Outdoor Ponds
Different types of ponds have been designed and experimented with for microalgae cultivation, which vary in size, shape, materials used for construction, and mixing device. Large outdoor ponds can be unlined, with a natural bottom, or lined with unexpensive materials such as clay, brick, or cement, or expensive plastics such as polyethylene, PVC sheets, glass fiber, or polyurethane.
Unlined ponds suffer from silt suspension, percolation, and heavy contamination, and their use is limited to a few algal species, and to particular soil and environmental conditions.
Also natural systems such as euthrophic lakes or small natural basins can be exploited for microalgal production, provided suitable climatic conditions and sufficient nutrients. Examples are the numerous temporary or permanent lakes along the northeast border of Lake Chad, where
Arthrospira sp. grows almost as monoculture, and is collected for human consumption by the
Kanembou people inhabiting those areas.
Arthrospira sp. naturally blooms also in old volcanic craters filled with alkaline waters in the Myanmar region. Production began at Twin Taung Lake in 1988, and by 1999 increased to 100 tons per year. About 60% is harvested from boats on the
surface of the lake, and about 40%is grown in outdoor ponds alongside the lake. During the blooming season in the summer, when the cyanobacterium forms thick mats on the lake, people in boats collect a dense concentration of spirulina in buckets.
Arthrospira is harvested on parallel inclined filters, washed with fresh water, dewatered, and pressed again. This paste is extruded into noodle like filaments which are dried in the sun on transparent plastic sheets. Dried chips are taken to a pharmaceutical factory in Yangon, pasteurized, pressed into tablets ready to be sold. Another cyanobacterium to be used as a food supplement is A
phanizomenon flos-aquae, which since the early 1980s has been harvested from Upper Klamath Lake, Oregon, and sold as a food and health food supplement.
In 1998 the market for
A. flos-aquae as a health food supplement was about $100 million with an annual production greater than 10
6 kg (dry weight). A. flos-aquae blooms are often biphasic, with a first peak in late June to early July, and a second peak late in September to mid-October. The harvested biomass is screened and centrifuged to remove small extraneous material. The algal concentrate is then gravity-fed into a vertical centrifuge that applies high centrifugal force to separate cells and colonies, removing about 90% of the remaining water. At this stage the algal product is 6–7% solids. Once concentrated, the product is chilled to 2°C and stored before being pumped to the freezers. The frozen algae is then put into storage boxes and shipped to the freezer facility for storage. When needed, the frozen product is shipped to an external commercial freeze drying facility to be freeze dried into a powder containing 3–5% water content. This final product is processed into consumable products such as capsules or tablets.
Natural ponds that do not necessitate mixing, and need only minimal environmental control, represent other extensive cultivation systems.
The largest natural ponds used for commercial production of microalgae are
Dunaliella salina lagoons in Australia. Western Biotechnology Ltd. operates 250 ha. of ponds (semi-intensive cultivation) at Hutt Lagoon (Western Australia); Betatene Ltd., a division of Henkel Co. (Germany), operates 460 ha. unmixed ponds (extensive cultivation) at Whyalla (South Australia). Both facilities produce biomass for β-carotene extraction. Other facilities use raceway culture ponds, such as those operated by Cyanotech Co. in Hawaii and Earthrise farms in California for the production of
Haematococcus and
Artrosphira biomass. In both cases large raceway ponds from 1000 to 5000 m
2 are adopted, with stirring accomplished by one large paddle wheel per pond. Raceway pond are also used for intensive cultivation of
D. salina by Nature Beta Technologies Ltd. in Israel.
The nutrient medium for outdoor cultures is based on that used indoors, but agricultural-grade fertilizers are used instead of laboratory-grade reagents. However, fertilization of mass algal cultures in estuarine ponds and closed lagoons used for bivalve nurseries was not found to be desirable as fertilizers were expensive and it induced fluctuating algal blooms, consisting of production peaks followed by total algal crashes. In contrast, natural blooms are maintained at a reasonable cell density throughout the year and the ponds are flushed with oceanic water whenever necessary. Culture depths are typically 0.25–1 m. Cultures from indoor production may serve as inoculum for monospecific cultures. Alternatively, a phytoplankton bloom may be induced in seawater from which all zooplankton has been removed by sand filtration. Algal production in outdoor ponds is relatively inexpensive, but it cannot be maintained for prolonged period and is only suitable for a few, fast-growing species due to problems with contamination by predators, parasites, and more opportunistic algae that tend to dominate regardless of the species used as inoculum.
Furthermore,
outdoor production is often characterized by a poor batch to batch consistency and unpredictable culture crashes caused by changes in weather, sunlight, or water quality. As stated earlier, at present, large-scale commercial production of microalgae biomass is limited to
Dunaliella,
Haematococcus,
Arthrospira, and
Chlorella, which are cultivated in open ponds at farms located around the world (Australia, Israel, Hawaii, Mexico, China). These algae are a source for viable and inexpensive carotenoids, pigments, proteins, and vitamins that can be used for the production of nutraceuticals, pharmaceuticals, animal feed additives, and cosmetics. Mass algal cultures in outdoor ponds are applied in Taiwanese shrimp hatcheries where
Skeletonema costatum is produced successfully in rectangular outdoor concrete ponds of 10–40 tons of water volume and a water depth of 1.5–2 m.
Photobioreactors
An alternative to open ponds for large-scale production of microalgal biomass are photobioreactors. The term “photobioreactor” is used to indicate only closed systems that do not allow direct exchange of gases or contaminants between the algal culture they contain and the atmosphere. These devices provide a protected environment for cultivated species, relatively safe from contamination by other microrganisms, in which culture parameters such as pH, oxygen and carbonic dioxide concentration, and temperature can be better controlled, and provided in known amount. Moreover, they prevent evaporation and reduce water use, lower CO
2 losses due to outgassing, permit higher cell concentration, thus reducing operating costs, and attain higher productivity.
However, these systems are more expensive to build and operate than ponds, due to the need of cooling, strict control of oxygen accumulation, and biofouling, and their use must be limited to the production of very high-value compounds from algae that cannot be cultivated in open ponds. Different categories of photobioreactors exist, such as axenic photobioreactors; tubular or flat photobioreactors; horizontal, inclined, vertical, or spiral; manifold or serpentine photobioreactors; air or pump mixed; single phase, filled with culture suspension, with gas exchange taking place in a separate gas exchanger, or two-phase, with both the gas and the liquid phase contained in the photostage.
The use of these devices dates back to the late 1940s, as a consequence of the investigation on the fundamental of photosynthesis carried out with
Chlorella. Open systems were considered inappropriate to guarantee the necessary degree of control and optimization of the continuous cultivation process. From the first vertical tubular reactors set up in the 1950s for the culture of Chlorella under both artificial light and sunlight, several types of photobioreactors have been designed and experimented with. Most of these are small-scale systems, for which experimentation has been conducted mainly indoors, and only few have been scaled up to commercial level. Significantly higher photosynthetic efficiencies and a higher degree of system reliability have been achieved in recent years, due in particular to the progress in understanding the growth dynamic and requirements of microalgae under mass cultivation conditions. Notwithstanding these advances,
there are only few examples of photobioreactor technology that has expanded from the laboratory to the market, proving to be commercially successful. In fact, the principle obstacle remains the scaling-up
phase, due to the difficulties of transferring a process developed at the laboratory scale to industrial scale in a reliable and efficient way. Two of the largest commercial systems in operation at present are the Klötze plant in Germany for the production of
Chlorella biomass and the Algatechnologies plant in Israel for the production of
Haematococcus biomass. Both plants utilize tubular, pump-mixed, single phase photobioreactors; in particular, the Klo¨tze plant consists of compact and vertically arranged horizontal running glass tubes of a total length of 500,000 m and a total PBR volume of 700 m
3. In a glasshouse requiring an area of only 10,000 m
2 an annual production of 130–150 tons dry biomass was demonstrated to be economically feasible under Central European conditions.
Other industrial plants actually operating are the plant built in Maui, Hawaii (USA) by Micro-Gaia Ltd. (now BioReal, Inc. a subsidiary of Fuji Chemical Industry Co., Ltd.), which is based on a rather complex design, called BioDome
TM, for the cultivation of Haematococcus; the rigid, plastic tubes photobioreactor of AAPS (Addavita Ltd., UK) and the flexible, plastic tubes photobioreactor of the Mera Growth Module (Mera Pharmaceuticals, Inc., USA).
Culture of Sessile Microalgae
Farmers of abalone (
Haliotis sp.) have developed special techniques to provide food for the juvenile stages, which feed in nature by scraping coralline algae and slime off the surface of rocks using their radulae. In culture operations, sessile microalgae are grown on plates of corrugated roofing plastic, which serve as substrate for settlement of abalone larvae. After metamorphosis, the spat graze on the microalgae until they become large enough to feed on macroalgae. The most common species of
microalgae used on the feeder plates are pennate diatoms (e.g.,
Nitzchia,
Navicula). The plates are inoculated by placing them in a current of sand filtered seawater. Depending on local conditions, the microalgae cultures on the plates take between 1 and 3 weeks to grow to a density suitable for settling of the larvae. As the spat grow, their consumption rate increases and becomes greater than the natural production of the microalgae. At this stage, the animals are too fragile to be transferred
to another plate and algal growth may be enhanced by increasing illumination intensity or the addition of fertilizer.
