Better Red Than Dead

Some cyanobacteria strain for photosynthetic efficiency at the red end of the light spectrum.

The plant world is green around us- why green, and not some other color, like, say, black? That plants are green means that they are letting green light through (or out by reflection), giving up some energy. Chlorophyll absorbs both red light and blue light, but not green, though all are near the peak of solar output. Some accessory pigments within the light-gathering antenna complexes can extend the range of wavelenghts absorbed, but clearly a fair amount of green light gets through. A recent theory suggests that this use of two separated bands of light is an optimal solution to stabilize power output. At any rate, it is not just the green light- the extra energy of the blue light is also thrown away as heat- its excitation is allowed to decay to the red level of excitation, within the antenna complex of chlorophyll molecules, since the only excited state used in photosynthesis is that at ~690 nm. This forms a uniform common denominator for all incoming light energy that then induces charge separation at the oxygen reaction center, (stripping water of electrons and protons), and sends newly energized electrons out to quinone molecules and on into the biosynthetic apparatus.

The solar output, which plants have to work with.

Fine. But what if you live deeper in the water, or in the veins of a rock, or in a mossy, shady nook? What if all you have access to is deeper red light, like at 720 nm, with lower energy than the standard input? In that case, you might want to re-engineer your version of photosynthesis to get by with slightly lower-energy light, while getting the same end results of oxygen splitting and carbon fixation. A few cyanobacteria (the same bacterial lineage that pioneered chlorophyll and the standard photosynthesis we know so well) have done just that, and a recent paper discusses the tradeoffs involved, which are of two different types.

The chlorophylls with respective absorption spectra and partial structures. Redder light is toward the right. Chlorophyll a is one used most widely in plants and cyanobacteria. Chlorophyll b is also widely used in these organisms as an additional antenna pigment that extends the range of absorbed light. Chlorophylls d and f are red-shifted and used in specialized species discussed here. 

One of the species, Chroococcidiopsis thermalis, is able to switch states, from bright/white light absorbtion with normal array of pigments, to a second state where it expresses chlorophylls d and f, which absorb light at the lower energy 720 nm, in the far red. This "facultative" ability means that it can optimize the low-light state without much regard to efficiency or photo-damage protection, which it can address by switching back to the high energy wavelength pigment system. The other species is Acaryochloris marina, which has no bright light system, but only chlorophyll d. This bacterium lives inside the cells of bigger red algae, so has a relatively stable, if shaded, environment to deal with.

What these and prior researchers found was that the ultimate quantum energy used to split water to O2, and to send energized electrons off the photosystem I and carbon compound synthesis, is the same as in any other chlorophyll a-using system. The energetics of those parts of the system apparently can not be changed. The shortfall needs to be made up in the front end, where there is a sharp drop in energy from that absorbed- 1.82 electron volts (eV) from photons at 680 nm (but only 1.72 eV from far-red photons)- and that needed at the next points in the electron transport chains (about 1.0 eV). This difference plays a large role in directing those electrons to where the plant wants them to go- down the gradient to the oxygen-evolving center, and to the quinones that ferry energized electrons to other synthetic centers. While it seems like more waste, a smaller difference allows the energized electrons to go astray, forming chemical radicals and other products dangerous to the cell. 

Summary diagram, described in text. Energy levels are described for photon excitation of chlorophyll (Chl, left axis, and energy transitions through the reaction center (Phe- pheophytin), and quinones (Q) that conduct energized electrons out to the other photosynthetic center and biosynthesis. On top are shown the respective system types- normal chlorophyll a from white-light adapted C. thermalis, chlorophyll d in A. marina, and chlorophyll f in red-adapted C. thermalis. 

What these researchers summarize in the end is that both of the red light-using cyanobacteria squeeze this middle zone of the power gradient in different ways. There is an intermediate event in the trail from photon-induced electron excitation to the outgoing quinone (+ electron) and O2 that is the target of all the antenna chlorophylls- the photosynthetic reaction center. This typically has chlorophyll a (called P680) and pheophytin, a chlorophyll-like molecule. It is at this chlorophyll a molecule that the key step takes place- the excitation energy (an electron bumped to a higher energy level) conducted in from the antenna of ~30 other chlorophylls pops out its excited electron, which flits over to the pheophytin, then thence to the carrier quinone molecules and photosystem I. Simultaneously, an electron comes in to replace it from the oxygen-evolving center, which receives alternate units of photon energy, also from the chlorophyll/pheophytin reaction center. The figure above describes these steps in energetic terms, from the original excited state, to the pheophytin (Phe-, loss of 0.16 eV) to the exiting quinone state (Qa-, loss of 0.385 eV). In the organisms discussed here, chlorophyll d replaces a at this center, and since its structure is different and absorbance is different, its energized electron is about 0.1 eV less energetic. 

In A. marina, (center in the diagram above), the energy gap between the pheophytin and the quinone is squeezed, losing about 0.06 eV. This has the effect of losing some of the downward "slope" on the energy landscape that prevents side reactions. Since A. marina has no choice but to use this lower energy system, it needs all the efficiency it can get, in terms of the transfer from chlorophyll to pheopytin. But it then sacrifices some driving force from the next step to the quinone. This has the ultimate effect of raising damage levels and side reactions when faced with more intense light. However, given its typically stable and symbiotic life style, that is a reasonable tradeoff.

On the other hand, C. thermalis (right-most in the diagram above) uses its chlorophyll d/f system on an optional basis when the light is bad. So it can give up some efficiency (in driving pheophytin electron acceptance) for better damage control. It has dramatically squeezed the gap between chlorophyll and pheophytin, from 0.16 eV to 0.08 eV, while keeping the main pheophytin-to-quinone gap unchanged. This has the effect of keeping the pumping of electrons out to the quinones in good condition, with low side-effect damage, but restricts overall efficiency, slowing the rate of excitation transfer to pheophytin, which affects not only the quinone-mediated path of energy to photosystem I, but also the path to the oxygen evolving center. The authors mention that this cyanobacterium recovers some efficiency by making extra light-harvesting pigments that provide more inputs, under these low / far-red light conditions.

The methods used to study all this were mostly based on fluorescence, which emerges from the photosynthetic system when electrons fall back from their excited states. A variety of inhibitors have been developed to prevent electron transfer, such as to the quinones, which bottles up the system and causes increased fluorescence and thermoluminescence, whose wavelengths reveal the energy gaps causing them. Thus it is natural, though also impressive, that light provides such an incisive and precise tool to study this light-driven system. There has been much talk that these far red-adapted photosynthetic organisms validate the possibility of life around dim stars, including red dwarves. But obviously these particular systems developed evolutionarily out of the dominant chlorophyll a-based system, so wouldn't provide a direct path. There are other chlorophyll systems in bacteria, however, and systems that predate the use of oxygen as the electron source, so there are doubtless many ways to skin this cat.

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