1-1 Global warming and energy shortage
Past years, the industrial revolution brought with it rapid economical development and a great improvement in our standard of living. As a result, it has also become an enormous burden on nature. The problem of global warming contributed by greenhouse gas is receiving great attention by the worldwide people.
Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, those absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth’s surface, and then this property causes the greenhouse effect. Greenhouse gases are essential to maintaining the current temperature of the Earth. Planet Earth is habitable because of its location relative to the sun and because of the natural greenhouse effect of its atmosphere [Karl, 2003]. Earth's most abundant
greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (NOx), ozone (O3), and chlorofluorocarbons. The most important greenhouse gases are water vapor and carbon dioxide [Treut et al., 2007]. Human activities intensify the blanketing effect through the release of greenhouse gases. For instance, the amount of carbon dioxide in the atmosphere has increased by about 31% from the 1961 to 1990, and this increase is known to be due to human activities, primarily the combustion of fossil fuels and removal of forests.
In the researches, the level of carbon dioxide in pre-industrial period is 280 parts per million by volume (ppmv), and the current level of carbon dioxide increased to more than370 ppmv today. Thus, humankind has dramatically altered the chemical composition of the global atmosphere with substantial implications for climate. Figure 1-1 shows the amount of CO2
is still elevating as an average increasing rate in these years and the global average surface heating approximates that of the increases of carbon dioxide [Houghton et al., 2001; Karl and
Trenberth, 2003]. The greenhouse gases trap outgoing radiation fromthe Earth to space, creating a warming of the planet.
The burning of fossil fuels has especially emitted large amounts of CO2, into the atmosphere, which is causing a problem known as greenhouse effect and global warming [Michiki, 1995]. Over ten times more CO2 is fixed by plants into biomass, and annually released by decomposers and food chains, than is emitted to the atmosphere due to the burning of fossil fuels. Human activity is already directly and indirectly affecting almost half of the terrestrial biological C cycle. Management of even a small fraction of the biological C cycle would make a major contribution to mitigation of this greenhouse gas [Hughes and Benemann, 1997].
CO2 is responsible for well over half of the total warming potential of all greenhouse gases.
Globally about 20 billion tons of fossil CO2 are emitted each year from the burning of fossil fuels, and another 2 to 8 billion tons are released through human-mediated oxidation of the biosphere: intensive agriculture, deforestation and other unsustainable practices.
Cumulatively, net biosphere CO2 emissions due to human activities over the past century have rivaled emissions from fossil CO2 sources. Indeed, human activities are currently impacting a large fraction, approaching half, of the total annual terrestrial primary biological productivity of our planet, estimated at 500 billion tons of CO2 fixed annually [Vitousek, 1994; Hughes and Benemann, 1997].
To solve this international problem, some researches of biological CO2 fixation and utilization have been carried out to develop those technologies in which CO2 is fixed by utilizing microorganisms such as bacteria and microalgae and converting it into useful substances [Michiki, 1995]. Electric power generation is responsible for roughly one third of fossil CO2 emissions. Direct CO2 mitigation processes are those that reduce fossil CO2
emissions from specific power plants. Direct biological CO2 mitigation processes include the cultivation of microalgae on flue-gas or captured CO2, and the co-firing of wood with fossil fuels.
Another international problem is energy crisis which was also caused may be
over-consumption of fossil energy. Majority of the world energy needs are supplied through petrochemical sources, coal and natural gases, with the exception of hydroelectricity and nuclear energy, of all, these sources are finite and at current usage rates will be consumed shortly [Srivastava and Prasad, 2000; Meher, 2006]. Diesel fuels have an essential function in the industrial economy of a developing country and used for transport of industrial and agricultural goods and operation of diesel tractor and pump sets in agricultural sector.
Economic growth is always accompanied by commensurate increase in the transport. The high energy demand in the industrialized world as well as in the domestic sector and pollution problems caused due to the widespread use of fossil fuels make it increasingly necessary to develop the renewable energy sources of limitless duration and smaller environmental impact than the traditional one. Global consumption of petrodiesel raised in an average of 16.1 million tons/year among 1990 and 2003 [IEA, 2006]. If the tendency keeps on, the worldwide demand for petrodiesel will increase to 1383 million tons/year by 2050. The global supply of petroleum is finite and expected to peak between 5 and 30 years from 2008 [Pahl, 2005; Pin Koh, 2007], after which demand will inevitably outstrip production. This has stimulated recent interest in alternative sources for petroleum-based fuels. The
alternative fuel must be technically feasible, economically competitive, environmentally acceptable, and readily available. One possible alternative to fossil fuel is the use of oils of plant origin like vegetable oils, tree borne oil seeds and microalgal oils for biodiesel
production. This alternative diesel fuel can be termed as Meher et al. reported [2006].
1-2 Microalgae
Microalgae are diverse group of prokaryotic and eukaryotic photosynthetic microorganisms that grow rapidly due to their simple structure. It is estimated that the biomass productivity of microalgae could be 50 times more than that of switch grass, which is the fastest growing terrestrial plant [Demirbas, 2006; Nakamura, 2006]. They are single- cellular photosynthetic microorganisms that convert sunlight, water and carbon dioxide to microalgal biomass.
The primary producers of oxygen and consumer of CO2 in aquatic environments are algae, especially planktonic microalgae. Microalgae are microscopic in size and grow in liquid culture, nutrients can be maintained at or near optimal conditions potentially providing the benefits of well-controlling. Microalgae can perform continuous productivity similar to microbial fermentation. From these points of view, the study aims to search and screen microalgae from the natural sources like oceans and lakes, and to establish the most
appropriate culture conditions and then evaluate the photosynthesizing capabilities thereof.
However, many microalgae are exceedingly rich in oil [Chisti, 2007; Banerjee, 2002], which can be converted to many products such as renewable fuels using existing technology. The utilization fields of microalgae can be categorized as follows [Michiki, 1995].
1. Energy-generating substances such as hydrocarbons, hydrogen, and methanol.
2. Foods and chemicals such as proteins, oils and fats, sterols, carbohydrates, sugar, alcohols.
3. Other chemicals colorants, perfumes, vitamins, physiologically-active substances.
Elementary techniques including separation and purification have been studied and evaluated.
These characteristics of microalgae are potential for green-house gas reducing and
system in which lipids and biodiesel from microalgae are separated and purified.
1-3 Microalgae for reducing pollution and renewable fuels
Photosynthetic microorganisms, microalgae, hold the key to realize the effective system of CO2 fixation and utilization. Particularly, about one half of global photosynthesis and oxygen production is accomplished by marine microalgae. They play an important role in CO2 recycling through photosynthesis, which is similar to higher plants in O2-evolved systems (PSI and PSII system).
Furthermore, technology development of microalgae culture that makes it possible to convert the formations resulting from photosynthesis into energy substances such as fuel oil.
The study also seeks to develop technologies that separate and refine a variety of useful substances [Michiki, 1995]. Microalgae are sunlight-driven cell factories that convert carbon dioxide to potential biofuels, foods, feeds and high-value bioactives [Metting and Pyne,1986; Schwartz, 1990; Kay, 1991; Shimizu, 1996,2003; Borowitzka, 1999; Ghirardi et al., 2000; Akkerman et al., 2002; Banerjee et al., 2002; Melis, 2002; Lorenz and Cysewski, 2003; Metzger and Largeau, 2005; Singh et al., 2005; Spolaore et al., 2006; Walter et al., 2005; Chisti, 2007].
Notwithstanding, microalgae can provide several different types of renewable biofuels.
These include methane produced by anaerobic digestion of the algal biomass [Spolaore et al., 2006; Chisti, 2007]; biodiesel derived from microalgal oil [Roessler et al., 1994; Sawayama et al., 1995; Dunahay et al., 1996; Sheehan et al., 1998; Banerjee et al., 2002; Gavrilescu and Chisti, 2005; Chisti, 2007] and photo-biologically produced biohydrogen [Ghirardi et al., 2000; Akkerman et al., 2002; Melis, 2002; Fedorov et al., 2005; Kapdan and Kargi, 2006;
Chisti, 2007]. Using microalgae as a source of fuel is not a new idea [Chisti, 1980-1981;
Nagle and Lemke,1990; Sawayama et al., 1995; Chisti, 2007], but it is now being paid much attention because of the escalating price and exhausting of petroleum and, more significantly, the emerging concern about global warming that is associated with burning fossil fuels [Gavrilescu and Chisti, 2005].
1-4 Microalgae cultivation
Grow ability, chemical composition of microalgae, lipid content and lipid composition of microalgae are influenced by environmental conditions, such as culture medium, light,
temperature, carbon dioxide and so on [Richmond, 1986; Tomaselli et al., 1988, James et al., 1989; Oliveira et al., 1999; Renaud et al., 2002]. Producing microalgal biomass is generally more expensive than growing crops. Photosynthetic growth requires light, carbon source, water and inorganic salts. Temperature must remain generally within 20 to 30°C. To minimize expense, biodiesel production must rely on freely available sunlight, despite daily and seasonal variations in light levels.
1-4-1 Growth medium
Growth medium must provide the inorganic elements that constitute the algal cell.
Essential elements include nitrogen (N), phosphorus (P), iron and in some cases silicon.
Minimal nutritional requirements can be estimated using the approximate molecular formula of the microalgal biomass, that is CO0.48H1.83N0.11P0.01. This formula is based on data
presented by Grobbelaar [2004]. Sea water supplemented with commercial nitrate and phosphate fertilizers and a few other micronutrients is commonly used for growing marine microalgae [Molina Grima et al., 1999]. Growth media are generally inexpensive.
Nutrients such as phosphorus must be supplied in significant excess because the phosphates added complex with metal ions, therefore, not all the added phosphorus is bio-available.
Silicon is specifically used for the growth of diatoms which utilize this compound for production of an external shell. Micronutrients consist of various trace metals and the vitamins thiamin (B1), cyanocobalamin (B12) and sometimes biotin. Two enrichment media that have been used extensively and are suitable for the growth of most algae are the Walne medium and the Guillard’s f/2 medium. Commercially available nutrient solutions may reduce preparation labor. The complexity and cost of the above culture media often excludes their use for large-scale culture operations. And there are alternative suitable enrichment medium for mass production of microalgae in large-scale extensive systems contain only the most essential nutrients and are composed of agriculture-grade rather than laboratory-grade fertilizers.
1-4-2 Nitrogen contents
Organisms use carbon and nitrogen for the important nutrient source of energy and
construction of cell structure. Other than carbon, nitrogen is quantitatively the most element in algal nutrition. Nitrogen supply is essential for preparation of algal culture medium for most microalgae. Microalgae are usually able to use nitrate, nitrite, ammonia, or other organic nitrogen sources such as urea. In practical, the preferred nitrogen source is ammonia or urea, either of which is economically more favorable than nitrate or nitrite, which is more expensive.
The growth capacity is usually the same with these nitrogen sources. The average nitrogen requirement for many green algae is approximately 5-10% of the dry weight or 5-50mM [Becker, 1994]. Nevertheless, nitrogen amounts are considerably variable, since the nitrogen content in the culture medium can be operated to produce nitrogen-limitation algal cells. Nitrogen limitation affects photosynthesis by reducing the efficiency of energy collection due to loss of chlorophyll a and increases in non-photochemically active carotenoid
but may cause accumulation of numerous carbon compounds, i.e. polysaccharides or lipids.
A low supply of nitrogen may result in a low respiration rate and an increase in the lipid reserves in most microalgae [Becker, 1994].
The major nonpolar lipids reported in algae are the triglycerides and hydrocarbons, and the major polar lipids classes include phospholipid, cardiolipin, diphosphatidylglycerol and glycolipids [Volkman et al., 1989 Dembittsky et al., 1991, Behrens and Kyle, 1996]. It is widely known that the main storage lipid class, triglycerides, accumulates in response to exhaust of nitrogen supply and during the stationary phase of a batch culture in most species [Richardson et al., 1969; Gordillo et al., 1998]. At the same time, the amounts of nitrogen supply may make difference in the fatty acid compositions of total lipid production and the length of fatty acid chains [Makrides et al., 1995; Regnault et al., 1995].
1-4-3 Light
Solar light is the energy source of the algal culture system for photosynthetic metabolism and is one of the major factors to determine the efficiency of the whole system. There are some environmental factors affect the lipid composition and growth of algae such as temperature, irradiation, and nutrient status [Thompson et al.., 1996]. Light drives
photosynthesis reaction and in this regard intensity, spectral quality and photoperiod need to be considered. Light intensity plays an important role, but the requirements vary greatly with the culture depth and density of the microalgal culture. However, light intensities that were adequate or optimal for growth in the log phase to declining growth phase can become stressful in stationary phase and lead to a condition known as photo-inhibition. It is
important that while the measured light intensity within the culture will decrease with increasing biomass if the incident illumination is maintained relatively high and then a large proportion of cells may become stressed, and the photo-inhibited culture can be pushed into
preferable for many species to halve or further reduce the incident light intensity when cultures enter stationary phase to avoid photo-inhibition.
At higher depths and cell concentrations, the light intensity must be rose up to penetrate through the culture medium. For example, 1000 lux is suitable for Erlenmeyer flasks, and 5000-10000 lux light is necessary for larger volumes. On the one hand, there are several studies of culturing Botryococcus brauni as well as Chlorella sp. by continuous light, and the other hand some studies are applying dark and light photoperiod for growing Chlorella sp.
[Hogetsu and Miyachi, 1970; Maeda et al, 1995; Sawayama et al., 1995; Allard and Templier, 2000; Sato et al., 2003; Achitouv et al., 2004; Miao and Wu, 2006]. In order to complement and utilize the solar light at the maximum efficiency, many important studies of algae culture aim to develop a highly-efficient light collection device that minimizes the light loss due to reflection and absorption in the visible light area directly influencing the CO2 fixation capability of microorganisms [Michiki, 1995].
1-4-4 Temperature
The optimal growth temperature for phytoplankton cultures is generally between 20 and 24°C, although this may vary with the elements of the culture medium, the species and strain diversity. Microalgae do not have the ability to regulate their internal temperature, so the temperature may in turn affect the microalgae growth and substrate utilization rate [Esener et al., 1981]. Most species of cultured microalgae generally grow in appropriate temperatures between 16-27°C. Temperatures lower than 16°C will decrease the growth rate, whereas those higher than 35°C are lethal for a number of species. If necessary, microalgal cultures can be cooled by a water flow of cooling system over the surface on the culture vessel or by controlling the air temperature with refrigerated air - conditioning units.
The influences of temperature on the biomass composition, nutrient requirement, nature of
metabolism and the metabolic reaction rate were well-known [Pirt, 1971; Novak, 1974; Mayo and Noike, 1996]. High growth temperature leads to different production of lipids and carbohydrates [Tomaselli et al., 1988; Oliveira et al., 1999; Renaud et al., 2002]. However, other studies have found that the response of microalgal composition to high and low growth temperatures varies from species to species. High growth temperature has been associated with increases in protein content and decreases in carbohydrate, and lipid in some
species[Thompson et al., 1992; Renaud et al.,1995], but these workers found no overall trend in gross biochemical composition for all species under study. We known temperature is a major effect on the types of fatty acids produced by microalgae. In some microalgal species, decreasing of growth temperature can bring about increasing of the unsaturated to saturated fatty acids [Ackman et al.,1968; Satu and Murata, 1980; Mortensen et al.,1988; Thompson et al., 1992; Oliveira et al., 1999; Renaud et al., 2002]. However, the response to growth temperature varies from species to species, with no overall consistent relationship between temperatures and chemical composition.
However, for the large scale of outdoor culturing system, microalgal culture could be heated over 40°C by sunshine in some extremely hot places. Microalgal growth is highly inhibited at high temperature, so there are many studies for searching the microalgae good in thermo-stability and productivity at high temperature for special cultivation.
1-4-5 Carbon dioxide
Microalgae need inorganic carbons to perform photosynthesis. Both CO2 and HCO3- are potential sources of carbon for photosynthesis in microalgae. While CO2 can diffuse through the microalgal cell membrane, HCO3- needs specific transport mechanisms to enter the algal cell [Raven and Johnston, 1991; Beer, 1994; Moroney and Somanchi, 1999].
Figure 1-2 shows the model for CO2 concentration in eukaryotic microalgae. Microalgal
diffusion from the air. The natural CO2 concentration in air (0.03%) is too low to sustain optimal growth and high productivity. Microalgae growing in fresh water with low salinity and at nearly neutral pH must be supplied with additional carbon to ensure satisfactory growth.
Although intensive mixing may increase the entrance coefficient for CO2 from the air into the culture, in most microalgal cultures, additional carbon mainly in the form of CO2-enriched air is supplied [Becker, 1994]. There is also studies showing the utilization of organic carbon for heterotrophic or mixotrophic cultures.
CO2 addition furthermore buffers the water against pH changes as a result of the
CO2/HCO3- balance. High CO2 aeration will result in pH decreasing of culture solution.
High CO2 concentration aeration may be a harmful effect on the microalgal cells growth, but increasing of microalgal cell density could promote the growth capacity of microalgae in the cultures aerated with higher CO2 concentrations [Chiu et al., 2008]. Therefore, the adequate amount of CO2 supplied must not be inhibitory effects on microalgal cell growth inhibition;
otherwise lipid content will decrease. It was reported that the carbon assimilation of lipid synthesis was decreased with decrease of pH [Yung and Mudd, 1966, Chiu et al., 2008].
Higher CO2 aeration may cause the decrease of lipid content, but the increase of biomass may improve the lipid productivity.
1-4-6 pH value
The pH value in the medium in algal system is known to influence the biomass regulation, ion transport system and metabolic rate [Guffanti et al., 1984; Mayo and Noike, 1994]. So controlling pH in culture medium will be one of the most critical factors affecting biomass and lipid production of microalgae. The pH value of the culture medium may be a simpler, indirect method for determining the degree of the cell growth of microalgae because the pH gradually rises as bicarbonate added to the culture medium is dissolved to produce CO2,
However, pH of algal cultures can be affected by several factors such as medium composition, buffering capacity of the medium, CO2 dissolution efficiency, temperature (influences solubility of chemical compounds) and metabolic activity of the algal cells.
Almost pH value of the medium in algal culture is usually neutral or slightly acidic values.
The main reason is to avoid precipitation of microalgal cells and several major elements.
The main reason is to avoid precipitation of microalgal cells and several major elements.