Microalgae, the wonderful gift of nature
Microalgae as sustainable source of fuels and Energy
Microalgal biomass is a potential source for biofuels and bioenergy production. The CO2 fixed by microalgae during photosynthesis are converted mainly into macromolecules lipid, carbohydrate and proteins. First two macromolecules serve as the principal source for biofuels and energy generation. Biofuels could be liquid, gas or solid fuels such as biodiesel, bioethanol, biogas and biohydrogen.
Biodiesel
Biodiesel is biodegradable, non-toxic, and renewable fuel that is most compatible with petroleum diesel and can directly be used in existing diesel engine as neat or blended with diesel. Biodiesel is usually defined as methyl (or ethyl) esters of fatty acids of microalgal lipids. Microalgal lipids are reacted with a short chain alcohol (methanol or ethanol) in presence of a catalyst to form biodiesel (fatty acid ester). Four types of catalysts have been used for algal biodiesel conversion are alkali, acid, lipase enzyme, and heterogeneous catalysts. The lipid content microalgae range from 1.2% to 56.1%. Therefore, selection of specific microalgae with higher lipid content would be the best candidate for biodiesel production. The fatty acids chain length of most microalgae is favorable, while degree of their unsaturation may be a barrier to become suitable biodiesel feedstock because higher level of fatty acids desaturation impacts negatively to the oxidative stability, cetane number and heat of combustion. Therefore, use of lipids containing astaxanthin of microalga Haematococcus pluvialis in a biodiesel blend can significantly increase the oxidative stability of biodiesel. Overall, biodiesel is a more complex fuel containing a mixture of fatty acids esters with different chain lengths and unsaturation level. These properties are crucial for biodiesel suitability and are largely dependent of microalgal strains as well as their growth conditions during lipid accumulation.
Bioethanol
Bioethanol are produced from several food-crops such as, corn, sugar cane, sugar beets, potatoes, sorghum, cassava and lignocellulosic spent biomass. Any starchy biomass can be converted to sugars through enzymatic process and/or fermentation of 6-carbon sugars to bioethanol. To meet the global demand of bioethanol from the above land-based food crops, there would be a shortage of arable lands for food crops for human sustainability as well as ecological imbalance. Thus microalgal carbohydrates can serve as an alternative feedstock for bioethanol production. This is because of the fact that microalgae can be cultivated sustainably using non arable land and non-potable water and throughout the year irrespective of depending on local weather conditions. Using biorefinery concept, the spent-biomass after lipid extraction (for biodiesel production) can be used as source of carbohydrate for fermentation with ethanologenic yeast or bacteria. Several microalgae like Chlorella vulgaris, C. reinhardtii UTEX90 accumulate high content of starch (35-45% of dry weight biomass) as their reserve material apart from cellulose and hemicellulose. There are some microalgae identified that produce bioethanol by self-fermentation of their carbohydrates. Photoautotrophic conversion of CO2 to bioethanol was also shown in genetically engineered cyanobacteria where the genes for pyruvate decarboxylase (pdc) and alcohol dehydrogenase II (adh) were introduced from the bacterium Zymomonas mobilis.
Biogas
The gaseous mixture (methane, CO2 and a small amount of H2S) obtained by anaerobic digestion of organic matter (microalgae biomass) is called biogas. This renewable biogas can suitably be used for cooking, heating or run a car. Microalgal biomass containing carbohydrates, proteins and lipids can be hydrolyzed to soluble sugars, amino acids and fatty acids as raw-materials for microbial processes like acidogenesis, acetogenesis and methanogenesis. In the final step, methanogenic bacteria convert the hydrolyzed products into biogas. The biogas quality is determined by the relative content of methane, which purely depends on the substrate and their fermentation conditions. Microalgae including cyanobacteria produces high (61% to 67%) amount of methane. Unlike other biogas feedstocks, microalgal biomass is better feedstock for biogas production because microalgae contain low amount of sulfurated amino acids and thus produces significantly low amount of H2S.
Biohydrogen
Production of biohydrogen at laboratory-scale using cyanobacteria and green microalgae have been possible long-time ago, but the present demand for renewable energy necessitates its industrial production. Various technologies involved in biohydrogen production are: biophotolysis, indirect biophotolysis, photo-fermentations, dark-fermentation and combination of photosynthetic and fermentation processes in an integrated system. Biohydrogen has several advantages such as, eco-friendly, efficiency, sustainable, renewable, and no CO2 emission during its production as well as during its point use. Several cyanobacteria were reported to produce hydrogen as a by-product of atmospheric nitrogen fixation.