The Blue Planet, the “Pale Blue Dot”, as Carl Sagan put it. That’s our planet, Earth. But not everything is blue on the surface, there is also green, tons of it, a living green. Life may seem complicated at a first glance, but its basic requisites are, in fact, only energy and matter. And organisms have evolved different ways to get both. Photosynthesis is one of them and, perhaps, the most influential evolutionary invention in the history of our planet. While Animals, Fungi and most Bacteria struggle in a dramatic survival battle for food, photosynthetic organisms live a more “comfortable” live, creating their own meals and obtaining energy through them.

Energy and matter

But how does this work? For both photosynthesis and chemosynthesis (what animals do), the goal is the same: to create useful structures made out of carbon, the so-called organic compounds. As an element, carbon is perfect to create complex and durable compounds that can be used to form structures or store energy. Animals or fungi normally obtain them from other organisms, and rearrange them to suit their purposes. Every time you eat, organic matter is disarranged and rearranged to allow your growth and function. The origin of the organic compounds, then, does not rely on us, but on photosynthesis. 

As a quick recap, what photosynthesis does is using energy to assemble carbon molecules into organic matter. For that, three ingredients are needed. One is carbon, which in most cases comes in the form of CO₂ a gas product of respiration, the universal process where organic matter is broken to obtain energy. With that, the bricks have been obtained.  The second is called the “electron donor”, a molecule which is sacrificed to obtain energy for the process. The most common of them is water (water your plant, please), but more exotic elements, like iron (in some Chlorobiacea, Bacteria), have also been used by different organisms. The third is light. Photosynthesis uses the energy of photons (the elementary particle responsible for light and related to the electromagnetic force) which our Sun is constantly emitting, as an energetic switch. With both light and the electron donor, energy has been obtained. 

With the help of light receptors (pigments, such as Chlorophyll), a photosynthetic organism captures light and induces the breaking of the “electron donor”. This creates three products: protons, electrons and oxygen. Protons accumulate in a structure of the plants cell, electrons “refill” the chlorophyll, which loses an electron in the process, and oxygen is the result of breaking the “electron donor”. The accumulation of protons results in a osmotic imbalance that finally concludes in the formation of ATP, the biological energy coin (see the image below).

Types of light, types of stars

Capturing protons via pigments is therefore indispensable, and is the whole reason behind why photosynthetic organisms have the colours they have. Green is the dominant colour in our planet, and to understand this fact we have to look to our Sun. It is clear that this is not an arbitrary colour, and that the light emitted by the Sun plays an important role. Our star is what is known as a G-type Star, a small sized yellow-like machine of atomic fusion. G corresponds to one of the types of stars in the Morgan-Keenan series, one of the most common ways to classify stars. In this system, stars are given the letters OBAFGKM according to their emission spectrum.

And now the question is. “what is an emission spectrum”?. Well, light is not homogeneous. The colours we perceive (red, blue), as well as ultraviolet, X-rays and others are indeed different types of lights, differentiated by their wavelength. Light is ubiquitous, and, in fact, most types of light are invisible to our eyes. Inside a star, atoms are continuously reacting, changing between energy states. When an atom or molecule goes from a high energy state, to a lower one, it releases energy in the shape of light. The type of light produced (its wavelength), depends on the material that has experienced this change and its own original energetic state. A star can have several types of atoms and molecules reacting, which creates different sets of lights. This set of lights emitted by a star is known as the emission spectrum. The Morgan-Keenan series just classifies all known stars into this cathegories, related to their emissium spectrum.

So, if the Sun is a G type star, which light does it produce? Let’s take a prism, for example, reminiscing Isaac Newton (or Dark Side of the Moon by Pink Floyd, as you prefer). With a prism you can easily break down the light of our star. If you try this at home, just a side note:  not all possible colours are represented there, many can’t be seen by the human eye, and other can’t even be said to be colours. In addition, not all colors are represented equally. If we take a more sofisticated method, we can see that the emission spectrum of the sun produces light from Ultraviolet (wavelength: 200nm) to Infrared (2500nm) and peaks specially around 500nm, which corresponds to visible light (around a blueish-greenish colour) (see image below). 

This means that the Sun sends us mainly light that organisms can see (not surprise, here, it would be strange for organisms to see mainly IR, where the emission is very low), in particular green light. A lot of green light. And our plants are green. Solved, we can go home.

A problem of green 

Nonono, our plants are green because they reflect green light. Every time light impacts an object, some wavelengths get absorbed, while others are reflected, and therefore can be seen (a black object, for example, normally captures most colour wavelengths).  This means that most photosynthetic organisms are not using this light, the most abundant light coming from our Sun (a not-so intelligent design, I must say).

But this is not completely true, either. Plants can, in fact absorb green light, but not with their most dominant pigment, the chlorophyll, but with complementary ones, like carotenoids. Carotenoids only reflect orange, and are responsible, for example, for the colourful leaves in spring/autumn in the southern/northern hemisphere.  

The reason why photosynthetic organisms use mainly chlorophyll, despite being suboptimal and having better options, is not completely understood, though. Two hypothesis for that have taken the lead: the purple planet and the light protection hypothesis.

The purple Planet is a hypothesis that states that before our Green planet, another type of photosynthesis dominated the planet: archean photosynthesis. You may have heard photosynthesis evolved just once. And that is true for oxygenic photosynthesis: the first organisms to perform that were cianobacteria, which were engulfed in a complicated evolutionary history of fagocitosis that led to the origin of the algae and plant cells. Archean photosynthesis, on the other hand, is not particularly spread. They normally dwell in salt mines, where they use rhodopsins to capture light. This pigment is, in fact, best at absorbing green light, and thus the best pigment to get light from the Sun. It is possible that the first cianobacteria could not compete with archea, and had to change to absorb of other types of light. (if you want to know more about ancient Earth and photosynthesis, check Seaweeds for sceptics (I)).

The second hypothesis is less complicated, and postulates that chlorophyll does not absorb green light in order to not become oversaturated with light (yes, too much light can be bad for a plant). By taking blue and red light mainly, it avoids collapsing the electron chain reaction due to a heavy luminic input.

Other worlds’ colours

In any case, theoretically, in any other planet, plants should be expected to have the colour that best matches the light from their star. There are millions of planets waiting to be discovered, just in our Galaxy, and many of these may be habitable. How would photosynthesis work in these planets? That’s what we want to answer in the next and last part of this article.

The last decade has brought us the discovery of hundreds of exoplanets thanks to telescopes such as the Kepler, and to the systems of transit and radial velocity. A transit happens when a planet crosses its star. Even if very little, the light of the star at that moment flickers, and this can be noticed by our telescopes. In a similar way, the gravitational pull of a planet can change the direction and speed of a stars movement, and that too can be used as evidence of an exoplanet (called radial velocity).

However, no more than 20 are considered to be into the ¨habitable zone¨, a region in a star system where water can remain in its liquid form. Stars offering this ¨habitable zone¨ are mainly of the types G (the Sun), K and M. These two represent the ¨coldest¨ class of stars, whose radiation is low enough to not automatically destroy organic matter, as it happens in the stars of type O to F. There is where scientists want to find photosynthesis, and to find live. 

To simplify things, first let’s imagine these organisms are similar to the ones on Earth, with a planet protected from high energy light (like UV) and that those organisms are adapted to get the best light from their star.

The three star system of Centauri

Let’s start with Proxima Centauri. Proxima Centauri is a star part of the tri-solar system known as Alpha Centauri, which also includes Rigil Kentaurus A, the most massive of all, and Rigil Kentaurus B. The two massive stars in this three system may not have an habitable zone, but Proxima Centauri, an M type, has.   

Discovered in 2016 through the radial velocity system, Proxima Centauri B is a small planet orbiting this faint reddish star, and is probably the closest potential habitable exoplanet, just around 4 years-light away.

Proxima Centaury only has 1% of the total luminosity the Sun can give, and most of this light is in the infrared zone. This faint light means that any plant or algae living in Proxima Centauri B would need any help available to absorb as much light as possible. The perfect colour for this job is then, black. Black absorbs most coloured light and even some infrared, and is common in the tree of life through several pigments, like melanin.

The plants in Proxima Centauri b, therefore, could be black. But would that be enough to capture sunlight? Probably not. These organisms would probably be in a struggling battle for survival, maximizing their surface as much as possible to obtain enough light. There are several ways to do so, and the simplest one is being small and spherical, the ideal optimum of surface/volume. Colonial or even multicellular organisms could also exist, as long as they would be able to capture enough sunlight to survive. As in Earth, we could expect organisms in Proxima Centauri b to have originated in water. Under water, light is limited to the upper layers, and at the same tame, refracted, which makes light come from many different directions. Millions of black centaurean “algae”, then could live on the upper layers of those seas, even creating gigantic dark blooms, trying to capture as much light as possible. 

But on land, things are different. On land, photosynthetic organisms cannot move by currents, and light is focused in certain directions at certain hours. This could lead to an extreme competition for light, as any shadow would reduce the available dim light from Proxima Centauri, to nothing, and lead to a certain dead. Driven by competition, and the fact that Proxima Centauri b is a smaller planet than our Earth, with a lower gravitational pull, plants there may be able grow to unprecedented heights. Many of them could develop strategies for maximum spread (for example floaty and easily dispersing seeds, able to remain in the atmosphere for long), or to avoid near competition (poisonous compounds, long lasting lives with spread canopies…) or complex canopies.  If light is such an scarce source, how can an organism maximize its light intake? Nature has shown us that curves and invaginations, specially those following a fractal structure, can deeply increase surface while remaining the same apparent length and thus maximizing light intake per volume. That’s how the plants of Proxima Centauri b could be, but this description is already speculative enough.

All of this, if life in such a planet is even possible. Scientists have pointed out that such a planet, due to its low mass and proximity to its star,  would be similar to Mercury, with one side always facing its star, while the other in perpetual darkness. These are not very good news for the habitability of such systems, but given that red stars are the most common in our Galaxy (although we cannot normally see them with naked eye), the probability that this problem could be solved by a mix of currents, atmospheric gases and other phenomena certainly exists.

So far, most discovered “habitable” exoplanets orbit around this type of stars. Other candidates with such companions, are GLIESE-667 at 23 light years, and Kapteyn-b, at around 13 light years.

Kepler, see your name in the skyes

And then, there is… Kepler-442b. Discovered in 2015 by the Kepler telescope, this planet orbits around a K type star, also known as a Goldilock type star, the best stars for the origin and evolution of life, even better than our Sun. K type stars emit less UV-light, which easily destroys organic matter and last for longer, thus allowing more time for life to appear. The light emitted by this type of star, then, would be similar to that of the Sun, but with less UV radiation and a peak in a more yellowish-orange colour.

If we ideally think on a system where photosynthetic organisms would try to capture this type of light, we can imagine them being of a blue-cyan colour, disregarding this type of light in favour of the abundant orange light. Interestingly, no pigment on Earth absorbs mainly orange light. The closest could be phycocianine, that absorbs mainly around yellow and orange light, but neglects mostly any other wavelength. In nature it’s only found as a complementary pigment in some cyanobacteria, so probably life in Kepler-442b would have something better than this.

So far, only 2 exoplanets in the habitable zone have been discovered around Goldilock stars and both, including Kepler-442b, are around 1000 light years away….which is not the best…but could be worse. The blue plants of Kepler-442b will have to wait longer, then.

Conclusion

Of course, if Earth has taught us something is that the history of life can be more complex than we normally imagine. The discovery of exoplanets opens a slight door in the field of Astrobiology, a field that so far, has no real organism to research on, but, whenever it finds one, it would completely revolutionize our conception of life in this planet.

The last sections of this article are highly speculative, and probably don’t follow the strict scientific criteria we have imposed in our Onelephantsandbacteria articles. This article was inspired by a Scientific American article by Nancy Y. Kiang, published in 2008. At that point in time, exoplanets were still scarce. You can read it whole here

In the next issue…

After this interlude, the plan is to finally finish Strangers on the Animal tree and the Scientific Imaginary creatures and start other subjects. Amongst my notes there are words such as Ediacara, ants, religion or self-awareness. Let’s see what the future holds.

Some bibliography