Enclosed bioreactors for the mass cultivation of photosynthetic microorganisms: the future trend

The Effects of Photobioreactor Type on Biomass and Lipid Production of the Green Microalga Monoraphidium pusillum in Laboratory Scale

Mass production of microorganisms, algae among them, for new bioactive compounds and renewable innovative products is a current issue in biotechnology. The greatest challenge of basic research on this topic is to find the best solution for both physiology and scalability. In this study, the main goal was to highlight the contradictions of physiological and technological optimization in the same, relatively small, laboratory scale. The green alga Monoraphidium pusillum (Printz) Komárková-Legnorová was cultured in a conventional Erlenmeyer flask (as air bubbled in a tank-type photobioreactor) and in a hybrid (fermenter type + helical tubular type) photobioreactor of the same volume (2.8 L). Higher cell numbers from 1.7–2.3-fold, 2–2.8-fold higher dry masses, and 1.9–2.6-fold higher total lipid contents (mg·L−1) were measured in the tank reactor than in the hybrid reactor. Cultures in the conventional tank reactor were characterized with better nutrient utilization (42.8–77.7% higher phosphate uptake) and more diverse lipid composition than in the hybrid reactor. The study highlights that well-scalable arrangements and settings could be not optimal (or unsuitable in some cases) from a physiological point of view. The results suggest certain developmental directions for complex, well-scalable devices and highlight the importance of testing the gained physiological optima on these systems.

Attached cultivation for improving the biomass productivity of Spirulina platensis

To improve cultivation efficiency for microalgae Spirulina platensis is related to increase its potential use as food source and as an effective alternative for CO2 fixation. The present work attempted to establish a technique, namely attached cultivation, for S. platensis. Laboratory experiments were made firstly to investigate optimal conditions on attached cultivation. The optimal conditions were found: 25 g m(-2) for initial inoculum density using electrostatic flocking cloth as substrata, light intensity lower than 200 μmol m(-2) s(-1), CO2 enriched air flow (0.5%) at a superficial aeration rate of 0.0056 m s(-1) in a NaHCO3-free Zarrouk medium. An outdoor attached cultivation bench-scale bioreactor was built and a 10d culture of S. platensis was carried out with daily harvesting. A high footprint areal biomass productivity of 60 g m(-2) d(-1) was obtained. The nutrition of S. platensis with attached cultivation is identical to that with conventional liquid cultivation.

Cultivation of green alga Botryococcus braunii in raceway, circular ponds under outdoor conditions and its growth, hydrocarbon production

The present study focused on cultivation, seasonal variation in growth, hydrocarbon production, fatty acids profiles of Botryococcus braunii (LB-572 and N-836) in raceway & circular ponds under outdoor conditions. After 18days of cultivation the biomass yield and hydrocarbon contents were increased in both raceway and circular ponds. The fat content was found to be around 24% (w/w) with palmitic and oleic acids as prominent fatty acids. Hydrocarbons of C(20)-C(30) carbon chain length were higher in raceway and circular ponds. Maximum biomass yield (2gL(-1)) and hydrocarbon content (28%) were observed in Nov-Dec. In case of B. braunii (N-836) after 25days of cultivation the biomass yield was 1gL(-1) and hydrocarbon content was 27%. Supplementation of 0.1% NaHCO(3) in the medium resulted in biomass yield of 1.5gL(-1) and hydrocarbon content of 30% compared to control.

A flat inclined modular photobioreactor for outdoor mass cultivation of photoautotrophs

A flat inclined modular photobioreactor (FIMP) for mass cultivation of photoautotrophic microorganisms is described. It consists of flat glass reactors connected in cascade facing the sun with the proper tilt angles to assure maximal exposure to direct beam radiation. The optimal cell density in reference to the length of the reactor light path was evaluated, and the effect of the tilt angle on utilization of both direct beam as well as diffuse sunlight was quantitatively assessed. The mixing mode and extent were also optimized in reference to productivity of biomass. The FIMP proved very successful in supporting continuous cultures of the tested species of photoautotrophs, addressing the major criteria involved in design optimization of photobioreactors: Made of fully transparent glass, inclined toward the sun and endowed with a high surface-to-volume ratio, it combines an optimal light path with a vigorous agitation system. The maximal exposure to the culture to solar irradiance as well as the substantial control of temperature facilitate, under these conditions, a particularly high, extremely light-limited optimal cell density. The integrated effects of these growth conditions resulted in record volumetric and areal output rates of Monodus subterraneus, Anabana siamensis, and Spirulina platensis. (c) 1996 John Wiley & Sons, Inc.

Photosynthetic productivity of conical helical tubular photobioreactor incorporating Chlorella sorokiniana under field conditions

The photosynthetic performance of a conical, helical tubular photobioreactor (HTP) incorporating Chlorella sorokiniana was investigated under conditions of high temperature and light intensity during midsummer in an outdoor environment. Although the culture medium temperature exceeded 40 degrees C for approximately 5 h each day, peaking at 47.5 degrees C under sunny conditions, a photosynthetic productivity of 30.0 g x m(-2) (installation area) x day(-1) and a photosynthetic efficiency of 8.66% [photosynthetically active radiation (PAR), 400-700 nm] were achieved. A maximum photosynthetic productivity of 33.2 g x m(-2) x day(-1) was achieved on a sunny day, when solar energy input was also maximal (11.5 MJ x m(-2) x day(-1) [PAR]). On the other hand, a maximum photosynthetic efficiency of 9.54% was obtained on a day that was rainy in the morning and cloudy in the afternoon, and there was relatively little solar energy input. The average daily photosynthetic efficiency over the two culture periods (August 4 to 7 and August 10 to 13, 1999) was 7.25%. Thus, a high level of photosynthetic performance was achieved in the conical HTP incorporating Chlorella sorokiniana despite the fact that culture medium temperature was not controlled. The use of Chlorella sorokiniana in the conical HTP should be a good choice to produce microalgal biomass during the summer under field conditions.

Evaluation of photobioreactor heat balance for predicting changes in culture medium temperature due to light irradiation

Microalgal photosynthesis requires appropriate culture medium temperatures to achieve high photosynthetic performance and to maintain production of a high-quality biomass product. Enclosed systems, such as our conical, helical tubular photobioreactor (HTP), can accomplish high photosynthetic efficiency and the small amount of culture medium used by these systems means that the culture medium temperature may be effectively controlled. On the other hand, because a high ratio of surface area to culture medium volume leads to rapid heating under the illumination condition and substantial heat loss at night, maintaining a suitable culture medium temperature is necessary to achieve efficient, commercially practical biomass production. In order to predict changes in the culture medium temperature caused by changes in solar irradiance and ambient temperature, it is necessary to understand the heat balance within the photobioreactor. We therefore investigated the heat balance in three major parts (photostage, degasser, and helical heat exchanger) of our conical HTP, analyzed the time-dependent changes in medium temperature at various room temperatures and radiant energy inputs, and predicted changes in the culture medium temperature based on the characteristics of heat transfer among the three parts. Using this model, the predicted changes in culture medium temperature were very similar to the changes observed experimentally in the laboratory and under field conditions. This means that by calculating the time-dependent changes in the culture medium temperature, based on measurements of solar energy input and ambient temperature, we should be able to estimate the energy required to maintain the culture medium temperature within a range where photosynthetic performance of microalgae is high.

Enhancement of microalgal growth by using perfluorocarbon as oxygen carrier

The contribution of green microalgae has been recognized in the production of useful products such as chemicals, fatty acids, proteins, carotenoids, pigments, polysaccharides, and pharmaceuticals. One of the challenges to the development of profitable bioproduct markets is the design, development, modeling, and evaluation of cost-effective production systems. The photobioreactors for microalgae can be either open or closed systems in large-scale production. In various situations, it has been reported that closed photobioreactors offer many advantages over open systems. These advantages include lower contamination, higher biomass densities, and better process control. Nevertheless, for the scale-up of enclosed tubular photobioreactors, the accumulation of oxygen as a photosynthetic byproduct has been a major limitation because it severely inhibits growth of microalgae. In this study, we use the distinctive feature of liquid perfluorocarbons (PFCs) as a carrier that efficiently removes the accumulated oxygen from algal medium phase in the spinner culture flasks. The results show the growth kinetics of microalgae cultivated by this approach is significantly improved.

Photosynthetic production of microalgal biomass in a raceway system under greenhouse conditions in Sendai city

The photosynthetic productivities of the marine microalgae Chlorophyta sp. and freshwater microalgae Chlorella sp. were investigated in a raceway system under greenhouse conditions in Sendai city. The system was constructed with a surface area of 0.986 m2 and equipped with paddle wheels (8 fins). The semicontinuous batch culture experiment was carried out for 4 months from June to October 1998 in Sendai city. The productivity of Chlorophyta sp. was ranged from 4 to 13 g.m(-2).d(-1) and the average was 8.2 g.m(-2).d(-1), which corresponded to the photosynthetic efficiency of 4.15% (PAR). The productivity of Chlorella sp. was ranged from 7 to 21 g.m(-2).d(-1) and the average was 13.2 g.m(-2).d(-1), which corresponded to the photosynthetic efficiency of 6.56% (PAR). These results indicate that stable microalgal cultivation with comparatively high photosynthetic efficiency could be obtained in a raceway system under greenhouse conditions in Sendai city located in the northern part of Japan.

Prediction of dissolved oxygen and carbon dioxide concentration profiles in tubular photobioreactors for microalgal culture

A model is developed for prediction of axial concentration profiles of dissolved oxygen and carbon dioxide in tubular photobioreactors used for culturing microalgae. Experimental data are used to verify the model for continuous outdoor culture of Porphyridium cruentum grown in a 200-L reactor with 100-m long tubular solar receiver. The culture was carried out at a dilution rate of 0.05 h-1 applied only during a 10-h daylight period. The quasi-steady state biomass concentration achieved was 3.0 g. L-1, corresponding to a biomass productivity of 1.5 g. L-1. d-1. The model could predict the dissolved oxygen level in both gas disengagement zone of the reactor and at the end of the loop, the exhaust gas composition, the amount of carbon dioxide injected, and the pH of the culture at each hour. In predicting the various parameters, the model took into account the length of the solar receiver tube, the rate of photosynthesis, the velocity of flow, the degree of mixing, and gas-liquid mass transfer. Because the model simulated the system behavior as a function of tube length and operational variables (superficial gas velocity in the riser, composition of carbon dioxide in the gas injected in the solar receiver and its injection rate), it could potentially be applied to rational design and scale-up of photobioreactors. Copyright 1999 John Wiley & Sons, Inc.

Vertical thin-layer photoreactor for controlled cultivation of cyanobacteria

Nostoc sp. was cultivated in an air-lift reactor with continuous recirculation of the head gas phase that aerated and agitated the cyanobacterial suspension at regulated flow rates. The supply of inorganic carbon for growth was coupled with pH control, in the range of 7.7 to 8.1, by intermittent sparging of CO2-head gas mixtures. The formation of irregular bubbles with swirling motion at the photostage of the reactor promoted efficient CO2 transference in dense populations of Nostoc sp. (1.1 g/l) when bubbling at flow rates of 10 l/min. Biomass productivity was almost six-fold higher in the photoreactor (16.4 mg/l.h) than in a conventional system (2.8 mg/l.h). The exponential growth phase of cultures in the photoreactor amounted to 60% of the total growth period.

Recent advances in microalgal biotechnology

In the last ten years, several reviews (Benemann et al, 1987; Goldman, 1979a; Richmond, 1986b; Soeder, 1980) and books (Borowitzka and Borowitzka, 1988b; Richmond, 1986a; Lembi and Waaland, 1988) have been published on the historical background of mass cultivation of microalgal biomass, and its possible commercial applications. This review presents a brief description of the concept of microalgal biotechnology, and describes some of the recent developments, mainly in the application and commercial development of this relatively new biotechnology, Finally, an attempt is made to indicate those areas where current research and development are paving the way for future applications.