Lipid productivity in a fast-growing green alga and its use as a 'whole cell' fuel additive

Sofie Secher1, Joanna Szaub1, Leslie May2 and Saul Purton1

1) Department of Structural & Molecular Biology, University College London, UK 2) Syngenta Ltd., Jealott's Hill Research Station, Bracknell, UK

The exploitation of microalgal lipids for biodiesel production will require strains that meet a number of basic criteria including high biomass productivity, high lipid content, and tolerance to various biotic and abiotic stresses. Our analysis has identified the freshwater alga, Chlorella sorokiniana as a promising candidate. Unlike most green algae, C. sorokiniana is capable of active growth at temperatures as high as 40 °C and with doubling times of less than three hours. Although the cell volume (and hence cell biomass) is significantly less than that of Chlamydomonas, a comparison of biomass productivity per day reveals a 5-fold higher productivity with the former after 2 days under optimum conditions. The axenic cultivation of C. sorokiniana does not require vitamins or other co-factors and can be achieved in a simple minimal medium. Furthermore, growth is unaffected by the addition of up to 0.3 M NaCl indicating that brackish water could be used in large-scale cultivation. However, growth is influenced by the choice of nitrogen source in the medium, with ammonium resulting in a superior growth rate as compared to nitrate; and nitrite unable to support growth. As in the case of other algal species, a reduction of nitrogen in the medium results in the accumulation of triacylglycerides (TAGs), with the amount of TAGs obtained inversely proportional to the N concentration. Recently, we have sought to increase TAG yields further through mutation of the starch biosynthesis pathway, and are currently characterizing a collection of starch-deficient mutants. Finally, since cell harvesting, dewatering, lipid extraction and trans-esterification are all major challenges in the exploitation of algal lipids as biodiesel, we are collaborating with fuel specialists at UCL to determine whether whole cells of TAG-enriched C. sorokiniana can be blended directly with conventional diesel, thereby replacing a proportion of the fuel with a bioenergy component.

e-mail address of presenting author: sofie.secher.09@ucl.ac.uk