Thursday, September 30, 2010

Energetics in the Hypotheses on the Origin of Life

Energy is integral to the existence of life; thermodynamic laws affirm this fundamentality. From the viewpoint of energetics, any hypothesis of life’s origins has to indicate the energy source(s) that could account for the (a) formation of reduced carbon compounds and (b) polymerization reactions. However, the field of abiogenesis (the study of life’s origins) does not always take this fundamental principle into account.

Abiogenesis hypotheses are divided into two broad ‘groups:’ “metabolism first” and “replication first.” The replication first" scenarios—including the RNA World concept—do not focus on primordial but instead focus on the chemistry of synthesis and mechanisms of information processing in replicating cycles. In contrast, the “metabolism first” concept does explicitly address the problem of energetics. Later I will address some of the hypotheses about the source of energy that drove the origin of life and what molecules are capable of utilizing energy and how.

First we must begin with the most famous experiment involving the creation of organic molecules from abiotic compounds and its limitations. This was the Miller and Urey model. In this experiment Miller and Urey mixed methane, hydrogen, ammonia and water vapor, added “sparks” which stimulated lightning and were able to form amino acids.

As amazing as this was, we now know that it has severe limitations for its historical plausibility. The reason is that this experiment was based on the assumption that the early Earth had a reducing atmosphere—with large amounts of hydrogen and almost no oxygen. 
However, many scientists no longer believe that early Earth had a reducing atmosphere, and instead believe it had a neutral atmosphere. They propose a composition of mainly carbon dioxide, with smaller amounts of nitrogen and hydrogen, similar to the modern atmospheres of Mars and Venus.

This means that the Miller-Urey experiment under the new atmospheric assumptions, does not produce amino acids.

This creates a dilemma for how life could have plausibly began, chemically or physically, because of the reality of energetics. Life needs energy. Living organisms can only exist if there is a source of energy flow like solar radiation or chemical reactions.

However, in order to reduce carbon dioxide in a carbon dioxide-rich atmosphere, there needs to be a source of electrons in order to create complex compounds.

Indeed, this is where other hypotheses of the energetics of abiogenesis come into play.

Many of these hypotheses believe that life began deep in the ocean. It has been proposed that the oxidation of FeS to FeS2 at the sea floor was used to drive the reduction of CO2 or CO. Let’s look first at the possible reduction of CO2.

It is known that the free energy of this redox transistion is sufficient to drive the reduction of CO2 (at least under some conditions), however so far all attempts to reduce CO2 at the expense of FeS oxidation have failed to yield measurable CO2 reduction under simulated conditions. The reason for this failure is that the reducing potential of the FeS/FeS2 redox pair is higher than the redox potential needed to drive Co2 reduction at a high enough rate; thus thermodynamics alone is not enough.

But what about CO?

The reduction of CO by FeS has been reported but only at unphysiologically high temperatures. Besides, CO is not a major part of the atmosphere now, nor (most likely) ever was. But even if it could rise in primordial settings though the reduction of CO2, however, this is not thermodynamically favorable and thus was very unlikely to have occurred.

Other theories involve marine hydrothermal systems which answer the question of energetics by proposing geochemical energy sources. This model depends on the formation of cellular building blocks from inorganic reactants. But the question from the energetics viewpoint is, can these anabolic processes yield energy? Amend and McCollom proposed that redox reactions occurred at “the interface between two end-member fluids—low temperature (25 °C), mildly acidic (pH 6.5), relatively oxidized (Eh -0.30 mV) seawater and moderately hot (140 °C), alkaline (pH 9), reduced (Eh -0.71 mV) hydrothermal vent fluid.”

They demonstrated that biomass synthesis is most favorable at moderate temperatures, where the energy contributions from HCO3- and H+ in seawater coupled to the reducing power in hydrothermal fluid are optimized. The models show that the net synthesis of cellular building blocks may yield small amounts of energy.

In a similar scenario, put forward by Russell and co-workers, hydrogen and hydrocarbons were produced below the sea floor in complex "serpentinization" reactions and then brought to the surface by hydrothermal fluids. The proposed energy source is the pH gradient across the inorganic membranes between alkaline hydrothermal fluids and the acidic primordial ocean. This concept is analogous to the transmembrane proton gradients on the membranes of modern bacteria cells, however, the evolution of these gradiants seems to be a relatively recent evolutionary advance.

Some also suggest the role of Radiation Chemistry. This concept relates the understanding that any energy able to create ionized and excited molecules can be responsible for the formation of carbon compounds also containing other elements with the fact that there were much higher levels of ionizing radiation on early Earth. Scientists of this field claim that radiation played a role in the formation of organic compounds because radiation abstracts electrons from single compounds, and forms free radicals which can create ew combinations and form compounds of higher molecular weight.

Finally, a new and increasingly acknowledged hypotheses explaining the energetics of the origin of life is the role of zinc sulfide.

Zinc has a unique ability to store the energy of light. Indeed, this is why it is used in sunscreen (as zinc oxide) and in glow-in-the-dark items. This unique ability is also important in possibly solving the conundrum of how to provide energy in a neutral atmosphere; because Zinc, once illuminated by UV light, can efficiently reduce carbon dioxide. This is similar to what plants do. Notably, zinc sulfide particles precipitate only at deep-sea hydrothermal vents today, however, it is proposed that life may have originated at photosynthetically-active, porous structures made of zinc sulfide, similar to today’s deep-sea hydrothermal vents. This means that life would have begun on Earth’s surfaces so that there was access to UV light. Previously, many scientists thought that UV light was a hindrance to the origin of life because of its ability to degrade compounds, however, the fact that UV light in combination with zinc sulfide, allows for the reduction of CO2 makes it a very interesting hypothesis for a primordial energy source. To back up their hypothesis, the scientists who propose this new theory of the origin of life contend that they “have found that proteins that are considered ‘evolutionarily old’ and particularly those related to handling of RNA specifically contain large amounts of zinc.”

There appear to be many ideas for the role of energetics in the origin of life. Indeed, this is a hot field at the moment and will continue to be important because of fundamentality of energy to the existence of life.






  1. Hi Kelsey, thank you for this very informative post--I particularly found the information on Zinc-mediated carbon reduction to be new, and an interesting way to get away from the "undersea vent" model for the genesis of energy-producing reactions in the first life-forms.

    On that score, I'm wondering if you could comment on some of the theories that were hot when I last was aware of the work in this area. For example, there was a series of papers in the 1980s-1990s by Wachterhauser, postulating chains of reactions involving using Iron sulfide and hydrogen sulfide and producing citric acid. A weirder proposal by Adam was that the energy source came from radioactive decay of Uranium in the sands of primordial beaches. Have these been discredited?

  2. So, I was just looking a little more into the research that Wachterhauser did regarding an autotrophic origin of life in which an energy flux provided by chemical reactions at liquid-solid interfaces was used for carbon fixation. Wachterhauser specifically suggested that a primitive metabolism evolved at the surface of pyrite minerals from the reduction of carbon dioxide using hydrogen sulfide over ferrous sulfide as the reducing agent. The number and diversity of these molecules would have grown autocatalytically by carbon fixation, leading to the self-organization of cyclic chemical reactions, producing more and more elaborate products.
    HOWEVER, current researchers have pointed out that although the autotrophic models for the origin of life are in theory experimentally feasible, preliminary attempts to test these theories have succeeded in producing only simple organic molecules - no more complex then butane. Furthermore, none of these reactions has been shown to be autocatalytic which is a crucial requirement to start real chemical evolution.
    So, from what I've found, I don't think that the Wachterhauser mechanism is still considered to be a realistic model for the energy-producing reactions in first biotic organisms.