondi-

Fig. 3 Small size commercial photobioreactor of 25 l with tubular, tion for optimal photosynthesis. Deficiencies will cause
sterilizable design disturbances in metabolism and disproportionate produc-
tion of intermediates of photosynthesis. Deviations from
the optimum pH, osmotical conditions and salinity will
reaching inhibitory concentrations. The complete avoid- cause physiological reactions and productivity problems.
ance of photorespiration, however, remains unsolved. Therefore, these easily controllable conditions should
Oxygen may become a problem in algal cultures of be maintained in optimum ratios in photobioreactors.
high cell densities not only because of the limitation of Organic carbon sources seem to be important for some
the rate of photosynthesis. In cultures of microalgae mixotrophic or even pure heterotrophic microalgal biomass
optimally supplied with CO2 the O2 production can easily production systems. Therefore, organic wastes as well as
reach concentrations of up to 40 mg/l. Upon radiation pure simple organic substances like acetic acid or various
with appropriate energy, oxygen radicals may develop sugars have been investigated for their possible use in
during the respiratory gas exchange and cause toxic microalgal production.
effects on cells due to membrane damage. Superoxide
dismutases and other O2 radical neutralizing systems may
have a protective effect. In high-cell-density microalgal Turbulence
cultures with optimum growth, species-specific O2 evolu-
tion rates between 28 and 120 mg O2/(g DW h–1) were Photoautotrophic nano- and microplankters live in their
recorded. Many algal strains cannot survive in significantly natural environment at a density of 103 cells/ml and at
O2-oversaturated milieu longer than 2–3 h. High tempera- distances of more than 1,000 µm between cells. Thus,
tures and PFDs, combined with CO2 limitation, will in high-cell-density microalgal cultures of up to 109
intensify the physiological inhibitory processes (Fig. 3). cells/ml, the natural conditions are not suitable for
CO2-concentrations usually have to be kept within high productivity (Borowitzka and Borowitzka 1988;
narrow margins. While a 0.03% CO2 content in air is Grobbelaar 2000).
suboptimal for plant growth, most plants will tolerate
CO2 concentrations only up to 0.1%. However, for many
strains of microalgae it was observed that they tolerate Photobioreactors of pilot to industrial scale
up to 12% CO2 at a temperature of 35°C. At present,
partial pressure of O2 (pO2) in microalgal suspensions For aquaculture applications, many taxonomically very
inside both open and closed photobioreactors may be different species of microalgae are produced, mostly
reduced only by (1) increasing turbulence, and (2) O2 locally. Both advanced open systems in the form of plastic
stripping with air. Both approaches imply an “unsolved sleeves, plastic pillars and containers are used, in many
dilemma” in the reactor system: Although an intensive cases as one-way systems. But because of the high costs
search for membranes suitable for gas exchange is of these systems combined with poor productivity,
underway, no breakthrough has yet been reported. At closed systems like tubular PBR (Biocoil, Biofence,
present, sufficient CO2 transitions on membranes can be ultrathin sheets) are gaining economic importance
provided only under pressure, otherwise the transition (Richmond 2000).
proceeds slowly along the gradient. Production using conventional fermenters with
An alternative may be the relatively simple injection microalgae-related organisms like Crypthecodinium
of small amounts of pure CO2 into high-cell-density under heterotrophic conditions appears to be of economic
microalgal cultures with biomass concentrations of 8–10 g interest for the production of polyunsaturated fatty acids.
dry weight (DW)/l (Straka et al. 2000). However, this method involves routine techniques of
292

Fig. 4 Natural lake in Myanmar producing Spirulina-biomass

Fig. 5 700 m3 glass tube photobioreactor producing Chlorella

heterotrophic biotechnology and not photobioreactors. biomass
Even today the approximately 5,000–6,000 t/a dry biomass
of microalgae produced worldwide originates mainly closed tubular PBR systems in Germany (Dilow 1985;
from open systems. These open systems (Fig. 4) include Borowitzka and Borowitzka 1988; Apt and Behrens
natural waters as well as unstirred but constructed ponds 1999).
and circular or raceway ponds. After an upscaling period of nearly 3 years, a tubular
Aphanizomenon and Nostoc, two rather uncommon system on an industrial scale was established in the year
genera, are exclusively harvested from natural waters; 2000 near Wolfsburg, Germany (Fig. 5). This closed
Aphanizomenon only in the US for health food, Nostoc system is apparently the largest PBR to have gone into
in many Asian countries for food. successful production. It consists of compact and vertically
The green alga Haematococcus has attained a total of arranged horizontal running glass tubes of a total length
approximately 50 t/a for astaxanthin production using a of 500,000 m and a total PBR volume of 700 m3. In a
combination of closed and open systems. This alga has glasshouse requiring an area of only 10,000 m2 an annual
great potential as a future crop for aquaculture. production of 130–150 tonnes dry biomass was demon-
Dunaliella is produced in natural ponds in Australia strated to be economically feasible under Central European
and in smaller amounts in the Ukraine, in earthwall-lined conditions.
unstirred ponds in Australia and also in raceway-ponds
in Israel and the United States. In all cases carotinoids
are the prime products. Conclusion
In terms of overall production Spirulina (Arthrospira)
seems to be the most important microalgal species and For the mass culture of microalgae, open pond systems
this is produced nearly exclusively in open systems, have mainly been the dominating systems up until now.
though there have been some efforts to establish a However, closed systems of light-distributing tube or
commercial Spirulina culture under temperate climatic plate design, known as photobioreactors, are now
conditions (Brouers 2000). The most famous, nearly increasingly finding new applications both for high-
natural open pond production facility was Lake Texcoco value products in pharmacy and cosmetics as well as for
in Mexico with considerable Spirulina production in the aqua- and agricultural uses.
past. This facility was closed in 1994/95 because of
problems with contamination and pollution. In recent
years crater lakes in Myanmar (Burma) have gained References
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