Biofuels’ 10 scariest challenges: Part 2 of 2

1. Photosynthetic limits

The ultimate challenge? Photosynthesis itself: the appallingly low rate at which plants convert sunlight to energy.

For example, corn checks in with a 1-2 percent efficiency rate. Raise photosynthetic efficiencies to 10-12 percent — and cost parity, food vs fuel, the cost of transporting biomass – issues begin to melt away.

Which brings us to the problem child, Rubisco, or by its full name ribulose-1,5-bisphosphate carboxylase oxygenase. Though obscure to the average citizen, it is not at all uncommon; in fact, it is the most abundant protein on earth.

It’s role: it is the enzyme that catalyzes the first step in the fixation of atmospheric carbon (for most plants, and also for cyanobacteria) – in short, it is a gateway to plant growth and carbon sequestration.

Though abundant, it is a slow, dim-witted enzyme if ever there was one. So slow that it fixes just three carbon molecules per second, and so dim-witted that it has trouble distinguishing between oxygen and CO2. Under many conditions, it will fix oxygen instead of CO2, in a process called plant respiration which causes carbon loss and robs the plant of growth opportunity.

There are two basic paths to meet the challenge. One, addressing photosynthetic efficiency through genetic engineering.

One problem? Hot and dry conditions. Turns out that more than 99% of plants, in sufficient conditions of heat and aridity, close down their little stomata pores — the portal for carbon to enter — to limit water loss. And, they stop fixing atmospheric carbon dioxide altogether and start fixing oxygen. The little devils.

It’s a problem ZeaKal has been addressing with its HME technology — they have found a means to increase the plant’s ability to store oil — and, ultimately, carbon — during normal production cycles — in a way that becomes accessible to the plant in the form of extra CO2.

Another route? Bypass photosynthesis altogether. Solazyme and others are going that route with heterotrophic microorganisms that derive their energy from consuming sugar — though ultimately, the sugar is photosynthesis dependent.

One alternative? Electrofuels. It’s an end-goal of ARPA-E’s Electrofuels project, which is scheduled to complete its first phase by years end. As the Electrofuels manifesto states:

“Most biofuels are produced from plant material that is created through photosynthesis, a process that converts solar energy into stored chemical energy in plants. However, photosynthesis is an inefficient process, and the energy stored in plant material requires significant processing to produce biofuels. Current biofuel production methods are also intensive and require additional resources, such as water, fertilizer, and large areas of land to grow crops. Electrofuels bypass photosynthesis altogether by utilizing microorganisms that are self-reliant and don’t need solar energy to grow or produce biofuels. These microorganisms can directly use energy from electricity and chemical compounds like hydrogen to produce liquid fuels from carbon dioxide (CO2).” In all, 14 Electrofuel projects are expected to complete this year under their ARPA-E grants.

And, last week we profiled the promise of quantum dot technology. With this one, it’s a technology that utilizes sunlight, CO2 and water. More importantly, when a photon, arriving on planet Earth after an eight minute journey from the Sun, happens to strike a quantum dot (instead of say, a plant’s light harvesting mechanism) — it produces one excited electron for every photon. There’s no limitation imposed by photovoltaic or photosynthetic efficiency.