Overcoming biological constraints to enable the exploitation of microalgae for biofuels
Highlights
► Physiological constraints to algal production and possible solutions are evaluated. ► Molecular constraints to algal biofuel production and possible solutions evaluated. ► Allelopathic interactions between algae and other microorganisms. ► Disease and infection, effects on algal productivity. ► Grazing; effects on productivity and possible control measures.
Introduction
Anthropogenic climate change, population growth and reduction in the remaining easily exploitable oil reserves have been amongst the most potent drivers to find novel energy supplies. Global developments of renewables (wind, wave, tidal and geothermal) have massive potential, but transport fuels, specifically for air travel, will require liquid fuels. Although biofuels have the potential to supply transport fuels, first generation plant-crop derived fuels are confined to agriculturally productive areas and can depend on freshwater supplies for irrigation. It is clear that further intensification of existing biofuel crop production could place unsustainable demands on these resources with significant implications for food production. Additional strategies are needed if biofuel production is to be sustainable at large-scale.
Microalgae have some clear advantages over “higher” plants, these include: their very high growth rates, their capacity to utilise a large fraction of the solar energy (in theory ∼10% of the total solar energy can be fixed into biomass) and can grow in conditions that are not favourable for terrestrial biomass growth (Carlsson et al., 2007). Future large-scale algal production facilities could also exploit abundant supplies of brackish and salt water by using marine micro-algae. Set against the high oil yields observed with algae are the economic costs for growth, harvesting and extraction of oil, which are high compared with biofuel crops (Brennan and Owende, 2010). Moreover, as with agriculture, nutrient supply in the form of fertilizers or organic waste would still be required for algal culture.
To date, the largest algal production facilities have focused on extremophiles, either on the basis of pH; Arthrospira/Spirulina (Benemann, 2003) for dietary supplements, or salinity for β-carotene production using Dunaliella salina (Ben-Amotz, 2004). In the above examples the product value is such that relatively low yields are commercially viable. However, in the case of biofuel a dry weight biomass yield in the region of 100 tonnes/ha/yr would be required for an open pond system to be economically viable at the time of writing (Carlsson et al., 2007). Although this is a major challenge, it is realistic, as yields of 60 tonnes/ha/yr have already been achieved for Pleurochrysis carterae and D. salina in field-scale production systems (Navid and Borowitzk, 2006). The reported oil yield for Pleurochrysis was 21.9 tonnes/ha/year, which compares favourably with the 4–5 tonnes/ha/yr obtained from oil palm, the highest yielding oil crop plant (Navid and Borowitzk, 2006, Sumathi et al., 2008). It is worth noting that this productivity from algae could meet annual US petroleum consumption with a land area of 427,000 square kilometres, close to that of California (US petroleum consumption was 18.8 million barrels per day for 2009, equivalent to approximately 935 million tonnes per year: US Energy Information Administration website, http://www.eia.doe.gov/energyexplained/index.cfm?page=oil_home#tab2).
There is clearly a great deal of potential and over the past few years a large number of academics and commercial groups have started to work in the field, virtually creating a new biotechnological sector. However, if algal biofuels are to compete against traditional fossil fuels significant challenges must be overcome. This paper outlines some of the key biological constraints to be addressed (Table 1) and suggests possible routes forward, particularly with respect to strain selection.
Section snippets
Addressing physiological constraints on productivity
This paper focuses on photoautotrophic growth for biofuel production, where the key to production is photosynthetic fixation of atmospheric CO2 for generation of a renewable fuel. However, algal cultivation could in addition be directly coupled with CO2 elimination/sequestration from industrial flue gases derived from fossil fuels.
Molecular constraints and solutions
Future algal biotechnological exploitation will be facilitated by genomics and metabolomics, accelerated in turn by advances in enabling technologies such as high-throughput sequencing. The latter procedure is generating an expanding list of fully sequenced genomes (Table 3) and can also provide valid transcriptomic comparisons of gene expression in micro-algae (Miller et al., 2011).
Some of the strains listed in Table 3 have become genetic models that can be readily transformed such as
Interaction with other organisms
Although a range of algae are grown commercially many, for example those used in aquaculture, are produced in relatively small quantities for niche applications. However, over the past 30 years a considerable amount of developmental work has been focused on the mass culture of two algae: Arthrospira/Spirulina produced for a range of products and Dunaliella, which is produced for its carotenoids. These organisms have been grown in large (1– >200 Ha), open culture systems in uni-algal cultures.
Conclusion
As outlined above there are a number of biological “pinch points” to any algal biofuel process. Having the “optimal” production strain and the underpinning scientific understanding to optimise productivity are the starting points of future large-scale algal biofuel production. Selection of appropriate production strain(s) will be the key to the future success of algal biofuels and “Intelligent screening” will use key criteria such as: biomass productivity, oil yield/composition, disease/grazer
Acknowledgements
The authors acknowledge funding for the BioMara project (www.biomara.ac.uk). The Biomara project is supported by the European Regional Development Fund through the INTERREG IVA Programme, Highlands and Islands Enterprise, Crown Estate, Northern Ireland Executive, Scottish Government and Irish Government.
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