Algal Diversity and Commercial Algal Products Author(s): Richard J. Radmer Source: BioScience, Vol. 46, No. 4, Marine Biotechnology (Apr., 1996), pp. 263-270 Published by: American Institute of Biological Sciences Stable URL: http://www.jstor.org/stable/1312833 Accessed: 19/09/2009 20:49 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=aibs. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected]. American Institute of Biological Sciences is collaborating with JSTOR to digitize, preserve and extend access to BioScience. http://www.jstor.org
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Algal Diversity and Commercial Algal ProductsAuthor(s): Richard J. RadmerSource: BioScience, Vol. 46, No. 4, Marine Biotechnology (Apr., 1996), pp. 263-270Published by: American Institute of Biological SciencesStable URL: http://www.jstor.org/stable/1312833Accessed: 19/09/2009 20:49
Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unlessyou have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and youmay use content in the JSTOR archive only for your personal, non-commercial use.
Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained athttp://www.jstor.org/action/showPublisher?publisherCode=aibs.
Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printedpage of such transmission.
JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with thescholarly community to preserve their work and the materials they rely upon, and to build a common research platform thatpromotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected].
American Institute of Biological Sciences is collaborating with JSTOR to digitize, preserve and extend accessto BioScience.
New and valuable products from diverse algae may soon increase the already large market for algal products
Richard J. Radmer
lgae are an extremely diverse group of organisms that make up the lower phylogenetic
echelons of the plant kingdom. A precise definition of this group is elusive; they share many obvious characteristics with higher (land) plants, whereas their distinguishing features from other plant groups are varied and more subtle (Bold and Wynne 1985). Most of the algae are photosynthetic (like higher plants) or are closely related to organisms that are. Algae perform roughly 50% of the photosynthesis on this planet (John 1994) and thus are instru- mental in supporting the biosphere.
In this article, I review the char- acteristics and classification of al- gae and highlight the profound di- versity of its members. I then describe some commercially available prod- ucts derived from algae and a few product areas in which algae may make a significant contribution in the near future.
Diversity of algae The algae have long been recog- nized as a heterogeneous group of organisms, ranging from the micro- scopic blue-green algae, which are closely related to Gram-negative bacteria, to the large, complex kelps, which can exceed 10 m in length. Recent molecular biology data sup-
Richard J. Radmer is president and founder of Martek Biosciences Corpo- ration, 6480 Dobbin Rd., Columbia, MD 21045. ? 1996 American Institute of Biological Sciences.
Production methods
range from
low-technology ocean
farming to cutting-edge manufacturing
port the idea that algae are a group of organisms that have indepen- dently acquired chloroplasts (intra- cellular bodies that contain the pho- tosynthetic machinery; Gibbs 1992).
The taxonomic classification of algae is still the source of some dis- agreement; Bold and Wynne (1985) summarize 14 such classifications, in addition to their own. Algae are generally grouped into more than a dozen major groups, primarily on the basis of pigment composition, storage products, and a variety of ultrastructural features. Table 1 fea- tures one such classification scheme.
More recently, the techniques of molecular biology have been used to determine the affiliations of the vari- ous algal groups and their relation- ships to other taxonomic groups. Some interesting and surprising re- sults of these studies are shown in Figure 1. Note that the various algal groups are scattered all over the map. The blue-green algae (also known as cyanobacteria) are prokar- yotes closely related to many com- mon bacteria. These algae are also considered to be the progenitors of the chloroplasts of some higher al-
gae and plants (Gibbs 1992). At the other end of the spectrum, green algae are closely related to higher plants. Dinoflagellates and eugle- noids have certain characteristics that are intermediate between prokaryotes and eukaryotes, and the term mesokaryotes is sometimes applied to these algae (Lee 1989). These mesokaryotes may be more closely related to the red algae and the slime molds than to other algal groups. Although molecular meth- ods of phylogenetic analysis are new and controversial, the data to date indicate that the algae are heteroge- neous at the molecular level.
The extensive phylogenetic diver- sity of algae shown in Figure 1 is reflected in many aspects of their existence. A few examples include:
Growth mode. Figure 2 illustrates the multiple means by which vari- ous algae can derive their metabolic energy. Although a defining charac- teristic of algae is their ability to grow photosynthetically at the ex- pense of light energy, some algae (a few percent) are able to thrive in the dark at the expense of food sources such as sugar. This so-called het- erotrophic growth mode is similar to that used by yeast, fungi, and bacteria. The use of these growth modes varies. Some algae can use only a single growth mode: they are the obligate phototrophs and obligate heterotrophs. Other algae are more versatile: facultative phototrophs/ heterotrophs are able to switch from one growth mode to the other, al- though they are generally likely to
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Table 1. Major groups of algae according to Lee (1989). Three minor groups are not tabulated. Nuc = nuclear characteristics (Prokaryote, Mesokaryote, or Eu- karyote); Chl = chlorophyll type; PBP = phycobiliproteins (present in three groups). Data on oils from Lee et al. (1985).
Characteristics
Common name
Blue-green algae Red algae Green algae Euglenoids Dinoflagellates Cryptophytes Golden algae Haptophytes Diatoms Yellow-green algae Chloromonads Brown algae
Nuc
Pro Eu Eu Meso Meso Eu Eu Eu Eu Eu Eu Eu
Chl
a a, d a, b a, b a, c2 a, c, a, c1, c2 a, c1, c2 a, c1, c2 a, c a, c a, c1, c2
Figure 1. Phylogenetic scheme based on the analysis of ribosomal RNA sequences by Wainright et al. (1993), modified by Hecht (1993). Common names, rather than scientific names, are used in an attempt to clarify the relationships for the nontaxonomist. Major phylogenetic groups are all capitalized.
grow better in one of the two modes than the other. Interestingly, some algae are able to blend these two growth modes, for example, simul- taneously deriving metabolic energy from light and cellular building blocks from carbon sources such as sugars. These hybrid growth modes, often referred to as mixotrophy, provide certain algae with the means to make maximal use of light and
fixed carbon sources. (See Kaplan et al. [1986] for a more detailed de- scription of the various growth modes of algae.)
Structural diversity. Algae appear in nature as single-celled organisms, as colonies of similar or identical cells, as filaments (both branched and unbranched), as membraneous thalli, and as complex multicellular
structures (as exemplified by the kelps). In this article, I divide algae into two structural groups-the microalgae and the macroalgae. Microalgae are best seen under a microscope; the individual organ- isms are often less than 1 mm in their largest dimension. Macroalgae, on the other hand, can be seen with the unaided eye. Less obvious, but perhaps more fundamental, differ- ences in the internal cellular struc- tures of various algae have also been described (Lee 1989). For example, differences in the organization of the genetic material reflect the fact that some algae are prokaryotes whereas others are eukaryotes.
Ecological diversity. The genetic and phenotypic diversity of algae is mani- fest in their nearly ubiquitous distri- bution in the biosphere. Algae com- monly grow in fresh water and seawater, and several species grow in extremely high-salt environments, such as the Great Salt Lake, Utah, and the Dead Sea, in Israel. Within these aqueous habitats, some algae grow within a few hundred microme- ters of the water surface, others in- habit the subsurface water column, and a few thrive at the limits of the photic zone (which is often 200-300 m below the surface). Algae also grow in soils (from rich humuses to austere desert sands), inside rocks, in snow fields, and in more exotic locations, such as the fur of sloths and polar bears. Finally, algae can either be free living or exist in asso- ciation with other organisms, as in the case of lichens.
Metabolic diversity. Algae produce many different and unusual bio- chemical compounds, including fats, sugars, pigments, and bioactive com- pounds. The diversity of algal me- tabolism is manifest in some un- usual ways, such as in:
* Fish oils. These oils are currently being promoted for their beneficial health effects, primarily on the basis of their high content of omega-3 fatty acids. Interestingly, fish can- not efficiently synthesize these fatty acids and, like humans, must obtain them primarily from their diet. Thus, fish oils are, in reality, algal oils that have been accumulated through the
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food chain. * Toxins. Okadaic acid and its de- rivatives cause a human affliction called diarrheic shellfish poisoning, which results from the ingestion of shellfish containing these com- pounds. These bioactive compounds were initially isolated from sponges; however, subsequent work has dem- onstrated that the okadaic acid com- pounds are produced by algae (spe- cifically, dinoflagellates) closely associated with the sponges (Shimizu 1993).
Large number of species. Algae rep- resent a large and virtually unex- plored group of organisms. Even though the phycological community is small compared with many other scientific disciplines, new species are being identified at a rate of approxi- mately one per week (Radmer and Parker 1994). A recent analysis by John (1994) suggests that the roughly 36,000 known species of algae represent only approximately 17% of the species existing today. These numbers indicate that there are currently more than 200,000 algal species worldwide.
Algal products The modes of production of com- mercial products from algae range from, at one extreme, the harvesting of wild stands of macroalgae with minimal postharvest processing, to, at the other extreme, the intensive cultivation of microalgae in closed culture systems (i.e., fermentation systems or their photosynthetic equivalent) with extensive post- harvest processing. Thus, the pro- duction of algal products spans the gamut from low-technology ocean farming to cutting-edge pharmaceu- tical-like manufacturing. The major algal products are currently pro- duced by what is called the seaweed industry. This industry is based on the harvest and use of macroalgae (i.e., seaweeds), primarily brown algae, the largest and most conspicu- ous of the macroalgae, and red al- gae, a diverse algal group.
In the following sections I de- scribe a variety of algal products, grouped on the basis of their pro- duction mode and product type. Economic data are also presented,
Carbon dioxide Light Water
Mixotrophy ALGA r ----------------
Heterotrophy
Sugars, fats, etc. Oxygen
Figure 2. Schematic diagram of the growth modes used by various algae.
Table 2. Products from macroalgae. From Jensen (1993), except agarose data were estimated by the author. Phycobiliprotein data were provided by A. Tsetsis, Martek Biosciences Corp., Columbia, Maryland, personal communication.
although their availability is spotty and their reliability and position within the distribution chain (e.g., wholesale versus retail) varies for many of the product areas. I also include a few products and product ideas that are in development but not currently available commer- cially.
Foods from macroalgae. The major foods derived from macroalgae are shown in Table 2. The algal biomass for these products is derived from wild, managed, or cultivated stands of macroalgae that undergo a mini- mal amount of processing after har- vest. In these cases, the product is the biomass itself, rather than chemi- cals extracted from the algae. Any postharvest processing serves only to clean and preserve the intrinsic character of the alga.
Nori. The major algal product in the world today is nori, the algal blade (called a thallus) of certain species of the red macroalga Porphyra.
Nori is a primary constituent of sushi, a Japanese food item that is becoming increasingly popular in the West. Because of its history and commercial success, this product provides a good example of the macroalgal production procedures currently available.
The seaweed industry is often described as a cottage industry, and the methods employed in the har- vesting and drying of the seaweed are often small scale, traditional, and primitive (Naylor 1976). In the case of nori, however, modern tech- niques introduced in the 1960s have provided the means to rapidly in- crease production yields (Oohusa 1993). Nori cultivation is, in real- ity, a type of farming in which seed- like propagules, called conchospores, are seeded onto nori nets, which are hung in sheltered ocean areas. Be- fore the mid-1960s, nori cultivation was limited to shallow, sandy bays, where the nori nets could be hung between poles stuck in the bottom.
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After this time, the nets were often attached to surface buoys, so that deeper water could be used for cul- tivation. Around 1970, a system was developed to raise the nets out of the water, which allowed the controlled drying and temporary storage of the Porphyra thalli. These sophisticated growth systems and procedures, coupled with fast-growing cultiva- tion forms and mechanized methods for processing, have provided ample production capacity, and nori sup- ply has recently exceeded demand (Oohusa 1993).
Many nori products are avail- able; of these, toasted nori sheets are the largest product segment. With a market value of approxi- mately $2 billion and a product vol- ume of 40,000 tons per year (Jensen 1993), nori represents the most suc- cessful algal product group at the present time. Although nori is pri- marily consumed in Japan, Korea, and China, sales in other countries are rapidly increasing-US sales in 1991 were estimated to be $20- $25 million (Merrill 1993).
Wakame. Another major algal food product is wakame, a group of related foods derived from a brown alga known as Undaria pinnatifida. This macroalgal product has been cultivated commercially since the mid-1950s (Yamanaka and Akiyama 1993). Like nori, wakame is pro- duced mainly in Japan, Korea, and China, with Korea being the major producer. Wakame is more exten- sively processed after harvest than most other macroalgal biomass products. The most popular wakame product is boiled and salted, which results in the green product most preferred by consumers.
As with nori, the primary market for wakame products is Japan, where it is available in many forms (e.g., salted or dried cut) and is used as an ingredient in soups, salads, noodles, and the like. As of 1990, roughly 20,000 tons, with a market value of $600 million, were sold annually.
Kombu. The third major algal food product group is kombu, which is derived from Laminaria japonica and related species of brown macroalgae. These algae are col- lected during the summer, dried naturally, and then boiled. A vari- ety of kombu products are produced,
which can be served with meat, fish, or soups or as a vegetable (Lee 1989). The annual market for these prod- ucts is approximately $600 million.
Other macroalgal foods. Many other macroalgae are also used as human food. For example, the red macroalga Palmaria palmata, known as dulse, has been consumed by shoreline populations of northwest Europe since approximately the tenth century (Lahaye and Vigou- roux 1992). Another dulse, Rhody- menia sp., is harvested and con- sumed in parts of North America, particularly the Maritime Provinces of Canada (Naylor 1976), where it is promoted as a sea vegetable. Naylor (1976) briefly describes ap- proximately two dozen other macro- algal foods.
Industrial products from macroalgae: the Hydrocolloids. Table 2 also sum- marizes the uses and market values of the major polysaccharide prod- ucts derived from algae. These prod- ucts, also referred to as hydrocol- loids, make up the major industrial products derived from algae at the present time; their combined mar- ket value is well over $500 million. The marine macroalgae from which these products are prepared (certain species of red and brown algae) are harvested from the wild, from man- aged wild stands, and from culti- vated beds. Carrageenans and agars are obtained from different species of red algae. Alginates are obtained from species of brown algae. Unlike the food products described above, the macroalgal biomass for these products undergoes extensive extrac- tion and processing to yield the final product (Lewis et al. 1988). The extreme case is agarose, which is derived from agar (already a pro- cessed product) by extensive sepa- ration and purification (Renn 1994).
Alginates. The alginates are salts of alginic acid; these salts, and the sodium salts in particular, are also known as algin. They are polymers composed of D-mannuronic acid and L-guluronic acid monomers. The sequences and proportions of these constituents vary with the source of the algin. The major com- mercial sources of alginates are brown macroalgae, particularly spe- cies of Laminaria, Macrocystis, and
Ascophyllum. Alginates are typically recovered from the macroalgal bio- mass by extracting the insoluble al- ginic acid salts with hot alkali (so- dium carbonate). The sodium alginate is then separated from the insoluble seaweed residue by filtra- tion and purified (McHugh 1987).
The primary characteristic of al- ginates is their ability to form vis- cous solutions when dissolved in cold water, which provides thicken- ing, gel-forming, water-retaining, and suspending properties to solu- tions containing them. These fea- tures underlie their importance in food, industrial, and biotechnologi- cal applications. Approximately 27,000 tons of alginates, with a value of $230 million, were sold annually around 1990 (Jensen 1993).
Carrageenans. The carrageenans are a complex group of polysaccha- rides derived from red macroalgae. Their unifying characteristic is that they are all made up of galactose- related monomers (a-1,3-D-galac- tose and P-1,4-3,6-anhydro-D-ga- lactose) to which sulfate groups are attached. Three major types of car- rageenans, designated kappa, lambda, and iota, are used com- mercially. They are primarily de- rived from Eucheuma cottonii, Chondrus crispus, and Eucheuma spinosum. The carrageenans are typically recovered from the macro- algal biomass by extraction with hot water. Subsequent processing depends on the characteristics of the product desired (Stanley 1987).
Like the alginates, the carrageen- ans can be used to gel, thicken, sus- pend, and stabilize foods and other products. Approximately 15,500 tons of carrageenans, with a value of $100 million, were sold annually at the beginning of this decade (Jensen 1993).
Agars. The agars are mixtures of polysaccharides extracted from cer- tain red macroalgae. Like the carra- geenans, their unifying characteris- tic is that they are all composed of galactose-related monomers, in this case, D-galactose and 3,6-anhydro- L-galactose. The agars also contain varying amounts of sulfate, pyru- vate, and methoxy groups, the con- tent of which vary with the source of the macroalgal biomass and the sub- sequent processing procedures. They
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are derived primarily from species of Gracilaria, Gelidium, Ptero- cladia, Acanthopeltis, andAhnfeltia. Like the carrageenans, the agars are usually extracted with hot water. Subsequent processing steps serve to generate a concentrated filtrate, which is allowed to gel; this gel is then treated, dehydrated, and milled (Armisen and Galatas 1987).
The ability of agars to form stable gels that retain their characteristics under a range of conditions (e.g., temperature, humidity, and chemi- cal milieu) underlies their value in many applications. In addition to their use in foods, agars are the media of choice to grow and ma- nipulate microorganisms such as bacteria and yeast. Because of their special characteristics, agars can often command a substantially higher unit price than the alginates and carrageenans. Annual sales of these products amounted to approxi- mately $160 million, on a volume of 11,000 tons, circa 1990 (Jensen 1993).
Agaroses. The agaroses are highly refined, specialized macroalgal prod- ucts that have played a pivotal role in the biotechnological revolution (Renn 1990). These products are manufactured by isolating the less ionic fractions of agar under highly controlled conditions designed to minimize lot-to-lot variation. The individual products are targeted to a variety of specialty applications, each requiring specific quality as- surance protocols.
The main applications for the agaroses are in the biotechnology area, and these products are key elements in powerful techniques such as gene mapping (Renn 1990). Be- cause of the competitive and spe- cialized nature of this product area, market data are difficult to obtain; the value of more than $50 million per annum in Table 2 is a crude estimate. The unit value of some of these products can be impressive, ranging beyond $25,000/kg.
Other macroalgal products. In addition to the major products de- scribed, macroalgae provide several other products with significant com- mercial impact. For example, three agricultural products-seaweed meal, manure, and liquid fertilizer- each have annual sales of several
millions of dollars annually (see Table 2). Naylor (1976) describes some of these products and their dissemination and use.
An interesting group of high-value macroalgal products is the phyco- biliproteins. These protein-contain- ing pigments, which are unique to certain algae, serve as valuable fluo- rescent tags with many applications in high-technology areas, such as flow cytometry, fluorescence-acti- vated cell sorting, and histochemis- try (Glazer 1994). The major prod- uct in this area is R-phycoerythrin, which is currently derived from spe- cies of Porphyra, either cultured or harvested from the wild. Because R- phycoerythrin exists as part of a mixture of several different phyco- biliproteins, sophisticated separa- tion and purification procedures, usually involving chromatography, are required for the production of the pure product. The high cost of this process is balanced by the high value of the product as a biomedical reagent. The raw material (purified phycobiliprotein) currently sells for approximately $5000/g in a mod- est, $2-million market.
Products from microalgae grown in open mass culture. The mass cultur- ing of microalgae dates back to the 1940s. (Shifrin [1984] and Soeder [1986] provide interesting accounts of this effort.) Microalgal mass-cul- turing systems are generally com- posed of a series of connected shal- low channels (on the order of 30 cm deep) called raceways. The algal suspension contained in the race- ways is usually mixed by slowly moving paddles. The management of these ponds can be complex, and efficient algal production is at the mercy of prevailing weather condi- tions, probably even more so than macroalgal culture or traditional agriculture.
The economical harvesting of microalgae from mass culture ponds has historically been a problem that has hindered the commercial devel- opment of products. Dense cultures from highly productive ponds gen- erally contain only approximately 0.1% of small algal biomass par- ticles (Shifrin 1984). Even in this high-yield case, 1000 g of water must be handled for each gram of
algae harvested, with sophisticated (and expensive) techniques, such as flocculation, centrifugation, and microscreening, used to isolate the small algal particles. (Compare this case with the simple harvest of macroalgae and agricultural prod- ucts.) Thus, harvesting costs can contribute substantially to the cost of the overall process. Successful microalgal production overcomes this problem, either by bypassing it (as in the case of Spirulina) or by producing a product with a value high enough to justify these expenses (e.g., beta-carotene from Dunaliella).
Because of these pond manage- ment and harvesting limitations, a recurring problem in the develop- ment of microalgal products has been the high cost associated with cultur- ing and harvesting microalgae. Al- though there is likely to be case-by- case variability, a detailed study by Borowitzka (1992) indicated that microalgal biomass costs generally exceed $10/kg dry weight, exclud- ing such costs as downstream pro- cessing. These findings are reflected in the marketplace; only high-value products, such as beta-carotene, and certain algal biomass products that can command a high price, such as Spirulina, are viable candidates for commercialization. Indeed, these are currently the only commercially suc- cessful products in this category.
Spirulina. Spirulina is a filamen- tous blue-green microalga that has a long history of use in the human diet (see Richmond 1986). Species of Spirulina have been consumed as dried algae cakes since ancient times in areas such as Lake Chad (in Af- rica) and Lake Texcoco (in Mexico). This alga thrives under alkaline con- ditions and thus can grow as a rela- tively pure culture, because growth of most other algae and other or- ganisms is inhibited. This feature, coupled with Spirulina's tendency to float and'clump, provides a natu- rally occurring edible algal bloom that can be readily harvested with- out special techniques. Modern Spirulina production systems make use of these same features. High- alkalinity open ponds are inoculated and maintained to provide condi- tions for algal growth. The algal biomass is then harvested and dried.
In the United States, Spirulina is
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used primarily as a health food, a market niche in which it can com- mand a substantial price. There is a good deal of literature, both in peer- reviewed scientific journals and in the popular press, concerning the value and suitability of Spirulina in the human diet (e.g., Richmond 1986). The safety of Spirulina has been established through various toxicological studies (Belay et al. 1993). The annual commercial pro- duction of food-grade Spirulina was approximately 800 tons in the early 1990s (Belay et al. 1993), with re- tail prices often exceeding $100/kg (see Table 3).
Beta-carotene. The dominant pig- ment product currently manufac- tured from algae is beta-carotene. This fact is surprising for two rea- sons: Beta-carotene is a constituent of all higher plants, and thus is con- sumed as part of the normal diet, and it is also produced synthetically at a significantly lower price. Thus its viability in the marketplace de- pends on the consumer's perception that there are reasons to supplement the normal diet with this substance and that there are real differences between the natural and synthetic products. Both of these perceptions are based on data published in peer- reviewed journals, although firm conclusions are currently lacking. Beta-carotene is known to be an effective antioxidant, which is the basis for its reputation as a health- promoting food supplement (Bur- ton and Ingold 1984). The purported superiority of the natural versus syn- thetic beta-carotene is based, at least in part, on the different isomeric composition of the two products (e.g., Mokady et al. 1990).
Algal beta-carotene is produced by growing species of the salt-toler- ant green alga Dunaliella in ex- tremely high-salt environments, in open ponds or raceways, under high light. These growth conditions pro- mote the accumulation of beta-caro- tene to greater than 5% of the total cell mass. (Glycerol, a low-value product, is also produced as part of this process.) The extreme growth conditions used in this culturing pro- cess serve to drastically limit the contamination of the open culture by other organisms. In fact, these algal species, along with species of
Table 3. Products from microalgae. Sources are given in the text.
Product Use Market value (million $US)
Spirulina Food 80 Beta-carotene Nutritional 25
supplement Chlorella Food 100 Labeled Growth media 5 compounds
Spirulina, are the only algae rou- tinely cultured under open-culture conditions; in both cases, the ex- treme growth conditions serve ef- fectively to exclude other, compet- ing organisms.
The harvest of the carotene-rich Dunaliella cells from the large, di- lute culture medium represents a major effort and expense. The small algal cells must be separated from large quantities of high-salt solu- tion without disrupting the fragile, wall-less cells. This effort represents a major technological challenge, and several clever proprietary techniques have been developed.
After harvest, the high-carotene biomass is processed by a variety of procedures to generate a variety of products, ranging from crude high- carotene biomass to high-concen- tration beta-carotene in a vegetable oil carrier. Because of the substan- tial price difference between the natural and synthetic products ($1400/kg versus $600/kg; see Radmer and Parker 1994), the algal products can access only a small fraction of the $100-million beta- carotene market (Table 3; Boro- witzka 1992).
Products from microalgae grown with fixed carbon. A small percent- age of algae are capable of using fixed carbon, either in addition to light (mixotrophy) or in total dark- ness (heterotrophy; see Figure 2). From a manufacturing perspective, heterotrophic algal growth provides major advantages. Fermentation (the process of growing heterotrophs in closed systems) is a well-developed manufacturing technology with a vast experience base at a variety of production scales. The cost of pro- ducing simple products, such as single-cell protein (microbial bio- mass used as a food or feed addi- tive), can be less than $1/kg dry
weight (Crueger and Crueger 1989), an order of magnitude less than the cost of phototrophic growth. Equally important, large fermentation fa- cilities, with capacities of hundreds of thousands of liters, are available virtually worldwide.
Chlorella. The primary het- erotrophic algal product is Chlo- rella, a green microalga that has been produced and consumed in substantial quantities since the 1960s, primarily in the Far East. Chlorella biomass is produced in large quantities (hundreds of tons), often using a hybrid process in which the biomass is generated in a closed fermenter at the expense of glucose or acetate and is then "greened" by exposing the algae to light, either in open ponds or using transparent plastic tubes. (Soong [1980] gives an interesting account of this pro- duction process.) After harvest of the biomass by centrifugation, the algal product is dried, using conven- tional industrial procedures, and put into its final form.
Chlorella is promoted and sold primarily as a health food product in the form of tablets or powder. Like Spirulina, it can command a substantial price. The annual com- mercial production of food-grade Chlorella is at least 1000 tons (Iwomoto 1993, Soong 1980); with retail prices often exceeding $100/ kg, the total market value is prob- ably in excess of $100 million (Table 3).
Other fermentative products. Sev- eral other fermentatively produced algal products are currently in de- velopment. For example, Running et al. (1994) have developed a fer- mentation process for the produc- tion of L-ascorbic acid (vitamin C) using a selected strain of Chlorella pyrenoidosa. This process has not yet been commercialized.
Heterotrophic algal production also may play a significant role in the area of aquaculture feeds. Bi- valve and shrimp hatcheries require substantial quantities of algae on a continuous basis, and the produc- tion of these algae can be a major part of the total production costs. Currently, production costs for pho- tosynthetic algae used as feeds are probably greater than $160/kg dry weight (De Pauw and Persoone
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1988). Most of the algal feeds for these applications are generated and used in the same commercial entity (e.g., Fulks and Main 1991). A re- cent attempt by Celsys (a company in Cambridge, United Kingdom) to commercialize this area by supply- ing heterotrophically produced feeds was not successful. More recent work, notably by Gladue and Maxey (1994), suggests that this area may still hold promise.
Probably the most exciting recent development in the algal product area is the demonstration that valu- able omega-3 fatty acids, notably docosahexaenoic acid (DHA), can be efficiently produced using het- erotrophically grown algae (Kyle et al. 1992). DHA plays a vital role in infant nutrition. It is the predomi- nant structural fatty acid in the gray matter of the brain and retina and must be supplied, preformed, in the diet, particularly in the case of in- fants. DHA is present in human milk but not in most infant formulas cur- rently on the market. Despite the need for this microalgal product, DHA is currently available only in small (kilogram) quantities (for re- search) and as a component of some infant formula products in Europe. It is not available to consumers else- where.
Microalgae produced in closed pho- tosynthetic systems. Many biotech- nological applications of microalgae (and probably macroalgae as well) require the use of closed culture (Radmer and Parker 1994). For those cases in which the alga of interest cannot be grown heterotrophically, closed photosynthetic systems must be considered.
The photosynthetic growth of al- gae in closed systems has been prac- ticed at the laboratory scale for the better part of this century, and effi- cient pilot-scale production units have been developed as part of the US space program to supply food and atmosphere regeneration for astronauts. (Watson [1979] de- scribes some of the patent literature in this area.) However, the use of these production systems for com- mercial purposes is in its infancy. A primary impediment is the high cost of generating the algal biomass: Closed systems that make use of
sunlight encounter operating costs at least as high as those endured for open ponds (Borowitzka 1992), whereas those using artificial light are likely to accrue costs several- fold higher than the other production modes (Radmer and Parker 1994).
The only product area that cur- rently has a high enough value to offset the high costs of artificially illuminated closed photosynthetic algal culture is stable isotope-la- beled biochemicals. These products, produced under completely closed conditions in which the gas phase is recycled, are finding increasing use in the areas of protein structure eluci- dation (particularly as applied to ra- tional drug design) and noninvasive diagnostics (Cox et al. 1988). The major products in this area, uni- formly labeled 13C-glucose, growth media uniformly labeled with 13C and 15N, and labeled amino acids and other sugars, sell into a rela- tively small $5-million market (Table 3). The unit prices are gener- ally more than $100/g.
Conclusions
The group of organisms collectively referred to as algae is not a natural assemblage. Although most algae are capable of photosynthesis, this ca- pability was probably acquired by several diverse taxonomic groups independently and in various ways. These polyphyletic origins are re- flected in the profound diversity of this group.
Algae currently provide the basis for a large, multibillion dollar in- dustry that is largely invisible, par- ticularly to consumers in the West. A few notable points are:
* At the present time, food prod- ucts, sold primarily in the Far East, comprise the major use of algae when reckoned on the basis of known market value. * The major industrial use of algae is in the area of hydrocolloids; in aggregate, this business is a large one that has a major presence in the food industry. * Major components of the biotech- nological revolution have been based on the use of agar and agarose, and the continued development of spe- cialized agarose has been a vital
aspect in the development of these technologies. * Although the algal kingdom is extraordinarily diverse and may comprise more than 200,000 spe- cies, major algal products at present are based on only 10-20 algal spe- cies, almost all of them macroalgae.
The development of valuable products from algae is in its infancy. Historically, a major impediment in this endeavor has been the lack of suitable culture systems, so that the supply of raw materials was depen- dent on harvest from the wild. The development of culture systems for the production of macroalgae, pri- marily in the Far East, has resulted in a stable supply of many of the macroalgae of commercial value. Similar developments in the supply of microalgae are occurring today.
Algal molecular biology is not nearly as well developed as that of yeast or bacteria, and presently there is no publicly disclosed commercial development or production system that uses this technology. Similarly, the culture of macroalgal tissues as unicells or multicellular masses is a topic of current research interest, but no commercial product is made in this manner. One might expect the development of new algal prod- ucts to accelerate as these technolo- gies mature. The recent progress in the cryopreservation and cryo- storage of microalgae should greatly aid in the maintenance and perpetu- ation of high-yielding algal strains.
Although there has never been a reported case of a commercial pharmaceutical product discovered or produced through algae, extracts of many algae have shown interest- ing biological activities in a variety of bioassays, and many interesting and novel chemical entities have been isolated and characterized during the course of this work. Product development in this area, as in many others, has been hindered by our inability to culture the algae of in- terest under controlled conditions at a large scale. Progress in this area, coupled with the ever-growing tools of biotechnology, should facilitate the production of additional useful and valuable products from this di- verse and fascinating group of or- ganisms.
April 1996 269
Acknowledgments I thank Bruce Parker and Teri Watson for their help in preparing this article.
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