Have you ever started peeling an orange and instantaneously been overtaken by the beautiful smell of citrus? The cyclic monoterpene limonene can be thanked for that. The versatility of limonene is impressive and will only grow as more of it becomes available and more people find more uses for it. Limonene is currently found in beauty products, common household cleaners, and food. It can alleviate petrol dependence, as it shows promise towards use as a platform for plastics or as a biofuel.
Limonene is naturally synthesized in small amounts by several plants. We typically extract it from oranges via steam distillation. This method is time consuming and inefficient, hence the large price tag on a kilogram of limonene. Additionally, the current methods rely almost exclusively on production of oranges, a business that is filled with price fluctuations due to disease, pests, early freezes and natural disasters. This makes limonene production fickle and certainly not up to the task of providing the world with renewable fuel or plastics.
The current methods for limonene acquisition are primitive and dull in comparison to what we have in mind. We seek to alter the metabolic processes of the archaea Thermococcus kodakarensis in such a way that it produces limonene. While the idea of using microbes as cellular factories, or even using microbes specifically to produce limonene, is not novel, though the usage of an archaeal organism to do so is. Attempts with organisms such as Escherichia coli have proven inadequate as the acidic environment that limonene creates is toxic to the producing organisms. With yields often falling short of 1%, this method is hardly better than extraction from orange peels. T. kodakarensis offers a readily available production vessel that can survive in up to 20% limonene concentrations. Additionally, its metabolic pathways require only the addition of a single protein for synthesis of limonene.
Low production rates have kept limonene from being viewed as a viable source of energy and to that we say "when life gives you oranges, make a little bit of limonene, but when life gives you archaea, make massive amounts of limonene."