10.2 - Photosynthesis

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

Despite that many energy schemes noted above, not all processes theoretically permissible under the Second Law of Thermodynamics are commonly or easily available to living organisms. The methods for the performance of useful work used, say, in modern industry are generally not utilized by Earthly lifeforms. For instance, changes in temperature such as might result from combustion or nuclear reactions are not found in biology. Instead, the creatures on this world uniformly may be characterized as "chemodynamic machines," operating by chemical rather than by thermal energy.

This is not a serious restriction. As Dr. E. Broda at the Institute of Physical Chemistry in Vienna points out, nutritionists have observed that "surprisingly high yields of useful energy can be obtained from food." Comparing chemical and thermal systems, Broda continues: "With a yield of 25% as observed, the {equivalent} temperature difference works out as 105 °C. Hence, if the body was a heat engine, local temperatures of at least 310 K (body temperature) + 105 K (food-conversion temperature) = 415 K would be needed."¹⁰¹³

While we recall that arguments on the basis of temperature alone cannot rule out the possibility of life, this example serves to illustrate the superior competitiveness of chemodynamic as opposed to strictly thermodynamic energy systems. We conclude, perhaps somewhat chauvinistically, that nonexotic chemical energy systems will normally be the method of choice for the majority of extraterrestrial lifeforms. Naturally, the most convenient and abundant source of usable energy for most ETs will be their sun.

Given the problem of designing a metabolic system, starting from the sole assumptions that (1) a chemical framework (2) powered by sunlight must be used, we quickly arrive at two logical conclusions.

First, the simplest organisms in a planetary ecology will be those capable of tapping the given energy source directly. These lifeforms accumulate energy from photons received from the sun, absorb any needed inorganic matter that happens to be lying around, and put the two to work in an integrated biochemical system. Because they are able to harvest energy straight from the original source all by themselves, such creatures are called "autotrophs."

Second, we might imagine another kind of lifeform which cannot tap the energy source directly. This class of organisms is either too lazy or too incompetent to manufacture its own food. So what powers them? Instead of patiently accumulating solar energy, these larcenous "heterotrophs" pirate energy-riches from the complacent autotrophs. Since there is no honor among thieves, we would also expect to find heterotrophs stealing energy from each other as well. An entire chain of robbery would develop, with the strong taking from the weak, the stronger taking from the strong, and so forth.

With a few minor variations, this is the basic scheme of life on Earth. The autotrophs are our plant life, which take up carbon dioxide and convert it to carbohydrates and other energy-rich goodies. The heterotrophs are the animals.

Clearly, the organization of an ecology into two major groupings (producers and consumers) is not at all arbitrary but follows logically from the twin assumptions stated earlier. It is difficult to imagine an easier or more elegant solution to the fundamental bioenergetic problem. Although other ecological systems may exist, the dual autotroph/heterotroph arrangement is probably the preferred technique for chemically-based, solar-energized metabolizers.¹⁴²⁸

Each year about 150 billion tons of carbon are taken in by the autotrophic plants on this planet and are combined photosynthetically with some 25 billion tons of hydrogen (split from the oxygen in water) to make carbohydrates. In the process, 400 billion tons of oxygen are set free. On the average, a typical molecule of carbon dioxide wends its way through the system once every 200 years; each O₂ cycles less frequently, perhaps once every 2000 years.⁹⁹⁷

Of course, it is not absolutely necessary for alien autotrophs and heterotrophs to participate in a carbon cycle biochemistry powered by the breakdown of water to oxygen. While the photosynthetic process itself is so simple as to suggest a certain measure of universality, there is nothing sacred about which chemicals are recycled. In fact, there are a number of other systems in use today right here on Earth.

One alternative to the photosynthetic H₂/O₂ cycle of which humanity is a part is the H₂/H₂SO₄ process of the sulfur bacteria -- an entirely different oxidation-reduction system than the one we use. Purple sulfur bacteria take in hydrogen sulfide (H₂S) and oxidize it to sulfuric acid (H₂SO₄). Desulfovibrio, another class of sulfur bacteria, completes the cycle by reducing the acid back to the original hydrogen sulfide gas.

Many other systems are in use on Earth besides this "sulfur cycle." There is an H₂/H₂O cycle, a CH₄/CO₂ cycle, an NH₃/N₂ cycle, and so forth. Microorganisms on this planet are capable of metabolizing such peculiar and diverse substances as selenium, iron sulfide, arsenic, thiosulfate ion, cyanides, and methanol. But the main hangup with using any of these exotic non-oxygenic systems to power large extraterrestrial organisms is their relative inefficiency.

Most are at least an order of magnitude less energetic than the water/oxygen cycle which dominates the biochemistry of Earth.

Because they are so woefully inefficient, non-oxygen-cycle lifeforms are significantly out-competed in most terrestrial environments and "have been driven to the fringes of life-as-we-know-it."^1390,1651 Nevertheless, there have been many valiant attempts to design viable extraterrestrial ecologies around various alternatives, notably by Asimov,¹³⁵⁸ Clement,²⁹² Glasstone,⁷² Mitz,¹⁴²⁴ Salisbury,¹⁶⁵⁸ and Vishniac et al.³¹³

If photosynthetic activity is extremely useful if not essential on other worlds, what is the best way to do it? Although there are many other molecules at work, chlorophyll predominates on Earth. Chlorophyll, the green active pigment in plants, is a member of a general class of carbon compounds known to biochemists as porphyrins.

Porphyrins are very simple ring-shaped molecules which have been produced in many prebiotic synthesis experiments,¹⁵⁹⁰ and which are believed capable of autocatalyzing their own production. Once formed, the porphyrin ring has enormous stability against decomposition. This may help to explain why these substances are so widely distributed on Earth today.

The porphyrin pigment chlorophyll has a single magnesium atom located in dead center. The exact function of this metal atom has yet to be clarified, but it is believed to play a crucial role in trapping and utilizing the energy of incoming photons used in photosynthesis.

If alien autotrophs use porphyrins too, will they be restricted to green, magnesium-based chlorophyll?

A few have argued that we should consider only the most abundant metallic elements in Earth’s crust -- say, the top 99% in abundance -- as candidates for the central atom.²³⁷⁴ If we buy this assumption, then a fairly good case can be made for the exclusivity of magnesium porphyrins in any water-solvent oxygenic biochemistry.^1423,2399

Of course, this is only a plausibility argument -- one which utterly fails if alternative liquid media (other than water) are considered. And even in water, despite the many points in favor of Mg, some doubt remains. Other possibilities may be open to ETs.

While photochemists have so far been unable to produce a substitute porphyrin complex "which involves relatively large storage acts" per photon of energy absorbed,⁹⁹³ it is well-known that zinc (Zn) porphyrin complexes are capable of undergoing reversible photochemical oxidation-reduction reactions similar to those exhibited by Mg-porphyrins.^1422,1423

One chlorophyll near-analogue, called zinc tetraphenylporphyrin, has shown weak photoactivity.⁹⁹³ Other zinc porphyrins, although admittedly rare on this planet, have been found in several organisms including Rhodopsuedomonas apheroides, the diphtheria bacillus, various mammalian organs, and in leaf tissue homogenates.^994,1069 Copper porphyrins have also been found in the diphtheria bacillus, and other substitutions using nickel, cobalt, or manganese are remotely possible but seriously questioned.^1422,1442

But perhaps we are being overly restrictive. What, after all, is so magical about the porphyrins? True, they arise in prebiotic experiments, and true, they are relatively simple molecules and they get the job done. But maybe there exist other equally suitable substances that could stand in for chlorophyll in alien plant biochemistries.

It seems difficult for many to believe that porphyrins are the best-suited class of molecules for the photosynthetic function. In fact, according to Bernard Pullman of the Institut de Biologie Physico-Chimique, University of Paris in France, "it certainly is not."³¹⁵ George Wald, Carl Sagan, and countless others have pointed out that chlorophyll actually absorbs light rather poorly in the green portion of the spectrum -- paradoxically, the very region where solar radiation is most intense. It is likely that a variety of dyes other than chlorophyll could have been used by plants.

Several alternative pigments are known in terrestrial biology to participate in the process of light absorption. For example, the carotenoids -- found in many species of bacteria, algae, and higher autotrophs -- absorb primarily blue light (which has more energy per photon) and thus are red, orange, or yellow in color. Carotenoids have no metal atoms and contain no porphyrin-like substances. Another category of terran photopigments are the phycobilins, which give both the red and the blue-green algae their distinctive, vivid color.

In the purple "halobacteria" (salt-loving), chlorophyll is entirely replaced by another Earthly photosynthetic pigment called bacteriorhodopsin. This substance has a deep purple color and is chemically related to rhodopsin, the photosensitive pigment called "visual purple" found in the rods of all mammalian eyeballs.

Research has suggested that this pigment may be selectively more advantageous in certain specific environments. This is particularly true under conditions of intense sunlight, elevated temperature, high salinity, and low oxygen concentrations.²⁴⁰² As a possible photosynthetic agent for extraterrestrial plant life, purple bacteriorhodopsin is less efficient (by one-third) but chemically simpler than chlorophyll.

Going still farther afield, why must complicated organic compounds be used at all? It has been amply demonstrated that the oxides of titanium (white), tungsten (canary yellow), and zinc (white) all possess high photosensitizing activity in oxidation-reduction reactions comparable to the activity displayed by chlorophyll. And these particular pigments are known to store light-energy in stable terminal products.²³⁷⁴ This may be useful for biology.

Finally, it is also well-known that many carbon-based organisms are capable of utilizing silicon and germanium to varying degrees. Why could not alien autotrophs, instead of sporting leaves impregnated with chlorophyll, sprout thin platelets of "organic photocells" analogous to the solar cells used by NASA to power spacecraft? Water could be split up by some electrolytic process, and the hydrogen thus liberated incorporated into useful energy-rich molecules.

Any of these substances could serve as photosynthetic pigments for alien plants. When human explorers reach out to other worlds, they may discover beautiful white, blue, red, yellow, orange, purple, glittering steel-gray -- yes, even green! -- landscapes of thriving vegetation.