ExxonMobil invests in algae for biofuel

The current interests and investments for production of biofuels using various species of algae are based on promises to achieve an economically sound process. The pros of the biology of algae are enthusing with CO2 as themain substrateand sunlight as energy source. However, there are intrinsic limitations, which most reports disregard in their descriptions of the potential of algae as a source for biofuels. Two important features (and there might be more) are the need for growth limiting conditions to enhance lipid accumulation and the very low cell concentration obtained i.e. very dilute cell suspensions (and the handling of a lot of water). Here are some results from the literature to illustrate these features. The home take message is to look at the whole picture and not just the exciting advantages. Algae accumulate large quantities of lipid as storage materials, but they do this when under stress and growing slowly (Rittman 2008). A key factor for oil accumulation is high light intensity and nitrogen deficiency (Solovchenko et al. 2008). In the comprehensive report of Sheehan et al 1998 it was stated “The common thread among the studies showing increased oil production under stress seems to be the observed cessation of cell division. While the rate of production of all cell components is lower under nutrient starvation, oil production seems to remain higher, leading to an accumulation of oil in the cells. The increased oil content of the algae does not to lead to increased overall productivity of oil. In fact, overall rates of oil production are lower during periods of nutrient deficiency. Higher levels of oil in the cells are more than offset by lower rates of cell growth. (Sheehan J, Dunahay T, Benemann J & Roessler P (1998). A Look Back at the U.S. Department of Energy’s Aquatic Species Program: Biodiesel from Algae NREL/TP-580-24190). They also state that “The factors that most influence cost are biological …� Independent of which mass growth mode is chosen, raceway ponds or tubular photobioreactors, the final cell concentration is very low. A cell concentration of 4 gram dry weight per liter was reported for autotrophic growth of microalgae in a photobioreactor facility and 0.14 gram dry weight per liter in raceway ponds (Chisti 2007). The cell concentration of oleaginous yeast, grown heterotrophically was reported to be 106.6 gram dry weight per liter in a 15 l stirred tank fermentor (Li et al. 2007). Due to these biological limitiations the production of a low value/high volume product using microalgae is still far from being economically viable. Can research change these biological limitations? Chisti Y (2007). Biodiesel from algae. Biotechnol. Adv. 25: 294-306. Li YH, Zhao ZB & Bai FW (2007). High density cultivation of oleaginous yeast Rhodosporidium toruloides Y4 in fed batch culture. Enzyme Microb.Biotechnol. 41:312-317. Rittman BE (2008). Opportunities for renewable bioenergy using microorganisms. Biotechnol. Bioengng. 100: 203-212. Slovchenko AE, Khozin-Goldberg I, Didi-Cohen S, Cohen Z & Merzylak MN (2008). Effects of light intensity and nitrogen starvationon growth, total fatty acids and arachidonic acid in the green microalga Parietochloris incise. J. Appl. Phycol. 20:245-251 Stefan Rokem Ph.D. Dept. Microbiology & Molecular Genetics, IMRIC, The Hebrew University of Jerusalem Israel