Microalgae

Microalgae cover all unicellular and simple multi-cellular microorganisms, including both prokaryotic microalgae and eukaryotic microalgae. 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

They can grow almost anywhere, requiring sunlight and some simple nutrients, although the growth rates can be accelerated by the addition of specific nutrients and sufficient

aeration.

Microalgae can either be autotrophic or heterotrophic. The former requires only inorganic compound such as CO₂, salts and light energy source for growth while the latter which is non-photosynthetic requires external source of organic compounds for nutrients as energy source. Sometimes microalgae can be autotrophic or heterotrophic when it exists in different condition, and it is called as mixotrophilic. It performs photosynthesis as the main energy source, though both organic compounds and CO2 are essential. Amphitrophy, subtype of mixotrophy, means that organisms are able to live either autotrophically or

heterotrophically, depending on the concentration of organic compounds and light intensity

available. Photoheterotrophycally, also known as photoorganitrophy, photoassimilation, photometabolism, describes the metabolism in which light is required to use organic

compounds as carbon source. The photoheterotrophic and mixotrophic metabolisms are not well distinguished, in particular they can be defined according to a difference of the energy source required to perform growth and specific metabolite production.

Microalgae are present in all existing earth ecosystems, not just aquatic but also terrestrial, representing a big variety of species living in a wide range of environmental conditions. It is estimated that more than 50,000 species exist, but only a limited number, of around 30,000, have been studied and analyzed. While among the 10,000 species which are believed to exist, only a few thousand strains are kept in collections, a few hundred are investigated for chemical content and just a handful are cultivated in industrial quantities [Richmond, 2004; Olaizola, 2003]. This collection attests to the large variety of different microalgae available to be selected for use in a broad diversity of applications, such as value added products for pharmaceutical purposes, food crops for human consumption and energy source.

Many research reports and articles described many advantages of using microalgae for biodiesel production in comparison with other available feed stocks, like microalgae are considered to be a very efficient biological system for harvesting solar energy for the production of organic compounds. Microalgae are non-vascular plants, lacking complex reproductive organs, and many species of algae can be induced to produce particularly high concentrations of chosen, commercially valuable compounds, such as proteins, carbohydrates, lipids and pigments. Microalgae are microorganisms that undergo a simple cell division cycle.

The farming of microalgae can be grown using sea or brackish water, and microalgae biomass production systems can easily be adapted to various levels of operational or technological skills.

Production of biodiesel and other bio-products from microalgae can be more

environmentally sustainable, cost-effective and profitable, if combined with processes such as wastewater and flue gas treatments. In fact various studies demonstrated the use of microalgae for production of valuable products combined with environmental applications. In addition, depending on the microalgae species various high-value chemical compounds may be extracted such as pigments, antioxidants, β-carotenes, polysaccharides, triglycerides, fatty acids, vitamins, and biomass, which are largely used as bulk commodities in different

industrial sectors. These materials are utilized in pharmaceuticals, cosmetics, nutraceuticals, functional foods, and biofuels fields, and the application of microalgae in producer country are listed in Table 1-1 [Lorenz et al., 2000; Hejazi et al., 2004; Pulz et al., 2004; Ratledge, 2004; Spolaore et al., 2006a; Loubiere et al., 2009]. For examples, some microalgae have been exploited for millennia (Nostoc in China and Arthrospira in Chad and Mexico).

Currently, they have several applications from human and animal nutrition to cosmetics and the production of high-value molecules (fatty acids, pigments, and stable isotope

biochemicals). Recently the utilization of microalgae can serve enormous purposes which are potential and feasible. Some possibilities currently being considered are listed below [Mata et al., 2010].

 Microalgae can provide feedstock for several different types of renewable fuels such as biodiesel, methane, hydrogen, ethanol, among others. Algae biodiesel contains no sulfur and performs as well as petroleum diesel, while reducing emissions of particulate matter, CO, hydrocarbons, and SOx. However emissions of NOx may be higher in some engine types [Delucchi, 2008].

 Removal of CO₂ from industrial flue gases by microalgae bio-fixation, reducing the GHG emissions of a company or process while producing biodiesel [Wang et al., 2008].

 Wastewater treatment by removal of NH4+

, NO3

-, PO4

3-, making algae to grow using these water contaminants as nutrients [Wang et al., 2008].

 After oil extraction the resulting algae biomass can be processed into ethanol, methane, livestock feed, used as organic fertilizer due to its high N:P ratio, or simply burned for energy cogeneration (electricity and heat) [Wang et al., 2008]

 Combined with their ability to grow under harsher conditions, and their reduced needs for nutrients, they can be grown in areas unsuitable for agricultural purposes

independently of the seasonal weather changes, thus not competing for arable land use, and can use wastewaters as the culture medium, not requiring the use of freshwater.

 Depending on the microalgae species other compounds may also be extracted, with valuable applications in different industrial sectors, including a large range of fine chemicals and bulk products, such as fats, polyunsaturated fatty acids, oil, natural dyes, sugars, pigments, antioxidants, high-value bioactive compounds, and other fine

chemicals and biomass [Li et al., 2008a; Li et al., 2008b; Raja et al., 2008].

 Because of this variety of high-value biological derivatives, with many possible commercial applications, microalgae can potentially revolutionize a large number of biotechnology areas including biofuels, cosmetics, pharmaceuticals, nutrition and food additives, aquaculture, and pollution prevention [Rosenberg et al., 2008; Raja et al., 2008].

Heterotrophic and mixotrophic cultivation could be a possible avenue of research. The genetic improvement of microalgae strains is also a present challenge. The use of transgenic microalgae for commercial applications has not yet been reported but holds significant promise. Modified strains could overproduce traditional or newly discovered algae

compounds and also serve to express specific genes that cannot be expressed in yeast. This could be of great importance for the production of hydrogen, for example. However, a successful drug discovery is the most promising aspect of microalgae biotechnology because the potential is immense although screening remains limited [Tramper et al., 2003].