E. Molina Grima

RECOVERY OF MICROALGAL BIOMASS AND METABOLITES: PROCESS OPTIONS AND ECONOMICS

Commercial production of intracellular microalgal metabolites requires the following: (1) large-scale monoseptic production of the appropriate microalgal biomass; (2) recovery of the biomass from a relatively dilute broth; (3) extraction of the metabolite from the biomass; and (4) purification of the crude extract. This review examines the options available for recovery of the biomass and the intracellular metabolites from the biomass. Economics of monoseptic production of microalgae in photobioreactors and the downstream recovery of metabolites are discussed using eicosapentaenoic acid (EPA) recovery as a representative case study.

Photobioreactors : light regime, mass transfer, and scaleup

Design and scaleup of tubular photobioreactors are discussed for outdoor culture of microalgae. Culture productivity is invariably controlled by availability of light, particularly as the scale of operation increases. Thus, light regime analysis is emphasized with details of a methodology for computation of the internal culture illumination levels in outdoor systems. Supply of carbon dioxide is discussed as another important feature of algal culture. Finally, potential scaleup approaches are outlined including promising novel concepts based on fundamentals of the unavoidable light-dark cycling of the culture.

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.

Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae: assessment of design and performance

A methodology is presented for designing photobioreactors with tubular loop solar receivers in which the fluid is circulated by an airlift device. The design method effectively combines the relevant aspects of external irradiance-dependent cell growth, oxygen accumulation in the solar loop, oxygen removal in the airlift device, and hydrodynamics of the airlift system that determine the flow velocity through the solar receiver. The design approach developed was used to model and build a outdoor photobioreactor. A compact degasser in the airlift section eliminated dead zones and dark zones, while achieving complete separation of gas and liquid. The measured gas–liquid hydrodynamics, mass transfer, and culture productivity were consistent with the model predictions. The reactor was tested with continuous culture of the microalga Phaeodactylum tricornutum at various liquid velocities through the tubular solar receiver. A biomass productivity of (or ) was obtained at a dilution rate of . Solar receiver linear liquid velocities of 0.50 and gave similar biomass productivities, but the culture collapsed at lower velocities. An adverse effect of high dissolved oxygen concentration on productivity was observed. Oxygen accumulation could be reduced by increasing the liquid velocity and this enhanced the biomass yield.

Downstream processing of algal polyunsaturated fatty acids.

Abstract Little information exists on recovering polyunsaturated fatty acids from microalgae; however, methods for concentration and purification of PUFAs from fish oil have been extensively reported. This review examines recovery and purification of microalgae derived PUFAs, but techniques developed for use with fish oil are also reviewed as being potentially useful for concentration and purification from microalgae. The two main techniques for concentrating and purifying—urea fractionation and high performance liquid chromatography—are discussed in depth and attention is focused on the process developed by the authors for obtaining highly pure PUFA. Other potentially useful techniques, such as supercritical fluid extraction and lipase-catalyzed processing are detailed.

Recovery of pure B-phycoerythrin from the microalga Porphyridium cruentum

Phycoerythrin is a major light-harvesting pigment of red algae and cyanobacteria that is widely used as a fluorescent probe and analytical reagent. In this paper, B-phycoerythrin and R-phycocyanin in native state, from the red alga Porphyridium cruentum were obtained by an inexpensive and simple process. The best results of this purification procedure were scaled up by a factor of 13 to a large preparative level using an anionic chromatographic column of DEAE cellulose. Gradient elution with acetic acid-sodium acetate buffer (pH 5.5) was used. In these conditions both 32% of B-phycoerythrin and 12% of R-phycocyanin contained in the biomass of the microalgae was recovered. B-phycoerythrin was homogeneous as determined by sodium dodecyl sulfate-poly-acrylamide gel electrophoresis (SDS-PAGE), yielding three migrating bands corresponding to its three subunits, consistent with the (alpha beta)(6)gamma subunit composition characteristic of this biliprotein and the spectroscopic characterization of B-PE (UV-visible absorption and emission spectroscopy; steady-state and polarization fluorescence), is accompanied. Finally, a preliminary cost analysis of the recovery process is presented.

A model for light distribution and average solar irradiance inside outdoor tubular photobioreactors for the microalgal mass culture

A mathematical model to estimate the solar irradiance profile and average light intensity inside a tubular photobioreactor under outdoor conditions is proposed, requiring only geographic, geometric, and solar position parameters. First, the length of the path into the culture traveled by any direct or disperse ray of light was calculated as the function of three variables: day of year, solar hour, and geographic latitude. Then, the phenomenon of light attenuation by biomass was studied considering Lambert-Beers law (only considering absorption) and the monodimensional model of Cornet et al. (1900) (considering absorption and scattering phenomena). Due to the existence of differential wavelength absorption, none of the literature models are useful for explaining light attenuation by the biomass. Therefore, an empirical hyperbolic expression is proposed. The equations to calculate light path length were substituted in the proposed hyperbolic expression, reproducing light intensity data obtained in the center of the loop tubes. The proposed model was also likely to estimate the irradiance accurately at any point inside the culture. Calculation of the local intensity was thus extended to the full culture volume in order to obtain the average irradiance, showing how the higher biomass productivities in a Phaeodactylum tricornutum UTEX 640 outdoor chemostat culture could be maintained by delaying light limitation. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 701-714, 1997.

Biotechnological production of lutein and its applications

Lutein is an antioxidant that has gathered increasing attention due to its potential role in preventing or ameliorating age-related macular degeneration. Currently, it is produced from marigold oleoresin, but continuous reports of lutein-producing microalgae pose the question if those microorganisms can become an alternative source. Several microalgae have higher lutein contents than most marigold cultivars and have been shown to yield productivities hundreds of times higher than marigold crops on a per square meter basis. Microalgae and marigold are opposite alternatives in the use of resources such as land and labor and the prevalence of one or the other could change in the future as the lutein demand rises and if labor or land becomes more restricted or expensive in the producing countries. The potential of microalgae as a lutein source is analyzed and compared to marigold. It is suggested that, in the current state of the art, microalgae could compete with marigold even without counting on any of the improvements in microalgal technology that can be expected in the near future.

Modeling of biomass productivity in tubular photobioreactors for microalgal cultures: effects of dilution rate, tube diameter, and solar irradiance

A macromodel is developed for estimating the year-long biomass productivity of outdoor cultures of microalga in tubular photobioreactors. The model evaluates the solar irradiance on the culture surface as a function of day of the year and the geographic location. In a second step, the geometry of the system is taken into account in estimating the average irradiance to which the cells are exposed. Finally, the growth rate is estimated as a function of irradiance, taking into account photoinhibition and photolimitation. The model interconnects solar irradiance (an environmental variable), tube diameter (a design variable), and dilution rate (an operating variable). Continuous cultures in two different tubular photobioreactors were analyzed using the macromodel. The biomass productivity ranged from 0.50 to 2.04 g L-1 d-1, and from 1.08 to 2. 76 g L-1 d-1, for the larger and the smaller tube diameter photobioreactors, respectively. The quantum yield ranged from 1.1 to 2.2 g E-1; the higher the incident solar radiation, the lower the quantum yield. Simultaneous photolimitation and photoinhibition of outdoor cultures was observed. The model reproduced the experimental results with less than 20% error. If photoinhibition was neglected, and a growth model that considered only photolimitation was used to fit the data, the error increased to 45%, thus reflecting the inadequacy of previous outdoor growth models that disregard photoinhibition. Copyright 1998 John Wiley & Sons, Inc.

Production of lovastatin by Aspergillus terreus: effects of the C:N ratio and the principal nutrients on growth and metabolite production

Production of lovastatin and microbial biomass by Aspergillus terreus ATCC 20542 were influenced by the type of the carbon source (lactose, glycerol, and fructose) and the nitrogen source (yeast extract, corn steep liquor, and soybean meal) used and the C:N mass ratio in the medium. Use of a slowly metabolized carbon source (lactose) in combination with either soybean meal or yeast extract under N-limited conditions gave the highest titers and specific productivity ( ∼0.1 mg g −1 h −1 ) of lovastatin. The maximum value of the lovastatin yield coefficient on biomass was ∼30 mg g −1 using the lactose/soybean meal and lactose/yeast extract media. The optimal initial C:N mass ratio for attaining high productivity of lovastatin was ∼40. The behavior of the fermentation was not affected by the method of inoculation (fungal spores or hyphae) used, but the use of spores gave a more consistent inoculum in the different runs.