According to a story on the Nature magazine news site scientists have finally figured out the structure of the last of three hydrogenase enzymes.
Why is this important? Because hydrogenase enzymes are the key to splitting hydrogen from water to create carbohydrates in the natural world. Green plants and some microbes and algae use this process. It has long been assumed that, if we understand the structure and operation of these enzymes, we will be able to produce artificial photosynthesis and through that process and an endless supply of fuel in the form of carbon/hydrogen compounds.
The process of photosynthesis is illustrated by the graphic here. Plants take in water through their roots and carbon dioxide through their leaves. They use the energy provided by sunlight to process water and carbon dioxide onto sugar (carbohydrates) which the plants use as food to grow. The byproduct is oxygen which the plants release into the atmosphere. The hydrogenase enzymes in the leaves of the plant serve as the catalyst to make all this possible.
We can turn the sugars made by plants into fuel such as alcohol or biodiesel or even, through further refining, into synthetic gasoline, diesel, or natural gas. The reason that we haven't been doing this to date is that we haven't understood the structure of hydrogenase enzymes completely enough to replicate them and their function. Now however, with this better understanding of the structure of these enzymes, we are one huge step closer to being able to produce artificial photosynthesis.
In an idealized setup, we could pull carbon dioxide out of the air and use solar energy to do the same thing that plants do, combine this CO2 with water thereby creating sugars that we could turn into fuel. The only by-product of this process would be oxygen which we would release back into the atmosphere. When we burn the fuels that we produced in this way we would release carbon dioxide and water as exhaust (just as we do now).
These two components would then become the raw materials for our artificial photosynthesis plants and so would be removed from the environment turned back into fuel again. It is a replication of the natural cycle on the Earth as it is and is therefore the ultimate "green" fuel source, and one which will allow us to go on burning hydrocarbon fuels and maintain our storage and distribution infrastructure "as-is". The faster that we can develop this process, the faster our CO2 pollution and fuel supply woes will be over, forever.