Future prospects of microalgal biofuel production systems

Review

Feature Review

Future prospects of microalgal biofuel production systems Evan Stephens1, Ian L. Ross1, Jan H. Mussgnug2, Liam D. Wagner3, Michael A. Borowitzka4, Clemens Posten5, Olaf Kruse2 and Ben Hankamer1 1

The University of Queensland, Institute for Molecular Bioscience, St. Lucia, Qld 4072, Australia University of Bielefeld, Department of Biology, Algae Biotechnology & Bioenergy, Bielefeld, NRW D-33501, Germany 3 The University of Queensland, School of Economics, St. Lucia, Qld 4072, Australia 4 Murdoch University, School of Biological Sciences and Biotechnology, Algae R & D Center, Murdoch, WA 6150, Australia 5 University of Karlsruhe, Institute of Life Science Engineering, Bioprocess Engineering, Karlsruhe, BW D-76131, Germany 2

Climate change mitigation, economic growth and stability, and the ongoing depletion of oil reserves are all major drivers for the development of economically rational, renewable energy technology platforms. Microalgae have re-emerged as a popular feedstock for the production of biofuels and other more valuable products. Even though integrated microalgal production systems have some clear advantages and present a promising alternative to highly controversial first generation biofuel systems, the associated hype has often exceeded the boundaries of reality. With a growing number of recent analyses demonstrating that despite the hype, these systems are conceptually sound and potentially sustainable given the available inputs, we review the research areas that are key to attaining economic reality and the future development of the industry. Politics of renewable energy technology development The importance of developing CO2-neutral fuel sources has been highlighted by the detailed modelling of climate change effects [1], its global [2] and national economic impacts [3] and the increasing competition for fossil fuel reserves [4,5] (Figure 1). Of these, climate change appears to be the most time-constrained driver of renewable energy technology development. This is because a reduction in CO2 emissions of 25–40% by 2020 and 80–90% by 2050 is predicted to be required to limit global temperature increases to less than the 2 8C limit agreed at the 2009 Copenhagen Climate Change Summit (see also [1–3]). Fossil fuel supplies extend beyond this time window but are by definition finite. Consequently policy makers are under increasing pressure to develop and deploy clean energy technologies, although there is a heated debate over the most effective political mechanisms for implementation. This article reviews the potential and limitations of microalgal biofuel systems in this context, which we hope will also provide an informed guide to policy development. The already apparent problem of reducing CO2 emissions is compounded by the prediction that the global Corresponding author: Hankamer, B. ([email protected]).

population will increase from 6.6 billion in 2008 to 9.2 billion by 2050 [11] and by the fact that the resultant increase in fuel use will be further exacerbated by the increasing energy demands of the rapidly expanding economies of China and India. Figure 1 shows the compounding effects of increasing the global population and economic growth (1.5–3% pa) on the depletion of both ‘proven fossil fuel reserves’ (1P reserves: observed and marketable reserves of oil, gas, coal and nuclear) and far less certain and more costly ‘ultimately recoverable’ reserves (the sum of 1P; and the increasing less certain 2P, 3P and 1C reserves) [6]. A 1% increase in energy efficiency per year is also included in these calculations. The important conclusion highlighted by Figure 1 is that if we rely solely on fossil fuels to supply global energy demand then proven reserves (1P) would be predicted be completely depleted between 2069 and 2088. As the recent oil spill in the Gulf of Mexico has shown, the extraction of 1P reserves is already technically challenging and likely to become increasingly costly. Consequently reliance on 2P, 3P and 1C reserves must be regarded as increasingly insecure. However, current data suggests that based purely on the estimated reserves and a stable population of nine billion people beyond 2050 even these reserves would only secure fossil fuel supply until 2084–2112 at economic growth rates of 1.5–3% pa. A major transition to renewable energy before 2030 should therefore be supported to ensure an orderly changeover from finite reserves and to address the more pressing constraints of climate change [12]. A major problem for policy makers and industry is how best to facilitate the transition to a renewable energy future [13] and what role biofuels should play in the energy mix. Currently almost all renewable energy technologies (e.g. photovoltaic, solar thermal, geothermal, wind and wave power) are designed to produce electricity, which accounts for only a third of the current global energy market [14]. By contrast, biofuel systems offer the best studied and closest to market potential renewable solution for supplying the global liquid fuel demand, which makes up two-thirds of the global energy market [14], by producing biodiesel or aviation fuel, methane, butanol, ethanol or hydrogen. Although electricity has certain advantages

5541360-1385/$ – see front matter . Crown Copyright ß 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2010.06.003 Trends in Plant Science, October 2010, Vol. 15, No. 10

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Figure 1. Predicted rates of global fossil fuel depletion: The depletion of total fossil fuels (oil, gas, coal and uranium) calculated on the basis of proved (1P) and Ultimately Recoverable Reserves (URR), assuming 1.5 and 3% economic growth even with energy efficiency improvements of 1% per annum (which will be hard to achieve). Global population is modelled as increasing from 6.7 billion (2008) to 9.2 billion by 2050 and then stabilizing at nine billion. The depletion rate is based on 100% fossil fuel use. Supplementing energy supply with renewable energy will extend supply. Data sourced from [4–11].

for transportation, fuels are likely to be required to power heavy machinery (e.g. mining equipment), haulage, shipping and the aviation of the future. Consequently a more complex energy mix is expected to emerge, determined by political, geographical, socio-economic and technology considerations. Overview of biofuel technologies The photosynthetic processes of crops, grasses, trees, algae and cyanobacteria all capture solar energy and store it as chemical energy in a wide range of feedstocks (e.g. starch, sugars and lipids) that can be used for the production of biofuels. Established ‘first generation’ biofuel systems based on crop plants such as sugarcane (Saccharum spp.), oil palm (Elaeis oleifera), sugar beet (Beta vulgaris), rapeseed (Brassica napus), soya beans (Glycine max), wheat (Triticum spp.) and corn (Zea mays) are already extensively used for the production of biofuels such as ethanol, diesel and methane, most notably in Brazil, the USA, South-East Asia and Europe. However, with an increasing global population and extensive droughts in major grain exporting regions (e.g. Australia), pressure on food supplies has resulted in growing concern and has led to a heated ‘food versus fuel’ debate [15]. A new generation of biofuel systems that do not require arable land is therefore being developed. The most important of these include lignocellulosic processes which convert cellulose-based products from plants into liquid fuels. Sorghum, Myscanthus, Camelina, switchgrass (Panicum virgatum) and poplar trees (Populus spp.) are currently the most prominent ‘non-food’ plant candidates for these approaches [16–18]. Nevertheless, the success of these systems is dependent on research and development of energy-efficient manufacturing processes, typically enzymatic lignin digestion processes, although chemical digestion methods are also under investigation. Although the resultant demand for large amounts of enzyme appears to

be a surmountable challenge, ultimately this technology might also contribute to food versus fuel concerns because of its current dependence on suitable land, much of which is presently forested. This in turn could lead to a forest versus fuel issue [19], unless waste products from established agricultural or forestry systems are exclusively used, or feedstocks produced on non-arable land can be developed. Although these crops can be grown on non-arable land, their productivity remains linked to soil fertility and water supply, and the scale of cultivation required (see below) to make a meaningful contribution towards global energy consumption will inevitably require lands that are currently used for food production or forestry. Micro-algal biofuel systems Micro-algal biofuel systems, which produce fuels from single-celled microalgae (eukaryotes or cyanobacteria) have the advantage that their productivity is not dependent on soil fertility, and thus could theoretically be scaled to make a substantial contribution to global demand without increasing the pressure on arable land or important forest ecosystems. Many microalgae can be grown in saline water and are able to produce a wide range of feedstocks for the production of biofuels (Figure 2), including biodiesel, methane, ethanol, butanol and hydrogen, based on their efficient production of starch, sugars and oils [20,21]. Because they absorb CO2 from both atmospheric and (in some cases) industrial sources during growth, microalgae can also contribute to carbon capture. Carbon storage can theoretically then be achieved by pyrolysing the waste biomass remaining after fuel production to produce a charcoal-like product (Biochar) suitable for long-term storage [22]. Biochar can also be used as a fuel in its own right, reducing dependency on coal, or marketed as a soil additive [23]. However, although microalgal biofuel systems can ‘in theory’ eliminate both the food versus fuel and potential 555

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Figure 2. Overview of next generation microalgal biofuel production. Inputs (light, CO2, water and nutrients) are used to cultivate microalgae for the production of feedstocks (H2, oil, sugars and starch) required for biofuel production (H2, diesel, ethanol and methane).

forest versus fuel problems, to date no microalgal biofuel systems have achieved economic viability. Recently, Emily Waltz [24] reported that despite a prevalent and excessive enthusiasm which she describes as ‘algae fervour’, there are companies that are actively developing these technologies towards commercial operation. Indeed, initial investment into microalgal biofuels [25] has continued to increase rather than waning despite the first closure of an early start-up company [26]. Based on recent economic case studies [27] on stand-alone microalgal biofuel production models and on a model that co-produces high-value products (HVPs), this investment appears sensible. Sensitivity analyses in these models identified the relative effects of various economic drivers, the key ones being construction costs, biomass productivity and the price of the dominant product (and its yield in the case of high value co-products). The process – constraints and key factors Solar energy The first parameter that should be established before advocating any renewable technology is that there is sufficient renewable energy to drive it and to supply a reasonably large fraction of global energy demand. Of the renewable energy sources available, solar energy is by far the largest with 5700 times the global energy demand [28] reaching the surface of the Earth which is 510 072 000 km2 (Table 1), and is referred to here as 100% (CIA, The World Factbook, https://www.cia.gov/library/publications/theworld-factbook/geos/countrytemplate_xx.html) every year (average annual level 170 W m–2). Approximately 29.2% of the surface of the Earth is land (148 940 000 km2), which can be subdivided into an arable (19 824 000 km2, 3.9%) and a non-arable (129 116 000 km2, 25.3%) component (https:// www.cia.gov/library/publications/the-world-factbook/ fields/2097.html?countryName=&countryCode=®ion Code=n). If average incident solar energy (at 170 W m–2) could be captured with 100% efficiency, only 88 201 km2 (or 0.017% of the surface of the Earth) would be required to supply the total current global energy demand. However, most natural plant ecosystems have a solar energy-to-biomass conversion efficiency of 1% [29]. Furthermore they only use the photosynthetically active radiation (PAR) of the solar spectrum (350–700 nm for green plants; extending to 900 nm for purple bacteria), which approximates to 40% of the incident solar energy [30]. Natural ecosystems, 22 050 556

000 km2 in size (4.3% of the surface of the Earth) would therefore be required to supply global energy demand (Table 1). At 350 W m–2 and 650 W m–2 irradiation in temperate and tropical regions global energy demand could be produced using 1.13–2.10% of the Earth’s surface (29– 54% of arable land). Theoretically sugar cane and other potential fuel crops could achieve maximum light to biomass conversion efficiencies of 8%. If this biomass is fully converted to energy without parasitic losses (which is infeasible) this would reduce land requirements to (3.6–6.4% of arable land). However, approaching this mark will not only be difficult to achieve but with the increasing food demands of a growing population, first generation biofuel systems in general are expected to play a more limited role in supplying future fuel needs. By contrast, microalgae can theoretically be cultivated on non-arable land and are already reported to have achieved light-to-biomass conversion efficiencies of 1–4% in conventional open pond systems [31]. Significantly higher productivities have been reported with closed photobioreactors [29,32–36], but their high cost currently remains a barrier to commercialization for low-value commodity products such as fuels (see Bioreactor capital cost considerations). With 3% efficiency (and 350–650 W m–2) they would therefore require 0.38–0.7% of the surface of the Earth (1 922 334 to 3 570 048 km2). More importantly this comprises only 1.5–2.7% of non-arable land for the production of the total global energy demand. To put this into context, such an area is equivalent to 1.5–2.8 times the area of forests logged between 1990 and 2005 (1 255 000 km2) [37]. This demonstrates that whereas first generation biofuel systems and second generation lignocellulosic processes will be limited by the availability of suitable land, micro-algal biofuel systems can be located predominantly on non-arable land or, potentially, on the ocean surface (361 132 000 km2). Although this suggests an increased flexibility of microalgal systems in terms of land availability, and it allows for a more widely distributed fuel production system than exists currently, they are still constrained by the availability of necessary resources (e.g. CO2, nutrients and adequate water). The collective microalgal cultivation of this area could use the carbon emissions from coal-fired power plants (or other carbon sources) in excess of 1 107 MW capacities (8 1010 T CO2 y–1). Finally, if biofuels are seen as providing one of the seven ‘energy wedges’ proposed by Pacala and Socolow

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Table 1. Projected area requirements for biofuel production Surface area

km 2

Earth surface area Sea surface area Land surface area - Non-arable land - Arable Land

510 072 000 361 132 000 148 940 000 129 116 000 19 824 000

Global energy demand Global energy demand (2008)

PJ a 472 857

Global solar energy supply Average annual solar irradiation m–2 @ 170 Wm–2 Annual global solar irradiation @ 170 Wm–2 Annual solar irradiation / Global energy demand (2008)

PJ 0.0000053611 2 734 557 201 5783

Global solar radiation @ 170 Wm–2 At annual global average solar irradiation levels (m–2) @ 170 Wm–2 Annual solar irradiation (km–2) @ 170 Wm–2

PJ 0.0000053611 5.36 Area (km2) 88 201 882 012 2 940 040 8 820 120

PJ (for PARb) 0.0000021444 2.14 Area (km2) 220 503 2 205 030 7 350 100 22 050 299

100% conversion efficiency 10% conversion efficiency 3% conversion efficiency 1% conversion efficiency

PJ 0.0000110376 11.04 Area (km2) 42 841 428 406 1 428 019 4 284 058

PJ (for PARb) 0.0000044150 4.42 Area (km2) 107 101 1 071 015 3 570 048 10 710 145

100% conversion efficiency) 10% conversion efficiency) 3% conversion efficiency) 1% conversion efficiency)

PJ 0.0000204984 20.50 Area (km2) 23 068 230 680 768 933 2 306 800

PJ (for PARb) 0.0000081994 8.20 Area (km2) 57 670 576 700 1 922 334 5 767 001

Area Area Area Area

required required required required

for for for for

global global global global

energy energy energy energy

demand demand demand demand

(km–2) (km–2) (km–2) (km–2)

@ @ @ @

100% conversion efficiency 10% conversion efficiency 3% conversion efficiency 1% conversion efficiency

Temperate regions @ 350 Wm–2 Annual solar irradiation (m–2) @ 350 Wm–2 Annual solar irradiation (km 2) @ 350 Wm–2 Area Area Area Area


for for for for




(km2) (km2) (km2) (km2)

@ @ @ @

Tropical & subtropical regions @ 650 Wm–2 Annual solar irradiation (m–2) @ 650 Wm 2 Annual solar irradiation (km 2) @ 650 Wm–2 Area Area Area Area


for for for for




(km2) (km2) (km2) (km2)

@ @ @ @

% of world surface area 100.0 70.8 29.2 25.3 3.9

PAR c

0.04 0.43 1.44 4.32 PAR

0.02 0.21 0.70 2.10 PAR

0.01 0.11 0.38 1.13

a

Petajoule (=1015 J) Percentage world surface area required to supply global energy demand c PAR = 40% of solar irradiation b

[38], the required land area would be 0.2% of the surface of the Earth, 0.7% of the non-arable land, or 180 000 km2. Economic analysis suggests that currently microalgal biofuel systems are dependent on the production of coproducts (e.g. biochar, pigments and nutriceuticals) for profitability [27]. Ultimately however, stand-alone microalgal biofuel production systems might well be preferable. This is because fuels are commodity products and have large markets, whereas high-value co-products tend to have smaller markets. Consequently, although co-production is important to an emerging microalgal biofuel industry, fuel and HVP production do not scale well unless multiple HVPs can be produced and the fuel feedstocks (e.g. oil) from each system are pooled. Economically viable, stand-alone systems in a mature biofuels market are likely to require increased solar energy to fuel conversion efficiencies: for example, increasing from current levels (20 g m–2 d–1 with an oil content of 25% dry weight) to 50 g m–2 d– 1 and 50% oil content [27]. Grown under normal conditions microalgae have been shown to have low calorific values of between 18–21 MJ kg–1, and maxima of up to 29 MJ kg–1 at an oil content of 63% [39]. Assuming negligible metabolic

and process losses, for a 2.5-fold increase in growth rate, a 2.5-fold increase in energy conversion is required. Furthermore, raising biomass oil composition from 25% w/w (23 MJ kg–1) to 50% w/w (28 MJ kg–1) would require a 1.2fold (i.e. 28/23) increase in energy conversion. Therefore to increase production from 20 g m–2 d–1 (25% oil) to 50 g m–2 d–1 (50% oil) would require a total efficiency gain of approximately threefold and would return an approximately fivefold increase in total extractable oil. At a biomass productivity of 20 g m–2 d–1 with an energy density of 23 MJ kg–1 (i.e. 25% oil), produced under an average radiation of 22 MJ m–2 d–1 solar energy, an achievable baseline is 2.1% photosynthetic conversion efficiency (PCE). An efficiency of 6.4% is a sensible target for economically viable stand-alone biofuel systems [27]. This is well within the theoretical maximum (see below). Considering the energy content of the biomass and end products, the net energy balance (NEB) of the system is also of importance in the context of sustainability and profitability. The anaerobic digestion systems proposed in previous economic modelling [20,40] could generate additional energy (through methane production), and 557

Review nutrient and carbon recycling. Nutrient recycling, although not economical at low productivity levels, will become increasingly important in future high performance systems as media costs make up a greater ongoing cost component [27] and issues such as general fertilizer pricing and phosphorous limitation [41] are likely to worsen in the future. Other issues can provide a strong impetus for nutrient recycling. For example large-scale biogas production through fermentation of microalgal biomass, offers the potential to recapture a large proportion of the supplied nutrients. Nutrient recycling can also potentially improve the NEB of the system through reduced emission load from fertilizer manufacture, and reduced transport related energy inputs. One promising approach that could also potentially increase economic viability for microalgal biofuel production is the secretion of oil from microalgal cells [42] (Next Generation Fuels and Chemicals, http:// www.syntheticgenomics.com/what/renewablefuels.html) and the ‘milking’ of oil from continuous microalgal cultivations. Oil secretion traits (e.g. as in Botryococcus braunii) could in principle be engineered into more productive strains, effectively simplifying the harvesting and extraction processes in a similar way to existing bacterial and algal milking approaches. Milking systems of this type have already been used to extract valuable products such as compatible solutes from semi-continuous bacterial cultures [43,44] and from microalgal cultures [45–47]. However, the extraction of oil requires special two-phase bioreactors (or equivalent technology) in which excreted oil is extracted from microalgal cultures using a solvent interface [48]. Future development Improving solar energy conversion efficiency Most microalgal ponds have a solar energy conversion efficiency of 1–4% under normal operating conditions and higher efficiencies can be achieved with closed photo-bioreactor systems [49,50]. Given that the theoretical maximum photosynthetic efficiency for converting solar energy to oil and hydrogen is considered to be approximately 8–12%, respectively [30,51–54], there is considerable margin for improvement, which is being targeted through accelerated breeding programs and genetic modification. Furthermore, with careful management of water chemistry, it might be possible to achieve higher levels of efficiency at higher CO2 concentrations because O2 and CO2 act as competing substrates for RuBisCo. Solar energy conversion efficiency can be improved at several levels through synergies between biology and engineering. For example, light scattering at the bioreactor surface must be minimized to maximize the light entering the bioreactor [51,55]. The next level of optimization is at the illuminated culture surface where an excess of solar energy is available to the exposed microalgae, and specifically to their chlorophyll-binding light-harvesting antenna systems. These antenna systems are easily saturated under high-light conditions [56,57], resulting in the photo-protective dissipation of excess energy as heat and fluorescence. Up to 95% of the incident solar energy can be dissipated (i.e. wasted) via these mechanisms [57]. By 558


contrast, cells deeper in the culture are light-limited, reducing the overall solar energy conversion efficiency of the bioreactor, particularly at high culture densities [58,59]. To overcome this limitation and improve efficiency, rapid mixing cycles on a millisecond time scale, can be used to move cells between light and dark zones in the reactor (the ‘flashing light effect’), matching the energy captured by the antenna systems during the short illuminated phase, to the limiting rate of carbon fixation, which continues during the dark phase. In addition to improving mass transfer efficiency [60], mixing also reduces photoinhibition [61–65]. These observations have driven a trend of developing high-density, rapid-mixing, thin-film reactors that include flat plate and helical collectors, and have enhanced solar energy conversion efficiencies [58,66–70]. However, these developments come at a cost, both in terms of construction and the energy required for mixing systems with increased surface area to volume ratios. Some of these ‘costs’ are now being overcome through the use of gas transfer by membranes, which avoids energy consuming bubbling [50]. Theoretically, however, cheap low-efficiency systems and expensive high-efficiency systems can be equally profitable, and the market will ultimately determine the best system solutions for a given process and location. Biological solutions are also contributing to a reduction in the energy required. Specifically, biological strategies are aimed at reducing the size of the chlorophyll-binding photosynthetic light-harvesting antenna systems so that each photosystem obtains only the light that it needs, rather than an excess [55,57,71,72] (Table 2). The advantage of this is that the light that is normally wasted as heat and fluorescence at the pond or reactor surface penetrates deeper into the culture and so increases the overall efficiency of the system (currently 2 times) [55,56]. As light penetration is improved, the energy required for mixing to induce the flashing light effect can also be reduced. Furthermore. these small antenna cell-lines make it possible to refine bioreactor designs by reducing their surface area to volume ratios and to support higher cell densities than is possible with conventional cell lines. Both factors reduce the cost of biofuel production. The third approach being taken to improve light capture efficiency is to collect the incident radiation from a given area (e.g. with Fresnel lens collectors), and distribute it over a much larger area of side-illuminating foils or fibres inside a closed bioreactor [33,97]; however, material costs for these optical devices must be kept low and optical efficiencies have to increase to make such systems economically viable. Combining the strategies of bringing the ‘algae to light’ and bringing ‘light to the algae’ are plate reactors with inbuilt light-conducting structures [97]. Examples of scaled-up systems of this type have been reported [98,99]. In general, closed photo-bioreactors have been used for the production of HVPs but require substantial cost reductions per unit biomass to be economical. Consequently, further design to optimize light penetration, hydrodynamics, and gas transfer [100] should contribute towards the development of cheaper low-energy-consuming bioreactors.

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Table 2. Antenna engineering strategies in selected microalgae Species Chlamydomonas reinhardtii

Strain B1 (psbA) 4D1c (DS521) A4d (psbA + DS521)

Strategy Mutagenesis (metronidazole and DCMU)

Chlorella vulgaris

WT

Low temperature 5 8C

Chlorella vulgaris Dunaliella salina

WT WT

High light or high excitation pressure High light or high excitation pressure

Synechocystis PCC 6714

PD-1

Comparison of pigment-deficient mutants with wild-type strains

Chlamydomonas reinhardtii

Cbs3

Chlamydomonas perigranulata

LHC-1

Insertional mutagenesis of the Cao gene Mutagenesis (UV)


npq2 lor1

Reduced carotenoids leads to smaller PSII antennae


WT

Scenedesmus obliquus Chlamydomonas reinhardtii

WT Stm3

High light or high excitation pressure High CO2, high light Targeted mutagenesis


Stm3LR3

Targeted RNAi repression of all LHC genes


tla1 tlaX tla1-CW+ Stm6-Glc4-T7

Mutagenesis (DNA insertion)

Chlamydomonas reinhardtii Cyclotella sp.

CM1 CM1-1

De-regulation of LHC translation repression Mutagenesis (ethylmethylsulfonate and UV)

Effect Antenna reduction of up to 90% (PSII) and n/a (PSI) B1 Tot Chl = 75%, a/b = NS A4d Tot Chl = 25%, a/b/ = 3 Chl a:b 2-fold higher Chl 80% lower than at 27 8C Antenna reduction Antenna reduction of 88% (PSII) and 60% (PSI) Photoinhibition reduced but FMAX constant indicating that D1 protein activity does not decline At 2000 mmol photon m 2 s 1 the productivity was 50% higher in mutant PD-1 Antenna reduction of 70% (PSII) and unchanged PSI Chl reduction of 25% (high light) to 60% (low light) PMAX was 50% higher in continuous culture Greater light intensity was required to saturate photosynthesis of the mutant compared with wild type Antenna protein mRNA reduction of 80–95% Antenna reduction of 10% Reduced NAB1 leads to increased antenna size Subunit-specific mRNA reduction of 74.0–99.9% Total chlorophyll reduction of 70% Reduced absorption cross-section, increased scattering cross section

Refs [73–76]

Antenna reduction of 10–17%

[95]

40–56% reduction in Chl-a Is 2–3 times greater However, no increase in culture productivity

[96]

[77] [77] [78–80] [81–83]

[84,85] [86,87]

[88]

[89] [90] [91] [55]

[57,92–94]

Abbreviations: Chl: chlorophyll; D1: the D1 reaction centre protein of Photosystem II; DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea; FMAX: maximum fluorescence; Is: light intensity where photosynthesis saturates; NS: not significant; PMAX: maximum productivity; PS: photosystem; PSI: photosystem I; PSII: photosystem II; Tot chl: total chlorophyll; a/b: chlorophyll A/B ratio; and WT: wild type.

Cell densities of 10 g dry weight L–1 are realistic for wellmade closed reactors, compared with typical open pond values of up to 1 g L–1 [100]. This is a considerable advantage for downstream dewatering processes and represents a considerable energy saving because the amount of water that must be pumped through a filter or a centrifuge is inversely proportional to biomass concentration. A survey of these issues, along with current reactor designs, is reported in [50–52,67,97,101,102]. At laboratory-scale concentrations, cell densities of up to 40 g L–1 have already been achieved (C. Posten, unpublished). Such high cell densities are also useful for the production of biogas because it removes the need for a solid–liquidseparation step before fermentation. Bioreactor capital cost considerations The capital cost of construction is a primary economic variable for these systems [27], with land costs; site preparation, earthworks and levees; geotextiles and materials all prone to significant variation depending upon site selection and the chosen project execution plan. At levels

of 200 ha, which has been proposed as a potential economic threshold for the size of microalgal biofuel plants under the conditions modelled in [27], construction costs per hectare have a much greater economic influence than the capital cost for downstream processing. However, it is conceivable that cooperatives of smaller farms could combine biomass harvests for collective extraction and fuel processing. Clearly there is the capacity for both models to operate based on regional and economic conditions. High value products Until biomass yields (and ideally net oil productivity) can be significantly improved, or the price of energy rises dramatically, the co-production of HVPs is necessary to generate additional income streams [27]. This accurately reflects the current international focus on bio-refining of HVPs of which there are many potential candidates [103]. The production of HVPs such as pigments or high-grade lipids using current systems is usually not optimal under the conditions yielding optimal biomass [104–106]. Both capital and operating costs for downstream processing of 559

Review HVPs can be higher than when processing biomass for other end products, so the price–yield aggregate remains the dominant factor in the economic sensitivity of HVP coproduction [27]. Temperature O2 and CO2 regulation Other key considerations with regard to bioreactor cost and design are the control of temperature, dissolved O2 and CO2 concentrations. Temperature Algal species only grow optimally within a specific temperature range. The high temperatures that typically accompany high incident-light levels often result in growth inhibition or death. Open ponds are effectively cooled by evaporation, limiting the upper temperature to about 40 8C. Closed bioreactors also operate well in sunny areas, as long as the mean temperature between day and night does not exceed the temperature limit. In hot areas, closed bioreactors require additional water for evaporative cooling, the use of heat exchangers, the reflection of infra-red radiation or light dilution. Light dilution has the added advantage of improving productivity. The cost of this additional complexity will probably have to be compensated for by increased levels of productivity or the coproduction of HVPs. The relationship between light levels and temperature, particularly during early morning, can dramatically affect biomass productivity [107,108]. Consequently warming cultures in early daylight hours can be as important as cooling during peak radiation periods. However, the active or passive warming of ponds or bioreactors can reduce the NEB unless waste heat can be used. Thus, geographic location is likely to remain an important production variable. O2 and CO2 The dissolved O2 level must also be carefully controlled because in excess it can result in inhibition of carbon fixation due to the oxidase action of RuBisCo. Dissolved CO2 levels must also be carefully regulated (e.g. to maintain stable pH levels). Technically, closed reactors might have the advantage of reduced CO2 losses (outgassing) but again costs must be kept under control. It is anticipated that membrane-based technologies for CO2 transfer will be used in the future, given that they have the potential to improve the energy balance of the system. Regulated CO2 supply can also be used to regulate the pH of the culture, and where sparging is employed it can be used to assist in mixing. Optimizing media for biomass and biofuel production – carbon, nutrients and pH Together with light and carbon, nitrogen and phosphorous are the most important inputs for biofuel production. Under photoautotrophic conditions, the photosynthetic machinery requires solar energy to fix CO2 into organic molecules. However, biomass production can be greatly improved if reduced forms of carbon can be taken up from the media by the microalgae and used for biomass production during photoheterotrophic growth, constituting an indirect form of solar energy harvesting. Connecting micro560


algal biofuel production systems to nutrient-rich wastewater streams (e.g. waste streams from the sugar industry) has considerable and near-term commercial advantages for biomass production (e.g. biomass can be increased during the night) while providing additional social benefits in the form of treated water [109–114]. A major focus of the microalgal biofuels industry is to develop photoautotrophic production systems. However, there are configurations (e.g. use of wastewater resources, or industrial carbon sources) in which photoheterotrophic growth can offer economic benefits and in which organic carbon sources can be recycled. Achieving optimal growth, whether photoautotrophic or photoheterotrophic, requires the correct nutrient composition. An ideal medium is nutrient sufficient and eliminates excess [115–119]. This maintains optimal production as well as minimal cost and waste. This is best achieved by incorporation of elemental balance analyses. The development of optimized media is highly specific for individual species and for the generation of desired products (e.g. nitrogen depletion increases oil content) [115,120–123]. Of the typical macroelements (nitrogen, phosphorous, potassium, calcium, magnesium and sulfur), nitrogen and phosphorus tend to be the most limiting. Iron is generally the most limiting micronutrient; for diatoms, silicon is also essential. Importantly, it is becoming increasingly clear that current methods and rates of phosphate usage are not sustainable [41] and consequently nutrient recycling will become increasingly important. Nutrient recycling The fermentative digestion of the biomass to methane also produces a nutrient-rich residue that can be recycled back to the bioreactors or ponds [124,125] and is a relatively well-understood technology. Although fermentation treats the biomass, it does not treat the water, which ideally should also be recycled to the ponds or sea unless a wastewater stream is available. Only when biomass is exported from the facility does the mineral content have to be replaced. Another area of importance for recycling research is the removal of inhibitory waste products [126– 128] and pathogens [129–133]. Many algae and cyanobacteria excrete growth-inhibiting products into their environment which affect other organisms (heteroinhibition) and/ or themselves (autoinhibition). Compounds that result in autoinhibition need to be removed from the recycled water stream. By contrast, heteroinhibition, also known as algal antibiosis or algal allelopathy [134], could potentially be exploited in the future as a response to issues of contamination from bacteria and fungi, other undesirable algal types, and even algal grazers [135]. Further understanding of these biochemistries and other microbial interactions will be imperative for any future usage of strategies that cultivate mixed microbial communities. Early reports of growth-inhibiting compounds produced by algae [136–138] have led to continued research which has implicated free fatty acids (FFAs) as the main products causing autoinhibition [126–128]; they could also have allelopathic potential. Cell wall degradation products have also been reported to be inhibitory during cryopreservation procedures [139]. Their impacts on actively growing cultures

Review are as yet unknown. There are numerous other inhibitory exometabolites produced by algae and cyanobacteria, such as cyanobacterin [140–142] and fischerellin [143,144]. However, these are primarily allelopathic and generally do not cause autoinhibition. Sensitivity to growth-inhibiting FFAs varies greatly amongst different algae and cyanobacteria and can range from zero effect to total lethality in a concentration-dependent manner. Thus bioassays similar to those developed by Volk et al. [145] should be incorporated into strain-selection processes and pilot-scale cultivations to determine the strains that are most compatible with the intended production methods. Where growth inhibition proves to be significant, it has been shown that it can be offset by nutrient replacement [68,70,146], periodic ultrafiltration of the media [147], or adequate dilution rates in continuous cultivation [148]. Contamination There is now extensive evidence that open pond systems can be run for more than six months without significant contamination using a wide range of microalgae [149]. Prolific strains of Chlorella, for example, are often dominant because they outgrow their competitors (and indeed can often be contaminants themselves in Arthrospira cultures or other microalgal strains). Extreme halophiles such as Dunaliella salina are also dominant in their optimal environments because they do not encounter much competition at high salinities. However, in the context of the wider microalgal industry, contamination issues are still of significant interest. The classic example is cultivation of the freshwater Chlorophyte Haematococcus pluvialis for astaxanthin recovery (or oil as reported by Huntley and Redalje [150]). Contamination can consist of competitive or pathogenic bacteria and fungi, viral infection, algal grazers such as rotifers, or microalgal competitors such as Chlorella and Scenedesmus. Although contamination with Chlorella might not be a major threat to economics in oil and feedstock production, it could easily cripple the economics in astaxanthin or other HVP production models, and this contamination issue could also affect a range of biofuel and HVP candidates. Theoretically, contamination can be better managed in closed photobioreactors, although both open and closed systems might become susceptible to contamination during long periods of continuous cultivation. Consequently there is an increasing focus on preventing contamination that could result in major losses in productivity in established systems. Modified cultivation regimes can reduce contamination issues, for example, increased inoculum sizes from high density, closed bioreactors and reduced retention time in open ponds [150]. Promising biochemical strategies are also emerging, but careful strain isolation and characterization would need to be coupled to a detailed understanding of the preferred cultivation parameters of individual microalgal candidates and of the expected contaminants in the region. Shifts in single environmental parameters (e.g. pH, temperature, solar irradiance or osmolarity) can be used to apply a selection pressure, favouring dominance of the desired species over competing cyanobacterial or algal strains [151–156]. More research is required on pathogenic contaminations (from pathogenic bacteria, fungi and


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viruses); however, some interactions between algae and bacteria [129], fungal pathogens [132,133] and algal viruses and grazers have been recently reported [130,131]. Conclusion The 2009 United Nations Climate Change Conference in Copenhagen perhaps best summarizes current international policy on climate change mitigation; there is a clear recognition that international action is needed to stay within a 2 8C temperature increase threshold, even though the course of action to achieve it remains highly contentious. One reason for this is that scientists and technologists now have to demonstrate the existing benefits of each of the plethora of solutions under development, as well as their future potential, so that policy makers can develop the necessary frameworks required to facilitate political progress. Of the clean energy technologies being developed, almost all target the electricity market (e.g. photovoltaic, solar thermal, geothermal, wind and wave power), which currently only accounts for 33% of global energy demand. However, to secure future fuel supplies (66% of global energy), biofuels represent almost the only viable option. Micro-algal biofuel systems have strengths in terms of delivering clean and sustainably produced fuels for the future while eliminating the food versus fuel and forest versus fuel concerns associated with first generation biofuels and lignocellulosic processes based on wood feedstocks. This conclusion is supported by detailed economic feasibility studies that demonstrate that microalgal production systems have considerable economic potential [27], not only for the production of fuels but also for food supplements (for a rapidly increasing population) and the production of a wide range of HVPs. Microalgal coproduction systems are therefore well placed to offer significant potential for near-term community benefit that will assist the maturation of a stand-alone industry over the next decade. Acknowledgments We gratefully acknowledge the support of the Australian Research Council (DP0877147 and LP0883380), and the Australian Government, through the Asia-Pacific Partnership on Clean Development and Climate, Deutsche Forschungsgemeinschaft and the Bundesministerium fu¨r Bildung und Forschung.

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