It's been nearly a dozen years since I came to America from Japan. I nostalgically recall the hot springs there, with people relaxing in the steamy water, as well as wild monkeys sinking down to their neck in the pools, sporting frozen whiskers and heads covered in snow.
But there is a lot more life in Japan's hot springs than human beings and monkeys. I would like to describe some organisms that frequent these hot springs, not for relaxation, but because they actually require such hot springs (or a similar environment) to survive.
Observations of bacteria-like organisms in hot springs were made early in the 1900s, but their isolation, cultivation, characterization, and biochemical study only really began in the 1960s. The organisms currently holding the record for living at the highest temperatures have been discovered in geothermal springs in the ocean. Some examples are Pyrolobus fumarii (235 F), Pyrodictium occultum (230 F), and Methanopyrus kandleri (230 F). They belong to a group of organisms called archaea that look like bacteria, but have features of both prokaryote (non-nucleated cells, such as bacteria) and eukaryote (nucleated cells, such as found in animals and plants) life.
Archaea is considered one of the most primitive groups of life on the Earth today, partly because when life started, the Earth was much hotter than now, and only organisms like archaea could have existed. For this same reason, many scientists think that thermophilic archaea are the best examples of what life was like on the Earth billions of years ago.
My favorite archaeon is called Sulfolobus shibatae, which was isolated from hot springs in Beppu, Japan. It grows best at 181 F, and requires highly acidic (pH 3.0) conditions. We grow Solfolobus in glass flasks in the laboratory, but in the hot springs they grow sticking to the surfaces of rocks and sand. In the real world temperatures fluctuate from day to night, and from summer to winter. These fluctuations are the most common type of stress for an organism like Sulfolobus shibatae, which does not have any way of controlling its body temperature (as we can). How does Sulfolobus shibatae cope with the big temperature fluctuations in its environment?
Life has evolved mechanisms to expand the limits of tolerance to extremes of temperature. At 50 degrees above its normal growth temperature, 80% of Sulfolobus cells die in two hours. However, when the temperature is kept at higher than normal, but lower than lethal, temperatures for one hour before raising it to the lethal temperature, 50% of Sulfolobus cells survive at the lethal temperature for five hours. This phenomenon is called acquired thermotolerance, and it is observed in all kinds of cells, including common bacteria like Escherichia coli and human cells grown in the laboratory.
My work includes the study of what Sulfolobus cells do to cope with exposure to lethal temperature. One of their adjustments is to modify and remake the lipids in the cell membrane to produce forms that are more stable at higher temperatures. Another is to produce a large amounts of a specific kind of protein called heat shock proteins. Their function is still under investigation. Some researchers think the heat shock proteins bind to other cellular proteins and prevent their deterioration by heat. Our research group believes that one of the heat shock proteins made by Sulfolobus shibatae stabilizes its membrane by binding to it and reducing its permeability, like plastering holes in a wall.
Life flourishes in many environments, including some in which human beings could not possibly survive. To discover extraterrestrial life, it will be useful to know the physical and chemical limits for life on Earth and the underlying biophysical laws. This is a good way to start, but we should keep in mind that the biophysical laws on Earth for extraterrestrial life could be significantly different.