cantor trees
Image from Steve Bowers
Trees on Chorus use a purplish pigment for photosynthesis

On Old Earth the dominant photosynthetic organisms split water to release oxygen. This is the most common sort of photosynthesis on inner system gardenworlds. However, water is not the only source, and oxygen is not the only possible result.

From the point of view of a photosynthetic organism the whole point of the exercise is to convert light energy to chemical energy. The best way to do this is to find a hydrogen donor for the process. The results include hydrogen ions, energetic electrons and the substrate (which is usually discarded). The resulting high-energy electrons are extraordinarily useful and can be used to run a whole series of biochemical reactions. These are usually run in a sort of cascade effect; the electrons power one reaction, lose a bit of energy, are passed on to another reaction, lose a bit more, go to another, and so on, until as much of the available energy as possible has been captured. The hydrogen ions are very useful, too. A hydrogen ion is small and highly mobile, and fits naturally into the "background" of hydrogen ions found in any aqueous solution. That is true not only for water but for ammonia, hydrochloric acid, hydrogen sulphide or most of the other solvents that are alternatives to water in xenobiochemistry.

The general chemical reactions involved in this first process are called "light reactions" because they are driven by photons. The details vary considerably, of course, depending on the substrate used and on the evolutionary history of the life form in question. In terragen plants and cyanobacteria this set of reactions is carried out by two systems called photosystems I and II. If only one of these systems is used, no oxygen is involved at all - just energetic compounds (ATP). If both are present then oxygen is released and the hydrogen ions are employed elsewhere in the organism's biochemical processes. Non-terragen organisms usually have something analogous. Some other terragen organisms use hydrogen sulphide as a substrate. This process liberates elemental sulphur which is either ejected or retained as grains within the organism's cells. Other organisms, with other origins, have analogous biochemical pathways to this. These all release an oxidizing chemical as a waste product; whatever is left of the source compound after its hydrogen atoms hve been removed. The release of oxygen or some other byproduct is a side-effect, and a rather inconvenient one at that. If it is a strong oxidizer, like oxygen or chlorine, it is dumped into the environment. If it is weaker, like sulphur or some complex organic compounds, it might be stockpiled within the organism itself, possibly for later use.

Once hydrogen ions have been liberated they can be used to produce other compounds, using all or some of the energy from the first reaction. Most commonly the hydrogen is used to fix carbon for producing carbohydrates and various other complex chemicals. Carbon is the favoured recipient because of the variety of compounds that can be produced. These compounds may be used later either as fuel for respiration (anaerobic or otherwise) or as a structural component in the cells. On rocky inner-system planets the source of carbon is usually carbon dioxide, which is stable at those temperatures and becomes the most common carbon-containing compound in the atmosphere and hydrosphere over a wide range of possible planetary conditions (as may be seen on Venus, Earth and Mars in Solsys). Old Earth biochemists once called the set of reactions that fixed carbon the "dark reaction" because the process can take place without the presence of light. In terragen plants or cyanobacteria this is the Calvin-Benson cycle; analogous processes are found elsewhere. Chemosynthetic organisms use processes like this to build organic matter with hydrogen ions and energetic electrons derived from chemical processes.

The most common sort of byproduct of photosynthesis, at least at earth-like temperatures and on a rocky inner-system world, is oxygen. This is because the most common hydrogen donor is water. Water is ubiquitous as it is made of the first and third most common elements in the universe. In the outer system it is often found as ices or clathrates. In the inner parts of a system it will be outweighed by rocky materials but it is generally more common than any other possible hydrogen donor by one, two or several orders of magnitude simply because oxygen is so common. So, at temperatures that allow liquid water, an oxygen-bearing atmosphere is likely to be the final result if life arises. Hydrogen sulphide is another possible donor - in fact there are entire groups of terragen organisms that use it - but sulphur is much less common than oxygen in the first place. Also, once some organism develops the ability to use water as a hydrogen donor the atmosphere begins to fill with oxygen. Hydrogen sulphide does not survive in an oxygenated atmosphere, and so hydrogen sulphide photosynthesizers are relegated to anaerobic niches once the biosphere reaches equilibrium. On very large inner system planets that have retained a significant volume of hydrogen, hydrogen gas itself may be the donor. What follows when water-based photosynthesis arises depends on the proportion of hydrogen in the atmosphere. If the relative amount of hydrogen is small the final result is as on Earth, with an atmosphere that contains significant amounts of oxygen and hydrogen photosynthesis relegated to a few anaerobic niches. If the amount of hydrogen is larger, then the atmosphere will never achieve an earth-like composition, since any oxygen released soon combines with the planet's reducing hydrogen atmosphere. Such anaerobic life-bearing worlds may never give rise to energetic animal-like bionts.

The following are some typical final results of photosynthetic processes, given various substrates:

H2O + CO2 ---> CH2O + O2
H2S + CO2 ---> CH2O + 2S
H2 + CO2 ---> CH2O
2HCl + CO2 ---> CH2O + Cl2

The general pattern is that hydrogen is stripped from a donor to produce a carbohydrate of some sort. In terragen organisms, this is usually a sugar, from which other compounds are evolved by other processes. That is also the most common pattern in non-terragen biochemistry. The first of these, releasing oxygen, is one of the most common in the universe for the reasons that have been given. The second is also usually found as a minor element once oxygen-producing photosynthesis takes hold on a planet, as is the third, unless the planet is large and begins with a significant fraction of hydrogen in the atmosphere. The fourth is extremely rare because hydrogen chloride is rarely found in sufficient quantity on garden worlds (it is unknown in terragen organisms), though it may be a parallel process to oxygen-evolving photosynthesis in some places (see Chlorine Worlds).

Image from Steve Bowers

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Development Notes
Text by Stephen Inniss
Initially published on 17 November 2004.