Algal enzyme synthesizes gasoline building blocks

Deciphering the mechanism of the photoenzyme Fatty acid photodecarboxylase (FAP), a research team co-headed by Frederic Beisson from University Aix-Marseille claims to have made a big step toward industrial-scale production of biofuels from vegetable oils. Using medium-chain fatty acids as a starting point, FAP from the microalgae Chlorella variabilis exhibited an ultrafast autocatalytic reaction that created C5–C11 hydrocarbons that can be used as biofuels. 

The natural photoenzyme, which was discovered only six years ago, harnesses light energy to produce hydrocarbons for a variety of potential uses. Recent studies suggested that wild-type FAP has primarily adapted to form long-chain hydrocarbons (C16 to C18), the researchers succeeded in creating medium chain hydrocarbons, when they used C8-fatty acids as a substrate. They observed that these were  produced up to ten times faster than long-chain hydrocarbons in cultures of C. variabilis as well as genetically engineered E. coli bacteria.

Using time-resolved spectroscopy, which involved illuminating and monitoring fluorescence changes over time, the researchers found that when they started synthesis with medium-chain fatty acids, FAP used n-alkane products formed earlier in the process to initiate an autocatalytic effect. The existing n-alkanes mimicked the missing section of the long chain, thereby enhancing FAP’s production of medium-chain hydrocarbons. They noted that catalytic activity was particularly high when converting C8 fatty acids, a size range that had been previously overlooked as a substrate for the reaction.

However, if we look at the CO₂ balance of the process, a total of 9 moles of CO2 per mole of C8 fatty acid are released during the production and subsequent combustion of the hydrocarbons - one mole of CO₂ during the decarboxylation of the fatty acid and 8 moles during the combustion of the octane. Although the process is bio-based, it is anything but CO₂-neutral.

A theoretically more promising way to produce CO₂-neutral or negative biofuels is being pursued by the US start-up Phytonix, which turns one tonne of CO2 into about 550 litres of n-butanol, 16% of which can be added to fuels in the USA.

Since the EU Commission is about to decide to ban the registration of new vehicles with combustion engines in favour of e-cars, for which there is currently too little regeneratively generated electricity available, developers see the development of biofuels for aviation and heavy goods traffic as more promising.