Everyone knows that we as humans literally owe the air we breathe to the greenery around us. As school children we learned that plants (as well as algae and cyanobacteria) perform the all-important biological "magic trick" known as photosynthesis, which helps generate the atmospheric oxygen we take in with every breath.
Plants, algae and cyanobacteria alter our planet in a way that only life can: they use photosynthesis to completely change the composition of the Earth's atmosphere. Since the days when dust devils on Mars were suspected to be the seasonal variation of vegetation, photosynthesis has been considered a key to identifying the presence of life on other planets.
Both atmospheric oxygen (in the presence of liquid water) and the reflectance spectrum of plant leaves produce signs of life—dubbed "biosignatures"—that can be seen from space. Therefore, photosynthetic biosignatures are a priority in the search for life on planets in distant solar systems. The big question is, will extrasolar photosynthesis use the same pigment as on Earth?
A broader range of light wavelengths
The process of photosynthesis is obviously more than simple magic. In basic terms, photosynthetic organisms take in CO2, water (H2O) and light energy to produce sugars (i.e. the food that makes plants a staple of our diet). During this process, photosynthetic organisms use a photopigment called chlorophyll a (Chl a) to split water molecules and produce oxygen. [Top 10 Poisonous Plants]
Until recently, scientists thought Chl a was the only photopigment used in oxygenic photosynthesis. Chl a uses photons in visible light at wavelengths of 400-700 nm.
According to NASA postdoc Steve Mielke, lead author of a new study, "It was assumed that, due to the stringent energy requirements for splitting water molecules, longer wavelengths of light (which have lower energy) could not be used for oxygenic photosynthesis."
That assumption changed in 1996 when Hideaki Miyashita and colleagues discovered a cyanobacterium named Acaryochloris marina that uses chlorophyll d (Chl d) instead of Chl a to perform oxygenic photosynthesis with photons from visible light through to wavelengths up to 740 nm in the near-infrared (NIR).
This discovery raised many questions about the wavelengths of light required for photosynthesis. Scientists wondered how difficult it was for A. marina to power biochemical reactions with low energy photons. It survives in environments where there is little visible light, because it gets the photons left over by Chl a organisms.
However, could A. marinabe regularly unsuccessful in using the longer-wavelength photons, and could its ability to use NIR be inefficient, at the edge of what the molecular mechanisms of oxygenic photosynthesis are able to handle? Or could these unique organisms actually thrive on low-energy photons?
New research has shown that A. marina doesn't struggle at all when living on low-energy photons. In fact, the cyanobacterium is just as efficient or more so in storing energy as organisms that rely on Chl a for photosynthesis.
Mielke and collaborators used a technique called pulsed time-resolved photoacoustics (PTRPA) to compare the photosynthetic abilities of A. marina to a Chl a cyanobacterium named Synechococcus leopoliensis. PTRPA involves laser pulses at controlled wavelengths and allowed the team to measure the efficiency of photon energy storage (energy stored vs. energy input) of cyanobacterial cells.