Procesos de Rendición de Cuentas en Instituciones de Educación

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into account all the different possible approaches and the specie-specific behavior of microalgae strains under given conditions. For microalgae cultivation several factors should be taken into account simultaneously, i.e. light availability and intensity, land topography, climatic conditions, water supply and access to the carbon source and other nutrients (Demirbas, 2011; Mata et al., 2010). Furthermore, biomass also depends on the mode of cultivation including photoautotrophic, heterotrophic and mixotrophic production, types of culture (open and closed systems), culture strategies (batch or continuous culture), inhibitors concentration, mixing, dilution rate, depth and harvests frequency (Gallardo Rodríguez et al., 2010). Nevertheless, for all of them, big efforts should be done in order to optimize the main variables according to specific production plant location. This must be accompanied by the design of robust and stable cultures, able to mitigate changes in environmental conditions without affecting microalgae growth and lipid productivity.

Under favorable conditions of growth, the algae can double their biomass within 24 h (Chisti, 2008). The growth rate directly affects the concentration of the metabolites of interest; for metabolites associated with growth, increased cell concentration will lead to greater final concentration of the product. Moreover, the yield of fatty acids and their composition varies between different strains, so it is needed to select those that best correspond to established standards. If the microalgae will grow in culture with little control over environmental conditions, it is necessary to ensure that they can respond positively to these changes, so productivity would be not affected significantly (Doran, 1995).

Durability and life cycle studies are important for long term operating plants. On the other hand, if sustainability problems want to be avoided, a detailed study on use of land, water, and nutrients are required, for which the predictable risks and impacts must be identified (Campbell et al., 2011; Yang et al., 2011). The economics of microalgae technology is very dependent of the scale and scaling-up of the process is still a main issue. Conversion rates of lipids into biodiesel reduce when scale increases. If a technically successful scale-up is achieved, some other aspects related with plant construction would become of interest. For a big production plant, larger markets of raw

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materials and products are required, as well enough skilled personnel. This significantly reduces the plant location options, but also in case of a successful plant establishment bigger positive impacts would be expected.

2.8.1. Light and carbon source

Under natural growth conditions, photosynthetic yield a of microbial system is affected by sunlight exposure and CO2 concentration (both natural free sources). The theoretical photosynthetic yield of microalgae is around 6%-7% of total solar energy. However, this yield is limited by availability of sunlight because of light-dark cycle and seasonal variations. This limitation can be overcome by artificial sunlight or fluorescent lamps implementation (Muller-Feuga et al., 1998; Yeh et al., 2010); or construction of microalgae-based industries in tropical countries where light is more stable over the year. Fifty percent of biomass dry weight of microalgae is approximately carbon by weight, (generally derived from carbon dioxide). Most microalgae can tolerate high levels of CO2 with a theoretical yield of roughly 513 tons of CO2 to produce 280 tons of dry biomass per ha-1y-1. Chlorococcum littorale, a marine alga, can utilize up to 40 percent CO2 concentration (Iwasaki et al., 1998). Chlorella strains from hot springs are used for biological fixation of carbon dioxide from industrial flue gases (Sakai et al., 1995). Therefore, in commercial scale, power plant exhaust can be applied for microalgae biomass production.

Metabolism of microalgae also determines the biomass concentration and cost of biodiesel production. As previously mentioned microalgae have several different modes of metabolisms (e.g. autotrophic, heterotrophic, mixotrophic, photoheterotrophic) and can make metabolic shift to cope with variable environmental conditions. Usually phototropic production is feasible for commercial production of microalgae biomass, and commonly deploys for open pond and closed photobioreactor system. However in autotrophic culture it is hard to attain high density of microalgae biomass and lipid content (Chen and Johns, 1991).

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Heterotrophic growth of microalgae is independent of light and use organic substrate as carbon source thus offers the more opportunities to increase cell density, productivity of algal biomass, and cellular lipid content (Miao and Wu, 2004; Xu et al., 2006). This is noteworthy that heterotrophic production lowers the harvesting cost (Chen and Chen, 2006), but the use of glucose or acetate as carbon source is costly. To resolve the drawback of high carbon source cost, crude glycerol a cheap resource derived from biodiesel production processes, can be used as carbon substrate (Liang et al., 2009). Mixotrophic microalgae have successful alliance of photosynthetic and heterotrophic metabolism. The capability of mixotrophs to process organic substrates or carbon dioxide as carbon source depends on several factors including the concentration of carbon substrates, and also light intensity in the growth medium. For example, C.

protothecoides, a mixotropic microalgae shift its metabolic process from

photoautotrophic to heterotrophic in response change of organic carbon source (glucose) and reduction of the inorganic nitrogen source in the medium (Miao and Wu, 2004). It infers that in mixotrophic cultivation, there is less loss of biomass during the dark phase.

2.8.2. Temperature and pH

Temperature is one of the most limiting factors among the environmental parameters governing the activities and growth rate of microalgae in open and closed system (Park et al., 2011). The optimal temperature range is generally between 25-35°C. Many microalgae can even tolerate temperatures around 15°C. Temperature affects, among others aspects, the types of fatty acids produced by these cells. Usually by lowering the temperature, the amount of saturated fatty acids increases, but it is not necessarily true for all species of microalgae (Renaud et al., 2002).

The pH of the algal system affects the biomass regulation, photosynthesis rate, availability of phosphorous to microalgae and species competition. pH influences toxicity of free ammonia to living algal cells by altering the ratio of free ammonia and ammonium ion (Y. Azov, 1982), and also directly influencing the metabolic rate of microalgae in

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