Long before the first plants, bacteria were using sunlight to form energy-storing molecules. Now, the machinary that some bacteria use to make this happen has been mapped in unprecedented detail, possibly teaching us some tricks on how to capture sunlight ourselves.
For all our advances in solar cell technology, there are still ways in which nature does it better. A blending of biologically inspired photosynthesis and technologies of our own design could be humanity’s ultimate power source. However, plant photosynthesis has proven hard to untangle, so Dr Raimund Fromme of Arizona State University turned to something simpler, the photosynthetic bacterium Heliobacterium modesticaldum. (Heliobacteria – from Helios, the Sun – should not be confused with stomach ulcer-inducing Helicobacteria).
"To truly and fully understand photosynthesis, one has to follow the process of converting light into chemical energy," Fromme said in a statement. "This is one of the fastest chemical reactions ever studied, which is part of what makes it so hard to study and understand." The duration of photosynthetic reactions are measured in the trillionths of seconds.
Heliobacteria thrive in muddy soils around hot springs, using hydrogen sulfide instead of water and releasing sulfurous gasses instead of oxygen. Plants collect sunlight at visible wavelengths, but heliobacteria use near-infrared light, allowing them to thrive in locations where higher energy light is scarce.
What makes heliobacteria easier candidates to study is that they have a single reaction center where photosynthesis occurs, while plants use two reaction centers to a cell. Easier does not mean easy, however, as Fromme’s team spent years purifying proteins from the reaction center of bacteria collected from Icelandic hot springs and growing crystals suitable for X-ray diffraction. Even once this had been done, it took two years to work out the three-dimensional structure from the diffraction patterns the team were getting.
The results, Fromme said, “proved everyone's initial prediction on the heliobacteria's RC was wrong.” Fromme reports in Science that heliobacteria reaction centers are almost perfectly symmetrical and lack a quinone compound vital to the function of one of the plant centers and some bacterial reaction centers, but have 60 chlorophylls – far more than expected.
The relative simplicity of heliobacteria reaction centers has led to the suspicion they resemble those of Earth's first photosynthesizing organisms. That’s hard to prove, but Fromme and his co-authors think it is likely to be true, with dual reaction centers with differing roles being a more recent development, something evolutionary biologists think occurred at least three times.
Artificial reaction centers are likely to be some way off, but if they can be produced, we could make solar cells that turn sunlight directly into biofuels that can be stored more easily than electricity. Fromme’s work is a vital step down that path.