Occasionally, a scientific discovery can have such an effect that its immediate impact appears to be not all that worthwhile. But those with the ability to see what it might lead to can observe just what massive changes will result from it These kinds of findings end up rewriting fundamental sections of school textbooks and one such thing may have just been uncovered.
Let’s Talk Photosynthesis
To begin, we should discuss the practice of photosynthesis. In direct and simple terms, this chemical process involves the transformation of light energy into physical chemical energy for use by living organisms. The mechanisms to do so can become complicated due to the structures involved, but let’s first understand the currently known type of photosynthesis. Its systems are differentiated entirely by the wavelengths of light that they are able to process and are named photosystem I and II based on the order in which each was found.
Photosystem I is capable of absorbing wavelengths to a max around 700 nanometers (nm) and photosystem II works at 680 nm, with both starting from around 430 nm.These photosystems are physically a protein complex that does the photochemistry themselves and are primarily found in the chloroplasts in plants, though cyanobacteria lack these larger structures and simply keep their photosystems within their cytoplasmic membrane around the cell to better absorb lightwaves.
To reiterate and expand, the photosystems rely on chlorophyll-A molecule P700 (and P680). Funnily enough, it is photosystem II that begins work first, taking the energy from light photons and from pigment light absorbers, using it to oxidize water molecules, and create electrons in the process. When these electrons are handed over to photosystem I, it uses them and further gathered light energy to then create ATP and send everything along to the Calvin cycle that eventually results in making sugars as an energy storage system.
The important thing to take away from all of this is that, according to conventional knowledge, photosynthetic organisms could only absorb light energy within the visible light spectrum and they are capped at around 700 nm in the low-powered red end of the spectrum. That end 700 region has been called the “red limit” and there’s only been one exception ever discovered that is able to surpass it and expand the available energy intake.
That second method of photosynthesis would belong to the cyanobacteria Acaryochloris, a single species that lives in the ocean underneath an invertebrate known as the green sea-squirt. By existing underneath this semi-translucent organism, the cyanobacteria only have the near-infrared light spectrum reaching them, the rest being absorbed or reflected by the tissues of the sea-squirt. This cyanobacteria uses the molecule chlorophyll-D in order to properly consume and synthesize energy from this kind of light. However, since this was a specialized situation involving symbiosis with another organism, it wasn’t considered all that revelatory by the scientific community.
Over the past few years, yet another molecule has been found to exist in the photosystems, chlorophyll F. While it was found to potentially be able to absorb more into the heat-making parts of the near-infrared beyond 700 nm, it was such a small proportion of overall pigment production at less than 10% that its role was considered minimal. It was thought to just be a light harvester that required heat in the surrounding environment to allow it to send that energy to chlorophyll A for processing, so not really affecting the red energy limit.
A New Paradigm
Now, in a collaboration between research centers in the UK, Italy, France, and Australia, a new aptitude has been found that could revolutionize the extent of photosynthesis and what this biological mechanism means for life as a whole both here and in the broader universe. While working with Chroococcidiopsis thermalis and growing the extremophile cyanobacteria under 750 nm far red light (FRL), they found that the wavelengths absorbed by the organism began to shift to higher spectra.
The peak of the bell curve moved all the way up to 709 nm on average for the three types of chlorophyll (A, D, and F), with each of their endpoints going as far as 740 nm, 753 nm, and 820 nm, respectively. This suggested that both of the photosystems had fundamentally changed in how they functioned and took up light energy. Fluorescence tests showed that these FRL consumers were present in both of the protein complexes making up the photosystems.
Together, this showcases a third type of photosynthesis, completely distinct and broader than the second system found only in the one species. This third type appears to be a successful process in all water-based cyanobacteria. Perhaps even more beyond that. Interestingly, rather than replacing all of the chlorophyll-A with the new version, like what is done by Acaryochloris with chlorophyll-D, the rest of the cyanobacteria appear to keep their 90% makeup of A and only a small component of chlorophyll-F. But don’t be confused, it is the latter that does all the heavy lifting under conditions where only FRL is available.
The chlorophyll-F acts as the primary electron donor to both photosystem I and II in such instances and helps link together the different antenna systems for wavelength gathering. In many ways though, this mechanism works the same as with Acaryochloris, so the scientists suggest that, together, these may make up a new “second red limit”, but one quite a bit beyond the original.
Life Finds A Way
Of course, the situations where cyanobacteria would need to use this are rare, largely isolated environments where deep shade only allows far red light to leak through. Even so, that this is biological possible and so widespread opens up many options for greater energy capacity. If plants could be given these additional chlorophyll systems, they could more easily grow even in shade. At the same time, this can also expand what kind of bacterial life might be found on other worlds and locations with little light in the visible spectrum may no longer be an obstacle to life surviving and thriving.
Either way, this discovery will likely mean a re-writing or a major addition to the textbooks on the chapter about photosynthesis, changing every photo that says photosynthesis can only be done in the visible range. What greater impacts this knowledge will provide in the future will have to be determined when we get there.
Photo CCs: Photosynthesis (9423341059) from Wikimedia Commons