Michael A. Borowitzka

Commercial production of microalgae: ponds, tanks, tubes and fermenters

The commercial culture of microalgae is now over 30 years old with the main microalgal species grown being Chlorella and Spirulina for health food, Dunaliella salina for β-carotene, Haematococcus pluvialis for astaxanthin and several species for aquaculture. The culture systems currently used to grow these algae are generally fairly unsophisticated. For example, Dunaliella salina is cultured in large (up to approx. 250 ha) shallow open-air ponds with no artificial mixing. Similarly, Chlorella and Spirulina also are grown outdoors in either paddle-wheel mixed ponds or circular ponds with a rotating mixing arm of up to about 1 ha in area per pond. The production of microalgae for aquaculture is generally on a much smaller scale, and in many cases is carried out indoors in 20-40 1 carboys or in large plastic bags of up to approximately 1000 1 in volume. More recently, a helical tubular photobioreactor system, the BIOCOIL™, has been developed which allows these algae to be grown reliably outdoors at high cell densities in semi-continuous culture. Other closed photobioreactors such as fiat panels are also being developed. The main problem facing the commercialisation of new microalgae and microalgal products is the need for closed culture systems and the fact that these are very capital intensive. The high cost of microalgal culture systems relates to the need for light and the relatively slow growth rate of the algae. Although this problem has been avoided in some instances by growing the algae heterotrophically, not all algae or algal products can be produced this way.

Limits to Growth

Urban, industrial and agricultural wastewaters contain up to three magnitudes higher concentrations of total nitrogen and phosphorous, compared with natural water bodies.1 Normal primary and secondary treatment of these wastewaters eliminates the easily settled materials and oxidizes the organic material present, but does not remove the nutrients which will cause eutrophication of the rivers or lakes into which these wastewaters may be discharged. Tertiary treatment of the effluent is therefore required, and both chemical and physical methods which are used are very expensive. Oswald2 estimates that the relative cost of tertiary treatment to remove PO 4 3− , NH 4 + and NO 3 − is about 4 times the cost of primary treatment. Higher orders of treatment, such as quaternary treatment required to remove refractory organics and organic and inorganic toxicants and quinary treatment to remove inorganic salts and heavy metals, are 8 to 16 times as expensive as primary treatment. Algae can be used as a biological alternative tertiary treatment and also for the removal of heavy metals and possibly other toxic substances.3,4 The possibility exists that the algae produced in these systems can be used as animal feed supplements,5,6 or be composted. The use of waste-grown algae may ultimately also have application in closed cycle life-support systems,7,8 or may be used in conjunction with power stations, not only to treat wastewaters, but also to act as a CO2 sink for the amelioration of the impact of greenhouse gases.9–13

High-value products from microalgae—their development and commercialisation

Microalgae (including the cyanobacteria) are established commercial sources of high-value chemicals such as β-carotene, astaxanthin, docosahexaenoic acid, eicosahexaenoic acid, phycobilin pigments and algal extracts for use in cosmetics. Microalgae are also increasingly playing a role in cosmaceuticals, nutraceuticals and functional foods. In the last few years, there has been renewed interest in microalgae as commercial sources of these and other high-value compounds, driven in part by the attempts to develop commercially viable biofuels from microalgae. This paper briefly reviews the main existing and potential high-value products which can be derived from microalgae and considers their commercial development with a particular focus on the various aspects which need to be considered on the path to commercialisation, using the experience gained in the commercialisation of existing algae products. These considerations include the existing and potential market size and market characteristics of the product, competition by chemically synthesised products or by ‘natural’ compounds from other organisms such as fungi, bacteria, higher plants, etc., product quality requirements and assurance, and the legal and regulatory environment.

Microalgae as sources of pharmaceuticals and other biologically active compounds

In the last decade the screening of microalgae, especially the cyanobacteria (blue-green algae), for antibiotics and pharmacologically active compounds has received ever increasing interest. A large number of antibiotic compounds, many with novel structures, have been isolated and characterised. Similarly many cyanobacteria have been shown to produce antiviral and antineoplastic compounds. A range of pharmacological activities have also been observed with extracts of microalgae, however the active principles are as yet unknown in most cases. Several of the bioactive compounds may find application in human or veterinary medicine or in agriculture. Others should find application as research tools or as structural models for the development of new drugs. The microalgae are particularly attractive as natural sources of bioactive molecules since these algae have the potential to produce these compounds in culture which enables the production of structurally complex molecules which are difficult or impossible to produce by chemical synthesis.

An economic and technical evaluation of microalgal biofuels

In her News Feature “Biotech’s green gold”, Emily Waltz details the ‘hype’ being propagated around emerging microalgal biofuel technologies, which often exceeds the physical and thermodynamic constraints that ultimately define their economic viability. Our calculations counter such excessive claims and demonstrate that 22 MJ m−2 d−1 solar radiation supports practical yield maxima of ∼60 to 100 kl oil ha−1 y−1 (∼6,600 to 10,800 gal ac−1 y−1) and an absolute theoretical ceiling of ∼94 to 155 kl oil ha−1 y−1, assuming a maximum photosynthetic conversion efficiency of 10%. To evaluate claims and provide an accurate analysis of the potential of microalgal biofuel systems, we have conducted industrial feasibility studies and sensitivity analyses based on peer-reviewed data and industrial expertise. Given that microalgal biofuel research is still young and its development still in flux, we anticipate that the stringent assessment of the technologys economic potential presented below will assist R&D investment and policy development in the area going forward.

Microalgae for aquaculture: Opportunities and constraints

The aquaculture of macroand micro-algae is a valuable global industry. Macroalgae are farmed for their hydrocolloids as well as for food (Abbott, 1996; Bixler, 1996), and microalgae are cultured commercially for use as health food and as a source of valuable chemicals such as betacarotene (Belay et al., 1994; Borowitzka, 1994). Microalgae are also an important food source and feed additive in the commercial rearing of many aquatic animals, especially the larvae and spat of bivalve molluscs, penaeid prawn larvae and live food organisms such as rotifers which, in turn, are used to rear the larvae of marine finfish and crustaceans. The importance of algae in aquaculture is not surprising as algae are the natural food source of these animals. Although several alternatives for algae exist such as yeasts and microencapsulated feeds (Jones et al., 1987; Nell, 1993; Heras et al., 1994; Nell et al., 1996), live algae are still the best and the preferred food source. The decline in fish stocks and in the catch from ‘wild’ fisheries in recent years has lead to an ever increasing focus on aquaculture. The increased importance of aquaculture is well illustrated by the shrimp industry. The world shrimp supply increased from 1925 103 t in 1984 to 3080 103 t in 1994, an increase of 60% (Ling et al., 1997). The bulk of this increase was in cultured shrimp, which increased 420% in the same period to a total of 921 103 t in 1994 which represents 29.9% of the total harvest. With increasing aquaculture of animal species there is an increasing need for suitable microalgae in the production of these animals. This paper will review the main problems and constraints faced by aquaculturalists in algal production and will consider the main advances being made to improve algal supply for aquaculture. Table 1. Microalgal species commonly used in aquaculture and the animals to which they are usually fed.

Sustainable biofuels from algae

There is currently great interest in microalgae as sources of renewable energy and biofuels. Many algae species have a high lipid content and can be grown on non-arable land using alternate water sources such as seawater. This paper discusses in detail the issue of sustainability of commercial-scale microalgae production of biofuels with particular focus on land, water, nutrients (N and P) and CO2 requirements and highlights some of the key issues in the very large scale culture of microalgae which is required for biofuels. The use of genetically modified algae is also considered.

Future prospects of microalgal biofuel production systems.

Climate change mitigation, economic growth and stability, and the ongoing depletion of oil reserves are all major drivers for the development of economically rational, renewable energy technology platforms. Microalgae have re-emerged as a popular feedstock for the production of biofuels and other more valuable products. Even though integrated microalgal production systems have some clear advantages and present a promising alternative to highly controversial first generation biofuel systems, the associated hype has often exceeded the boundaries of reality. With a growing number of recent analyses demonstrating that despite the hype, these systems are conceptually sound and potentially sustainable given the available inputs, we review the research areas that are key to attaining economic reality and the future development of the industry.

Diurnal lipid and mucus production in the staghorn coral Acropora acuminata

Net 14C-accumulation into lipids of Acropora acuminata was rapid and increased with light intensity. Dark 14C-incorporation was less than 1% noon maximum. Structural lipids were the first radioactively labelled lipid types showing linear 14C-uptake kinetics. Storage lipids showed non-linear, power-curve kinetics for 14C-uptake. The rate of 14C-incorporation into triglycerides and wax esters was maximal during early afternoon and at midday, respectively. Electron microscopic evidence is given for zooxanthellae being primary sites for synthesis of lipids which are exuded from chloroplasts and transferred to animal tissues. Free lipid droplets and crystalline inclusions (wax ester) were common in animal tissues, the inclusions being often associated with mucus-producing cells. The diurnal rate of mucus production was constant. However, 14C-mucus-lipid production showed a light-dependent diurnal pattern and accounted for 60 to 90% total 14C of mucus during periods of photosynthetically-saturating light. Here, 14C was primarily associated with wax esters which were always present in the mucus-lipid. 14C-triglycerides occur in mucus released only during the day. Lipid and mucus synthesis is discussed in relation to the carbon budget of A. acuminata, in which mucus represented a loss of 40% net C fixation.

Algal biotechnology products and processes: matching science and economics

Several microalgae, such as species ofChlorella, Spirulina andDunaliella, are grown commercially and algal products such as β-carotene and phycocyanin are available. The main focus of algal biotechnology continues to be on high value fine chemicals and on algae for use as aquaculture feeds. This paper provides the outline for a rational approach in evaluating which algae and which algal products are the most likely to be commercially viable. This approach involves some simple market analysis followed by economic modelling of the whole production process. It also permits an evaluation of which steps in the production process have the greatest effect on the final production cost of the alga or algal product, thus providing a guide as to what area the research and development effort should be directed to. An example of this approach is presented and compared with other models. The base model used here gives a production cost of microalgal biomass at about AS 14 to 15 kg−1, excluding the costs of further processing, packaging and marketing. The model also shows that some of the key factors in microalgal production are productivity, labor costs and harvesting costs. Given the existing technology, high value products such as carotenoids and algal biomass for aquaculture feeds have the greatest commercial potential in the short term.