Lake 2010: Wetlands, Biodiversity and Climate Change
PROSPECTS OF DIATOMS AS THIRD GENERATION BIOFUEL
Shilpi Samantray, Aakanksha, Supriya Guruprasad, Ramachandra T.V.
Energy & Wetland Research Group, Centre for Ecological Sciences, Indian Institute of Science,
Bangalore 560012, India
ABSTRACT :
Increased use of conventional energy sources has lead to the scarcity of crude oil, enhanced CO2 emission
resulting in ocean acidification and global warming. Today the entire world is looking forward for more cleaner
and sustainable energy sources. Cost-effective renewable energy sources and other energy alternatives are the
need of the hour. Microalgae have proven to be a promising source to meet the current energy demand and
providing a pollution free environment to the future generation. Their high lipid content increases the oil
production 10-200 times, when compared to oil seed crops. They neither need high cultivation space nor have to
compete with food production and water use. Diatom is one such unicellular, photosynthetic microalgae which
contributes to 25% of global primary productivity, with a peculiarity of siliceous cell wall, which consists of
pair of frustules and girdle bands that protect and withhold the size of the oil droplets and helps in capturing the
light needed for biosynthesis. Their lipid content enhances manifolds with myriad kinds of environmental stress
responses as nutrient stress. Furthermore, emerging new technologies for oil extraction and its bioconversion by
transesterification and direct reduction of oils to hydrocarbons at super critical temperatures aids in the
downstream processing and separation. The extract obtained from various processes can be used as a feedstock
for biodiesel. And this emphasizes the utility of diatoms for energy generation and curbing green house gas
emissions. Such microalgal-derived biofuels could progressively substitute a significant proportion of the fossil
fuels required to meet the growing energy demand.
INTRODUCTION :
Diatoms are photosynthetic eukaryotes, within the class Bracillariophyceae in Heterokont division, found
throughout the world’s ocean and freshwater systems. They are believed to have arisen around 200 million years
ago following a secondary endosymbiotic event between a red eukaryotic alga and a heterotrophic flagellate.
They reside inside a glass-like silica shell, which are also called as frustules. These frustules exhibit two
asymmetrical sides with a split between them. Because of this, the group is known as diatoms. This review
presents an overview of the need of biofuel, the advantages of diatoms as a feedstock for biofuels, the biodiesel
extraction process, the lipid content and their interaction with biotic community.
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Lake 2010: Wetlands, Biodiversity and Climate Change
Diatoms are major players in biomass production and the sinking of atmospheric greenhouse gas. Estimates
indicate that these photosynthetically active organisms are responsible for 20–25% of total terrestrial primary
production and approximately 40% of annual marine biomass productions making them the most dominant
group of organisms sequestering carbon from the atmosphere. They have influenced the global climate by
changing the flux of atmospheric carbon dioxide into the oceans. Diatoms are normally brownish or yellowish in
color as they have chlorophyll A , chlorophyll C and carotenoid fucoxanthin, which is present in their plastids.
Using these pigments, diatoms algae produce their food through photosynthesis and acts as a great provider of
oxygen. Lipid production by diatom is an important process in the aquatic ecosystem. Diatom lipids have been
suggested as a potential diesel fuel substitute with an emphasis on the neutral lipids due to their lower degree of
unsaturation and their accumulation in algal cells at the end of growth stage. Their high productivity and the
associated high lipid yields make them an attractive option for renewable energy.
NEED FOR BIOFUEL :
At present, the interest in renewable fuels is rising because of the global need to reduce emission of greenhouse
gases, the increasing prices for fossil energy sources and to provide an assured supply of fossil energy in the
future. Biofuels play an important role in diversifying energy supplies to meet the world’s growing energy
needs. Biofuels that would replace fossil fuels could end our dependency on the depletion of global natural
resources and reduce our carbon footprint on the planet. As a replacement of petroleum fuel it is much cleaner
alternative. The First Generation Biofuel includes food crops like Brassica napus (commonly known as
rapeseeds), Elaeis guineensis (commonly known as oil palm), Glycine max (commonly known as soybean);
which have a demerit of requirement for high cropping area and their competition with other food crops and
water use. The oil production was not enough to meet the rising demand. This paved the way for the researchers
to look for another alternative, called as Second Generation Biofuels which included the non-food crops like
Jatropha curcas, Panicum virgatum (commonly known as Switchgrass), Miscanthus giganteus, non-food parts
of the plants etc. These plants do not compete with food production, but they required large cultivation area and
regular water supply for their growth. The oil production was higher and the cultivation area requirement was
lower when compared to the first generation biofuels, but the yield was not enough to meet the increasing
demand of fuels. Sustainable production of renewable energy is being hotly debated globally since it is
increasingly understood that first and second generation biofuels, primarily produced from food crops and nonfood feedstock are limited in their ability to achieve targets for biofuels production, climate change mitigation
and economic growth. These concerns have increased the interest in developing Third Generation Biofuels
produced from non-food feedstock such as microalgae, which potentially offer greatest opportunities in the
longer term and holds the greatest promise to grow the biofuels industry to large scale. Algae grow naturally
along rivers, the seashore, and in wastewater. They consume CO2, convert it to oil, proteins, carbohydrates and
other useful products and emit only oxygen to our atmosphere. They can produce 100 times more oil per acre
than traditional oil crops, which can be converted directly to biodiesel. The lipid content of planktonic algae is
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Lake 2010: Wetlands, Biodiversity and Climate Change
considerably higher than of terrestrial plants. Table 1 shows the comparison of sources of three generation of
biofuels with respect to their oil yield (L/ha) and the required cultivation area (M ha).
Table 1 :
Comparison of sources of three generation of biofuels
Crop
Oil yield (L/ha)
Land area needed (M ha)*
Corn
172
1540
Soybean
446
594
Canola
1190
223
Jatropha
1892
140
Coconut
2689
99
Oil Palm
5950
45
Microalgae
136,900
2
*For meeting 50% of all transport fuel needs of the United States
Advantages of Third Generation Biofuel Over First and Second Generation :
•
Higher photosynthetic efficiency.
Terrestrial plants convert only 0.5% of solar energy into plant biomass, where as microalgae’s
photosynthetic efficiency can exceed 10%.
•
The processing steps for conversion of biomass into liquid fuel is much simpler in case of algae.
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Lake 2010: Wetlands, Biodiversity and Climate Change
•
Extremely rapid growth rates , approximately 1-3 doubling per day.
•
Can thrive in waters of widely varying salinities and chemical composition.
•
Can grow on marginal lands (eg – arid, desert and semi-arid lands), that are not suitable for
conventional agriculture.
•
Annual biomass productivity of microalgae per unit land area greatly exceeds that of terrestrial plants.
•
Harvesting rates can be modified.
•
Continuous year round production, avoiding the strong seasonality of terrestrial crop production.
•
Reduces greenhouse gas emission.
•
Can utilize nitrogen and phosphorous from a variety of waste water sources ( eg – agricultural run-off,
concentrated animal feed operations, and industrial and municipal wastewaters), providing the
additional benefit of wastewater bioremediation.
GENOMICS OF DIATOMS :
Recent analysis of a diatom genome reveals that it has genes derived from plant, animal and bacterial lineages,
thus proving it to have a mosaic nature. The recent completion of the first genome sequence from a marine
centric diatom, Thalassiosira pseudonana, have provided unprecedented insights into diatom evolution and
physiology that likely reflect the novel evolutionary origins of diatoms and their specialized ecological
adaptations. It was found that the nuclear genome is a composite of ‘animal-’ and ‘plant-like’ genes. And they
perform fatty acid metabolism in both peroxisomes and mitochondria. And they have an integrated functional
urea cycle which leads to nitrogen metabolism. Advances in molecular genomics are facilitating the use of
diatom-specific genes as a modern biotechnology tool. Transporter genes present in diatom help in
bioremediation of water contaminated with heavy metals or enriched in phosphate and nitrogen. With
availability of diatom genome sequence information, it is now possible to clone specific diatom transporters and
evaluate their substrate affinity and specificity. Many genes, however, of green algae origin have also been
identified, in the genomes of diatoms, indicating more complex gene acquisitions. A specific database, the
Diatom EST Database, is devoted to information about diatom expressed sequence tags and the genome
sequences. Eleven diatom genomes have received sequencing attention, but till date only three genomes of
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Lake 2010: Wetlands, Biodiversity and Climate Change
diatoms have been completely sequenced. About 57% of the genes found in pennate have homologues in centric
and both have acquired a remarkable number of bacterial genes (after secondary endosymbiosis). The genomes
revealed an array of genes that are putatively involved in silicon biochemistry. These include genes encoding
silicic acid transporters, many spermidine and spermine synthase-like enzymes, silaffins and frustulins (casing
glycoproteins). Not much genetic engineering has been done to alter or increase fatty acid production by
diatoms, although mutagenesis has been successfully applied to obtain mutants with altered unsaturated fatty
acid composition. Despite the growing number of completed microalgae genome sequences, only a few
examples of genetic engineering of the metabolism for the production of biofuels are reported. The complexity
of fatty acid metabolism may be one reason. A recent review suggested Amphora (Bacillariophyta) as an
interesting microalgae, at least in terms of lipid productivity.
BIOFUELS :
Biofuels refers to the solid, liquid or gaseous fuels that are predominantly produced from biorenewable
feedstocks. They are non polluting, locally available, accessible, sustainable and reliable fuels. Liquid biofuel
can replace the petroleum fuel. The flow chart below shows the Petroleum and Biorenewable based transport
fuels.
Diesel Fuel
Fig.1 Petroleum and Biorenewable based transport fuels
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Lake 2010: Wetlands, Biodiversity and Climate Change
BIODIESEL FROM DIATOMS :
Biodiesel can be defined as the monoalkyl esters of vegetable oils, animal fats or algal biomass. But vegetable
oils, animal fats or algal biomass as alternative engine fuels are extremely viscous. The viscosity ranges from
10-17times greater than petroleum diesel fuel. There are four primary ways to make biodiesel, direct use and
blending, micro emulsions, thermal cracking (pyrolysis) and transesterification. The most commonly used
method is Transesterification. Transesterification of parent oil is done to achieve a viscosity close to that of
petro-diesel. This chemical conversion of oil to its corresponding fatty ester (biodiesel) is called
transesterification.
A simple reaction takes place during biodiesel transesterification reaction ,
Triglyceride + 3 Monohydric alcohol → 3 Monoalkyl esters (Biodiesel) + Glycerol
This is an equilibrium reaction where triglycerides can be processed into biodiesel, usually in the presence of a
catalyst. The catalyst can be alkaline, acidic or enzymatic (lipase enzyme). Alkali-catalyzed transesterification is
about 4000 times faster than acid-catalyzed reaction. It is carried out at approximately 60°C under atmospheric
pressure. Alkalis like sodium and potassium hydroxide are commonly used as commercial catalysts. Use of
lipases offers important advantages, but is not feasible because of the high cost of the catalyst.
Parent oil used in making biodiesel consists of triglycerides in which fatty acid molecules are esterified with a
molecule of glycerol. Transesterification produces methyl esters of fatty acids (biodiesel) and glycerol. The
reaction occurs stepwise: triglycerides are first converted to diglycerides, then to monoglycerides and finally to
glycerol. Methanol and oil do not mix, so the reaction mixture contains two liquid phases. Methanol is used in
the process as it is the least expensive, when compared to the other alcohols.
Transesterification of oil to biodiesel. R1-3 are hydrocarbon groups
Transesterification requires 3 mol of alcohol for each mole of triglyceride to produce 1 mol of glycerol and 3
mol of methyl esters. Industrial processes use 6 mol of methanol for each mole of triglyceride. The large excess
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Lake 2010: Wetlands, Biodiversity and Climate Change
of methanol ensures that the reaction is driven in the direction of methyl esters, i.e towards biodiesel. Biodiesel
is recovered by repeated washing with water to remove glycerol and methanol.
Fig. 2 Process of extraction of biodiesel from diatom lipid using base catalyst
LIPIDS OF DIATOMS :
Lipids act as storage molecules for energy production. The compositions of total lipids vary from species to
species and is also affected by the climatic conditions. Generally the conditions favoring the maximum lipid
production seemingly are not compatible with those required for yield optimization.
Both polar lipids and non polar (neutral) lipids are found in diatoms. The neutral lipids can be used for the
production of biodiesel. The major lipid components in all diatom species, that is used for biodiesel production
is Triacylglycerides (TAG). The concentration of TAG changes from species to species, depending on the the
environmental conditions. Environmental parameters like pH, temperature, light, nitrogen, carbon, silicon,
phosphorous, iron, salt concentration etc affect the lipid composition of the diatoms. Stress condition ceases the
growth of the species, thereby reducing the biomass content but enhances the lipid composition. The major
stress conditions like limitation of silicon and nitrate, mostly stimulates the lipids accumulation in diatoms. So
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to maximize the TAG content we need to grow the diatom in nutrient-deficient media. In case of nitrate
deficiency, where there is insufficient N for protein production necessary for growth, excess carbon from
photosynthesis is channeled into storage molecules such as TAGs and thus making it a rich source of oil. The
commercial use of diatoms depends on the diversity of lipid composition and TAG accumulation.
BIOTIC INTERACTIONS :
Aquatic organisms have been shown to produce a variety of bioactive compounds, mainly secondary
metabolites. There is evidence showing that chemical signals produced by aquatic organisms are involved in
many processes, such as predator defence, competition, resource foraging and reproduction. Chemicals
produced by aquatic organisms, and especially micro-organisms, have received increasing attention in the last
decade for their role in shaping interactions and communities. It has been seen that diatoms, unicellular algae
and other microbes like bacterias in aquatic ecosystems produce a wide range of bioactive metabolites. Cell to
Cell Signaling is the mode of communication between the different species of micro-organisms. Using the
signaling mechanism the microbes were able to sense their environment and react appropriately. Studies on
microbial mats of North Sea have shown uncommon modes of living and extraordinary structures like spherical
objects covering a community of cyanobacterial, diatoms and chemoorganotrophic bacteria. Fig 1 clearly
depicts the microbial mats found in North Sea. The potential role of bacteria has been observed to produce
signal substances which influence on aggregate and sphere formation. It was observed that the cyanobacteria
show some chemotactic response to the bacterial products which leads to the massive development of diatoms
inside the spheres. This work was performed at laboratory level and it successfully produced huge number of
microbial spheres, having diatoms as the most abundant species. Such increased diatom population can help in
enhancing the biomass content and lipid production.
Fig 3. Microbial mats of diatoms, cyanobacteria and chemoorganotrophic bacteria
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Lake 2010: Wetlands, Biodiversity and Climate Change
CONCLUSION :
Geologists claim that much crude oil comes from diatom, which indeed produce oil and can solve the
productivity gap by getting 10-200 times more oil, compared to oilseed crops. As derived from diatom biomass,
the biodiesel obtained doesn’t contribute to atmospheric CO2 emission. It can be used in existing diesel engines
without modification. It is non-toxic and highly biodegradable. Complete combustion of biofuel occurs giving it
a cleaner burn. Their rapid growth rate leads to the high per acre yield, which is 7-31 times greater than the next
best crop, palm oil. Their high photosynthetic efficiency, simpler processing steps, rapid growth rate, annual
biomass productivity, continuous year round production makes them a better alternative than the terrestrial
crops. Keeping all these advantages in mind we can conclude saying that diatom based biofuel is a cleaner
alternative and hold great promise for the successful replacement of petroleum fuel. Existing demand for liquid
transport fuels could be met sustainably with biodiesel from diatoms. Diatom biomass needed for production
of large quantities of biodiesel could be grown in photo-bioreactors, but a rigorous assessment of the economics
of production is necessary to establish competitiveness with petroleum-derived fuels. Achieving the capacity to
inexpensively produce biodiesel from diatoms is of strategic significance to an environmentally sustainable
society.
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