Microalgae biotechnology

Substrate and nutrient manipulation during continuous cultivation of extremophilic algae, Galdieria spp. RTK 37.1, substantially impacts biomass productivity and composition

The extremophilic nature and metabolic flexibility of Galdieria spp. highlights their potential for biotechnological application. However, limited research into continuous cultivation of Galdieria spp. has slowed progress towards the commercialization of these algae. The objective of this research was to investigate biomass productivity and growth yields during continuous photoautotrophic, mixotrophic and heterotrophic cultivation of Galdieria sp. RTK371; a strain recently isolated from within the Taupō Volcanic Zone in Aotearoa-New Zealand. Results indicate Galdieria sp. RTK371 grows optimally at pH 2.5 under warm white LED illumination. Photosynthetic O2 production was dependent on lighting intensity with a maximal value of (133.5 ± 12.1 nmol O2 mgbiomass -1 h-1) achieved under 100 μmol m-2 s-1 illumination. O2 production rates slowed significantly to 42 ± 1 and <0.01 nmol O2 mgbiomass -1 h-1 during mixotrophic and heterotrophic growth regimes respectively. Stable, long-term chemostat growth of Galdieria sp. RTK371 was achieved during photoautotrophic, mixotrophic and heterotrophic growth regimes. During periods of ammonium limitation, Galdieria sp. RTK371 increased its intracellular carbohydrate content (up to 37% w/w). In contrast, biomass grown in ammonium excess was composed of up to 65% protein (w/w). Results from this study demonstrate that the growth of Galdieria sp. RTK371 can be manipulated during continuous cultivation to obtain desired biomass and product yields over long cultivation periods.

Reformation of dairy effluent-a phycoremediation approach

Microalgae are a unique renewable resource utilized since ages, serving as a reservoir for the production of various metabolites. In this study, dairy waste water (DWW) is used as the nutrient media for the cultivation of microalgae. This study focuses on the phycoremediation process of converting rich nutrients in the effluent into biomass and removing contaminants using microalgae. The specific growth rate reached the maximum of 0.55 day^-1 in Desmococcus olivaceous, followed by 0.39 day^-1 for Scenedesmus dimorphus, 0.23 day^-1 in DCS (consortia composing all three strains in equal ratio), and lastly 0.22 day^-1 in Chlorella vulgaris. The biomass productivity was 1.44 g L^-1 day^-1, 1.06 g L^-1 day^-1, 0.88 g L^-1 day^-1, and 0.65 g L^-1 day^-1 in D. olivaceous, S. dimorphus, C. vulgaris, and DCS, respectively. The COD and BOD removal percentage was 82.85% and 45.40% in D. olivaceous, 81.98% and 44.25% in C. vulgaris, 80.73% and 53.45% in S. dimorphus, and 80.10% and 43.10% in DCS, respectively. These results emphasize the promising role of algae in dairy effluent treatment, highlighting the effluent as a suitable medium for microalgae cultivation. It verifies the circular bio-economy concept where the treated wastewater is converted into value-added products.

Fourth generation biofuel from genetically modified algal biomass for bioeconomic development

Biofuels from microalgae have promising potential for a sustainable bioeconomy. Algal strains' oil content and biomass yield are the most influential cost drivers in the fourth generation biofuel (FGB) production. Genetic modification is the key to improving oil accumulation and biomass yield, consequently developing the bioeconomy. This paper discusses current practices, new insights, and emerging trends in genetic modification and their bioeconomic impact on FGB production. It was demonstrated that enhancing the oil and biomass yield could significantly improve the probability of economic success and the net present value of the FGB production process. The techno-economic and socioeconomic burden of using genetically modified (GM) strains and the preventive control strategies on the bioeconomy of FGB production is reviewed. It is shown that the fully lined open raceway pond could cost up to 25% more than unlined ponds. The cost of a plastic hoop air-supported greenhouse covering cultivation ponds is estimated to be US 60,000$ /ha. The competitiveness and profitability of large-scale cultivation of GM biomass are significantly locked to techno-economic and socioeconomic drivers. Nonetheless, it necessitates further research and careful long-term follow-up studies to understand the mechanism that affects these parameters the most.

Influence of microalgae wastewater treatment culturing conditions on forward osmosis concentration process

Forward osmosis is envisioned as a technology for microalgae concentration but fouling propensity during dewatering is currently a limiting factor that requires better understanding. The purpose of this study is to define the impact of microalgae culturing conditions on the downstream forward osmosis (FO) separation process-water recovery and microalgae harvesting. Chlorella vulgaris was cultivated in an outdoor lab-scale reactor fed with synthetic wastewater mimicking primary settled municipal influent under changing environmental conditions (temperature, solar radiation, nutrient balance) with varying hydraulic retention time. High efficiency of nutrient removal was achieved under all tested conditions but microalgae autoflocculation and lower rate of pollutant removal were observed with batches where culturing temperature (6.5-21 ^°C), solar irradiation rate (181 W/m²), and nitrogen/phosphorous ratio (2.9) were below the optimal range. Regarding FO concentration, high initial water fluxes (in the range of 18.2 to 19.5 L·m²·h^-1) and water extraction rate (60.1-83.9%) were observed in all subsequent FO concentration tests. Significant membrane fouling (microalgae deposition on surface) associated with poor biomass recovery from the FO cell was found to be dependent on exopolymeric substance accumulation, which was a response to non-optimal environmental culturing conditions.

Operational and economic aspects of Spirulina-based biorefinery

Microalgae biorefinery systems have been extensively studied from the perspective of resources, energy expenditure, biofuel production potential, and high-added value products. The genus Spirulina (Arthrospira) stands out among the microalgae of commercial importance. It accounts for over 30% of biomass produced globally because of high protein concentration and, carotenoid and phycocyanin content. Spirulina cultivation can be used to reduce greenhouse gases and for effluent treatment. Furthermore, its cellular morphology facilitates biomass recovery, which contributes to the process cost reduction. Spirulina biomass is widely applicable in food, feed, cosmetics, biofertilizers, biofuels, and biomaterials. A feasibility analysis of Spirulina biorefinery would provide specific information for the decision-making for the improvement of the Spirulina production process. In that context, this review aimed to present a parameter assessment to contribute to the economic viability of Spirulina production in a biorefinery system.

The Use of Microalgae for Coupling Wastewater Treatment With CO2 Biofixation

Production and emission of CO2 from different sources have caused significant changes in the climate, which is the major concern related to global warming. Among other CO2 removal approaches, microalgae can efficiently remove CO2 through the rapid production of algal biomass. In addition, microalgae have the potential to be used in wastewater treatment. Although, wastewater treatment and CO2 removal by microalgae have been studied separately for a long time, there is no detailed information available on combining both processes. In this review article, microalgae-based CO2 biofixation, various microalgae cultivation systems,¯ and microalgae-derived wastewater treatment are separately discussed, followed by the concept of integration of CO2 biofixation process and wastewater treatment. In each section, details of energy efficiency and differences across microalgae species are also given.

Microalgae and cyanobacteria modeling in water resource recovery facilities: A critical review

Microalgal and cyanobacterial resource recovery systems could significantly advance nutrient recovery from wastewater by achieving effluent nitrogen (N) and phosphorus (P) levels below the current limit of technology. The successful implementation of phytoplankton, however, requires the formulation of process models that balance fidelity and simplicity to accurately simulate dynamic performance in response to environmental conditions. This work synthesizes the range of model structures that have been leveraged for algae and cyanobacteria modeling and core model features that are required to enable reliable process modeling in the context of water resource recovery facilities. Results from an extensive literature review of over 300 published phytoplankton models are presented, with particular attention to similarities with and differences from existing strategies to model chemotrophic wastewater treatment processes (e.g., via the Activated Sludge Models, ASMs). Building on published process models, the core requirements of a model structure for algal and cyanobacterial processes are presented, including detailed recommendations for the prediction of growth (under phototrophic, heterotrophic, and mixotrophic conditions), nutrient uptake, carbon uptake and storage, and respiration.

Microalgae for biofuel production

Microalgae have been used commercially since the 1950s and 1960s, particularly in the Far East for human health foods and in the United States for wastewater treatment. Initial attempts to produce bulk chemicals such as biofuels from microalgae were not successful, despite commercially favorable conditions during the 1970s oil crisis. However, research initiatives at this time, many using extremophilic microalgae and cyanobacteria (e.g., Dunaliella and Spirulina), did solve many problems and clearly identified biomass productivity and harvesting as the two main constraints stopping microalgae producing bulk chemicals, such as biofuels, on a large scale. In response to the growing unease around global warming, induced by anthropogenic CO₂ emissions, microalgae were again suggested as a carbon neutral process to produce biofuels. This recent phase of microalgae biofuels research can be thought to have started around 2007, when a very highly cited review by Chisti was published. Since 2007, a large body of scientific publications have appeared on all aspects of microalgae biotechnology, but with a clear emphasis on neutral lipid (triacylglycerol) synthesis and the use of neutral lipids as precursors for biodiesel production. In this review, the key research on microalgal biotechnology that took place prior to 2007 will be summarized and then the research trends post 2007 will be examined emphasizing the research into producing biodiesel from microalgae.

Optimization of carbon dioxide supply in raceway reactors: Influence of carbon dioxide molar fraction and gas flow rate

Influence of CO2 composition and gas flow rate to control pH in a pilot-scale raceway producing Scenedesmus sp. was studied. Light and temperature determined the biomass productivity whereas neither the CO2 molar fraction nor the gas flow rate used influenced it; because pH was always controlled and carbon limitation did not take place. The CO2 molar fraction and the gas flow rate influenced carbon loss in the system. At low CO2 molar fraction (2-6%) or gas flow rate (75-100l·min(-1)) the carbon efficiency in the sump was higher than 95%, 85% of the injected carbon being transformed into biomass. Conversely, at high CO2 molar fraction (14%) or gas flow rate (150l·min(-1)) the carbon efficiency in the sump was lower than 67%, 32% of the carbon being fixed as biomass. Analysis here reported allows the pH control to be optimized and production costs to be reduced by optimizing CO2 efficiency.

Microalgal lipids biochemistry and biotechnological perspectives

In the last few years, there has been an intense interest in using microalgal lipids in food, chemical and pharmaceutical industries and cosmetology, while a noteworthy research has been performed focusing on all aspects of microalgal lipid production. This includes basic research on the pathways of solar energy conversion and on lipid biosynthesis and catabolism, and applied research dealing with the various biological and technical bottlenecks of the lipid production process. In here, we review the current knowledge in microalgal lipids with respect to their metabolism and various biotechnological applications, and we discuss potential future perspectives. The committing step in fatty acid biosynthesis is the carboxylation of acetyl-CoA to form malonyl-CoA that is then introduced in the fatty acid synthesis cycle leading to the formation of palmitic and stearic acids. Oleic acid may also be synthesized after stearic acid desaturation while further conversions of the fatty acids (i.e. desaturations, elongations) occur after their esterification with structural lipids of both plastids and the endoplasmic reticulum. The aliphatic chains are also used as building blocks for structuring storage acylglycerols via the Kennedy pathway. Current research, aiming to enhance lipogenesis in the microalgal cell, is focusing on over-expressing key-enzymes involved in the earlier steps of the pathway of fatty acid synthesis. A complementary plan would be the repression of lipid catabolism by down-regulating acylglycerol hydrolysis and/or β-oxidation. The tendency of oleaginous microalgae to synthesize, apart from lipids, significant amounts of other energy-rich compounds such as sugars, in processes competitive to lipogenesis, deserves attention since the lipid yield may be considerably increased by blocking competitive metabolic pathways. The majority of microalgal production occurs in outdoor cultivation and for this reason biotechnological applications face some difficulties. Therefore, algal production systems need to be improved and harvesting systems need to be more effective in order for their industrial applications to become more competitive and economically viable. Besides, a reduction of the production cost of microalgal lipids can be achieved by combining lipid production with other commercial applications. The combined production of bioactive products and lipids, when possible, can support the commercial viability of both processes. Hydrophobic compounds can be extracted simultaneously with lipids and then purified, while hydrophilic compounds such as proteins and sugars may be extracted from the defatted biomass. The microalgae also have applications in environmental biotechnology since they can be used for bioremediation of wastewater and to monitor environmental toxicants. Algal biomass produced during wastewater treatment may be further valorized in the biofuel manufacture. It is anticipated that the high microalgal lipid potential will force research towards finding effective ways to manipulate biochemical pathways involved in lipid biosynthesis and towards cost effective algal cultivation and harvesting systems, as well.

Evaluation of carbon dioxide mass transfer in raceway reactors for microalgae culture using flue gases

Mass transfer of CO2 from flue gas was quantified in a 100m(2) raceway. The carbonation sump was operated with and without a baffle at different liquid/gas ratios, with the latter having the greatest influence on CO2 recovery from the flue gas. A rate of mass transfer sufficient to meet the demands of an actively growing algal culture was best achieved by maintaining pH at ∼8. Full optimisation of the process required both pH control and selection of the best liquid/gas flow ratio. A carbon transfer rate of 10gCmin(-1) supporting an algal productivity of 17gm(-2)day(-1) was achieved with only 4% direct loss of CO2 in the sump. 66% of the carbon was incorporated into biomass, while 6% was lost by outgassing and the remainder as dissolved carbon in the liquid phase. Use of a sump baffle required additional power without significantly improving carbon mass transfer.

Production of biodiesel and biogas from algae: a review of process train options

Algae are an attractive biofuel feedstock because of their fast growth rates and improved land use efficiency when compared with terrestrial crops. Process train components needed to produce algal biofuels include (1) cultivation, (2) harvesting, and (3) conversion into usable fuel. This paper compares various process train options and identifies knowledge gaps presently restricting the production of algal biodiesel and algae-derived biogas. This analysis identified energy-intensive processing and the inability to cultivate large quantities of lipid-rich algal biomass as major obstacles inhibiting algal biodiesel production. Anaerobic digestion of algal biomass requires fewer process train components and occurs regardless of lipid content. In either scenario, the use of wastewater effluent as a cultivation medium seems necessary to reduce greenhouse gas emissions and maximize water use efficiency. Furthermore, anaerobically digesting algal biomass generated from low-technology wastewater treatment processes represents an appropriate technology approach to algal biofuels that is poorly investigated. Coupling these processes can improve global health by improving sanitation, while providing a cleaner burning biogas alternative to indoor biomass cooking systems typical of less-developed areas.

Light regime characterization in an airlift photobioreactor for production of microalgae with high starch content

The slow development of microalgal biotechnology is due to the failure in the design of large-scale photobioreactors (PBRs) where light energy is efficiently utilized. In this work, both the quality and the amount of light reaching a given point of the PBR were determined and correlated with cell density, light path length, and PBR geometry. This was made for two different geometries of the downcomer of an airlift PBR using optical fiber technology that allows to obtain information about quantitative and qualitative aspects of light patterns. This is important since the ability of microalgae to use the energy of photons is different, depending on the wavelength of the radiation. The results show that the circular geometry allows a more efficient light penetration, especially in the locations with a higher radial coordinate (r) when compared to the plane geometry; these observations were confirmed by the occurrence of a higher fraction of illuminated volume of the PBR for this geometry. An equation is proposed to correlate the relative light intensity with the penetration distance for both geometries and different microalgae cell concentrations. It was shown that the attenuation of light intensity is dependent on its wavelength, cell concentration, geometry of PBR, and the penetration distance of light.

Heterotrophic high cell-density fed-batch cultures of the phycocyanin-producing red algaGaldieria sulphuraria

Growth and phycocyanin production in batch and fed-batch cultures of the microalga Galdieria sulphuraria 074G, which was grown heterotrophically in darkness on glucose, fructose, sucrose, and sugar beet molasses, was investigated. In batch cultures, specific growth rates and yields of biomass dry weight on the pure sugars were 1.08-1.15 day-1 and 0.48-0.50 g g-1, respectively. They were slightly higher when molasses was the carbon source. Cellular phycocyanin contents during the exponential growth phase were 3-4 mg g-1 in dry weight. G. sulphuraria was able to tolerate concentrations of glucose and fructose of up to 166 g L-1 (0.9 M) and an ammonium sulfate concentration of 22 g L-1 (0.17 M) without negative effects on the specific growth rate. When the total concentration of dissolved substances in the growth medium exceeded 1-2 M, growth was completely inhibited. In carbon-limited fed-batch cultures, biomass dry weight concentrations of 80-120 g L-1 were obtained while phycocyanin accumulated to concentrations between 250 and 400 mg L-1. These results demonstrate that G. sulphuraria is well suited for growth in heterotrophic cultures at very high cell densities, and that such cultures produce significant amounts of phycocyanin. Furthermore, the productivity of phycocyanin in the heterotrophic fed-batch cultures of G. sulphuraria was higher than is attained in outdoor cultures of Spirulina platensis, where phycocyanin is presently obtained.

Minimization of carbon losses in pilot-scale outdoor photobioreactors by model-based predictive control

The optimization of carbon use in pilot-scale outdoor tubular photobioreactors is investigated in this study. The behavior of a 0.20-m(3) tubular photobioreactor was studied, with and without algae, by steady-state and pulse dynamic-response analysis experiments. A model of the system was obtained and implemented in a programmable control unit and was used to control the reactor under normal production conditions. Results showed that, using and on-off control, the mean daily CO(2) flow in the reactor was 0.86 g min(-1), 19.7% of this being lost. By using a predictive control algorithm the mean daily CO(2) flow was reduced to 0.74 g min(-1), with losses being reduced to 15.6%. In this case, pH tracking was not adequate, especially at the beginning and end of the daylight period, because the variation in solar irradiance was not considered. Taking solar irradiance into account resulted in better performance, with mean daily CO(2) flow reduced to 0.70 g min(-1), and carbon losses reduced to 5.5%. pH tracking was improved and valve actuation was reduced. Improvement of pH control reduced pH gradients in the culture, which increased the photosynthesis rate and biomass productivity of the system. Biomass productivity increased from 1.28 to 1.48 g L(-1) day-(1) when on-off control was replaced by model-based predictive control plus solar irradiance effect mode. Implementation of this methodology in outdoor photobioreactors can increase productivity by 15% and reduce the cost of producing biomass by >6%. Clearly, application of effective control techniques, such as model-based predictive control (MPC), must be considered when developing these processes.

Assessment of the production of 13C labeled compounds from phototrophic microalgae at laboratory scale

An integrated process for the indoor production of 13C labeled polyunsaturated fatty acids (PUFAs) from Phaeodactylum tricornutum is presented. The core of the process is a bubble column photobioreactor operating with recirculation of the exhaust gas using a low-pressure compressor. Oxygen accumulation in the system is avoided by bubbling the exhaust gas from the reactor in a sodium sulfite solution before returning to it. To achieve a high 13C enrichment in the biomass obtained, the culture medium is initially stripped of carbon, and labeled 13CO(2) is automatically injected on-demand during operation for pH control and carbon supply. The reactor was operated in both batch and semicontinuous modes. In semicontinuous mode, the reactor was operated at a dilution rate of 0.01 h(-1), resulting in a biomass productivity of 0.1 g l(-1) per day. The elemental analysis of the inlet and outlet flows of the reactor showed that 64.9% of carbon was turned into microalgal biomass, 34.9% remained in the supernatant mainly as inorganic compounds. Only 3.8% of injected carbon was effectively fixed as the target labeled product (EPA). Regarding the isotopic composition of fatty acids, results showed that fatty acids were not labeled in the same proportion, the higher the number of carbons the lower the percentage of 13C. Isotopic composition of EPA ranged from 36.5 to 53.5%, as a function of the methodology used (GC-MS, EA-IRMS or gas chromatography-combustion-isotope ratio mass spectrometry (GC-IRMS)). The low carbon uptake efficiency combined with the high cost of 13CO(2) make necessary to redefine the designed culture system to increase the efficiency of the conversion of 13CO(2) into the target product. Therefore, the possibility of removing 12C from the fresh medium, and recovering and recirculating the inorganic carbon in the supernatant and the organic carbon from the EPA depleted biomass was studied. The inorganic carbon of the fresh medium was removed by acidification and stripping with N(2). The inorganic carbon of the supernatant was recovered also by acidification and subsequent stripping with N(2). The operating conditions of this step were optimized for gas flow rate and type of contactor. A carbon recovery step for the depleted biomass was designed based on the catalytic oxidation to CO(2) using CuO (10 wt.%) as catalyst with an oxygen enriched atmosphere (80% O(2) partial pressure). In this way, the carbon losses reduced an 80.2% and the efficiency of the conversion of carbon in EPA was increased to 19.5%, which is close to the theoretical maximum. Further increase in 13CO(2) use efficiency is only possible by additionally recovering other labeled by-products present in the biomass: proteins, carbohydrates, lipids, and pigments.

A light distribution model for an internally radiating photobioreactor

Analysis of light energy distribution in culture is important for maximizing the growth efficiency of photosynthetic cells and the productivity of a photobioreactor. To characterize the irradiance conditions in a photobioreactor, we developed a light distribution model for a single-radiator system and then extended the model to multiple radiators using the concept of parallel translation. Mathematical expressions for the local light intensity and the average light intensity were derived for a cylindrical photobioreactor with multiple internal radiators. The proposed model was used to predict the irradiance levels inside an internally radiating photobioreactor using Synechococcus sp. PCC 6301 as a model photosynthetic microorganism. The effects of cell density and radiator number were interpreted through photographic and model simulation studies. The predicted light intensity values were found to be very close to those obtained experimentally, which suggests that the proposed model is capable of accurately interpreting the local light energy profiles inside the photobioreactor system. Due to the simplicity and flexibility of the proposed model, it was also possible to predict the light conditions in other complex photobioreactors, including optical-fiber and pond-type photobioreactors.

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.

Photosynthetic productivity of conical helical tubular photobioreactors incorporatingChlorellasp. under various culture medium flow conditions

The characteristics of the flow of culture medium significantly affects the photosynthetic productivity of bioreactors incorporating microalgae. Therefore, in order to optimize the performance of a conical helical tubular photobioreactor (CHTP) designed to be useful in practical applications, we characterized the flow pattern of the culture medium through the reactor. The effects of medium flow conditions on the photosynthetic productivity of Chlorella sp. were investigated using several different CHTP units with 0.50-m2 installation areas which were designed to vary the direction and rate of flow driven by airlift. In addition, the performance of two- and four-unit systems constructed by combining individual CHTP units was evaluated. We found that when medium flowed from the bottom to the top of the photostage, it exhibited smoother flow of culture medium than when flowing from top to bottom, which led to higher photosynthetic productivity by the former. Consistent with theoretical calculations, varying the lengths of vertical flow passages caused flow rates to vary, and higher flow rates meant smoother circulation of medium and better photosynthetic performance. Flow of medium through a four-unit CHTP system was similar to that in single units, enabling a photosynthetic productivity of 31.0 g-dry biomass per m2-installation area per day to be achieved, which corresponded to a photosynthetic efficiency of 7.50% (photosynthetically active radiation (PAR; 400-700 nm)). This high photosynthetic performance was possible because smoother medium flow attained in single units was also attained in the four-unit system.

Investigation of photobioreactor design for enhancing the photosynthetic productivity of microalgae

As photosynthetic efficiencies are relatively high at irradiation levels of <500 micromol m(-2) s(-1), photosynthetic productivity could be increased by redistributing strong light over a larger photo-receiving area using conical, helical, tubular photobioreactors (HTP). When Chlorella were exposed to light irradiation of 980 micromol m(-2) s(-1), the ratio of productivities was 1.00:1.13:1.23:1.66 for conical HTPs with cone angles of 180 degrees (flat type), 120 degrees, 90 degrees, and 60 degrees, respectively. This suggests that photo-redistribution technology is a highly effective and convenient approach for increasing the photosynthetic productivity of microalgae.

Carbon dioxide uptake efficiency by outdoor microalgal cultures in tubular airlift photobioreactors

The influence of solar irradiance and carbon dioxide molar fraction of injected CO(2)-air mixtures on the behavior of outdoor continuous cultures of the microalga Phaeodactylum tricornutum in tubular airlift photobioreactors was analyzed. Instantaneous solar irradiance, pH, dissolved oxygen, temperature, biomass concentration, and the mass flow rates of both the inlet and outlet oxygen and carbon with both the liquid and gas phases were measured. In addition, elemental analysis of the biomass and the cell-free culture medium was performed. The oxygen production rate and carbon dioxide consumption rate increased hyperbolically with the incident solar irradiance on the reactor surface. Carbon losses showed a negative correlation with the daily variation of the carbon dioxide consumption rate. The maximum CO(2) uptake efficiency was 63% of the CO(2) supplied when the CO(2) concentration in the gas supplied was 60% v/v. Carbon losses were >100% during the night, due to CO(2) production by respiration, and hyperbolically decreased to values of 10% to 20% in the midday hours. An increase in the carbon fixed in the biomass with the solar cycle was observed. A slight daily decrease of carbon content of the cell-free culture medium indicated the existence of carbon accumulation in the culture. A decrease in CO(2) molar fraction in the injected gas had a double benefit: first, the biomass productivity of the system was enhanced from 2.05 to 2.47 g L(-1) day(-1) by reduction of CO(2) inhibition and/or pH gradients; and second, the carbon losses during the daylight period were reduced by 60%. The fluid dynamics in the reactor also influenced the carbon losses: the higher the liquid flow rate the higher the carbon losses. By using a previous mass transfer model the experimental results were simulated and the usefulness of this method in the evaluation and scale-up of tubular photobioreactors was established.

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.