Feature

Life in Hot Water

Life on Earth may have begun in hot water billions of years ago.

Polly Carr and Roy Daniel

Researchers at Waikato University and the University of Glasgow are investigating some of the features which make marine hydrothermal systems a promising potential site for the emergence of life.

The Earth formed 4.5-5.0 billion years ago, and until about 4.4 billion years ago was subjected to "sterilising" meteoric impacts so powerful they were capable of vaporising entire oceans. For another 0.6 billion years, lesser impacts vaporised the upper layers of the oceans. The first unambiguous evidence for life is dated at about 3.5 billion years ago, by which time life was almost certainly widespread, so the time available for the origin of life from pre-biotic chemicals, and for its spread over the Earth, may have been as little as 500 million years.

Hydrothermal systems and deep-sea sediments are the only areas which would have been protected from those impacts vaporising the upper oceans. Any other sites would have required life to have emerged over a very much shorter period, possibly as little as 100 million years. Although we have no evidence for the actual time needed, given the incredible complexity of the simplest life-form compared with non-living organic chemicals, it is conservative to favour theories which have less restrictive time constraints.

The hot water of marine hydrothermal systems would have offered early developing organisms protection from the volatile nature of the Earth's surface. The water was also a source of relatively high concentrations of the organic and inorganic ingredients required to produce more complex cellular components, and provided a chemically reducing environment which assists the sort of chemical reactions involved in life.

Furthermore, both useful organic chemicals and emerging life-forms in an underwater location would be protected from the intense ultraviolet irradiation of the early Earth.

A reducing environment, which enables the flow of electrons in chemical reactions, is required for the synthesis of complex organic molecules vital for life. At the time of the origin of life, there were only very low concentrations of simple abiotic organic molecules, such as formaldehyde and glycine. Some were introduced by meteors, others formed by simple chemical processes. Evolution of the first cell required the synthesis of more complex organic compounds, which can only occur in a reducing environment.

It was originally believed that these reducing conditions were supplied by a high concentration of compounds such as ammonia and methane in the atmosphere at that time. However, the consensus now is that the atmosphere contained a large proportion of CO₂, producing just a slightly reducing environment, with acidic oceans.

Since organic molecules (simple and complex) are vital for life, some sort of "pocket" of suitable reducing conditions would be needed for life to arise. Such conditions could not have been found in the atmosphere, but may have occurred in the impact sites of comets (which would have been highly unstable for the synthesis of complex organics) or in submarine hydrothermal systems.

Life Bubbles Up

On-axis hot springs, known as black smokers, arise at the line of origin of ocean floor spreading, such as the mid-Atlantic ridge. The hydrothermal circulation generated by the heat of the line of spreading sets up adjacent off-axis circulation cells, which are longer-lived and at successively lower temperatures (depending on their distance from the axis) than the black smokers.

In off-axis hydrothermal systems, the water is saturated with carbon dioxide and contains nitrogen and a little hydrogen. As it circulates past catalytically active minerals at up to 200^oC, surprisingly high concentrations of acetic acid and ammonia could be produced, with micromolar amounts of ethanol, urea, formic acid, methanol, methenamine, ethanamine and acetaldehyde. Trace amounts of the simplest amino acid, glycine, would also be formed.

Considering the likely flow through these off-axis hydrothermal systems in a more tectonically active and hotter Earth, more than 100 million tonnes of acetate could have been formed each year by these systems. About 5% of this would be destroyed by eventual passage through the much hotter (up to 400^oC) on-axis systems. The corresponding synthesis of glycine would amount to about 10 tonnes per year.

We can also obtain evidence by looking back towards the characteristics of the "most primitive" organisms. The further back we go along the evolutionary tree of the bacteria, the more thermophilic the organisms are. One of the most thermophilic bacteria is Thermotoga, discovered by our group simultaneously with workers in the US and Germany in 1986, which was placed towards the base of the evolutionary tree. Very recently a new and even more thermophilic organism, named Aquifex, has been discovered. Aquifex is placed even further down towards the stem of the evolutionary tree, and it is also found to be more thermophilic when compared to less "primitive" forms.

Moreover, the most primitive members of the third kingdom of living organisms, the Archaea (which like the bacterial kingdom is made up entirely of simple single-celled organisms) are also thermophilic. Members of this kingdom are generally regarded as evolving more slowly than the other kingdoms, and so may resemble the most primitive organisms more closely than do other types of organism. It is notable that all the most extremely thermophilic organisms known are found in this kingdom, growing optimally at temperatures up to 112^oC), and a very high proportion of all the members of this kingdom are thermophiles.

Thus, although not all scientists agree, the current balance of opinion is firmly in favour of a high temperature origin of life.

Three Starting Points

Three different starting points have been suggested for the origin of the first "protocell".

The first proposal involves a lipid micelle. This is a simple lipid bilayer forming an enclosed structure. Bilayers and micelles are energetically favourable structures for many lipids, and form very readily.

The problem here is that although it is possible to synthesise some complex organic molecules with the conditions and components available at the time of the origin of life, there is no evidence for a mechanism for the synthesis of the hydrocarbons required for a lipid membrane.

The second proposed starting point suggests the development of a clay surface, such as montmorillonite, that adsorbs organic molecules. It is proposed that the random stacking system of the clay could hold "information" and act as a template for other layers. There have also been suggestions of a similar starting point involving crystalline surfaces, such as pyrite, with the same transfer of information via the random stacking nature of the crystalline sheets.

The problem with these ideas is the difficulty of getting from the clay or crystal to the first cell. This raises questions about how to bridge the gap between the open, two-dimensional structure and information system maintained by the clay or crystal, and the enclosed, three-dimensional systems found in even the most primitive life forms.

Our proposal bypasses these difficulties by suggesting enclosure by mineral membranes as the starting point for the first cell's evolution.

We suggest that iron sulphide membranes formed at the interface between the iron-containing, acidic, relatively cool ocean and the highly reduced sulphide-bearing, alkaline hydrothermal fluid emerging from off-axis hot springs at a temperature of 150^oC. These membranes could have formed vesicles which have the characteristics necessary for the formation of the first "protocell". They enclose, have the possibility of energy generation, and can carry out catalytic reactions on the membrane surface or within the membrane vesicle.

At first the iron sulphide membrane forms chimney-like structures. These structures then extend into spherical botryoids or vesicles. As these vesicles are further expanded, they "reproduce" by budding. We suggest that initially the membrane is inflated by the buoyant hydrothermal fluid, and that later expansion is facilitated by osmosis. Osmotic forces are possible as molecules inside the membrane are separated from those in the exterior environment, creating concentration gradients of various chemical components between the two solutions.

These membranes can be made in the laboratory, and remarkably similar structures can be seen in "fossil" hydrothermal springs.

Extension of the membrane vesicle can be regarded as embodying a rudimentary form of information transfer, in that the composition of the new membrane will depend upon the existing composition (and its particular propensity to adsorb or include molecules present in the immediate environment) and on the chemical nature of the local environment.

The membrane surface has the ability to adsorb organic molecules. This adsorbance partly determines the properties of the membrane. For example, the organics help to protect the membrane from mineralisation which leads to a loss of flexibility. The membrane is also strengthened by the addition of organic compounds to the system.

This latter proposal was recently demonstrated in the Thermophile Unit at the University of Waikato where the membrane was markedly strengthened by the addition of low concentrations (down to 100 parts per million) of organic molecules. The organics used included formaldehyde and methanol, and simple amino acids such as glycine and alanine. These simple molecules are likely to have formed abiotically in the environment at the time of the origin of life. If organics can adsorb to the surface then it is possible for the membrane to facilitate the concentration of vital organics, otherwise found in low concentrations, that are required for the metabolism of the first cell.

It is proposed that as the organic molecules adsorb to its surface, the membrane would become electrically insulated as well as strengthened. This proposal has recently been confirmed by work at Glasgow University which has shown that the electrical resistance of the membrane is similar to biological membranes and that, even in the absence of organics, the iron sulphide membrane will maintain a pH gradient of much greater than 1 for several days; in the presence of simple organic compounds, this is raised to 5.

The significance of this finding is that chemiosmosis -- the use of a pH gradient to generate energy for metabolism -- is ubiquitous among living organisms, which have special enzyme systems for making such gradients. Here, the gradient is supplied by the juxtaposition of the alkaline hydrothermal fluid (pH 8) with the acid ocean (pH 5), separated by the iron sulphide membrane. The hot spring thus provides a possible energy system as well as the organic feedstocks.

The ability of the membrane to separate charge also leads to the possibility of electron transfer and catalysis of redox reactions. This catalytic role for the membrane is so far untested, but the ubiquitous occurrence of iron-sulphur-linked enzymes in prokaryotic membranes, and especially in primitive prokaryotes is suggestive. Nevertheless, an overall test of the proposal will be the nature of the organic compounds which can be synthesised from credible prebiotic chemicals.

It is a long way from the iron sulphide vesicle to the first cellular life form but this proposal, while not purporting to solve the difficult issues of the origin of modern life's complex information and catalytic systems, does offer an experimentally testable proposal for the environment within which such systems developed.

Polly Carr is with Waikato University's Biological Sciences Department
Roy Daniel is with Waikato University's Biological Sciences Department.