Available online at www.sciencedirect.com
Algae biofuels: versatility for the future of bioenergy
Carla S Jones1,2 and Stephen P Mayfield1,2
The world continues to increase its energy use, brought about
by an expanding population and a desire for a greater standard
of living. This energy use coupled with the realization of the
impact of carbon dioxide on the climate, has led us to reanalyze
the potential of plant-based biofuels. Of the potential sources
of biofuels the most efficient producers of biomass are the
photosynthetic microalgae and cyanobacteria. These versatile
organisms can be used for the production of bioethanol,
biodiesel, biohydrogen, and biogas. In fact, one of the most
economic methods for algal biofuels production may be the
combined biorefinery approach where multiple biofuels are
produced from one biomass source.
Addresses
1
The San Diego Center for Algae Biotechnology, University of California
San Diego, 9500 Gilman Drive, MC0368, La Jolla, CA 92093, United
States
2
Division of Biological Sciences, University of California San Diego, 9500
Gilman Drive, MC0368, La Jolla, CA 92093, United States
Corresponding author: Mayfield, Stephen P ([email protected])
Current Opinion in Biotechnology 2012, 23:346–351
This review comes from a themed issue on
Energy biotechnology
Edited by James C Liao and Joachim Messing
Available online 19th November 2011
0958-1669/$ – see front matter
# 2011 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.copbio.2011.10.013
Introduction
Over the past 50 years, the world’s population has more
than doubled, coupled with an expectation of a higher
standard of living and an ever-increasing economic output
this has resulted in a large increase in primary energy
consumption, particularly the use of fossil fuel-derived
energy [1]. In 2010, world primary energy consumption
grew by 5.6%, the largest percentage growth in almost 40
years. This growth included an increase in the consumption of all major fossil fuels including oil, natural gas, and
coal [2]. This trend in increasing energy consumption is
expected to continue as the world’s population is projected to increase by an additional 1.4 billion people by
2030, and have an increase of 100% of the world’s real
income [1]. These increases will put enormous pressure
on the finite supply of fossil fuel-based energy, exacerbating global concerns over energy security, fossil fuelbased environmental impacts such as climate change, and
the rising cost of energy and food. All of these real
Current Opinion in Biotechnology 2012, 23:346–351
concerns support the need for the development of
alternative and renewable sources of energy [3].
Currently, the world consumes about 15 terawatts of
energy per year and only 7.8% of this is derived from
renewable energy sources. Yet, the total power of sunlight
hitting the surface of the Earth every year is about
85,000 terawatts [2,4]. However, replacing fossil fuelderived energy with renewable energy sources derived
from sunlight, such as wind, solar, hydro, or biomass
energy is a daunting task in large part because these
energy sources have a lower energy density, cannot be
controlled with an ‘on and off’ switch, and most are
considerably more expensive than what fossil fuels are
today [5]. In 2009, transportation represented 29% of the
end-use shares of total energy consumption in the US
with a significant portion of this consumption (approximately 80%) resulting from road transport using liquid
petroleum fuels [6,7]. The high energy density and ease
of transportation and storage of liquid petroleum transportation fuels make them difficult to replace with any
current commercially available sources of renewable
energy [7]. One potential answer to the replacement of
the unique characteristics of liquid transportation fuels is
to use the same resources that provided us with fossil
petroleum fuels originally: photosynthetic microorganisms producing bio-oils.
Sustainable sources of bioenergy
Photosynthetic organisms such as higher plants, algae,
and cyanobacteria are capable of using sunlight and
carbon dioxide to produce a variety of organic molecules,
particularly carbohydrates and lipids. These biomolecules
can be used to generate biomass or more directly through
extraction as a source of fuel known as biofuels. The value
of biofuels to meet energetic needs of the future, particularly transportation fuels, has continued to play a role in
the formation of US policy for a number of years, including the recent establishment of the Renewable Fuels
Standard in 2009 mandating the production of 36 billion
gallons of biofuels by 2022 to displace petroleum in our
transportation fuels mix [8,9].
Two of the most common biofuels currently produced are
ethanol produced from corn or sugarcane and biodiesel
produced from a variety of oil crops such as soybeans and
oil palm [9]. Ethanol production has flourished in the US,
rising 25% between 2000 and 2008 due to its use as a
gasoline additive and due to federal mandates and tax
incentives to fuel blenders. Today 30% of the corn
currently grown is used for ethanol production [9,10].
If corn ethanol was the sole source used to achieve the
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Algae biofuels Jones and Mayfield
2020 federal mandates for renewable fuel, than 100% of
the corn currently available in the US would be
required. To meet these mandates and maintain today’s
30% corn crop utilization would require an increase in
corn harvest by 423%, a number unlikely to be achievable in the next 10 years [8,9]. The dedication of
significantly higher amounts of the US corn crop to fuel
production could have devastating effects on food availability around the world where about 1.02 billion people
are already undernourished [11]. This food versus fuel
dilemma and the limited environmental savings associated with corn ethanol production led policymakers to
specify that a significant portion of the biofuels (21
billion gallons) in the renewable fuels mandates be
derived from noncorn starch products [8,9,12]. One
source of these biofuels may be ethanol derived from
sugarcane; however, although the domestic production
cost of sugarcane ethanol is 24% lower than corn ethanol, the transportation cost and the coproduct credits
associated with corn ethanol make sugarcane ethanol
17% more costly. Thus, sugarcane ethanol is also an
unlikely candidate for the displacement of significant
amounts of fossil fuels [10].
The high cost of sugarcane ethanol and the competition
with food from corn ethanol leave a large gap between
the current feasible production levels of ethanol and the
fuel requirements for the RFS2 mandate. To overcome
these limitations, lignocellulosic feedstocks are also
being developed as sources for the production of ethanol
[3]. Lignocellulosic feedstocks come in a wide range of
different plants, including agricultural waste products
such as corn stover, woody sources such as aspen and
dedicated energy crops such as hybrid poplar and
switchgrass [13]. Recent research has focused on understanding how the biochemical composition of these
crops, mainly the ratios of cellulose, hemicellulose, and
lignin, impact the efficiency of ethanol production, and
on methods to lower the costs of the enzymes and
pretreatments needed to release the fermentable sugar
components [11,13]. Currently, no commercial scale
cellulosic ethanol plants are in operation largely due
to the high price of production, almost twice that of corn
ethanol [13,14].
The displacement of transportation fuels by biofuels is
not limited to ethanol. Oil-seed plants such as soybean,
rapeseed, or palm oil, also offer the opportunity to produce biodiesel. However, once again these traditional oil
crops are used as food, and hence using them as fuel has
an impact on food availability [15]. Another source of
biodiesel that has recently had a lot of media exposure is
Jatropha curcas. J. curcas is a small tree that is considered
drought-tolerant and produces seeds that contain 20–40%
nonedible oil, therefore being noncompetitive with food
sources and agricultural land [16]. Although there is not an
agronomically developed strain of J. curcas for biodiesel
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347
production, research and breeding programs are focused
on using many modern techniques such as transcriptomics and near-infrared spectroscopy to identify traits
valuable in making this plant a dedicated oil-seed energy
crop [17,18].
In recent years, algae have become a focus in both
academic and commercial biofuels research. These
photosynthetic organisms are known to produce high
oil and biomass yields, can be cultivated within nonfreshwater sources including salt and wastewater, can
be grown on nonarable land, do not compete with common food resources, and they very efficiently use water
and fertilizers for growth [19]. However, the true hallmark
of these microscopic organisms is in fact their versatility
(Figure 1). Algae can tolerate and adapt to a variety of
environmental conditions, and are also able to produce
several different types of biofuels.
Algae bioenergy production options
Bioethanol
Bioethanol from algae holds significant potential due to
their low percentage of lignin and hemicellulose as compared to other lignocellulosic plants [20]. Algae can be
classified as either microalgae or macroalgae based on
morphology and size. Microalgae are microscopic organisms while macroalgae are typically composed of multicellular plant-like structure, like giant kelp. Although
macroalgae can look similar to land plants, these organisms in fact do not have the same lignin crosslinking
molecules in their cellulose structures because they grow
in aquatic environments where buoyancy allows for
upright growth in the absence of the lignin crosslinking
[21]. While having a low lignin content, macroalgae contain significant amount of sugars (at least 50%) that could
be used in fermentation for bioethanol production [22].
However, in certain marine algae such as red algae the
carbohydrate content is influenced by the presence of
agar, a polymer of galactose and galactopyranose. Current
research seeks to develop methods of saccharification to
unlock galactose from the agar and further release glucose
from cellulose leading to higher ethanol yields during
fermentation [22,23].
Microalgae are also being studied for bioethanol production. Green algae including Spirogyra species and
Chlorococum sp. have been shown to accumulate high
levels of polysaccharides both in their complex cell walls
and as starch. This starch accumulation can be used in the
production of bioethanol [20,24]. Harun et al. have
shown that the green algae Chlorococum sp. produces
60% higher ethanol concentrations for samples that are
pre-extracted for lipids versus those that remain as dried
intact cells [20]. This indicates that microalgae can be
used for the production of both lipid-based biofuels (see
below) and for ethanol biofuels from the same biomass as
a means to increase their overall economic value.
Current Opinion in Biotechnology 2012, 23:346–351
348 Energy biotechnology
Figure 1
Current Biofuels Sources
Oil Seed Plants
Corn & Sugarcane
Anaerobic
Fermentation of
Waste Materials
Lignocellulosic
Plants
Biodiesel
Bioethanol
Biogas
Biohydrogen
Ref. 15, 25, 27
Ref. 15, 20
Ref. 29, 38
Ref. 32, 37
Algae
Algal Versatility for Economic Viability
Current Opinion in Biotechnology
The RFS2 mandates the production of 36 billion gallons of biofuels by 2022. Currently many plant resources can be used to produce these biofuels;
however, many of them compete with food availability. Algae represent a single photosynthetic production system that is capable of extremely
versatile biofuels applications that will undoubtedly supplement other less versatile systems. References shown highlight their versatility.
Biodiesel
Algae biodiesel is another algal-derived biofuel with
future commercial feasibility. Many species of algae
produce large amounts of lipids as storage products, as
high as 50–60% of their dry weight. Upon transesterification, these lipids are chemically similar to other oilseed crop derived lipids making algae a very productive
potential source of biodiesel [25]. Despite this productivity, algae biodiesel is still not yet economically
competitive with petroleum diesel, with algal biodiesel
at US $1.25/lb compared to petroleum-based diesel at
US $0.43/lb [26].
for lipid extraction, the complex cell walls of algae
prevent this pressure extraction process [28]. Optimization of direct transesterification by chemically extracting and transesterifying lipids in the same step will
circumvent the need for a two-step process and can
still yield a quality biodiesel [26,28]. Although there are
still many processes that need to be optimized, the high
biomass and lipid productivity of algae warrants continued investment in research and development and
makes algae a strong candidate as a source of commercially viable biodiesel.
Biohydrogen
The cost of algae-derived biodiesel is proportional to the
species-specific efficiency of algae to sequester carbon
dioxide as lipids. Thus, microalgal bioprospecting has the
potential to greatly impact the future efficiencies, and
hence reduce cost, of algae biodiesel production [25].
In particular, bioprospectors look to find strains that are
not only high lipid producers but also have superior
growth and harvesting characteristics [15]. For instance,
Aravjo et al. found that some algal species including
Chaetoceros gracilis and Tetraselmis tetrathele can grow in
saline water, produce high levels of lipids, and respond to
sodium hydroxide inducing flocculation allowing for
easier harvesting [27].
Optimization of the transesterification process would
also greatly benefit the cost of algae biodiesel production. Unlike oil-seed crops that can be compressed
Current Opinion in Biotechnology 2012, 23:346–351
In the past few years most algae biofuel research has
focused on liquid fuels, particularly those that can
replace transportation fuels such as biodiesel and
bioethanol; however, algae are also a potential source
of commercial biohydrogen and biogas (biomethane)
used as a gas fuel or for electricity generation [29].
Macroalgae are a potential source of biomass for the
production of these gases due to their fast growth rates,
ability to grow in oceanic environments and their lack
of the structural lignin which is typically difficult to
digest [30]. Many species of macroalgae are known for
having high levels of carbohydrate, although in many
cases these carbohydrates consist of nonglucose monosaccharides such as galactose [30]. Recent research has
shown that the red algae Gelidium amansii and the
brown algae Laminaria japonica are both potential biomass sources for biohydrogen production through
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Algae biofuels Jones and Mayfield
anaerobic fermentation, but in the case of G. amansii,
an inhibitory by-product of acid hydrolysis (5-hydroxymethylfurfural) decreases the hydrogen production
rate by 50% due to noncompetitive inhibition
[30,31]. Thus, as bioprospecting of macroalgae for their
fermentative future continues, it will be necessary to
optimize pretreatment methods for maximum biohydrogen production.
Photosynthetic microalgae and cyanobacteria are also
able to directly produce biohydrogen through photofermentation in an anaerobic process involving oxidation of
ferredoxin by the hydrogenase enzyme [32]. However,
hydrogenases directly compete with many other metabolic processes for the partitioning of electrons, and not all
activities of hydrogenases function equally. Thus, a significant amount of recent research on microalgae photobiohydrogen production has focused on identifying robust
hydrogenase activities, understanding their interaction
with ferredoxin and other metabolic processes, and
genetically modify these interactions to increase the
efficiency of biohydrogen production [32,33,34].
Although hydrogen production from algae still seems
years away from commercial viability, continued progress
in this area shows its ultimate potential.
Biogas
Recently, microalgae have also become a topic of interest in the production of biogas through anaerobic
349
fermentation. The efficiency of biogas production has
been shown to be species-dependent based on relative
efficiency of cell degradation and on the presence or
absence of molecules that may inhibit methanogenic
archaea [29]. The production of biogas from algae may
also play an important role in bioremediation as harmful
algal blooms in lakes, ponds, or oceans can result in the
production of toxic secondary metabolites that can have
drastic effects on these ecosystems, and removing these
algae for biogas production can reduce these impacts
[35]. Currently, the production of biogas from algae is
still limited due to the need to heat the digesters and the
requirement for more land area and infrastructure to
produce the same amount of energy as can be obtained
for algae biodiesel [36].
Versatility of microalgae for economic success
The versatility of biofuel production from algae may
provide answers to both the economic hurdles and the
lifecycle challenges faced in renewable energy production.
By extracting more than one type of biofuel from algal
biomass or an additional coproduct, the value of the biomass increases while also offering additional offsets to the
environmental impacts. As mentioned above, the combined biorefinery concept can be used to increase ethanol
content from algae following extraction of lipids [20].
This concept can also be used in combination with biogas
and biohydrogen production, either by producing a valuable product before fermentation or by using the gaseous
Figure 2
Bioprospecting
Genetics
Breeding
Water
Nutrients
Pond Design
Water Management
Crop Protection
Strain
Development
Inputs
Production
Harvest
Dewatering - recycled to inputs
Direct H2 production
Co-Production
Extraction
Gasoline
Refining
Diesel
jet
Value added Co-products
Fuel
Extraction
Residual
Biomass
Anaerobic Digestion - Biogas
Nutrient recycling to inputs
Current Opinion in Biotechnology
General schematic of the algal biofuels production chain. The biggest challenges currently being investigated concerning this production chain are
shown in bold.
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Current Opinion in Biotechnology 2012, 23:346–351
350 Energy biotechnology
products of fermentation to power the process of producing
that high value product. In the first scenario, the high value
product can include biohydrogen produced anaerobically
just before anaerobic digestion for biogas production
[29,37,38]. In the second case, electricity generated from
biogas can be used to offset the energy requirements for
anaerobic digestion of microalgae during biogas production, agriculturally derived biogas can be used to provide a CO2 stream for algae growth and coproduct
production, and biogas can be used to power the cultivation
and lipid extraction process for algae biodiesel [36,39,40].
Regardless of the combined biorefinery concept chosen,
the economic viability and environmental sustainability of
the production of algae biofuels will depend on creating a
completely optimized and efficient overall utilization that
leaves little waste and uses every component of the algal
biomass.
References and recommended reading
Papers of particular interest, published within the period of review,
have been highlighted as:
of special interest
of outstanding interest
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Acknowledgments
SPM is a founder of and has a financial interest in Sapphire Energy an algal
biofuel company, but this work should not be considered to reflect the views
of Sapphire Energy. This report was supported by a grant from the US Air
Force #FA9550-09-1-0336 to SPM. CSJ is a San Diego IRACDA
Postdoctoral Fellow supported by NIH Grant GM06852.
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Current Opinion in Biotechnology 2012, 23:346–351