dunaliella salina ponds

Beta-carotene is currently utilized for a wide range of applications like nutritional supplement, agent for food fortification, as animal feed, food colorant etc. The high demand for b-carotene led to the development of its synthetic alternative. Considering its safety as food additive and better biological properties, natural beta-carotene is valued more than its synthetic counterpart. However, high costs are associated with natural product sources, basically due to costs involved in production and extraction processes. Limited stability to light, heat and pH variations plus issues relating to a lack of uniformity in water solubility are the other factors responsible for the higher price tag for natural product. 

Natural β-carotene has better physical properties than the synthetic entity in the presence of two isomers, the trans isomer and the 9-cis isomer. Synthetic b-carotene contains only the fully trans isomer, which has lower liposolubility and antioxidant activity than the 9-cis isomer, which is found exclusively in the natural environment (Gomez and Gonzalez, 2005). Unlike the synthetic, natural molecule is a mixture of cis (mainly 9-cis and 15-cis) and trans isomers. While carrots have almost only the trans isomer, Dunaliella salina based b-carotene may include 60 % of cis, depending on the culture conditions. There is evidence that the natural b-carotene owing to its isomeric composition, is better adsorbed by living organisms. Hence, the biological beta-carotene is normally targeted to dietetic, pharmaceutics and cosmetic markets. 

The introduction of natural b-carotene will have multiple impacts on b-carotene usage and markets. Today, the price of extracted and purified natural beta carotene is much higher than that of synthetic one. Diffusion and substantiation of knowledge about desired medical properties of natural beta carotene will increase demand for the product. Tartrazine a synthetic lemon yellow azo dye is used in fruit juice, drinks, pastries, cakes, desserts, biscuits, soft drinks, snack foods, sauces and confectionery, but very much linked to broncho constriction (Corder and Buckley, 1995), related asthmatic and a host of idiosyncratic reactions. Tanaka (2006) confirmed reproductive and neurobehavioural toxicity of tartrazine when administered to mice in the diet. The genotoxic activity of synthetic b-carotene is involved in carcinogenesis, while natural b-carotene is always of practical value in tumor prevention (Xue et al., 1998). Most synthetic colors have also been linked to an increased risk of cancer, skin disorders, migraines, hyperactivity in children and nausea.

With increasingly health conscious population, the b-carotene sector is expected to see a boom. The European diet encompasses greater amounts of fortified foods, and the preference for natural additives is emerging as a key consumer trend. The b-carotene segment is likely to witness further growth from the development of new applications and marketing approaches to widen the consumer base (www.frost.com). Food is the most important sector consuming β-carotene, especially as a color. The market value of b-carotene was projected to reach US $ 253 million by 2009 (http : //www.bccresearch.com), however broke the billion dollar barrier in the same period, A new report from Business Communications Company (BCC) projects annual 2.9% rise for the global market of all commercially used carotenoids for the incoming years, growing from $887 million to $1 billion.

The most common practice for the natural production of b-carotene is through culture of  unicellular biflagellate marine microalga, Dunaliella salina (Borowitzka, 1995). Dunaliella accumulates up to 14 % of dry weight as β-carotene. All existing commercial Dunaliella facilities grow the alga outdoors in open-air cultures at high salinity extensive culture systems (Australia and China) or in more intensive, paddle-wheel stirred raceway ponds (Israel and USA) (Del Campo et al., 2007). Several trials have been initiated to grow Dunaliella in closed photobioreactors, although up to date, none of these trials have taken production beyond the laboratory or small pilot scale plant (García- González et al., 2005). Open tanks represent the conventional method used in commercial production plants for Dunaliella (Borowitzka and Borowitzka, 1988; Ben-Amotz, 1999). The carotenoid content of the microalgal biomass is the determinant of market price for the latter. Therefore, the influence of the operating conditions on Dunaliella biomass quality is of major concern (Prieto et al., 2011). 

The production in open systems has many disadvantages related to factors governing outdoor productivity of photoautotrophic microorganisms (Richmond, 1991). The relatively long light-path (pond-depth) mandates maintenance of relatively dilute cultures that are difficult to harvest. Other issues include high CO₂ consumption with low efficiency, contamination problems, limited control of environmental factors such as temperature and the relatively large running costs due to the need for huge volumes of water, salt and land. Open systems has to consider issues like capital investment, energy costs, labour, water, nutrients, specific alga requirements, local climatic conditions and productivity. 

Economic viability of open unmixed pond systems seems to rest on low land costs, as well as on the fact that water is free other than for low energy pumping costs and that climate is close to optimum for Dunaliella culture. Production can thus be maintained all year round. Meanwhile, dry cell yield is very low, with concentration in cell suspension at less than 0.2 g β-carotene/m³. In reality commercial large-scale endeavors fall much short of such a high production rate and attain only 0.1 g β-carotene/m³ as annual average. The unit cost of producing Dunaliella Salina in an open pond system is about AUS$5 (US$3.14) for each kilogram of dry biomass (Professor Michael A. Borowitzka of Murdoch University at Singapore Algae World 2008 Conference, http://www.oilgae.com/blog/ ) 

The use of closed tubular photobioreactors presents a very interesting alternative to outdoor open tanks, since they offer high values of both photosynthetic efficiency and productivity (reduced light-path), lesser operational inputs (CO₂, water, salt, nutrients) and providing steady and controlled conditions (Borowitzka, 1999; Tredici and Zitelli, 1997). However, they are certainly more expensive to build and operate than the open systems. Photobioreactors, where the algae fluid remains in a closed environment enables accelerated growth and better control over environmental conditions. These glass or plastic enclosures, often operated under modest pressure, can be mounted in a variety of horizontal or vertical configurations and can take many different shapes and sizes. Rigid frameworks or structures are used to support the photobioreactor enclosures. Consequent to this, the myriad customized photobioreactor components result in high installation, capital, operational and maintenance costs for large-volume applications (i.e., 100 to 1000 acres). Production performance has been increased with photobioreactor, but at the expense of additional cost and complexity. So, one of the prime challenges of bioreactors is scaling them up. The experience with Dunaliella in tubular photobioreactors in 1990 atCartagena,Murcia,Spain, was bitter, costly lesson to learn with reactor upscaling. 

Comparing both the systems, with tanks the illuminated area of algae cells is limited to the upper photic zone only, with lots of optically dark zones with light and dark cyclic movement of cells. However, in unmixed open ponds, light radiation flux is homogenous due to a very shorter light path (depth of illuminated culture). Light utilization efficiency is poor with tanks and highest with pond cultures. Salinity increase is highly poor during culture detention phase from inoculation to harvest stage in tanks. But with ponds, salinity increase backed by natural wave action and salted brine / sabkha bottom leaching is faster. Tank productivity results are not realistic and are not practically scalable for mass cultivation system. Pond culture results are realistic and the economies of scale could be extrapolated for large scale industrial scale systems.

Stirring technology is energy and / or capital intensive. Wind rowing of algae in ponds is cheap and free. The amount of solar energy absorbed in tanks is less in tanks but high in open ponds. The open pond land not only acts as supporting structure for the system, but can also provide a degree of thermal management. Pond environment is supra high saline enough to resist competitors and predators. The halo-strength strategy helps Dunaliella to breed truly and proliferate easily. It is the ease of growth and commercial expanse cultivation ability at low cost that matters for calculation of commercial β-carotene production economics in the order of a 1000 ha algae farm. Low operating costs and multistage cultivation processes led to the success of commercial Dunaliella farms in Australia under nutrient-stressed, high-salt and high-light conditions. It has a synergy with solar gradient ponds, desalination, evaporation basins and salt harvesting. The generation of saline effluent, whether from desalination plants or other industries is normally viewed as a severe environmental problem. In Australia, in the Cookes plain Project, Dunaliella was introduced into concentrated brine waste (from Desalination plants) in 7.6 x 40 m plastic lined channels. Growth was exceptional with 400,000 cells/ ml. in eight weeks. Waste paradigm is thus shifted into resource recovery (Fisher, 1998). 

The unmixed open pond design meets the basic criteria of low construction using perimetric SRC-concrete blocks and maintenance costs while providing an optimal hydrodynamic and light environment for the algae. Pond sizes less than 2500 sq. mt. allow efficient wind-carriage of algae to and fro the path of actions.  Mixing prevents the settling of cells and avoids thermal and oxygen stratification in the pond. Larger the pond area, greater the amount of water lost by evaporation.  Shallow pond depths of 20 cms facilitates in increasing mixing velocities.

During culture, high humidity makes protein content lower than expected. The greater surface area to perimeter ratio obviously results in less erosion of the banks, less windrowing of algae and dilution by rain relative to pond volume. Proper cyclic planning demonstrated the feasibility of maintaining cultures under a regime of regular harvesting. Visibly, cells are often observed to concentrate at the surface of pond and be driven by wind to the shore of pond, and if this phenomenon could be exploited, it could significantly reduce the harvesting costs.  Large commercial application would include automated harvesting systems, allowing for nearly continuous collection. Fractionated air bubble injection and dissolved air floatation effects can surfacially skim algae biomass in record time. Bubbles rise to surface of liquid medium carrying electrostatically adsorbed flocs with them, forming a foam layer. An organic polyelectrolyte or polymer, can be further added in a concentration that is less than about 2% of the weight of  dry biomass to effect aggregation of the coagulated flocs into larger flocs. This dissolved air floatation skimming technology is cheaper and faster than currently available methods and retains many of the properties of the microalgae which are lost in conventional technologies. The system is simple to use and inexpensive to maintain. 

The diverse product lines that can be derived from algae would strengthen the desert agricultural sector. For its privileged climatic conditions, high irradiance and elevated number of sunlight hours, desert land is a suitable zone for the establishment of Dunaliella mass culture. The productive process combines characteristics to be directed as a soft biotechnology and can be an alternative to the exploitation of saline and other hypersaline habitats, extensive natural resources with scarce or negligible economic exploitation in this region. Marginal or erodible soils could be retired from traditional agricultural cultivation in these areas and be devoted to the growth of algae.  Algal culture would complement traditional agriculture and should be welcomed as a development that will strengthen the agricultural sector. From the standpoint of desert biosaline agriculture, the cultivation of algae for production of marketable products is a potentially profitable venture.

With the Middle East embarking upon Dunaliella culture, this opportunity marks an alternative exploitation strategy for brine waste utilization released from desalinization plants.  The limitations of the present cultivation procedures and dry β-carotene yield registered by existing players in arid zones, offers possible solutions and modifications to learn from their mistakes. Hypersaline waters are a resource worth its salt. Production costs have been the Achilles’ heel for investors and developers elsewhere the globe. But open architecture approaches with focus on low costing and design simplicity produces meaningful volumes involving natural CO₂ utilization and maintainability. Additionally, to overcome issues like lower shelf stability, product development and product differentiation are required to enhance the coloring attributes, stability, and bioavailability of the active component of natural colors. 

Summing up, effect of nutrients on the specific growth rate is normally a vital key to be elucidated in the open pond. A higher maximum cell number does not immediately mean a higher specific growth rate and vice versa. Carotenoid production is not correlated with location, maximum cell number or specific growth rate. Optimal harvesting cycle and dilution, pond depth and pond area are the vital factors. Solar energy absorption and β-carotene yield are highest in shallow outdoor pond designs alone. In order to decide the best performance for commercial carotenoid production, other factors that affect the efficiency of each system must also be considered, such as time and energy required for operations, harvesting, cleaning and refilling. The size of the volume to be handled in these operations strongly influences the energy consumption and the final cost (Prieto et al., 2011). Areal cell productivity and beta carotenogenic vigour are favourable only with vast expanse shallow pond cultivation system. Also, higher the salt strengths, greater the insurance of the carotene milking pan from adversaries (cyclonic rains, parasites, epizootics etc).  It is the ease of cheaper semi-continuous growth and commercial expanse cultivation ability at low cost that matters for calculation of commercial β-carotene production economics in the order of a 1000 haalgae farm. Earth, sun and shallow brine with Dunaliella (on barren saltern land available as carotenogenic, solar captor surface) works. Is earthen pond culture technology, the only way to beat high natural β-carotene prices? The answer is indisputably ‘Yes’.