GW Final April 1, 2008

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Carbon sequestration for mitigating hazardous effects of global warming

Adarsh K. Puri and T. Satyanarayana*

Department of Microbiology, University of Delhi South Campus, New Delhi, India

*Corresponding Author, E-mail: [email protected]

Abstract

The concentration of atmospheric CO2 has increased in an unprecedented manner after the

industrial revolution. The whole scientific community, politicians and industrialists

throughout the world are debating on the development of strategies to bring down the

atmospheric carbon levels to the acceptable limits. Carbon sequestration has emerged as the

most potential and effective way for mitigating global warming in the past few years.

Carbon capture and storage in ocean and deep geological formations are the promisingsolutions. The current focus now, however, is on novel ideas that ensure a leakage-proof and

cost-effective approach for long term and maximal storage of CO2. Capturing carbon by

biological means is not only a mean of sequestering carbon, but may also lead to the

production of useful products.

1. Introduction

The thawing permafrost (Kolchugina and Vinson, 1993), melting glaciers (U.N. Environment

Programme, 2008; Vaughan et al., 2003; Dyurgerov and Meier, 2000), raising sea levels (Meier

and Wahr, 2002), changing hydrological cycle (Held and Soden, 2006; Mirza, 2002), increasing precipitation (Schnur, 2002), declining crop productivity (Tan and Shibasaki, 2003; Ortiz et al.,

2008), early breeding of birds (Brown et al., 1999; Sydeman et al., 2001; Wormworth and

Mallon, 2007), vanishing coral reefs (Donner et al., 2005; Crabbe, 2007), increasing health

hazards (McMichael and Woodruff, 2004; Sutherst, 2004) are all predictable effects of the

hottest debatable phenomenon on earth called global warming. Global warming, frequently

referred to as climate change, is not just a theory or a distant threat. The 2007 Nobel peace prize

to Intergovernmental Panel on Climate Change (IPCC) and Albert Arnold (Al) Gore Jr. has made

every one realize the severity of the problem, and further, cleared all doubts being raised by

naysayers over its ‘reality’. The overwhelming agreement among the world’s prominent

scientists, governments and scientific bodies is that the Earth is heating up and that human

activities are largely to blame [IPCC, 2007; National Research Council (NRC) 2001]. The global

warming is expected to significantly disrupt the planet’s climate system. Minimization of

greenhouse gas emissions to acceptable limits is the intrinsic environmental responsibility of the

whole world.

1

mailto:[email protected]

mailto:[email protected]

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Over the last 200 years since the Industrial Revolution, most of the world’s energy has been

derived from burning finite resources of fossil fuels, mainly coal, oil and more recently, gas

(Ansolabehere et al., 2007). Fossil fuels account for 80% of the global energy demand (Table 1).

Table 1. Fuel sources and world energy demand [Source: Ansolabehere et al., 2007]

Sources Global energy demand met

Coal 25%

Natural gas 21%

Petroleum 34%

Nuclear 6.5%

Hydro 2.2%

Biomass and waste 11%

Geothermal, Solar and Wind 0.4%

During the process, a billion tons of carbon dioxide and other green house gases (GHGs) have

been spewed into the atmosphere. Energy sector accounts for the greatest share (36%) of carbon

dioxide emissions. A large 1000 Megawatt coal power station releases around 5.5 million tons of

CO2 annually (Evans and Furlong, 2003).

Earth’s atmosphere is essentially transparent to incoming radiation from the Sun, as sunlight

peaks in the visible part of the spectrum. On the other hand, thermal radiation from the Earth, inthe form of long-wavelength infrared rays, lies in the absorption spectrum of carbon dioxide and

other GHGs. These GHGs absorb radiation primarily in a very narrow frequency band (7-13µm),

while CO2 absorbs over a much larger (13-19µm) spectral range (Halmann and Steinberg, 1999).

This is why CO2 accounts for 21% of the greenhouse effect (after water vapour that accounts for

64%) than ozone (6%) and other trace gases (9%) (Barry and Charley, 1992; Marsh, 2001).

Moreover, carbon dioxide makes up 68% of the total greenhouse gas emissions (Harrington and

Foster, 1999).

The atmospheric CO2 concentration has increased from 280 ppm in 1800, the beginning of

industrial age, to 380 ppm today (IPCC, 2007; Takashaki, 2004; NOAA Climate Monitoring and

Diagnostics Laboratory, 2003), and without any mitigation, it could reach levels of 700-900 ppm

by the end of the 21st century, which could bring about severe climate change (Houghton et al.,

2001; IPCC, 2001). The annual CO2 concentration growth rate was larger during the last 10 years

(1995-2005 average: 1.9 ppm per year) than it has been since the beginning of direct atmospheric

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measurements. In fact, eleven of the last twelve years (1995-2006) rank among the twelve

warmest years since 1850 (IPCC, 2007; Hadley centre, 2006).

This abrupt imbalance has disturbed the Earth’s carbon cycle that is normally kept in balance by

the oceans, vegetation, soil and the forests. The most pressing technical and economic challenge

of the present time is to supply energy demand for the world economic growth without affecting

the Earth’s climate. That is why the current focus is on reducing fossil fuel usage and minimizing

the emission of CO2 in atmosphere (Evans and Furlong, 2003). In spite of the great advances

made in the field of renewable energy, it has not been possible to replace gas, coal and oil to

meet the current energy needs (Collins, 1998). If fossil fuels, particularly coal, remain the

dominant energy source of the 21st century, then stabilizing the concentration of atmospheric

CO2 will require development of the capability to capture CO2 from the combustion of fossil

fuels and store it safely away from the atmosphere (House et al., 2006).

The hazards of global warming have reached to a magnitude that irreversible changes in the

functioning of the planet are seriously feared. And mankind has been ruthless (harmful rather

useless)! It is, therefore, implacable for the whole scientific community to restore permissible

levels of CO2 by using the existing knowledge.

Carbon sequestration or carbon capture and storage (CCS) has emerged as a potentially

promising technology to deal with the problem of global warming. Several approaches are being

considered, including geological, oceanic, and terrestrial sequestration, as well as CO 2 conversion

into useful materials.

In this chapter, an attempt has been made to review the possible strategies for carbon

sequestration. An emphasis has been laid on biological ways of carbon sequestration due to the

drawbacks associated with ocean and geological sequestration approaches. This includes normal

photosynthetic ways of carbon fixation, exploiting enzymatic machinery and metabolic pathways

of microbes and use of various biomimetic approaches.

2. Gases contributing to global warming: Green House Gases (GHGs)

Molecules of various greenhouse gases trap the heat that is expected to escape from earth. The

extent of greenhouse effect contributed by different gases over a certain time frame is expressed

in terms of their individual Global Warming Potential (GWP) taking CO 2 as the reference gas

(Table 2).

The main greenhouse gases produced by human activity are carbon dioxide (CO2), methane

(CH4), nitrous oxide (N2O) and some halogenated compounds with high-GWP. Perfluorocarbons

(PFCs), sulphur hexafluoride (SF6) and hydrofluorocarbons (HFCs) were added to the list of

green house gases under the Kyoto Protocol to the United Nations Framework Convention on

Climate Change (UNFCCC) in 1997. Non-CO2 greenhouse gases are also a matter of concern

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owing to their significant contribution (≈30%) to the overall anthropogenic greenhouse effect

since preindustrial times (IPCC, 2001).

Table 2. Global warming potential of selected greenhouse gases.

Greenhouse Gases

(GHGs)

Source Average rate of

increase

(in % per year)

Global warming

potential

(GWP) †

Estimated

atmospheric

lifetime(in years)

Carbon dioxide (CO2) Fossil fuel combustion,

deforestation, changing land

use, biomass burning,

erosion

1.1-3.0Ψ 1 NR

Methane (CH4) Enteric fermentation in cattle

and insects, biomass

burning, waste burial, coal

mines, gas leaks, rice fields,

swamps and tundra

0.8-1.0* 21 12

Nitrous oxide (N2O) Aerosols, refrigeration and

air conditioning, plastic

foams, solvents, computer

industry, sterilants, medical

supplies

0.25 310 14

Sulphur hexafluoride (SF6) Electrical insulation,

semiconductor manufacture ,

magnesium foundries (cast)

7 23,900 3,200‡

Perfluorocarbons (PFCs) Primary aluminum

production and

semiconductor manufacture

1.3-3.2 6,500- 9,200 2,600- 50,000

Hydrofluorocarbons

(HFCs)

Man made alternatives to

ozone depleting substances

(ODSs)

High variation 140-11,700 1-260

Source: USEPA, 2006; IPCC, 2001.

Ψ Commonwealth Scientific and Industrial Research Organization, 2007.

† Million metric tons of CO2 equivalent over 100 year integration time (MMT CO2 Eq.).

* Milich, 1999; Glantz and Krenz, 1992.

‡ Fenhann, J. (2000)

NR = not reported because this values depends greatly on assumptions.

The amount of anthropogenic CO2 emitted to the atmosphere is much greater than any of other

greenhouse gases. As a result, CO2 makes the highest contribution to the greenhouse effectdespite its low GWP.

3. Carbon sequestration and its importance

Carbon sequestration can be defined as the capture and secure storage of carbon that would

otherwise be emitted to or remain in the atmosphere. The idea is to keep carbon emissions produced by human activities from reaching the atmosphere by capturing and diverting them to

secure storage or to remove carbon from the atmosphere by various means and storing it (DOE,

1999).

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Carbon sequestration could be a major tool for reducing carbon emissions from fossil fuels.

Much work, however, remains to be done to understand the science and engineering aspects and

potential of carbon sequestration options. Given the magnitude of carbon emission reductionsneeded to stabilize the atmospheric CO2 concentration, multiple approaches to carbon

management will be needed.

The natural carbon cycle is balanced over a long term, but dynamic over the short term.

Historically, acceleration of natural processes that emit CO2 is eventually balanced by the

acceleration of processes that sequester carbon, and vice versa. The current increase inatmospheric carbon is the result of anthropogenic mining and burning of fossil carbon, resulting

in carbon emissions into the atmosphere. Developing new sequestration techniques and

accelerating existing techniques would help in diminishing the net positive atmospheric carbon

flux.

4. General methods of carbon sequestration

4.1. Ocean sequestration

Oceans cover over 70% of the Earth’s surface with an average depth of about 3800 metres.

Depending upon the oceanic equilibrium with the atmosphere, a significant amount of captured

CO2 could be deliberately injected into the ocean at great depth, where it would remain isolated

from the atmosphere for centuries.

4.1.1. Direct injection of CO2

Direct ocean CO2 disposal, first suggested by Marchetti in 1977, is now the biggest hope to use

ocean as the largest sink for carbon sequestration purposes. A large literature is now available

which has brought significant technological developments (Kobayashi, 1995; Aya et al., 1997;

Brewer et al., 2000; Fer and Haugen, 2003) and improved our understanding the disposal of CO 2

directly into the ocean.

The research on ocean disposal options has mostly focused on predicting the behavior and the

dissolution time scale of the released CO2. Different scenarios of CO2 disposal in the ocean have

been proposed at various depths and in different forms in relation to the phase properties of CO 2

(Ozaki et al., 2001; Shitashima et al., 2008). CO2 can be released directly in to the ocean in any

of its physical forms- gas, liquid, solid or solid hydrate. But, it is important to study CO2 induced

density changes on the fluid dynamics of the ocean before its release. Dissolved CO 2 increases

the density of seawater (Fig.1) that affects its transport and mixing (Bradshaw, 1973; Song et al.,

2005). Density of Injected CO2 is also controlled by geothermal gradient, which varies from

0.02oC/m to 0.04oC/m. The rate of CO2 dissolution in the seawater depends upon its physical

form (gas, liquid, solid or solid hydrate), the depth and temperature of disposal, and the local

water velocities.

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Fig.1. Density profiles of liquid CO2 at 0

o

C and 10

o

C (solid curves), CO2-saturated seawater (dashed line), and seawater (dotted line). [Source: Fer and Haugen, 2003]

CO2 disposal in gas/liquid/solid phase

CO2 could potentially be released as a gas above 500m depth. However, due to lesser density of

gas bubbles than surrounding seawater, these bubbles tend to rise up on the surface, dissolving at

a radial speed of about 0.1 cm hr -1 (Teng et al., 1996). It is better to use CO2 diffusers to produce

smaller CO2 bubbles, which can dissolve completely before reaching the surface.

CO2 can exist as a liquid below roughly 500m to 2,500m depth. Due to lower temperature

(<9 oC), CO2 hydrate tends to form on the droplet wall. Under these conditions, the radius of

droplet would diminish at a speed of about 0.5 cm hr -1 (Brewer et al., 2002). Below roughly

3,000 m, liquid CO2 becomes denser than the surrounding seawater, and hence, tends to sink.

Similarly CO2 released in solid form is also denser and would sink to sea floor dissolving at a

radial speed of about 0.2 cm hr -1 (Aya et al., 1997). Large masses of solid CO2 reach sea floor

before complete dissolution.

CO2 Hydrate

CO2 hydrates (5.75 H2O. CO2) are nonstoichiometric crystalline compounds that form at high

pressure (greater than; 4.5 MPa) and low temperature (less than 9.85o C) by trapping CO2

molecules in hydrogen- bonded cages of H2O (Lee et al., 2002; Fer and Haugen, 2003). It can

form in average ocean waters below 400m depth. Being 15% denser than seawater (Wannamaker

and Adams, 2002), these hydrates tend to sink and dissolve rapidly into the relatively dilute

ocean waters at a speed similar to that of solid CO2 (about 0.2 cm/hr -1) [Rehder et al., 2004; Teng

et al., 1999]. It is, however, not feasible to release CO2 hydrates directly through the pipelines.

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That is why a paste like composite of hydrate and seawater has been proposed to be used for

sequestration purposes (Tsouris et al., 2004).

Liquid CO2 Lakes

When liquid CO2 is injected into a sea floor depression at a depth greater than 3,000 m (where itis denser than sea water), it accumulates as a stable large “lake" of CO 2. The dissolution of these

liquid CO2 lakes is retarded by formation of a thin hydrate layer over it (Ohsumi, 1993; Fer and

Haugen, 2003).

While investigating different kinds of discharge pipes for CO2 lake creation on sea floor,

Nakashiki (1997) proposed a ‘floating discharge pipe’ that was simple and less likely to be

damaged by wind and wave in storm conditions. Slurry of liquid CO 2 mixed with dry ice in

discharge pipe provides good conditions for lake formation (Aya et al., 1997).

4.1.2. Natural oceanic mineralization

Oceans can sequester so much of CO2 not only because of their large volume but also because

CO2 dissolves in water to form various ionic species that increases the total dissolved inorganic

carbon (DIC) of seawater. Total dissolved inorganic carbon (DIC) is the sum of carbon contained

in H2CO3, HCO3- and CO3

-2.

CO2 (g) + H2O H2CO3 (aq.) HCO3- + H+ CO3

-2 + 2H+

Ocean surface water is supersaturated with respect to calcium carbonate, while the deeper ocean

water would be with lower pH and remain under saturated. This makes organisms to produce

calcium carbonate particles (e.g. corals) in the surface oceans, which settle and dissolve in under

saturated regions of deep oceans.

5. Geological sequestration

Since the first use of CO2 for large-scale recovery of residual oil from Texas reservoirs in 1972,

the concept of using CO2 for beneficial purposes has got momentum. Long term operational

experience with geological formations, its substantial capacity as a CO2 sink (Table 3) and its

immediate availability has led to consideration of global warming problem through geological

sequestration (Klara et al, 2003). Geological formations include depleted oil and gas fields, deepsaline reservoirs and unminable coal seams.

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Table 3. Estimates of storage capacities for different geological reservoirs

Source: Gale, 2004.

Storage option Global capacityGt CO2 % of emissions to 2050

Depleted oil and gas fields 920 45

Deep saline reservoirs 400- 10,000 20-500

Unminable coal seams 20 <2

CO2 can be trapped in geologic formations by three principal trapping mechanisms (DOE, 1993):

(1) hydrodynamic trapping, where CO2 can be trapped under a low-permeability caprock like gas

reservoirs or aquifers, (2) solubility trapping, where CO2 can be trapped in a dissolved phase in a

liquid- like petroleum and (3) physical/mineral trapping, a relatively slower process which

involves conversion of CO2 in the form of calcium, magnesium or iron carbonates.

5.1. Depleted oil and gas reservoirs

A considerable amount of oil or gas is often present in ‘depleted’ oil and gas reservoirs following

rigorous primary recovery processes (Gentzis, 2000). CO2 is being injected into oil reservoirs as

an established and successful technique (over 80 projects worldwide; Wells et al., 2007) for

enhanced oil recovery (CO2-EOR). Injected CO2 reduces the interfacial tension between oil and

the reservoir rock, expands the volume of oil (oil swelling) thereby reducing its viscosity and

making its easier mobility towards the production well (oil recovery enhanced by 10-15%;

Davison et al., 2001). Alternatively, in situations where CO2 is immiscible with oil, CO2 is

injected to increase the reservoir pressure helping to push more oil towards the production well.Up to half of the injected CO2 is stored in the immobile oil remaining in the reservoir at the end

of production. The rest is collected from the production well and get re-circulated. This improves

the overall economics for sequestration projects (Holloway, 2005).

Gas fields have much higher primary recovery rates (80-95%) than oil fields (Gale, 2004). This

leaves a big void space in the reservoirs, which can be used for CO2 storage as a supercritical gas

for thousand of years. Similarly, the void space that had previously been occupied by oil and

natural gases is being used for large-scale sequestration of CO2.

5.2. Deep saline aquifers

A large amount of underground water filled strata (aquifers) is too salty to be used for agriculture

or human consumption. These aquifers can potentially be used as long-term CO 2 reservoirs (IEA,

2001; IPCC, 2005). CO2 injected (with techniques similar to those for gas and oil fields) into

these aquifers would displace brine and some of it would get partially dissolved (Nordbotten et

al., 2005). A part of the injected CO2 is also reported to react with calcite and aluminosilicates to

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form permanent carbonates. The best example of CO2 storage in deep saline aquifer is the

Sleipner project in the North Sea, which sequesters approximately 1 Mt CO2 annually (IPCC,

2005).

5.3. Deep unminable coal seams

Storing CO2 deep into unminable coal seams appears to be a good approach due to its value

added benefit of CO2 – enhanced coal bed methane (ECBM) recovery. Coal beds typically

contain large amounts of methane-rich gas (Holloway, 2005) that is adsorbed onto the surface of

the coal. CO2 adsorbs more strongly on the micropores of coal than methane (CH4). However, the

volumetric ratio of adsorbable CO2:CH4 depends on the type of coal. This ratio ranges from 1 for

anthracite to about 10 for lignite coal (IPCC, 2005). This can be exploited to lock CO2

permanently on the micropores of coal provided the coal is never mined. Over 100,000 tons of

CO2 has been successfully injected at Allison Unit in New Mexico, USA during a ECBM project

(Davison et al., 2001).

However, the scope of geological trapping is currently economically limited to point sources of

CO2 emissions that are near geological formation of choice. Continuous monitoring along with

exhaustive geophysical and geochemical study is needed to make sure the injected CO2 stays in

ground.

6. Drawbacks associated with artificial approaches of carbon sequestration

Permanence of the stored carbon through abiological sequestration methods is of great critical

concern. Ocean and geological storage of carbon dioxide is associated with future risk of leakage

from the site of injection (Oldenburg and Unger, 2003). Sequestered CO2 may leak back into the

atmosphere and impose future climate damages. If CO2 migrates out of the receiving geological

formation and rises to the surface, it could cause local ecological damage, primarily by

displacing soil gas and affecting plant roots. Moreover, upward migration of injected CO 2 could

contaminate hydrocarbon reservoirs or surface drinking water supplies. In rare cases, rapid

escape of CO2 may cause asphyxiation or toxicity risks to local animal and human populations.

The limnic eruption of CO2 during 1986 at Lake Nyos, West of Cameroon is the most evident

example, which killed more than 1700 people (Clarke, 2001).

Deep-sea organisms are highly sensitive to any environmental disturbances. Increased partial

pressure of CO2 (hypercapnia) and decreased pH of seawater caused by CO2 dissolution may

affect the whole marine biodiversity (Shirayama, 1997).

The scientific community is trying to get rid of leaky sequestration approaches. Novel concepts

are being contemplated to find the most environment-friendly way to sequester CO2. This

includes the art of exploiting natural biological ways of capturing carbon and storing it in the

most eco-compatible way.

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7. Biological ways of carbon sequestration

Biological systems have solutions to the most dreaded problems of all times. The photosynthetic

fixation of atmospheric CO2 in plants and trees could be of great value in maintaining a CO 2

balance in the atmosphere. Algal systems, on the other hand, being more efficient in

photosynthetic capabilities are the choice of research for solving global warming problem. The

biomass thus produced could be used as fuel for various heating and power purposes.

Mankind is indebted to microbes for bringing and maintaining stable oxygenic conditions on

Earth. A proper understanding of microbial systems and their processes will help in stabilizing

atmospheric conditions in future too. Investigations are in progress to exploit carbonic anhydrase

and other carboxylating enzymes to develop a promising CO2 mitigation strategy. Recent work

on biomimetic approaches using immobilized carbonic anhydrase in bioreactors has a big hope

for the safe future.

7.1. Exploiting Photosynthesis

7.1.1. Terrestrial carbon sequestration

The process of carbon assimilation by photosynthesis has made forests, trees and crops as the

major biological scrubbers of CO2. Terrestrial biomes are potential CO2 sinks (Table. 4).

7.1.1.1. Forest lands

Afforestation (Ozawa et al., 1995) and reforestation (Yokoyama, 1997) leads to a net increase in

plant carbon stocks. A young growing forest sequesters more carbon than a matured one. Forest

management can contribute to carbon sequestration by promoting forest growth and biomass

accumulation (Sedjo, 2006).

7.1.1.2. Agricultural lands

Croplands: Improved cropland management (including agronomy, nutrient management,

tillage/residue management and water management) has significant carbon sequestration

potential (Dendoncker et al., 2004; Smith, 2004). Worldwide adoption of best management

practices (BMPs) can sequester a considerable part of the lost carbon back into croplands (Lal et

al., 1998).

Table 4. Global potential carbon sequestration (CS) rates of terrestrial carbon sinks

Terrestrial carbon sinks Potential CS (GtC / year)

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Agricultural lands 0.85-0.90

Biomass croplands 0.5-0.8

Grasslands 0.5

Rangelands 1.2

Forests 1-3

Deserts and degraded lands 0.8-1.3

Terrestrial sediments 0.7-1.7

Boreal peatlands and other

wetlands

0.1-0.7

Total 5.65-10.1

Source: DOE, 1999.

Grasslands: Grasslands cover about 70% of the world’s agricultural area (Soussana and Luscher,

2007). Recent studies have suggested that tropical grasslands and savannas sequester

approximately 0.5 Gt of carbon annually (Scurlock et al., 1998). There are reports of increased

CO2 uptake in calcareous grasslands during day time at elevated CO2 levels (Amthor, 1995;

Stocker et al., 1997; Niklaus et al., 2000).

Range lands: Rangelands (including grasslands, shrub lands, deserts and tundra) occupy abouthalf of the world’s land area, and contain more than a third of above- and below-ground C

reserves (Allen-Diaz, 1996). Grazing and burning in rangelands have resulted in increased soil

organic carbon storage (Schuman et al., 2002; Rice, 2000)

7.1.1.3. Biomass croplands

This includes croplands, which apart from assimilating carbon and increasing soil organic matter

produce value added products (e.g. biofuels).

7.1.1.4. Urban shade trees

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Urban trees play a major role in sequestering CO2. One tree in urban area is equivalent to three to

five forest trees. The average sequestration rate of an urban tree of 50m2 crown area has been

estimated to be about 11-19 kg/year (Nowak, 1994; Akbari, 2002).

7.1.1.5. Soil sequestration

Plants assimilate carbon through the process of photosynthesis and return some of it to the

atmosphere through respiration. After the death and decomposition of plants, carbon in the form

of plant tissue is either consumed by animals or added to the soil as litter. The primary way that

carbon is stored in the soil is as soil organic matter (SOM) . SOM is a complex mixture of carbon

compounds, consisting of decomposing plant and animal tissue, microbes (protozoa, nematodes,

fungi, and bacteria) and carbon associated with soil minerals. Soils contain three times more

carbon than the amount stored in living plants and animals (Houghton et al., 1985). Increasing

the soil organic carbon (SOC) by 0.01% would nullify the annual increase in atmospheric carbon

due to anthropogenic CO2 emissions (Cole et al., 1996).

7.1.1.5.1. Role of microbial communities in soil

Microbial community structure and various microbial processes have been shown to directly

affect carbon sequestration in soil agro ecosystems. A thorough understanding of microbial

community structure and processes is required for enhanced carbon sequestration in agricultural

soils. A balance between microbial community dynamics and formation and degradation of

microbial byproducts maintains the soil carbon content. Soil microbes also indirectly influence C

cycling by improving soil aggregation, which physically protects SOM. Consequently, the

microbial contribution to C sequestration is governed by the interactions between the amount of

microbial biomass, microbial community structure, microbial byproducts, and soil properties

such as texture, clay mineralogy, pore-size distribution, and aggregate dynamics (Six et al.,

2006).

Fungi and bacteria have been found to be responsible for most of the carbon transformations and

long-term storage of carbon in soils. However, chances of persistent C storage are more in fungi

due to their complex chemical composition (Holland and Coleman, 1987; Guggenberger et al.,

1999) and higher carbon utilization efficiency. In fact, increased fungal to bacterial activity has

been shown to be associated with increased carbon stored in soil (Bailey et al., 2002).

7.1.1.6. Carbon concentrating mechanisms (CCM)

Photoautotropic organisms ranging from bacteria to higher plants have evolved with unique

carbon concentrating mechanism (CCM) in response to the declining levels of CO 2 in their

surrounding environment. It is proposed that ribulose-1,5-biphosphate carboxylase/oxygenase

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(Rubisco) co-evolved during the process. The organization of the carboxysomes in prokaryotes

and of the pyrenoids in eukaryotes, and the presence of membrane mechanisms for inorganic

carbon (Ci) transport are central to the concentrating mechanisms (Kaplan and Reinhold, 1999).

There can be different types of CCM based on the biochemical mechanisms in different

photoautotropic organisms such as C4 photosynthesis and crassulacean acid metabolism (CAM)

in terrestrial higher plants, active transport of inorganic carbon (C i) primarily in cyanobacteria

and CO2 concentration following acidification in a compartment adjacent to Rubisco found in

some eukaryotic algae (Moroney and Ynalvez, 2007).

Higher terrestrial plants having crassulacean acid metabolism (CAM) primarily capture CO 2

through PEP carboxylase located in the cytosol of their mesophyll cells. PEP carboxylase uses

bicarbonate as its primary substrate for fixation of CO2 into oxaloacetate, thus CO2 entering from

the external environment must be hydrated rapidly by a carbonic anhydrase (CA) and converted

to bicarbonate. C4 carboxylic acids such as malate or aspartate formed in the mesophyll cell

cytosol serve as the intermediate CO2 pool.

CCM found in eukaryotic algae relies on the pH gradient set up across the chloroplast thylakoid

membrane in the light. Light-driven photosynthetic electron transport sets up a pH around 8.0 in

chloroplast stroma and a pH between 4 to 5 inside the thylakoid lumen. Under these conditions,

bicarbonate is the predominant species of Ci in the chloroplast stroma, while CO2 is the most

abundant form of Ci in the thylakoid lumen. Bicarbonate transporters on thylakoid membrane are

proposed to help bicarbonate transport inside thylakoid lumen where it is converted into CO2

with the help of carbonic anhydrase (Pronina et al., 1981; Pronina and Semenenko, 1990;

Moroney and Ynalvez, 2007).

The CCM mechanisms make it possible for cells to enhance the delivery of CO 2 to ribulose-1,5-

biphosphate carboxylase/oxygenase (Rubisco) and limit the oxygenase activity of this enzyme

(Raven and Falkowski, 1999; Omata et al., 2001; Miyachi et al., 2003; Ogawa and Kaplan,

2003). Cyanobacterial CCM is proposed to have developed in the Proterozoic era in response to

falling pCO2 and rising pO2 levels. The CO2 taken up by the cyanobacterial cell is converted into

HCO3- by extracellular carbonic anhydrase that diffuses into the carboxysome. Here HCO3

- is

once again converted back into CO2 by intracellular carbonic anhydrase. This releases OH -,

which raises the sheath pH and hence provides optimum conditions for maximal sheath

calcification by cyanobacteria (Riding, 2006). CCM has significant role in capturing CO2 by the

process of biocalcification.

7.1.1.7. Algal photosynthesis

Photosynthesis is much more efficient in microalgae than in terrestrial C3 and C4 plants (Kodama

et al., 1993). This high efficiency is again due to the presence of both intracellular and

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extracellular carbonic anhydrases and the CO2 concentrating mechanism (Miyashi et al., 1997).

The present focus is on exploiting the ability of microalgae to convert solar energy and CO2 into

O2 and carbohydrates. Considerable efforts have been made for CO2 fixation along with valuable

material production by mass cultivation of algal cultures. To advance both the near- and long-

term development and applications of microalgae for biofixation of CO2 and GHG mitigation,

the U.S. Department of Energy (DOE) and EniTecnologie, the R&D arm of the Italian oil

company Eni, with the assistance of the IEA Greenhouse Gas R&D Programme, in Cheltenham,

Great Britain, organized the ‘International Network for Biofixation of CO 2 and Greenhouse Gas

Abatement with Microalgae’.

Microalgal mass cultures can use CO2 from power plant flue gases for the production of biomass.

The algal biomass thus produced can directly be used as health food for human consumption, as

animal feed or in aquaculture, for biodiesel production or as fertilizer for agriculture. A fast

growing marine green alga Cholococcum littorale is reported to tolerate high concentrations of

CO2 (Kodama et al., 1993 ). Yun et al. (1996 and 1997) used waste water containing phosphate(46 g m-3) from a steel plant to raise cultures of the photosynthetic microalga Chlorella vulgaris.

Flue gas containing 15% CO2 was supplemented further to get a CO2 fixation rate of 26 g CO2 m-

3 h-1. Research is in progress on the development of a novel photobioreactors for enhanced CO2

fixation and CaCO3 formation. CO2 fixation rate was increased from 80 to 260 mg l-1h-1 by

using Chlorella vulgaris in a newly developed membrane-photobioreactor (Cheng et al., 2006).

A novel multidisciplinary process has recently been proposed that uses algal biomass in a

photobioreator to produce H2 apart from sequestering CO2 (Skjanes et al., 2007).

Enhanced growth rate of marine macroalgae Gracilaria sp. and G. chilensis has been observed

by increasing CO2 concentration from 650 ppm to 1250 ppm (Gao et al., 1993). Gao and

McKinley (1994) proposed that this macroalgal culture could make important contributions to

both biomass production for chemicals and fuel and CO2 remediation.

7.2. Non-Photosynthetic ways of capturing carbon

7.2.1. Methanogenic and acetogenic bacteria

Nonphotosynthetic CO2 fixation occurs widely in nature by the methanogenic archaebacteria.

These are obligate anaerobes that grow in freshwater and marine sediments, peats, swamps and

wetlands, rice paddies, landfills, sewage sludge, manure piles, and the gut of animals.Methanogens are responsible for more than half of the methane released to the atmosphere.

These methanogenic bacteria grow optimally at temperatures between 20 oC and 95 oC. Carbon

monoxide dehydrogenase and/or acetyl-CoA synthase aid them to use carbon monoxide or

carbon dioxide along with hydrogen as their sole energy source.

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Waste gases from blast furnaces containing oxides of carbon were used for converting them into

higher- Btu (more calorific value) methane using thermophilic methanogens (Bugante et al.,

1989). A column bioreactor operated at 55 °C and pH 7.4 was used for the process. A mixture of

three culture of bacteria, viz. Rhodospirillum rubrum, Methanobacterium formidium and

Methanosarcina barkeri was used for complete bioconversion of oxides of carbon to methane(Klassen et al., 1990). Acetogenesis, on the other hand, is involved in the recycling of 10 to 20%

of the carbon on earth (Gollin et al., 1997).

7.2.2. Carbon sequestration using heterotrophic bacteria

The concept of CO2 fixation in certain representatives of heterotrophic bacteria was first

proposed by Wood and Werkman (1941). The idea, however, faced lot of criticism, as with many

new findings in the scientific world. While working on propionic acid bacteria they proposed

that CO2 and pyruvate combined to form oxaloacetate, which was later called as the Wood-

Werkman reaction.

The same pathway can be exploited now for capturing carbon using heterotrophic bacteria.

Carbonic anhydrases play a critical role in concentrating CO2 inside the cell. The capability of

carbonic anhydrases to convert CO2 in bicarbonate may be utilized by carboxylases such as

phosphoenolpyruvate (PEP) carboxylase and pyruvate carboxylase, to form oxaloacetate (Norici

and Giordano, 2002). Such anapleurotic pathway exists in organisms to compensate for the loss

of oxaloacetate siphoned off for the synthesis of amino acids of aspartate family (Fig. 2).

Heterotrophic bacteria having maximal carbonic anhydrase and phosphoenolpyruvate

carboxylase and/or pyruvate carboxylase titers may be raised in fermentors, and these can be

flushed with flue gases with CO2 concentration to produce useful metabolites such as

oxaloacetate and amino acids. Extensive research has been done on the production of glutamic

acid and lysine by Corynebacterium glutamicum. The presence of both phosphoenolpyruvate

carboxylase and pyruvate carboxylase and the PEP–pyruvate–oxaloacetate node (Sauer and

Eikmanns, 2005) makes this bacterium suitable for fixing carbon in the form of amino acids. An

increased bicarbonate supply by the action of carbonic anhydrases (in elevated CO2 conditions)

to phosphoenolpyruvate and pyruvate carboxylases may enhance their activity, thereby making

the conditions favorable for enhanced lysine production. Previous studies on PEP and pyruvate

carboxylase activity in relation to lysine production (Gubler et al., 1994; Jetten et al., 1994)

supports the assumption. Work is in progress in our laboratory at South Campus of the

University of Delhi to understand the effect of different levels of carbon dioxide on carbonic

anhydrase, phosphoenolpyruvate carboxylase and pyruvate carboxylase titres and hence their

overall effect on lysine production. Dual benefit of carbon sequestration along with useful

byproduct formation makes this approach very attractive.

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Fig. 2. Simplified TCA cycle showing the anapleurotic sequence involved in feeding of oxaloacetate via the activity

of Carbonic Anhydrase (PEPC: Phosphoenolpyruvate Carboxylase; PC: Pyruvate Carboxylase)

7.2.3. The Biomimetic Approach/ Biomimetic remediation of CO2

Biomimetic approach involves identification of a biological process or structure and its

application to solve a nonbiological problem (Bond et al., 2001). It has emerged as an

environment friendly process, which can be operated at near ambient temperature and pressure

with no costly CO2 concentration and compression steps.

Microbes, being widespread in nature, play a major role in chemical cycles that influence

atmosphere-hydrosphere composition and are extensively involved in the production and

accumulation of various sediments deep inside the oceans. Containing about 150,000 Gt of CO 2,

carbonate minerals constitute the Earth’s largest reservoir of CO2 (Liu et al., 2005). Bacteria are

the key organisms in the formation of microbial carbonates. Mineral carbonation has emerged as

a new carbon capture and storage technology in the past few years. The idea of applying

carbonation reactions for CO2 storage was proposed by Seifritz (1990). Carbonic anhydrases are

the fastest enzymes known for their capability and efficiency for converting carbon dioxide into

bicarbonates. Gillian M. Bond of New Mexico Tech, USA, started working on this enzyme for

mineral CO2 sequestration since 2001.

The process of carbon dioxide fixation can be carried out successfully with a stream of carbon

dioxide (from flue gases) in a bioreactor. Various methods for carbonic anhydrase

immobilization are being attempted for the development of an efficient biodegradable matrix that

can ensure maximal activity along with its long term use in bioreactors for sequestration

purposes. Carbonic anhydrase was recently immobilized in chitosan-coated alginate beads and

the mineralization was studied by FT-IR (Simsek-Ege et al., 2002). A novel trickling spray

16

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PC

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reactor employing immobilized carbonic anhydrase has been developed that enables

concentration of CO2 from the emission stream (Bhattacharya et. al., 2004). Carbonic anhydrase

is one of the fastest enzymes, which make mass transfer from the gas phase to aqueous phase.

This biocatalytic fixation of carbon could be the answer to tackle atmospheric pollution.

New cation sources are being identified for carbonate formation. Seawater is a good source for cations but its availability is limited to coastal locations only. Waste brines from industries and

produced waters from the oil and gas industries have emerged as promising sources of cations

for biomimetic carbonate formation. Development of an efficient process can sequester

approximately 3.49 Mt (millions of metric tons) CO2 /year utilizing about 2.07 Mt calcium ions

and 0.67 Mt magnesium ions from the waters produced from New Maxico and Permian Basin

regions of texas (Liu et al., 2005).

Most of the work on biomimetic sequestration uses carbonic anhydrase from animal sources.

However, there is a need for a thermophilic carbonic anhydrase that sustains high pressure if we

really want to use brine as the most favorable cationic source for mineral carbonation under inthe deep-sea environment. A gene encoding a putative β- type carbonic anhydrase in the

methanoarchaeon Methanobacterium thermoautotrophicum has been expressed in E. coli and

found to encode a thermostable (up to 75°C) carbonic anhydrase (Smith and Ferry, 1999). Its

activity at different hydrostatic pressures needs to be studied for its biomimetic applications in

carbon sequestration.

The biomimetic approach has now also been applied in relation to geological sequestration. A

“closed-loop” fossil-fuel carbon cycle has been proposed to be developed, in which microbial

consortium (comprising of methanogens) could be used to convert CO2 to methane at a

commercially useful rate. This can be used either in a geological setting (following injection of CO2 into depleted oil and gas well, saline aquifer, etc.) or above ground in rapid-contact reactors

(Beecy et al., 2000; Medina et al., 2001).

Possibility of an on-site scrubber that would provide a plant-by-plant solution to CO2

sequestration, apart from eliminating the concentration and transportation costs is the potential

advantage of the biomimetic approach.

Conclusions

Several novel concepts and techniques are being attempted for a safe and permanent capture of

CO2. Routine abiotic methods although appear promising at prima facie but costly concentration

and transportation steps along with future leakage risks have led to focus on new biotic methods.

Evolution has equipped plants and various domains of microbial life with different mechanisms

for carbon fixation. The present need is to exploit these biological mechanisms along with

existing biochemical engineering techniques for long term CO2 sequestration. Exhaustive study

needs to be done on various metabolic pathways that employ carboxylases. Behavior of enzymes

like carbonic anhydrase and Rubisco with gases other than CO2 in flue gas must be understood.

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8/3/2019 GW Final April 1, 2008


Despite finite sink capacity, biological approaches provide a natural and cost-effective method of

carbon sequestration. Biotic and abiotic approaches have their own merits and demerits, and they

are complementary and have the potential to mitigate the risks of climate change.

The World Environment Day slogan for 2008 ‘Kick the Habit! Towards a Low Carbon

Economy’ shows a growing concern and recognition that climate change has become thedefining issue of present era. Countries, companies and communities throughout the world are

focusing attention on reducing the greenhouse gas emissions.

Acknowledgements

We gratefully acknowledge the financial assistance form the Department of Biotechnology

(DBT), Government of India during the course of this manuscript preparation.

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