The Purple Hills of Proxima b

In our continuing look at biosignatures that could flag the presence of life on other worlds, we’ve sometimes considered the so-called ‘red edge,’ the sharp change in reflectance of vegetation that shows up in the near-infrared. It’s worth remembering that vegetation is the largest reflecting surface on Earth (about 60 percent of the land surface), with an increase in reflectance that shows up around 700 nm. As Alex Tolley explains below, the red edge may shift depending on the evolution of plant life and the variables, including light intensity but comprising many other factors, that would affect life on M-dwarf planets. These are the first whose atmospheres we’ll be seriously examining for biosignatures, and the question of how to extrapolate from Earth life to environments as exotic as these is complex. A Centauri Dreams regular, Alex reminds us that vegetative life may prove adaptable in ways that will surprise us.

by Alex Tolley

Artist’s conception of Proxima Centauri b. Credit: M. Kommesser

Xi Jinping University, New Beijing, Mars

NEWS RELEASE

FOR IMMEDIATE RELEASE

Contact Wendy Ho, Media Liaison.

wendy.ho@univxijinping.co.mars

Nov. 11, 2091

The first image of the surface ofProxima b is released today. Captured by the solar focal telescope (SoFoT), the 2-megapixel image captures the 3⁄4 illuminated planet’s surface. This is the first direct image of the planet that does not rely on using the EPSI inversion model for the reflected solar radiation. Both continents and oceans are clearly visible. The ocean appears wine-dark, and the continental landmasses are partially covered in a purplish color that is most probably vegetation. This vegetation contrasts with the orange-colored material of the deserts. There are no polar ice caps. The vegetated areas reach into the very high latitudes of both poles. The previous spectroscopic analysis had indicated that the atmosphere contained the biosignature gases oxygen and methane. The likely presence of vegetation suggests the source of this oxygen is photosynthesis. The color of the vegetation is likely due to the shift of the maximal light absorption into the red and near-infrared, as well as the lower flux of blue and green wavelengths of light in Proxima’s spectrum. This gives an orange cast to the landscape, with the bluer color of the local chlorophyll analog dimmed. The color difference seen is similar to that of undersea life on Earth which looks dark in the blue light at depth, but which are brightly colored in white light.

Professor Zhang Yong suggested that the photic zone in the oceans was likely quite shallow due to the low blue light emission of the star and that possibly the rate of photosynthesis was lower than on Earth. The Chinese star probe, New Dawn, is currently on its way to Proxima. Launched in 2077 it is expected to reach the Proxima system in 2099. The image of Proxima b is the first to have life confirmed visually and indicates that the probe will confirm life on the planet. The IAU will meet in December to select an official name for the planet and its features.

The abundance of M-dwarfs in the galaxy, as well as the M-dwarf, Proxima Centauri, having a rocky planet in the habitable zone (HZ) has renewed speculation of what adaptations photosynthetic plants might have to the red-shifted spectrum of these stars. Figure 1 below shows the difference in spectrum received by Earth compared to that of Proxima b. The red-shifted peak emission of Proxima is evident, as well as the high intensity of extremely short wavelength radiation compared to Earth.

Figure 1. Top-of-atmosphere full spectral irradiance received by Proxima b (black) and the Earth (red). An orbital distance of 0.0485 AU is assumed for Proxima b. [4]

Most speculation about the adaptation of plants concerns multicellular, terrestrial green plants that have clothed the surface of our continents. These plants are almost all green plants that contain chlorophylls a and b, whose peak absorption wavelengths are both in the blue and red end of the spectrum, and therefore reflecting wavelengths between these peaks, that we perceive as green (see figure 2). The adaptation of green plants to our sun’s spectrum leads to the speculation that plants on exoplanets around M-dwarfs will similarly adapt, with a dominant absorption that would extract more energy from the red end of the spectrum and therefore changing the apparent color of their leaves. Some have speculated that the absorption of more of the light spectrum that we are adapted to see would render the leaves almost black to our eyes.

Jack O’Malley-James, School of Physics and Astronomy, University of St Andrews:

“Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible.”

In this essay, I will argue that there is evidence that chlorophylls can evolve to trap much redder light, but that this is too simplistic a story when considering how plants may evolve in response to the spectrum of M-dwarfs like Proxima.

While green plants dominate the Earth today, there are other plants that use different combinations of chlorophylls. The red algae (Rhodophyta) use chlorophylls a and d, plus a red-colored accessory pigment, phycoerythrin. These ensure that the dominant absorption is at the blue end of the spectrum, and is adapted for their deeper marine habitats where the penetrating sunlight is blue filtered. Without white light to illuminate these algae, they appear black to our eyes. The brown algae, which includes the kelps, use chlorophylls a and c and an Accessory pigment, fucoxanthin. Again, the peak absorption wavelengths are at the blue end of the spectrum as befits their subsurface, marine habitats. All these plants, whether multicellular or unicellular, capture the energy of the sun to fix carbon via photosynthesis. This reduces the carbon of the carbon dioxide (CO2) with the hydrogen in water (H2O) to produce sugars and release oxygen (O2) as a byproduct. However, this is not the only method of photosynthesis.

Figure 2. The absorption spectra of chlorophylls a and b, found in green plants and green Algae.

Bacteria also trap electromagnetic radiation and fix carbon. However, they use either hydrogen gas (H2) or hydrogen sulfide (H2S). In the latter case, they release particulate sulfur. Both of these types of photosynthesis require anoxic conditions and the bacteria live in hot springs or vents where these 2 gases are available. Both plant and bacterial chlorophylls are based on tetrapyrrole molecules. The absorption peaks of which are changed by small modifications of the structure.

Figure 3. Tetrapyrrole ring for chlorophyll a.

Figure 4. The structure of chlorophyll a. This is the primary photosynthetic chlorophyll.

While terrestrial life has evolved under our sun, there is the question of whether these light-absorbing molecules can evolve to meet the different spectra of other stars. While there is interest in whether planets in the HZ of M-dwarfs can even evolve life, that life would be limited to low energy, unicellular chemotrophs unless they can harvest the longer wavelength energy of their star.

The first question is whether these core, tetrapyrrole, light-absorbing molecules have already evolved on Earth to trap longer wavelength light. The obvious place to look is with the bacteria that live in very low light environments yet still photosynthesize to fix carbon.

The answer is yes. Of the 8 known bacteriochlorophylls, bacteriochlorophyll b that is found in purple bacteria has peak absorption wavelengths in the infrared (IR) of 835–850nm and 1020–1040nm.

Figure 5. Photosynthetic and other light-absorbing pigments. Absorbance spectra of selected chlorophylls (Chls), bacteriochlorophylls (Bchls), and carotenoids showing the wavelengths of absorption peaks. A visible spectrum colorbar is shown at the top. The y-axis scaling is arbitrary. The Chl absorbance data are of extracted pigments in methanol solution from Chen & Blankenship (2011). The Bchl a and Bchl b data are of whole cells and originate from Cogdell & van Grondelle (2003). The beta-carotene (carotenoid) spectrum is of pigment extract in hexane from Dixon et al. (2005), while the lutein (carotenoid) is from Janik et al. (2008). Pigments dissolved in solvents have absorption peaks slightly shifted from those in cells. These data are publicly available on the Virtual Planetary Laboratory Biological Pigments Database (http://vplapps.astro.washington.edu/pigments). [3]

Relatively recent work by Faries [1] and ​ Vairaprakash [2] on bacteriochlorins, modifying their side groups has shown that the peak absorption wavelength can be pushed out into the near infra-red. The number of variants in the chlorin ring exceeds that found in nature, and shows how small changes can modify the peak absorption wavelength. Figure 6 shows the results of these experiments that aimed to understand exactly how to tailor these molecules for different peak absorptions, primarily for industrial purposes. Note that all the absorption spectra retain their double peaks at each end of the visible spectrum. It should also be noted that there is clearly a drift towards a secondary peak at the short wavelengths that get shorter as the maximum peak wavelength gets longer, however this appears bound by a lower limit around 360nm, near the violet to ultraviolet transition. Whether these modifications could be produced by natural biochemistry would be interesting. The reactions needed may be outside of the possible repertoire, or too complex to prove evolutionarily viable.

Figure 6. The expansion of the spectral window for the bacteriochlorins by placement of uxochromes at designated sites affords a commensurate increase in the photochemical tailorability of this class of nature-inspired molecules. Collectively, the results provide fundamental insights into the rational design and synthesis of red- and NIR-absorbing bacteriochlorins for solar-energy and life sciences research. [2]

This bodes well for the possible evolution of photosynthesis on planets around M-dwarfs. It indicates that life could evolve to harvest energy not just of M-dwarfs, but even brown dwarfs. This would at least ensure that non-oxygenic photosynthesis by bacteria is possible to fix Carbon.

Photosynthesis in green plants consists of 2 systems, photosystem I (PS I) and photosystem 2 (PS II). PS I is the earlier system to evolve and is the reaction that drives carbon fixation in nonoxygenic, photosynthetic bacteria. PS II evolved later and uses the addition of shorter wavelength, higher energy light, captured more by chlorophyll b to split the H2O molecule to liberate O2 as a byproduct before the reactions to fix carbon.

Experimental work [5] shows that some blue light is needed for photosynthesis to operate efficiently, although red light alone will allow suboptimal photosynthesis to occur. This is important, as it implies that even terrestrial green plants should be able to grow on an exoplanet in the HZ around an M-dwarf.

So far the story suggests that photosynthesis, both oxygenic and nonoxygenic (in bacteria) should be possible under M-dwarf light. The evolution of longer wavelength absorbing light in bacterial chlorophylls, both natural and experimentally, suggests that light trapping tetrapyrrole molecules or possible analogs, should allow the evolution of photosynthesis under the light spectrum of M-dwarfs.

While O’Malley-James (see earlier quote) speculated that plants on M-dwarf planets might use more chlorophylls to capture light at different peak wavelengths, we do not see such adaptations on Earth, even though plants live in varied light condition environments. All plants seem to produce just 2 chlorophylls, with chlorophyll a being the dominant one for photosynthesis.

The question is “Why don’t plants produce more than 2 chlorophylls?” The answer lies most probably in the cost of a larger genome to code for the extra pathways and enzymes to manufacture the chlorophylls and regulate them. Instead, plants have opted for simpler, less costly solutions. The brown algae encode the biochemistry to manufacture the accessory pigment fucoxanthin, and the red algae, the protein phycoerythrin as an accessory pigment. These seem to be a tradeoff that optimizes the capture of available light energy for growth and reproduction.

However, light-capturing molecules are not the whole story. Terrestrial, multicellular plants adapt to low light conditions by a number of strategies. These include:

1. Increasing the number of chloroplasts in their cells. This is why plants in the low light understory plants of tropical jungles tend to have dark green leaves.

2. Rather than expending energy creating lignin-rich trunks to raise the leaves of trees to the sunlight, vines simply raise their leaves by climbing those trunks, or other surfaces to reach more sunlight.

One of the main objections to life on M-dwarf exoplanets is the high ultraviolet (UV) flux. On Earth, the bacterium Deinococcus radiodurans has evolved robust biology to resist radiation damage. Other organisms might respond by remaining at depth in the oceans, or under the ice to avoid the UV intensity. If the UV is inconstant, they may adapt by other means to reduce exposure. One means is to stay unicellular and attach to a motile animal than can move to safety. On Earth, we have marine slugs that ingest algae to extract their chloroplasts for photosynthesis. Maintaining algal pouches like squid do for luminescent bacteria is also a possibility.

Figure 7. The sea slug, Elysia chlorotica, eats green algae. It retains the chloroplasts so that it can harness photosynthesis for extra energy.

It is often suggested that flares will strip the atmosphere off an M-dwarf planet. If life has evolved, atmosphere stripping would freeze the oceans, creating a thick ice crust. Photosynthetic life might be able to live below that crust, trapping the light that penetrated the icy crust, or even within the ice using the tiny liquid water fraction to allow metabolism. The conclusion I draw from our one example of Earth’s plant life, is that we should be careful in extending simple logic to speculate how plants might evolve on M-dwarf planets. Lifeforms optimize a myriad of variables to gain an edge, using different strategies to maximize gene replication. As plant breeders have long since learned, it is often not possible to optimize for one feature, such as increased seed size and production, without needing to alter other features. Light intensity and spectrum are not the only variables plants will experience on M-dwarf planets and plants will find a number of ways to adapt to those conditions, probably in ways we cannot foresee based on our experience.

A final note on detection. Because of the dominance of chlorophyll a on Earth, the reflectance of the Earth has a sharp increase at wavelengths just beyond its peak absorption. This is known as the “red edge” and is suggested as a possible biosignature. O’Malley-James and Kaltenegger [6] suggest that since the early photosynthetic cyanobacteria also have the same chlorophyll type, that the red edge signature could be used even for exoplanets at a much earlier stage of the evolution of life. However, the longer wavelength peak absorption of bacteria using different bacteriochlorophylls, and the possible evolution of longer wavelength absorbing chlorophylls suggests that this red edge may shift, perhaps into the IR. If so, some flexibility in the determination of any red edge biosignature should be entertained.

References

1. Faries, K. M., Diers, J. R., Springer, J. W., Yang, E., Ptaszek, M., Lahaye, D., … Holten, D. (2015). “Photophysical Properties and Electronic Structure of Chlorin-Imides: Bridging the Gap between Chlorins and Bacteriochlorins.” The Journal of Physical Chemistry B, 119 (24), 7503-7515. doi:10.1021/jp511257w.

2. Vairaprakash, P., Yang, E., Sahin, T., Taniguchi, M., Krayer, M., Diers, J. R., … Holten, D. (2015). “Extending the Short and Long Wavelength Limits of Bacteriochlorin Near-Infrared Absorption via Dioxo- and Bisimide-Functionalization,”​ The Journal of Physical Chemistry B Vol. 119 (12), 4382-4395. doi:10.1021/jp512818g

3. Schwieterman, Edward. (2018). “Surface and Temporal Biosignatures.”

10.1007/978-3-319-30648-3_69-1.

4. Ribas et al., “The full spectral radiative properties of Proxima Centauri,” Astronomy & Astrophysics Vol. 603, A58 (July, 2017).

https://www.aanda.org/articles/aa/full_html/2017/07/aa30582-17/aa30582-17.html

5. Hogewoning, S. W., Trouwborst, G., Maljaars, H., Poorter, H., Van Ieperen, W., & Harbinson, J. (2010). “Blue light dose-responses of leaf photosynthesis, morphology, and chemical composition of Cucumis sativus grown under different combinations of red and blue light,” Journal of Experimental Botany, 61 (11), 3107-3117. doi:10.1093/jxb/erq132

6. O’Malley-James, J. T., & Kaltenegger, L. (2019). “Expanding the Timeline for Earth’s Photosynthetic Red Edge Biosignature,”. The Astrophysical Journal, Vol. 879 (2), L20. doi:10.3847/2041-8213/ab2769

7. Berdyugina, S. V., & Kuhn, J. R. (2019). “Surface Imaging of Proxima b and Other Exoplanets: Albedo Maps, Biosignatures, and Technosignatures,” The Astronomical Journal Vol. 158 (6), 246. doi:10.3847/1538-3881/ab2df3

8. Stevenson, D. S. (2017). Life Jim, But Not as We Know It. In The Nature of Life and Its Potential to Survive (Springer), 193–236. doi:10.1007/978-3-319-52911-0_5