Written by: Vincent Mai
As concerns about greenhouse emissions continue to grow worldwide, ethical concerns on the impending threat of climate change becomes more apparent. It is predicted that over 68% of greenhouse gas (GHG) emissions is due to the release of carbon dioxide gas into the atmosphere (Brennan and Owende 2010; Ho et al. 2011; Kumar et al. 2011). Economic incentives enforced by carbon policies and regulations such as carbon taxes may provide even greater incentives to reduce carbon emissions in the future.
The potential for further policy in this direction shifts industries towards carbon neutral production lines. One potential solution to industrial carbon dioxide emissions is the sequestration of carbon directly from flue gases into microalgae grown in photobioreactors. This solution could potentially create carbon neutral flue gas producing factories due to the ability to sequester carbon dioxide gas into algae before it leaves the system.
Microalgae have unique characteristics that suggest they are suitable candidates for large scale carbon sequestration. Some examples of these is its ability to function in high concentrations of CO2 making to possible when utilizing CO2 from the flue gases produced by power plants. They also are fast growers that have been shown to be able to double their biomass volumes in 24 hrs. More specifically when tested at a flow rate of 0.3 L/min of air with 4 % CO2 concentration, most microalgal strains are able to achieve a carbon-fixation rate of roughly 14.6 gcm-2/day (Bhola, V., Swalaha, F., Kumar, R. R., Singh, M., & Bux, F. 2014).
This research paper will analyze the overall feasibility of using microalgae grown in photobioreactors to sequester carbon by looking at the potential strengths, weaknesses, and requirements for this technology to be implemented.
There are many characteristics of algae that make them attractive candidates for large-scale industrial carbon sequestration. One example is that cyanobacteria and algae do not require arable land, alleviating concern for competition with food crops (Nozzi, N. E., Oliver, J. W. K., & Atsumi, S. (2013). In addition, they also have the ability to grow in water with high levels of salt so there is no additional demand of fresh water, a major cost associated with land crops (Pokoo-aikins, G., Nadim, A., El-halwagi, M., & Mahalec, V. (2010).
Their ability to function in high concentrations of CO2 makes them particularly adept for utilizing CO2 from the flue gases produced by power plants. They also are fast growers that have been shown to be able to double their biomass in 24 hrs. More specifically when tested at a flow rate of 0.3 L/min of air with 4 % CO2 concentration, most microalgal strains are able to achieve a carbon-fixation rate of roughly 14.6 gcm-2/day (Bhola, V., Swalaha, F., Kumar, R. R., Singh, M., & Bux, F. (2014)).
Flue gases often are composed of CO2, water vapor, NOx, SOx and heavy metals such as nickel, vanadium and mercury with CO2 as a major component of industrial flue gases. Using flue gas to culture microalgae is advantageous as the carbon requirement will be provided for by the flue gas. The rate of carbon dioxide (CO2) sequestration is dependent on microalgal productivity, carbon content of selected strains and overall process efficiency.
A study by Stewart and Hessami (2005) investigated the carbon uptake rate of Synechocystis aquatilis and found this strain was able to sequester carbon at a rate of 1.5 g/L/day. Another study by (Cole, A. J., Mata, L., Paul, N. A., & Nys, R. (2014)) found over the 4 week growth trial that Oedogonium sequestered carbon at an average rate of 3.75 (+/-0.23) g/L/day1 , 2.94 (+/-0.48) g/L/day1 and 1.4 (+/-0.18) g/L/day1 in the 7.5, 8.5, and control pH treatments, respectively suggesting that photobioreactors are most effective at pH 7.5.
With further research involving genetic modification, the rate of carbon sequestration and overall hardiness of microalgae at previously unfavorable conditions show a high potential for improvements due to having a faster growth rate than terrestrial plants and a simple genetic background which is relatively easy to manipulate (Koksharova and Wolk, 2002). Specifically bombarding target cells with DNA-coated metal particles has shown to be an effective and highly reproducible method of transformation.
High-velocity tungsten microprojectile transformation has been applied in the transformation of nuclear and chloroplast of many algal species including C. reinhardtii, Volvox carteri, Chlorella sorokiniana, Chlorella ellipsoidea, Chlorella kessleri, Haematococcus pluvialis,Phaeodactylum tricornutum, and G. pectorale (Boynton and Gillham, 1993).
As promising as this technology sounds, it comes with hurdles that must be addressed. First off, flue gases from factories are often released at temperatures of around 120℃. This limits options to thermophilic species of algae or installing a heat-exchange system (Bhola, V., Swalaha, F., Kumar, R. R., Singh, M., & Bux, F. (2014)). Besides concerns about temperature, a study by Watanabe et al. (2000) showed that strains that grew well at CO2 concentrations between 5-10 %, but showed significant decreases in their growth rate and productivity when CO2 concentrations exceeded 20 %.
Another significant hurdle is that microalgae require other inorganic nutrients such as phosphorus and nitrogen to grow. This means that although algae would not compete with food crops for arable land, there may be competition for fertilizer requirements. One study of noted that target production greater than 10 billion gallons per year would require nutrient inputs that could double current fertilizer use (Pate R., Klise G., Wu B, 2011). Phosphate specifically use for fertilizer is sourced through mining, and is considered a limited resource.
Projections differ on when these mines will be exhausted, but the limitation is there(Nozzi, N. E., Oliver, J. W. K., & Atsumi, S. 2013). As for nitrogen, the Haber-Bosch processed may be utilized but nitrogen produced this way is not recycled, and may further contribute to climate change as nitrous oxide greenhouse gas; a gas 300 times more warming than carbon dioxide by mass (EPA).
Often it is economic factors that ultimately decide on the feasibility of a project, and this technology is no different. One major drawback to using closed system photobioreactors (PBRs) is that between the production costs (including labor and running costs), PBRs were significantly more expensive at $111/L when compared to open raceway ponds at $52/L (Alabi 2009). Open raceway ponds have shown to be effective in a study by Kadam (2001) demonstrating that a 1,000-ha open raceway pond could capture 210,000 t/yrCO2 of the 414,000 t/yrCO2 generated by a 50-MW power plant leading to a 50% reduction in CO2 from flue gas emissions.
Potential issues associated with open raceway ponds as opposed to closed-system PBRs is the risk of algal migration. Certain engineered traits of microalgae used for carbon sequestration such as tolerance of harsh conditions or enhanced growth rate, may facilitate the survival of genetically modified algae in sensitive ecosystems. A worst-case scenario of escaped algae is that they may produce toxins or become abundant enough to create harmful algae blooms more so than the algae native to the ecosystem.
As a precaution, J. Craig Venter (Synthetic Genomics) and other researchers have stressed that genetically modified algae should be programmed with “suicide genes” or other characteristics that would make it impossible for feral strains to survive outside of their intended environment (Allison A. Snow, Val H. Smith 2012).
Overall, there are many characteristics that make algae a unique and attractive species for large scale carbon sequestration. Despite the potential of this technology, their drawbacks are critical and must not be overlooked. The direction of research and genetic engineering suggests that future efforts with hardier strains of algae hold the potential for high rates of sequestration, but the costs of nutrients like nitrogen and phosphorus, and the production of PBRs still mean that this technology will be a relatively expensive solution. Perhaps future progress in policy and technology will make carbon sequestration through microalgae a viable investment. For now it remains too costly of a solution that still needs many more leaps in innovation before it may be widely used.
Alabi AO, Tampier M, Bibeau E (2009) Microalgae technologies and processes for biofuels/bioenergy production in British Columbia. The British Columbia Innovation Council, Winnipeg
Allison A. Snow, Val H. Smith; Genetically Engineered Algae for Biofuels: A Key Role for Ecologists. BioScience 2012; 62 (8): 765-768. doi: 10.1525/bio.2012.62.8.9
Bhola, V., Swalaha, F., Kumar, R. R., Singh, M., & Bux, F. (2014). Overview of the potential of microalgae for CO2 sequestration. International Journal of Environmental Science and Technology : (IJEST), 11(7), 2103-2118. Retrieved from http://login.proxy.seattleu.edu/login?url=http://search.proquest.com/docview/1665455715?accountid=28598
Boynton, J. E., Gillham, N. W., Harris, E. H., Hosler, J. P., Johnson, A. M., Jones, A. R., Randolph-Anderson, B. L., Robertson, D., Klein, T. M., Shark, K. B., and Sanford, J. C. (1988). Chloroplast transformation in Chlamydomonas with high velocity microprojectiles. Science 240, 1534–1538.
EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources. U.S. Environmental Protection Agency, Washington, DC, USA.
Kadam KL (2001) Microalgae production from power plant flue gas: Environmental implications on a life cycle basis. Technical report, National Renewable Energy Laboratory Contract No. DE-AC36-99-GO10337
Koksharova, O., and Wolk, C. (2002). Genetic tools for cyanobacteria. Appl. Microbiol. Biotechnol. 58, 123–137.
Nozzi, N. E., Oliver, J. W. K., & Atsumi, S. (2013). Cyanobacteria as a Platform for Biofuel Production. Frontiers in Bioengineering and Biotechnology, 1, 7. http://doi.org/10.3389/fbioe.2013.00007
Pate R., Klise G., Wu B. (2011). Resource demand implications for US algae biofuels production scale-up. Appl. Energy 88, 3377–338810.1016/j.apenergy.2011.04.023
Pokoo-aikins, G., Nadim, A., El-halwagi, M., & Mahalec, V. (2010). Design and analysis of biodiesel production from algae grown through carbon sequestration. Clean Technologies and Environmental Policy, 12(3), 239-254. doi:http://dx.doi.org/10.1007/s10098-009-0215-6
Wang, B., Wang, J., Zhang, W., & Meldrum, D. R. (2012). Application of synthetic biology in cyanobacteria and algae. Frontiers in Microbiology. doi:https://doi.org/10.3389/fmicb.2012.00344
Zheng, Y., Chen, Z., Lu, H., & Zhang, W. (2011). Optimization of carbon dioxide fixation and starch accumulation by tetraselmis subcordiformis in a rectangular airlift photobioreactor. African Journal of Biotechnology, 10(10), 1888-1901. doi:http://dx.doi.org/10.5897/AJB10.16