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Micro-algae for mitigating carbon dioxide

Micro-algae are a group of unicellular or simple multicellular fast growing photosynthetic microorganisms that can conserve CO2 efficiently from different sources, including the atmosphere, industrial exhaust gases, and soluble carbonate salts. Micro-algae act as a major system for converting atmospheric CO2 into lipids under sunlight and increase the output of algal oil. The enzyme acetyl Co-A carboxylase (ACCase) from micro-algae catalyses the key metabolic step in the synthesis of oil in algae.

See also: 'Biofuels from algae'.

Introduction top

About 3,000 species out of 200,000 species were found to be useful for sequestration of CO2 and the production of biodiesel (Keffar and Kleinheinz, 2002). Micro-algae are a promising alternative to CO2 mitigation by CO2 fixation, biofuel production, and wastewater treatments. CO2 fixation by photoautotrophic algal cultures has the potential to diminish the release of CO2 into the atmosphere, thereby helping to alleviate the trend toward global warming (figure 1). Biofuel is derived from microbes that can live on land unfit for crops and generate nearly engine-ready chemicals which are considered to be third generation biofuels (New Scientist, 2011).

illustration © climatetechwiki.org

Figure 1: A conceptual micro-algal system for combined biofuels production, CO2 biomitigation, and N/P removal from wastewater. Inputs: carbon source, CO2; nitrogen and phosphorus sources, N/P rich wastewater; energy source, solar energy. Outputs: low (source: Wang et al., 2008).

illustration © climatetechwiki.org

Figure 2: Microalgal biomass conversion to secondary products (modified from Tsukahara and Sawayama, 2005).

Micro-algae, when fed with CO2 and sunlight, produced large amounts of lipids and hence increase the output of algal oil. The enzyme Acetyl CoA Carboxylase (ACCase) from micro-algae helps to catalyse and transform CO2 in the synthesis of oils in algae.

Technological developments, including advances in photo bioreactor design, micro-algal biomass harvesting, drying, and other downstream processing technologies are important areas that may lead to enhanced cost-effectiveness and therefore, effective commercial implementation of the biofuel using a micro-algae strategy.

Feasibility of technology and operational necessities top

Micro-algae can fix carbon dioxide from different sources, which can be categorised as:

  1. CO2 from the atmosphere.
  2. CO2 from industrial exhaust gases (e.g., flue gas and flaring gas).
  3. Fixed CO2 in the form of soluble carbonates (e.g., NaHCO3 and Na2CO3).
  4. Can be grown in closed systems, which could result in savings of precious freshwater resources.

The systems for using micro-algae for CO2 sequestration involve the following sub systems:

1) The open pond system

The size of open pond micro-algae production systems typically ranges from 0.22-0.4ha (Pedroni et al., 2001). An even larger (900ha) single algae production system has been reported from Mexico City (Becker, 1994). Similarly, the Arizona Department of Environmental Quality reported an algal growing pond of 1,406ha in Florida (Arizona Department of Environmental Quality, 1995). Advantages for utilising the open pond system are low initial and operational costs. Disadvantages of open pond system are the enormous size of the area required, which is not affordable in many regions, and a high water requirement.

illustration © climatetechwiki.org

Figure 3: Algae production in open pond.

2) The closed photo-bioreactor system

Photo-bioreactors provide advantages such as large surface/volume ratios, a barrier to minimise contamination, a capacity to achieve a high density of biomass, a high biomass productivity, and therefore, high CO2 fixation rate (Rosello et al., 2007). The tubular photo-bioreactor is one of the most popular configurations of photo –bioreactors used in algal carbon sequestration process (Travieso et al., 2001).

Advantages

  1. The photo-bioreactor system has a higher potential productivity due to better environmental control and harvesting efficiency.
  2. Even though the open pond systems seem to be favored for commercial cultivation of micro-algae at present due to their low capital costs, closed systems offer better control over contamination, mass transfer, and other cultivation conditions.
  3. Closed photo-bioreactors require less fresh water than open ponds. However, cooling systems that utilize water may be needed to cool the reactors under excessively warm conditions, although poorer quality water may be utilised for the cooling.

Disadvantages

  1. Photo-bioreactors are highly uneconomic due to their prohibitive cost.
  2. Photo-bioreactors can be used only for micro algal strains that are easily harvested.

illustration © climatetechwiki.org

Figure 4: Close photo-bioreactor system.

3) Environmentally controlled system

Another strategy explored for CO2 sequestration use by algae is to build moderate environmentally controlled systems, such as greenhouses. Growers can control the environment inside greenhouses while construction costs are not as high as a photo-bioreactor with a solar collector system.

Status of the technology and its future market potential top

Advantages

  1. Micro-algal CO2 bio-mitigation can be made more economic, cost-effective, and environmentally sustainable, especially when it is combined with other processes such as wastewater treatment. The utilisation of wastewater for micro-algae cultivation will bring about remarkable advantages including the following:
    • Micro-algae have been shown to be efficient in nitrogen and phosphorus removal (Mallick, 2002), as well as in metal ion depletion, and combination of micro-algae cultivation with wastewater treatment will significantly enhance the environmental benefit of this strategy.
    • It will lead to savings by minimising the use of chemicals such as sodium nitrate, potassium and phosphorus as exogenous nutrients.
    • Micro-algae have much higher growth rates and CO2 fixation abilities compared to conventional forestry, agricultural, and aquatic plants (Li et al., 2008).
  2. Some micro-algae species, such as Chlorella, Spirulina and Dunaliella have commercial values. It is expected that commercial profit from biomass production will offset overall operational costs for CO2 sequestration.
  3. Species such as Chlorella can grow under 20% CO2 conditions, and therefore, they can use industrial exhaust gases for a CO2 source, and they can be used as a health food (Becker, 1994).
  4. Some micro-algae (eg. Dunaliella) use CO2 to produce secondary metabolites such as β-carotene, fertilisers, and biofuels as byproducts of economic importance. These products are used as food, medicine and cosmetic products. They also produce cost-effective biofuel (Graham and Wilcox, 2000).
  5. Micro-algae are also considered as multifunctional systems which are used as waste treatment, especially for the removal of nitrogen and phosphorus from effluents (Mallick, 2002) and in aqua culture farms, as well as being an environmental friendly technology.
  6. The high growth rate of micro-algae makes it possible to satisfy the massive demand for biofuels, using limited land resources without causing potential biomass deficit.
  7. Micro-algal cultivation in closed systems consumes less water than land crops.
  8. The tolerance of micro-algae to high CO2 content in gas streams allows high-efficiency CO2 mitigation (figure 5).
  9. Nitrous oxide release could be minimised when micro-algae are used for biofuel production.
  10. Micro-algal farming could be potentially more cost-effective than conventional farming.
  11. Micro-algal farming can be coupled with flue gas CO2 mitigation and wastewater treatment.

illustration © climatetechwiki.org

Figure 5: CO2 tolerance of various micro-algae species.

Disadvantages

  1. A low biomass concentration in the micro-algal culture must be maintained in order not to limit light penetration which in combination with the small size of algal cells makes the harvest of algal biomasses relatively costly.
  2. The cost of production is very high.

Present practice

Obtaining biofuels from micro-algae is a research topic at several locations around the world. However, commercial production does not yet exist. According to Wijffels and Barbosa (2010), current worldwide micro-algal manufacturing infrastructure can produce only about 5,000 tonnes of dry algal biomass per year, and that is devoted to extraction of high value products, such as carotenoids and omega-3 fatty acids for food and feed ingredients.

The main barrier is the enormous cost of production, as well as practical aspects, such as harvesting and drying. Wijffels and Barbosa (2010) estimate an area the size of Portugal would be needed to supply the transport fuel needs of Europe from micro-algae, so the scale of production would have to increase by three orders of magnitude. They also state that a concomitant decrease in the cost of production by a factor of 10 is needed.

Financial requirements and costs top

As per Schenk et al. (2008) and Benemann and Oswald (1996) the cost of algal oil production comes in the range of US$52–$91 per barrel. This estimate was based on 400 hectares of open ponds, using either pure CO2 or flue gas from a coal-fired power station and productivity assumptions of 30-60 g m−2 day−1 with 50% algal lipid yield. Such high yields are theoretically possible but to date have not been demonstrated. Another analysis (Huntley and Redalje, 2006) estimated algae oil production costs to be US$84 bbl. This scenario was based on the infrastructure cost assumptions utilising a hybrid system with an aerial productivity of 70.4 g m−2 day−1and 35% algal lipid yield.

References top

Arizona Department of Environmental Quality (1995) Arizona guidance manual for constructed wetlands for water quality improvement. Arizona Department of Environmental Quality, Arizona.

Becker, E. W. (1994) Microalgae: Biotechnology and microbiology. Cambridge University press, Cambridge, Great Britain. p293.

Graham, L. E. and Wilcox, L.W. (2000): Algae, Prentice-Hall, Inc., Upper Saddle River, NJ. pp 640.

Hanagata, N., Takeuchi, T., Fukuju, Y., Barnes, D. J. and Karube, I. (1992): Tolerance of microalgae to high CO2 and high temperature. Phytochemistry 31(10):3345-3348.

Keffar, J.E., and Kleinheinz, G.T. (2002): Use of chlorella vulgaris for CO2 mitigation in photobioreactor. J. Industr. Microbiol. and Biotech. 29:275-280.

Kodama, M., Ikemoto, H. and Miyachi, S. (1993): A new species of highly CO2-tolreant fast growing marine microalga suitable for high-density culture. Journal of marine biotechnology 1:21-25.

Li Y. Horsman M., Wu N, Lan CQ and Dubois-Calero N (2008): Biofuels from microalgae. Biotech Prog (in press) ASAP Article, DOI 10.1021/bp070371kS8756-7938(07)00371-2

Mallick N (2002) Biotechnological potential of immobilized algae for wastewater N, P and metal removal: a review. BioMetals 15:377–390.

Matsumoto, H., Shioji, N., Hamasaki, A., Ikuta, Y., Fukuda, Y., Sato, M., Endo, N. and Tsukamoto, T. (1995) Carbon dioxide fixation by microalgae photosynthesis using actual flue gas discharged from a boiler. Applied biochemistry and biotechnology 51/52, 681-692.

Miura, Y., Yamada, W., Hirata, K., Miyamoto, K. and Kiyohara, M. (1993) Stimulation of hydrogen production in algal cells grown under high CO2 concentration and low temperature. Applied biochemistry and biotechnology 39/40:753-761.

Miyairi, S. (1995): CO2 assimilation in a thermophilic cyanobacterium. Energy conversion and management 36(6-9):763-766.

Nagase, H., Eguchi, K., Yoshihara, K., Hirata, K. and Miyamoto, K. (1998): Improvement of microalgal NOx removal in bubble column and airlift reactors. Journal of fermentation and bioengineering 86(4), 421-423.

Nakano, Y., Miyatake, K., Okuno, H., Hamazaki, K., Takenaka, S., Honami, N., Kiyota, M., Aiga, I. and Kondo, J. (1996) Growth of photosynthetic algae Euglena in high CO2 conditions and its photosynthetic characteristics. ActaHort 440, 49-54.

New Scientist (2011) : The rush towards renewable oil. New Scientist 21 May 2011.

Pedroni, P., Davison, J., Beckert, H., Bergman, P. and Benemann, J. (2001): A proposal to establish an international network on biofixation of CO2 and greenhouse gas abatement with microalgae. Journal of energy and environmental research 1(1): 136-150.

Rosello Sastre R, Csogor Z, Perner-Nochta I, Fleck-Schneider P and Posten C (2007) Scale-down of microalgae cultivations in tubular photo-bioreactors—a conceptual approach. J Biotechnol 132:127–133.

Seckbach, J., Gross, H., Nathan, M. B. (1971): Growth and photosynthesis of Cyanidium caldarium cultured under pure CO2. Israel journal of botany 20, 84-90.

Travieso L, Hall DO, Rao KK, Benitez F, Sanchez E and Borja R (2001): A helical tubular photobioreactor producing Spirulina in a semicontinuous mode. Int Biodeterior Biodegrad 47:151–155.

Tsukahara K and Sawayama S (2005): Liquid fuel production using microalgae. J Jpn Petrol Inst 48:251–259.

Wang B., Li Y., Wu N. and Lan C Q. (2008): CO2 bio-mitigation using microalgae. Appl Microbiol Biotechnol 79:707-718.

Wijffels, R.H., and Barbosa, M.J. (2010). An outlook on microalgal biofuels. Science 329:796-799.

Yoshihara, K., Nagase, H., Eguchi, K., Hirata, K. and Miyamoto, K. (1996): Biological elimination of nitric oxide and carbon dioxide from flue gas by marine microalga NOA-113 cultivation in a long tubular photobioreactor. Journal of fermentation and bioengineering. 82(4):351-354.