Review Development of suitable photobioreactors for CO2 sequestration addressing global warming using green algae and cyanobacteria Kanhaiya Kumar a, Chitralekha Nag Dasgupta a, Bikram Nayak a, Peter Lindblad b, Debabrata Das a,��� a Department of Biotechnology, Indian Institute of Technology Kharagpur 721302, India b Department of Photochemistry and Molecular Science, Uppsala University, Sweden a r t i c l e i n f o Article history: Received 15 October 2010 Received in revised form 15 January 2011 Accepted 17 January 2011 Available online 1 February 2011 Keywords: Photobioreactor Green algae Cyanobacteria Carbon dioxide fixation Global warming a b s t r a c t CO2 sequestration by cyanobacteria and green algae are receiving increased attention in alleviating the impact of increasing CO2 in the atmosphere. They, in addition to CO2 capture, can produce renewable energy carriers such as carbon free energy hydrogen, bioethanol, biodiesel and other valuable biomole- cules. Biological fixation of CO2 are greatly affected by the characteristics of the microbial strains, their tolerance to temperature and the CO2 present in the flue gas including SOX, NOX. However, there are addi- tional factors like the availability of light, pH, O2 removal, suitable design of the photobioreactor, culture density and the proper agitation of the reactor that will affect significantly the CO2 sequestration process. Present paper deals with the photobioreactors of different geometry available for biomass production. It also focuses on the hybrid types of reactors (integrating two reactors) which can be used for overcoming the bottlenecks of a single photobioreactor. �� 2011 Elsevier Ltd. All rights reserved. 1. Introduction Global warming has been reached to an alarming level due to the change in global environment. Industries related to electricity generation, natural gas processing, cement, iron and steel manu- facturing, combustion of municipal solid waste are the major con- tributors of atmospheric CO2 because of their dependence on carbon sources like coal, oil, natural gas for fulfilling their energy requirement (Inventory of U.S greenhouse gas emissions and sinks: 1990���2008). According to the report of carbon dioxide information analysis center (CDIAC), CO2 emissions have increased from 3 met- ric tons in 1751 to 8230 metric tons in 2006. Alarming feature of CO2 emission can be understood by the trends of its presence in atmosphere at Mauna loa observatory (Hawaii, US) which shows 390 ppmv in 2010 compared to 280 ppmv in 1958. Keeling curve clearly indicates initially the slow and latter progressively faster rise in the concentration of CO2 (Tans, 2010). Sequestrations of CO2 from the industries are today���s demand in order to reduce the impact of CO2 on global warming. Sequestration strategies adopted so far can be broadly divided into physical and biological means. Physical means of CO2 sequestration has disadvantages, having high costs associated with it thereby need to develop the suitable technologies. Capturing, transporting and storing CO2 are also very expensive processes. Biological method of CO2 sequestra- tion is an alternative to physical methods. The use of algae for CO2 sequestration has several advantages: mitigating CO2, the major source of global warming as well as producing biofuels and other interesting secondary metabolites. One kilogram of algal dry cell weight utilizes around 1.83 kg of CO2. Annually around 54.9��� 67.7 tonnes of CO2 can be sequestered from raceway ponds corre- sponding to annual dry weight biomass production rate of 30��� 37 tonnes per hectare (Brennan and Owende, 2010). Algal biomass can be used for the production of biofuels (e.g. biodiesel, bioetha- nol, biohydrogen) and other commercially and scientifically impor- tant products like industrial biofilters, food products, water quality testing (Loubiere et al., 2009). The major problem associated with the biological use of CO2 are the high temperatures of flue gas and the presence of NOx, SOx as well as other impurities of the fossil fuel used. For the cultivation of algae for CO2 sequestration both open as well as closed systems are used. However, open system has disadvantage to control parameters like availability of light, agitation, pH, temperature and nutrient concentrations. Fluctua- tion in temperature and light availability due to diurnal cycles and seasonal variations are a major problem for open systems (Brennan and Owende, 2010). Use of open system for sole aim of CO2 sequestration is being downplayed because of the very low residence time of the sparged gas in the culture which gives very little time to algal biomass to sequester CO2 from flue gas. It is also susceptible for high contamination which reduces the biomass productivity and its use for the production of commercially impor- tant products. In a closed system the degree of control is very high and it is possible to control crucial parameters that influence the culture (Carvalho et al., 2006). 0960-8524/$ - see front matter �� 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.01.054 ��� Corresponding author. E-mail addresses: ddas.iitkgp@gmail.com, ddas@hijli.iitkgp.ernet.in (D. Das). Bioresource Technology 102 (2011) 4945���4953 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

This paper is mainly focused on factors affecting the sequester- ing of CO2 from industrial flue gas by microalgae and discussed about the various types of photobioreactors with different geome- tries and parameters implemented for CO2 sequestration and bio- mass production. It also gives a clear view on suitable photobioreactors for CO2 sequestration to be used in the future. 2. Microbiology Green algae and cyanobacteria (formally blue-green algae) comprise a vast group of photosynthetic organisms. They are ubiq- uitously distributed throughout the biosphere and grow under the widest possible variety of conditions from aquatic (freshwater to extreme salinity) to terrestrial places. Its uniqueness that separates them from other microorganisms is due to presence of chlorophyll and having photosynthetic ability in a single algal cell, therefore allowing easy operation for biomass generation and effective ge- netic and metabolic research in a much shorter time period than conventional plants. Well defined nucleus, a cell wall, chloroplast containing chlorophyll and other pigments, pyrenoid, a dense re- gion containing starch granules on its surface, stigma, and flagella are the major components of green algae (Michael et al., 2008). Fil- amentous colonies of cyanobacteria have ability to differentiate into different cell types like vegetative cells, akinetes, and hetero- cysts. General function of vegetative cells, akinetes and heterocysts are ability to carry out complete oxygenic photosynthesis, resis- tance for climate and having a potential to fix nitrogen, respec- tively. Heterocysts contain the enzyme complex nitrogenase which converts atmospheric nitrogen into ammonium, a unique capacity among photosynthetic oxygenic organisms. These are the only known prokaryotes having oxygenic photosynthesis for fixation of CO2 like eukaryotic algae and plants (Michael et al., 2008). 3. Biochemistry of CO2 fixation In a multistep process of photosynthesis plants and algae (green algae and cyanobacteria) fix CO2 into sugar using light and water as energy and electron source, respectively. The overall reaction for photosynthesis is given by: CO2 �� H2O �� light ! ��CH2O��n �� O2 The step of photosynthesis in which CO2 is converted into sugar with the help of ATP (adenosine-50-triphosphate) by the carboxyl- ase activity of the enzyme ribulose 1,5-bisphosphate carboxylase/ oxygenase (RuBisCO), is called as Calvin cycle (Nelson and Cox, 2005). Synthesis of one mole CH2O, requires a minimum of 8 mol of photons (quanta) each having 218 kJ of energy per mol. Photo- synthesis converts approximately 27% of solar energy into chemi- cal energy as it produces 467 kJ of energy per mol of CH2O as against 1744 kJ required per mol for its formation (Brennan and Owende, 2010). Concentration of CO2 in water in equilibrium with air is approximately 10 lM. However, since RuBisCO has low affin- ity for CO2, at the normal atmospheric level of CO2 (390 ppmv) it is only half saturated with the CO2. Moreover it also performs oxy- genase activity which produces glycolate 2-phosphate as the end product. It has no use to cell and its synthesis consumes significant amount of cellular energy and also releases previously fixed CO2 by the carboxylase activity of RuBisCO. The oxygenase activity of RuBisCO inhibits biomass formation of around 50% (Giordano et al. 2005). To overcome the low affinity of RuBisCO for CO2, most algae and cyanobacteria have different CO2 concentrating mecha- nisms (CCMs). CCMs activates only at low dissolved carbon con- centration. The maximum value of dissolved inorganic carbon till which it is active depends upon strain, pH, light availability, pread- aptation of cells etc. For example, in cyanobacteria Km(CO2) is 200 lM as against approximately 10 lM dissolved CO2 in water in equilibrium with air (Moroney and Somanchi, 1999). Similarly, in Chlorella ellipsoidea at pH 7.5, the minimum equilibrium dis- solved inorganic carbon (DIC) concentration at which high CO2 characteristics were maintained, i.e. transport was repressed, was 2100 lM, whereas the maximum equilibrium DIC concentration below which DIC transport was fully induced was 500 lM (Matsu- da and Colman, 1995). CCMs acts as an enhancer for higher growth rates in algae and hence can be used for improvement in photobi- oreactor productivity (Ramanan et al., 2010). The expression of the enzyme carbonic anhydrase (CA) has been associated with Table 1 Temperature and flue gas tolerance of various algal species (Ono and Cuello, 2004). Algal species Maximum temperature tolerance (��C) Maximum CO2% (v/v) tolerance Maximum SOx (ppm) tolerance Maximum NOx (ppm) tolerance References Cyanidium caldarium 60 100 ��� ��� Seckbach et al., 1972 Scenedesmus sp. 30 80 ��� ��� Hanagata et al., 1992 Chlorococcum littorale ��� 70 ��� ��� Ota et al. 2009 Synechococcus elongates 60 60 ��� ��� Miyairi. 1995 Euglena gracilis ��� 45 ��� ��� Nakano et al., 1996 Chlorella sp. 45 40 ��� ��� Hanagata et al., 1992 Chlorella sp. HA���1 ��� 15 ��� 100 Yanagi et al., 1995 Eudorina sp. 30 20 ��� ��� Hanagata et al.,1992 Dunaliella tertiolecta ��� 15 ��� 1000 Nagase et al., 1998 Chlamydomonas sp. MGA 161 35 15 ��� ��� Miura et al., 1993 Nannochloris sp. 25 15 ��� 100 Yoshihara et al., 1996 Tetraselmis sp. ��� 14 185 125 Matsumoto et al., 1995 Monoraphidium minutum 25 13.6 200 150 Zeiler et al., 1995 Spirulina sp. ��� 12 ��� ��� de Morais and Costa, 2007 Chlorella sp. T-1 35 ��� 20 60 Maeda et al., 1995 4946 K. 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