For many fuel purposes, alkanes are more desirable than the other biofuels already discussed. For example, jet fuel standards (Jet-A or JP-8) demand a fuel with high energy density, low viscosity, low freezing point and good physical-chemical compatibility. These criteria cannot be met with fuels such as ethanol or fatty acid methyl esters, biodiesel. Being able to directly make al­kanes would have a great payoff as these biofuels are "drop-in" fuels, able to directly substitute for presently used petroleum-based fuels as they could be used with existing infrastructure and would require no engine modification, etc.

Cyanobacteria, like some other bacteria, have long been recognized as being able to synthesize at least very small quantities of alkanes, which in fact can serve as a biogeochemical marker for their presence in the past (Han et al., 1968; Winters et al., 1969). This was taken advantage of in a recent demonstration of the heterotro­phic production of alkanes using a modified E. coli that expressed the alkane biosynthetic pathway from a cyanobacterium, consisting of an acyl-carrier protein reductase, which produces a fatty aldehyde, and an aldehyde decarbonylase (Schirmer et al., 2010). This allowed the production and secretion of a variety of C13—C17 alkanes and alkenes. Of course it would be desirable to actually do this in a cyanobacterium, and one study examined this through the heterologous expression of fatty acyl-CoA reductase in Synechocystis (Tan et al., 2011), which allowed the production of small quantities of aliphatic alcohols. The acc genes, encoding acetyl-CoA carboxylase (ACCase), which catalyses what is believed to be the rate-limiting step of fatty acid biosynthesis, were introduced into the genome in hopes of boosting alkane production, but only insignificant quantities were made. Further work is required to demonstrate significant alkane synthesis by a cyanobac­terium. However, it may prove difficult to greatly boost alkane synthesis in this oxygen-evolving organism as the critical enzyme, aldehyde decarbonylase, has recently been shown to be a di-iron enzyme with an unusual mechanism that requires anaerobic conditions for full activity (Das et al., 2011).

CONCLUSION AND OUTLOOK

As discussed in this chapter, recent studies have shown the great promise for biofuels production by cyanobacteria. Unique among possible biofuel producers, cyanobacteria combine the attributes of being able to carry out photosynthesis-driven carbon dioxide fixation and to be easily manipulated geneti­cally. The next few years should see advances in increasing the production rates and titers of the different demonstrated biofuels as well as perhaps the widening of the spectrum of possible biofuels. Nevertheless, for cyanobacterial systems to live up to their potential, a number of serious hurdles must be overcome. These include the development of reliable methods of stable cyanobacterial mass culture at high levels of productivity and the demonstration of cost — effective harvesting strategies. Harvesting presents a real dilemma no matter what the biofuel. If the biofuel is contained within the cell, then the biomass has to be removed from the culture medium, of which it is less than 1% by weight. If the biofuel is an excreted liquid, then this will necessarily be quite dilute and require substantial concentration. If the biofuel is a gaseous product, the culture will have to be enclosed in air­tight transparent material at a substantial cost given the large surface areas that would be required. Of course, the payoff to solving these problems would be enormous and this is likely to inspire future research and development in this area.