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 first generation biofuel systems which had the downside of resulting in food vs. fuel concerns, the associated hype has often exceeded the boundaries of reality. The introduction (Chapter 1) provides a broad overview of microalgal biofuel systems. Chapters 2 (published in Nature Biotechnology) and 3 (prepared for publication) present economic feasibility studies of integrated microalgal production systems for the production of oil or hydrogen (predominantly) and other products. They identify for each set of business models the primary technical and economic obstacles to the ongoing development and commercialisation of these technologies. In both cases economic feasibility was determined to be most sensitive to the capital costs, and to the productivity and price of the primary product/s. In the case of microalgal H2 production systems, the management of O2 levels throughout cultivation and the management of H2 post-production, were also critical aspects of economic feasibility. The models analyse the relative importance of a wide range of factors on economic viability by conducting sensitivity analysis. Factors analysed included the reduction of capital construction costs for microalgal oil production systems from US$143,500 ha-1 to US$100,000 ha-1, or the sale of microalgal biomass for US$1,000 T-1 rather than US$400 T-1) and how these can potentially increase economic feasibility, or relax the productivity/price targets that are required to achieve profitable systems.
While economic feasibility is clearly of critical importance for commercial development of integrated microalgal production systems, sustainability issues such as resource availability and social acceptance constitute equally important aspects for commercialisation. In Australia and other drought affected regions (e.g. south western U.S.A.) the availability of freshwater resources can be limiting in both agricultural and urban settings. While many marine and salt tolerant strains of microalgae exist that could potentially ameliorate the impact of water resource considerations, there are microalgae that produce specific products (e.g. Chlamydomonas reinhardtii for biohydrogen, or Haematococcus pluvialis for astaxanthin) or specific processes (e.g. utilisation of municipal wastewater for microalgal production) which are currently limited to freshwater applications. Consequently the development of a molecular toolkit which can facilitate the development of strains to tolerate salt and other stresses is of obvious benefit, and such research in higher plants is well established.
In this work, the capacity to engineer halotolerance in freshwater Chlamydomonas reinhardtii cc400 was examined through incorporating an over expression construct for ribosomal protein L13 from the marine variety of Chlamydomonas: Chlamydomonas sp. W80. A prominent halotolerant phenotype was evident in many of the tested transformants. Although this did facilitate nominal growth at moderate salt levels (+260mM NaCl), no detectable difference in photosynthetic efficiency was discernable between mutants and the parental strain. This result is consistent with previous claims that L13 conveys protection against dehydration. In addition, some clones were able to tolerate salt levels equivalent to sea water (600mM), but were not able to actively grow under these conditions. Though promising, these results are some way from a commercial application, and the remaining interest concerns the mechanism of action. Both ribosomal and extra-ribosomal functions are possible candidates for further analysis, while it should also be determined if simple over-expression of endogenous C.reinhardtii rpl13 can also induce this phenotype, or if the effect is specific to the sequence differences in C.W80 rpl13.