Thursday, August 20, 2015

Chemotrophs and Energy

In a previous post, it was discussed how a simple path for life might be comprised of an initial self-replicating molecule, leading through piecewise steps to the first pre-Archaea cell. Energy was not discussed. It was assumed instead that there would be free energy available in the initial duplication of the initial self-replicating molecule. However, going much beyond that might require than an energy source be tapped.

The only energy source available at such an early stage of life is chemical energy. Thus the initial self-replicating molecules, either before or after a film membrane was evolved, would need to become chemotrophs. There have been a few unconfirmed reports of radiotrophs, which gain energy from nuclear radiation, but they need to live at a nuclear reactor site.

On Earth, chemotrophs have been found which extract energy from hydrogen sulfide, elemental sulfur, ammonia, hydrogen, ferrous iron, and lower valence state manganese. Others consume methane. Most of the known chemotrophs use oxidative reactions, in other words, they are aerobic, even in the deep ocean. Since free oxygen was not present in the early atmosphere on Earth, and probably would not initially be on alien ‘habitable’ planets, life must have initially evolved with anaerobic chemotrophs. Some use H2 instead of O2. The two types known include those which consume sulfate and those which consume carbon dioxide. The sulfate-using Proteobacteria are found underground in sulfate rock formations, and the carbon dioxide ones, Methanopyrus, is found by undersea vents. These may be the two places where life using chemical power could originate. There are currently four other known anaerobic chemical pathways used by organisms to gain energy for life, inhabiting these and similar environments. These may be the first types of higher-energy cellular organisms on our planet.

Once the use of a chemical power reaction occurs, a boost in survivability would occur, and also a huge increase in what is possible to evolve. Prior to the first use of chemical power from one of these sources, only reactions which have surplus energy could be evolved. But when these chemical power reactions become part of the cell’s armamentarium, the next step is energy storage, in a chemical such as ATP. And once such energy is available it can be transferred from whatever chemical receives it in the cell to specific reactions in the cell, meaning a wide variety of energy-requiring reactions can take place. In short, the evolutionary step to allow a chemical reaction producing energy into the cell and the next one to store it somehow open up a vista of possibilities for evolution.

So far, there have been no difficult transitions envisioned in our hypothetical pathway to advanced life. Three conditions have been noted. One is the amino acid soup, another is a substrate that a self-replicating molecule could adhere to, and the third is an energy source, either in the solution or the substrate. At this point in the pathway, a long time is needed for the primitive cells to mutate and evolve sufficiently to start to produce cells similar to ones we are now familiar with. The self-replicating molecule at the origin of this, which turned into something having one coding for a gene, not synthesized with any intermediary, that produced cell wall molecules, has to slowly evolve to be a solely coding mechanism. There has to be the evolution of countless proteins that do countless jobs inside the cell. It is possible to say that a fourth condition is the continuation of a favorable environment for evolution of single cells.

These four conditions do not appear to be very unique demands that would reduce the likelihood that planets in a habitable zone would develop single-celled life. Organic molecules, a rock surface, some chemical reaction to produce energy, and aeons of time are the four. Fossils on earth show signs of early cells about four and a half billion years ago, only a few hundred million years after the planet coalesced. Chemical energy is nice, but it is only available in restricted locations, whereas solar photons are available everywhere except the ocean deeps and the subsurface. Fossil evidence implies that a couple of hundred million years after cells formed, some sulfate-reducing bacteria supplemented their energy source with a early type of photosynthesis, IR-based, which assisted in the sulfate energy cycle. These might arise on planets around K type stars as well as G stars like the sun.

The journey to visible photosynthesis, involving the generation of oxygen, took a billion and a half more years to achieve. Chlorophyll is a difficult compound to evolve. Once cells became higher-powered, even more evolutionary options arose, and complex cells, possessing internal membranes, arose first. Then intercellular communication had to be evolved which allowed multicellular life to arise.

The next environmental requirement is a land surface. Between the development of primitive cells and the emergence onto land there are no new conditions, only time. Everything is produced from the two energy sources, chemical and photonic, plus the various molecules and elements found in the ocean and the seabed, including along the coastlines. The only Great Filter concept here is duration and stability. The oxygen which is needed for even higher powered reactions is produced by the descendants of the primitive cells.

There are different elements used in some of the evolutionarily more recent biomolecules, but they do not show the attributes of a Great Filter than would prohibit life from passing a certain stage of growth on alien planets. The distribution of elements is determined by two processes which should be the same in the various planets of the galaxy. One is the original synthesis inside stars and supernovas; this is not planet-dependent. The other is the formation of the crust of the planet, and this is governed by the constituents that congeal to form the planet. There does not seem to be much reason to think that there would initially be, in the protoplanetary disk encircling a new star, much differentiation of elements. The differentiation comes later when light and volatile elements cannot be retained by a planet because of the interaction of the mean temperature and the gravity of the planet. Thus, for example, if there is iron on a planet, there should be a sprinkling of magnesium and manganese and other elements which might be incorporated by cells. So the appearance of trace elements in cellular biochemicals would not be a Great Filter.

To summarize, once some elementary conditions are met for primitive cells to form, nothing new is needed until land organisms spring up. Everything in the oceans that more complex cells need should have been there when primitive cells arose. Energy sources are one of the elementary conditions, ranging from a variety of chemical energy sources up to the spectrum of solar photons. Only time is a Great Filter after this. How much time is determined by the global rate of mutational experiments.

As noted elsewhere, the time needed for an evolutionary step on Earth is only a clue as to what time would be needed on another planet, because the mutational experiment rate might be different. The rate is proportional to two things, the prevalence of subjects, for example, cyanobacteria, and the rate of mutation per cell. If the mutations are caused by internal conditions, such as fluctuations inside a cell when it is in the process of replicating, and occur at a fixed rate, the prevalence of subjects is the only factor. The rate of internally generated mutations might not be equal on different planets. For example, a planet with an ocean at 50 degrees C might have a significantly higher mutation rate than one with an ocean at 10 degrees C. On the other hand, if the star is a G5, with more UV photons, there might be a larger externally caused mutation rate than if the star were a K5. Thus, comments about hot stars not being able to produce life are true, but the threshold where the line is drawn might be higher because of the effect of induced mutations.

Just to complete this discussion, if the mutation rate, externally caused by UV photons, is too high, extinction will be the result instead of a more rapid evolution. Most mutations are fatal. The immediate result of this is that there is a distribution function of evolutionary rate from mutations induced by photons, depending on the star type, the distance to the planet, the type and thickness of the atmosphere and the mean depth of the organisms. It would be interesting to see if Earth is at the apex of this curve, in other words, if this planet had the fastest evolutionary trajectory in the galaxy.

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