Biotechnology of algal biomass production: a review of systems for outdoor mass culture

Microalgae are very efficient solar energy converters and they can produce a great variety of metabolites. Man has always tried to take advantage of these proporties through algal mass culture. Despite the fact that many applications for microalgae have been described in the literature, these micro-organisms are still of minor economic importance. Industrial reactors for algal culture are at present, all designed as open race-ways (shallow open ponds where culture is circulated by a paddle-wheel). Technical and biological limitations of these open systems have given rise to the development of enclosed photoreactors (made of transparent tubes, sleeves or containers and where light source may be natural or artificial). The present review surveys advances in these two technologies for cultivation of microalgae. Starting from published results, the advantages and disadvantages of open systems and closed photobioreactors are discussed. A few open systems are presented for which particularly reliable results are available. Emphasis is then put on closed systems, which have been considered as capital intensive and are justified only when a fine chemical is to be produced.     

An investigation of a fertilizer-tap water medium for mass algal production in outdoor plastic-enclosed systems

The feasibility of using two fertilizers (urea plus superphosphate) in tap water as a medium for the mass culture of green algae (Scenedesmus and Ankistrodesmus) in outdoor plastic-enclosed minipond systems was investigated. Experiments in which the basic fertilizer-tap water medium was enriched with micro- and/or macronutrients revealed no nutrient deficiency symptoms in the algal biomass produced. Biomass production was found to be quantitatively related to the concentration of fertilizer added and maximal production (> 15 g/m(2) day) was achieved following the addition of 30 mg N/L (1.89 g N/m(2) day) and 4.5 mg P/L (0.28 g P/m(2)/day).     

Automatic on-line growth estimation method for outdoor algal biomass production

An on-line measuring procedure for estimating productivity in outdoor algal cultures was developed and tested experimentally. The procedure is based on a previously described method for on-line measuring net O(2) production rate (OPR). The data obtained by this method was found to correlate well with the conventional procedures for estimation productivity by measuring the changes in biomass concentration in the culture. The new procedure seems to be superior to the latter since it can be carried out in an almost continuous way and can give immediate indication on the productivity. OPR could be used to monitor on-line the photosynthetic and/or respiration activity in small research fermentors or in large-scale open systems outdoors.     

High algal production rates achieved in a shallow outdoor flume

A mass culture of Tetraselmis suecica grown in seawater enriched with only inorganic nutrients and CO(2) in a shallow outdoor flume containing foil arrays to effect systematic vertical mixing achieved average daily production rates of over 40 g ash-free dry wt (AFDW)/m(2) over periods as long as one month when grown on a three-day dilution cycle. Photosynthetic efficiencies associated with these high production rates averaged 8-11% based on visible irradiance. Operation of the system in a one-, two-, or four-day dilution cycle resulted in lower photosynthetic efficiencies of 6-7%. A remarkable feature of the three-day dilution cycle results was the fact that production on the third day after dilution averaged 60-70 g AFDW/m(2), and corresponding photosynthetic efficiencies averaged 13-19%. The high production rates and photosynthetic efficiencies achieved on the third day after dilution may have reflected the nonequilibrium nature of the production cycle and, in particular, the fact that the adaptation of the cells to changing light condition lagged behind light condition in the culture.     

Effect of CO2 enhancement in an outdoor algal production system usingTetraselmis

One of the objectives of microalgal culture is to provide reliable production technology for important live aquaculture feed organisms. Presented here are the results of experiments designed to provide a better understanding of the relationship between inorganic carbon availability and algal production.Our results suggest that through additions of CO₂ gas we were able to maintain sufficient dissolved carbon to stabilize outdoor algal cultures. Increases in the rate of addition of CO₂ increased levels of dissolved CO₂, total dissolved inorganic carbon (CO₂), and decreased pH in the growth medium. This translated into improved buffering capacity of the culture medium and higher growth rate. A minimum of 2.4 mM CO₂ was found necessary to maintain a maximal growth rate of 0.7 doublings/day. We also found that the increased productivity more than offsets the cost of adding the CO₂.     

Intensive outdoor algal cultures: How mixing enhances the photosynthetic production rate

Experiments were carried out to define the effect of mixing upon primary productivity. The method compares the production of a water column with ¹⁴C-labelled samples (i) along a classic vertical profile and (ii) with a rotating system moving between the surface and the bottom of the pond, which was 0·5 m deep. In all the experiments, photosynthesis was best in the moving flask. The effect of mixing is particularly important at sunrise and sunset hours: the vertical photosynthetic budget may be over 2·5 times that of homogeneous non-turbulent water. These results have clear implications for monitoring intensive outdoor algal cultures.     

Optimizing algal biomass production in an outdoor pond: a simulation model

A deterministic simulation model was developed to predict production rates of the marine prymnesiophyteIsochrysis galbana in an outdoor algal mass culture system. The model consists of photoadapation, gross photosynthesis and respiration sections. Actual physiological and biophysical laboratory data, obtained from steady state cultures grown under a wide range of irradiance levels, were used in calculating productivity. The resulting values were used to assess optimal operational parameters to maximize algal biomass production. The model predicted a yearly averaged production rate of 9.7 g C m^?2d^?1, which compared well with field data reported in the literature. The model evaluated the effect of pond depth and chlorophyll concentration on potential production rate in various seasons. The model predicted that a yearly averaged chlorophyll areal density of 0.65 g m^?2 will yield the maximal production rate. Chlorophyll areal density should be seasonally adjusted to give maximal production. This adjustment could be done either by changing pond depth or chlorophyll concentration. The model predicted that under optimal operational conditions, the diurnal respiration losses averaged 35% of gross photosynthesis. The calculated growth rate for maximal productivity ranged between 0.15 and 0.24 d^?1, suggesting an optimal hydraulic retention time of 6.7 and 4.2 d for various seasons.     

Mass algal culture in outdoor plastic-covered minipond systems

A comparative study was undertaken to evaluate the performance of algal cultures grown in plastic-covered outdoor minipond systems. It was demonstrated that production in enclosed miniponds was higher than in open ones on account of the stimulatory effects of elevated temperatures which developed in the enclosed systems. Although the plastic covering reduced the amount of light which was available for photosynthesis, this had minimal effects on production due to the light saturation characteristics of the algal cells. The practice of enclosing mass algal cultures with transparent plastic appears to show promise in semi-arid regions not only as a procedure to reduce evaporation, but also as one for increasing algal yields.     

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.     

A portable chamber for measuring algal primary production in streams

This paper describes a portable chamber that measures net primary production of stream periphyton using a ¹⁴C uptake method. The unique feature is that substrates are moved through water at a velocity of 20 cm s ⁻¹ rather than moving water over substrates. The chamber consists of a plexiglass cylinder that is 9 cm in height and 15 cm in diameter. On the top of the cylinder is a DC gearmotor powered by a 12 volt, deep cycle, marine battery. The motor turns a shaft that rotates a 13.3 cm plexiglass plate at a velocity of 20 cm s ⁻¹ . Small tiles (3.2 cm × 3.2 cm × 0.5 cm) that have natural algal assemblages are mounted on the rotating plate. After adding 500 ml of filtered stream water and 185 kBq (5 μCi) NaH¹⁴CO₃ to the chamber, the chambers are placed along a stream margin for 5 h. Measurements of ¹⁴C uptake by algae on the tiles provide estimates of net primary production (NPP). To assess the sensitivity and practicality of the chamber, algal primary production was measured in open and closed canopy sections of Kingsley Creek, Randallsville, New York. In autumn, primary production was higher in the open than closed canopy section and NPP was lower in spring in both sections probably because of scouring of algae due to snowmelt.     

Productivity of outdoor algal cultures in unstable weather conditions

In the outdoor cyclic fed batch cultures of Chlorella pyrenoidosa, some typical growth kinetics patterns in unstable weather conditions were observed. On cloudy days, the biomass output rate (R) was low, but the bioenergetic growth yield (Y) was generally high. In the cloudy morning-sunny afternoon condition, the values of Y were low, especially in the afternoon. In the sunny morning-cloudy afternoon condition, both R and Y were high. A few very high short-term Y values were measured during the cloudy the cloudy afternoon. (c) 1993 Wiley & Sons, Inc.     

Productivity of outdoor algal cultures in enclosed tubular photobioreactor

At quasi-steady-state outdoor cyclic fed batch cultures of Chlorella pyrenoidosa, the growth irradiance incidented on the tubular photobioreactor increased about fivefold between 9:00 a.m. and noon. Overheating of the cultures was observed, resulting in decreasing biomass output rate when culture temperature went above 40 degrees C. In cultures with temperature control, the quasi-steady-state output rate of all cultures increased throughout the day and leveled off in the late afternoon in high-density cultures. The daily area output rate was proportional to the density of the cultures. The specific growth rate of the light-limited cultures increased only marginally (20%) in the morning. (c) 1992 John Wiley & Sons, Inc.     

Developing microalgal oil production for an outdoor photobioreactor

Journal of Applied Phycology - In this paper the preparations are described to develop a production of oil rich microalgal biomass under south European conditions. Ten microalgal species were...     

Environmental indicators for sustainable production of algal biofuels

For analyzing sustainability of algal biofuels, we identify 16 environmental indicators that fall into six categories: soil quality, water quality and quantity, air quality, greenhouse gas emissions, biodiversity, and productivity. Indicators are selected to be practical, widely applicable, predictable in response, anticipatory of future changes, independent of scale, and responsive to management. Major differences between algae and terrestrial plant feedstocks, as well as their supply chains for biofuel, are highlighted, for they influence the choice of appropriate sustainability indicators. Algae strain selection characteristics do not generally affect which indicators are selected. The use of water instead of soil as the growth medium for algae determines the higher priority of water- over soil-related indicators. The proposed set of environmental indicators provides an initial checklist for measures of algal biofuel sustainability but may need to be modified for particular contexts depending on data availability, goals of stakeholders, and financial constraints. Use of these indicators entails defining sustainability goals and targets in relation to stakeholder values in a particular context and can lead to improved management practices.     

Process development and evaluation for algal glycerol production

This article and develops a process for large-scale production of glycerol by means of a hemophilic algae. The process is shown to be economically and technically feasible. Although the proposed process is extremely capital intensive, the total production cost is competitive with existing glycerol process. In addition, the overall energy requirement is much lower than that of the petrochemical process. This proposed process provides an alternative route for glycerol production that is minimally dependent on fossil fuels and is therefore, less sensitive to crude oil availability and price. The primary raw material carbon dioxide from stack gas, is an inexpensive and renewable resource. Maximal Utilization of solar energy is made not only in the glycerol synthesis steps but also in the product recovery system. Significant improvement in the process economics can be realized through further development of large-scale cultivation technology, and biomass distribution and collection machinery. Due to the labor intensive nature of the proposed algal process, it is particularly suitable for less developed nations with limited fossil fuel resources and lower labor costs.     

Modeling algal productivity in large outdoor cultures and waste treatment systems

A deterministic mathematical model was used to describe the production of green microalgae (Scenedesmus obliquus and Coelastrum sphaericum) in outdoor mass cultures. The model was calibrated against 16 months of temperature and irradiance measurements, during which time productivity measurements were made in up to five ponds with surface areas of up to 263 m². During this period rates of algal dry matter production varied between 1·7 and 16·92 g m⁻² day⁻¹. The model predicted productivity to within 4·2% of the observed rates, for the same period. Negative productivity values (loss of biomass) were calculated for the months from November to January. It was concluded that appreciable amounts of biomass could be produced for 7 months per year in temperate areas.Several assumptions were made during the construction of the model, especially with regard to loss factors, such as: respiration, release of exuded organic carbon and photo-inhibition. The latter was included as a separate factor in the model and is merely conceptual. Several applications of the model are discussed, one of which concerns the relation between areal density and productivity, where the optimal areal density for maximal productivity was calculated to be 38–41 g (dry wt) m⁻². A distinction was also made between cultures which were mainly autotrophic and waste systems. It was shown that the presence of gilvin and/or tripton would adversely influence productivities and that the contribution of these factors to vertical light attenuation would have to be measured in waste systems.     

Physiological and technological considerations for optimising mass algal cultures

The successful coupling between physiology andtechnology is central to the success of algalbiotechnology. Imperative is a proper understandingof the variables and their impacts on biomass and/orbiocompound production. The crux lies inphotosynthesis and the capturing of light energy atthe optimal rate for eventual maximal photochemistry(biosynthesis). It is in the hands of algalbiotechnologists to understand the dynamics andregulatory mechanisms of especially PSII (photosystemII) activity in order to advance this technologyfurther. Biophysical and technological optimisationand design aimed at maximising photon flux capture aresome of the avenues that needs be pursued. This needsto be augmented by molecular, biochemical andphysiological inputs. Unfortunately detailedsystematic analyses of the variables, theirinteraction and possible synergism have rarely beendone. The debate regarding the merits andproductivity in closed, either plate or tubular,vertical or horizontal, and open pond reactors need tobe resolved. Exciting developments regarding onlinemeasurements and feedback control for optimalproductivities are part of the solutions andapproaches that need to be followed. Multistagesystems that not only utilise autotrophic growth andstress components, but also combinedautotrophic/heterotrophic systems could providesolutions to specific production requirements. Theseand other important issues are addressed in theoverview. The challenges facing algalbiotechnologists and future research needs are also discussed.     

Probing green algal hydrogen production

The recently developed two-stage photosynthesis and H(2)-production protocol with green algae is further investigated in this work. The method employs S deprivation as a tool for the metabolic regulation of photosynthesis. In the presence of S, green algae perform normal photosynthesis, carbohydrate accumulation and oxygen production. In the absence of S, normal photosynthesis stops and the algae slip into the H(2)-production mode. For the first time, to our knowledge, significant amounts of H(2) gas were generated, essentially from sunlight and water. Rates of H(2) production could be sustained continuously for ca. 80 h in the light, but gradually declined thereafter. This work examines biochemical and physiological aspects of this process in the absence or presence of limiting amounts of S nutrients. Moreover, the effects of salinity and of uncouplers of phosphorylation are investigated. It is shown that limiting levels of S can sustain intermediate levels of oxygenic photosynthesis, in essence raising the prospect of a calibration of the rate of photosynthesis by the S content in the growth medium of the algae. It is concluded that careful titration of the supply of S nutrients in the green alga medium might permit the development of a continuous H(2)-production process.     

Wastewater treatment high rate algal ponds for biofuel production

While research and development of algal biofuels are currently receiving much interest and funding, they are still not commercially viable at today’s fossil fuel prices. However, a niche opportunity may exist where algae are grown as a by-product of high rate algal ponds (HRAPs) operated for wastewater treatment. In addition to significantly better economics, algal biofuel production from wastewater treatment HRAPs has a much smaller environmental footprint compared to commercial algal production HRAPs which consume freshwater and fertilisers. In this paper the critical parameters that limit algal cultivation, production and harvest are reviewed and practical options that may enhance the net harvestable algal production from wastewater treatment HRAPs including CO₂ addition, species control, control of grazers and parasites and bioflocculation are discussed.     

Extracellular polysaccharide production in outdoor mass cultures of Porphyridium sp. in flat plate glass reactors

This work concerns an attempt to develop large scalecultivation of Porphyridium sp. outdoors. Theimpact on cell growth and production of solublesulphated polysaccharides of light-path length (LP)was studied in flat plate glass reactors outdoors. TheLP of the plate reactors ranged from 1.3–30 cm,corresponding to culture volumes of 3–72 L. The sidewalls of all reactors were covered, ensuring similarilluminated surfaces for all reactors. Maximal daytemperature was maintained at 26 ±1 ^°C.Growth conditions of pH (7.5), stirring (withcompressed air) and mineral nutrients, were optimal.Maximal volumetric concentration of the soluble sulfated polysaccharide (1.32 g L^-1) was obtained in winter with the smallest light-pathreactor (1.3 cm ) at a cell density of 1.37 ×10¹¹cells L^-1. Under these conditions, theviscosity of the culture medium was also highest,being inversely proportional to the culture'slight-path. Highest areal concentration of solublepolysaccharides (60 g m^-2) and areal cell density(3.01 × 10¹²m^-2) was recorded in the 20 cmLP reactor, progressively lower values being obtainedas the light path became shorter. A similar patternwas obtained for the areal productivity ofpolysaccharides, the highest being 4.15 g m^-2day^-1 (considering the total illuminated reactorsurface), produced in the 20-cm LP reactor.The main sugar composition (i.e. xylose, galactose andglucose) of the sulfated polysaccharides was similarin all reactors. As viscosity increased with timeduring culture growth, there was a substantial declinein bacterial population. Cultivation throughout mostof the year provided good evidence that a light pathlength of 20 cm in flat plate reactors under theseconditions is optimal for maximal areal solublepolysaccharide production of Porphyridium sp.