Abstract
The last decade has witnessed a tremendous impetus on biofuel research due to the irreversible diminution of fossil fuel reserves for enormous demands of transportation vis-a-vis escalating emissions of green house gasses (GHGs) into the atmosphere. With an imperative need of CO2 reduction and considering the declining status of crude oil, governments in various countries have not only diverted substantial funds for biofuel projects but also have introduced incentives to vendors that produce biofuels. Currently, biodiesel production from microalgal biomass has drawn an immense importance with the potential to exclude high-quality agricultural land use and food safe-keeping issues. Moreover, microalgae can grow in seawater or wastewater and microalgal oil can exceed 50–60% (dry cell weight) as compared with some best agricultural oil crops of only 5–10% oil content. Globally, microalgae are the highest biomass producers and neutral lipid accumulators contending any other terrestrial oil crops. However, there remain many hurdles in each and every step, starting from strain selection and lipid accumulation/yield, algae mass cultivation followed by the downstream processes such as harvesting, drying, oil extraction, and biodiesel conversion (transesterification), and overall, the cost of production. Isolation and screening of oleaginous microalgae is one pivotal important upstream factor which should be addressed according to the need of freshwater or marine algae with a consideration that wild-type indigenous isolate can be the best suited for the laboratory to large scale exploitation. Nowadays, a large number of literature on microalgal biodiesel production are available, but none of those illustrate a detailed step-wise description with the pros and cons of the upstream and downstream processes of biodiesel production from microalgae. Specifically, harvesting and drying constitute more than 50% of the total production costs; however, there are quite a less number of detailed study reports available. In this review, a pragmatic and critical analysis was tried to put forward with the on-going researches on isolation and screening of oleaginous microalgae, microalgal large scale cultivation, biomass harvesting, drying, lipid extraction and finally biodiesel production.
Highlights
‘Algae-for-fuel’ has gained renewed interest during the last decade
Biodiesel Production with Microalgae crops, (iv) microalgae can grow in seawater, brackish water or even wastewater, (v) microalgae cultivation can be combined with waste CO2 sequestration, (vi) use of the spent biomass for other value-added products, etc
Multiple desaturases and elongases are another class of enzyme which are responsible for the production of polyunsaturated fatty acids (PUFAs) from acylCoA in the second stage of fatty acid biosynthesis, commonly can be called as TAG production pathway (Gimpel et al, 2015)
Summary
‘Algae-for-fuel’ has gained renewed interest during the last decade. Unlike terrestrial feed-stocks such as soybean, rapeseed, jatropha, etc., microalgae have been projected with various advantages as (i) algae cultivation does not need arable land, (ii) higher photosynthetic rate of microalgae than terrestrial plants, (iii) microalgal oil yield could significantly exceed the yield of the best oilseed. Cellular lipid content reaching above 50% of dry cell weight (dcw) has been seen in a number of microalgal species (Table 1). In Nannochloropsis sp., lipid content up to 60% (dcw) was observed with 15% CO2 sparging under nitrogen starvation and high light intensity (Jiang et al, 2011).
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