Microalgae or microphytes are photosynthetic microorganisms which vary in size from 2 to 200 micrometers, can form chains of cells, but do not form differentiated multicellular organism, are polyphyletic and can be found in a variety of saline and freshwater streams on our planet. The evolutionary history and taxonomy of microalgae is complex due to constant revisions because of new genetic and ultrastructural evidence [ref] The majority of microalgae are classified in the division of Chlorophyta, but can also be bacteria (Cyanobacteria), diatoms (Chromalveolata) and other protists, like the Chromista (Bahadar & Bilal Khan, 2013)]. Microalgae are also defined as tallophytes, which are plants that do not have vascular system, embryos, stems and leaves; have chlorophyll-a as a primary photosynthetic pigment and are among the oldest living microorganisms (Mutanda et al., 2011). It is estimated that exist about 200,000 to 800,000 algae species and so far 50,000 species have been described (Richmond, 2004).
A diverse range of these organisms are photoautotrophs, but some may grow under mixotrophic or heterotrophic conditions. The first only require inorganic compounds like CO2, salts and light as an energy source, while the heterotrophic are non-photosynthetic therefore requiring an external source of organic compounds as well as nutrients as an energy source (Brennan & Owende, 2010).
Microalgae have gathered much attention by the scientific community and industry in the last decades due to their large biotechnological potential for producing value added products for feed, food, nutraceutical, pharmaceutical industries and biofuels. In addition, these microorganisms can be used in environmental engineering such as for wastewater treatment, and for bio mitigation of CO2 in flue gases from coal-fired power stations (Zhou, Yuan, Chen, & Ochieng, 2015).
Research on microalgae for biofuels started in the 1920s in Germany where Chlorella strains were found to accumulate a large amount of lipids but technologies for producing microalgae have only been known for about 70 years (Gonzalez-Fernandez & Muñoz, 2017). First large scale cultivation was developed in the USA, Germany and Japan after 1948 but only in early 1960s that cultivation of Chlorella was first commercialized in Japan (Spolaore, Joannis-Cassan, Duran, & Isambert, 2006)(Ishika, Moheimani, & Bahri, 2017). Successive oil crises in the following decades gave researchers the opportunity to reopen the field with highly ambitious projects like the US Aquatic Species Project in the 1980s (Nrel, 1998).
Today, large microalgae cultivations have been rising in the last years across the world for varied applications, i.e. 750 ha pond in Hut lagoon, Western Australia and 400 ha in South Australia are used for commercial production of β carotene (Lin, 2005).
Microalgae are considered one of the most promising feedstocks for biofuels which include bioethanol, biobutanol, biodiesel, biohydrogen, and biomethane by biological or chemical processes. (Gonzalez-Fernandez & Muñoz, 2017).
They can produce bioethanol by fermentation and there is potential of using this alternative due to the fact that most biomass feedstocks which generate bioethanol such as corn, and sugar cane have a high value for food applications and require large quantities of land to be produced (Harun, Singh, Forde, & Danquah, 2010). Microalgae can produce biomethane and contain almost no lignin and lower cellulose, therefore showing a good process stability and high conversion efficiencies for anaerobic digestion (Vergara-Fernández, Vargas, Alarcón, & Velasco, 2008). Another biofuel that can be produced from these organisms is biohydrogen, either through green algae that produce hydrogen under anoxia conditions or as feedstock rich in carbohydrates for fermentative hydrogen production by anaerobic bacteria (Nagarajan, Lee, Kondo, & Chang, 2017). At last, the ability of some microalgae species to accumulate large quantities of lipids under nutrient-limiting conditions and their high photosynthetic rates has lead to major research efforts into maximizing lipid production for biodiesel applications (Bahadar & Bilal Khan, 2013; Gonzalez-Fernandez & Muñoz, 2017).
There are several advantages of using microalgae for biofuels compared to the first-generation biofuels. Algae has a high productivity of oil content per area, which greatly exceed those from the best oilseed crops (Zhou et al., 2015); e.i. average biodiesel production yield from microalgae can be 10 to 20 times higher than the yield obtained from oleaginous seeds and/or vegetable oils (Gouveia & Oliveira, 2009). Microalgae can can be cultivated in marginal lands and do not compete for arable land with conventional biomass crops. It has low demand of water quality and can be cultivated in saline water environments. Finally, it may utilize waste nutrient substances like nitrogen and phosphorus from various wastewater resources, therefore having the potential to bioremediate effluents (Zhou et al., 2015).
Microalgae can be a good alternative to the first generation biofuels. Recently, several entrepreneurs are attempting to produce biodiesel from microalgae commercially using the current technologies and worldwide, research and demonstration programs are being carried out to develop the technology needed to expand algal lipid production from a craft to a major industrial process. Although microalgae are not yet produced at large scale for bulk applications, recent advances present opportunities to develop this process in a sustainable and economical way within the next 10 to 15 years. (Wijffels & Barbosa, 2010).
This study aims to summarize the steps and state of the art technologies of the bioprocess in biodiesel production from microalgae as well as evaluate the progress that has been done so far in this field of research.
Originally published 15.10.2019