Siegel et al. ([ref.73]), in approximate simulations of Martian environments, studied tolerances of certain seed plants, such as cucumbers, corn, and winter rye, to low temperatures and lowered oxygen tensions. Lowered oxygen tensions enhanced the resistance of seedlings, particularly cucumber and rye to freezing, and lowered the minimum temperature required for germination. Germination of seeds in the absence of liquid water has also been studied. In this case, seeds of xerophytes have been suspended in air at 75-mm Hg pressure above water. The air was thus saturated. Germination was slow but did occur.
Siegel et al. (refs. [ref.73] and [ref.74]) found that the growth rate of several higher plants was enhanced by certain gases usually thought to be toxic, such as N2O. This finding is significant inasmuch as the presence of nitrogen oxides in the Martian atmosphere has been cited as evidence for the nonexistence of plants on that planet by Kiess et al. ([ref.75]). Exploratory survival tests showed that various mature plants, as well as the larvae, pupae, and adult specimens of a coleopteran insect, were undamaged when exposed to at least 40 hours of an atmosphere containing 96.5 percent N2O, 0.7 percent O2, and 2.8 percent N2.
Lichens are of interest because of their ability to survive and thrive under extreme environmental conditions on Earth. Biological activity of slow-growing lichens was detected by metabolic gas exchange, CO2 detection being especially convenient. Siegel points out that this method is sensitive and nondestructive, to be preferred to staining techniques, which at present are limited because they are only semiquantitative, subjective, and destructive of the lichen.
A Russian study of simulated planetary environments has been performed with good simulation but for periods of only 2 to 6 hours. Comments on simulation experiments made by Zhukova and Kondratyev ([ref.69]) are presented as follows:
On the basis of modern conceptions on Martian conditions it is difficult to imagine that higher forms of animals or plants exist on the planet. A Martian change of seasons similar to that of our planet empowers us to think that there is a circulation of an organic substance on Mars, which cannot exist without participation of microbic forms of life. Microorganisms are the most probable inhabitants of Mars although the possibility is not excluded that their physiological features will be very specific. That is why the solution of the problem concerning the character of life on Mars is of exceptional interest. But still the answer to this question can be verified only by simulating Martian conditions, taking into account the information obtained from astrophysicists.
Experiments aimed at creating artificial Martian climatic conditions have been started quite recently; their number is not large since they cannot be combined with the results of numerous experiments investigating the effect of extreme factors on microorganisms. The result of the effect of such physicochemical parameters of the medium as pressure, sharp temperature changes, the absence of oxygen and insolation, depends on their combination and simultaneity. These examples convincingly show that while simulating Martian conditions one should strive to the most comprehensive complex of simultaneously acting factors. The creation of individual climatic parameters acting successively leads to absolutely different, often opposite results. It should be mentioned also that refusal to imitate insolation and the performance of experiments with specimens of soil which itself has protective effect on cells of microorganisms, but not with pure culture of bacteria, are usual shortcomings in the bulk of studies on this problem.
It appears that organisms from Earth might survive in large numbers when introduced to Martian environment. Whether these organisms will be capable of growth and explosive contamination of the planet in a biological sense or not is highly questionable. The likelihood of an organism from Earth finding ideal conditions for growth on Mars seems extremely low. However, the likelihood of an organism from Earth serving as a contaminant for any life-detection device flown to Mars for the purpose of searching out carbon-based life is considerably higher. The chance that life has originated and evolved on Mars is a completely separate question and much more difficult to answer.
It would be interesting to attempt to determine possible evolutionary trends which might occur on a planet by means of selection of organisms in a simulated planetary environment. Rapid genetic selection combined with radiation and chemicals to speed up mutation rate under these conditions should reveal possible evolutionary trends under the planetary environmental conditions. This could be attempted after the planetary environments are more accurately defined.
EXTREME AND LIMITING ENVIRONMENTAL PARAMETERS OF LIFE
The question of the existence of extraterrestrial life is one of the most important and interesting biological questions facing mankind and has been the subject of much controversial discussion and conjecture. Many of the quantitative, and even qualitative, environmental constituents of the planets also are still subjects of controversy and speculation. Best guesses about a relatively unknown planetary environment, combined with lack of information about the capabilities of Earth life to grow in extreme environments, do not provide the basis for making informed scientific estimates.
Life on Earth is usually considered to be relatively limited in its ability to grow, reproduce, or survive in extreme environmental conditions. While many common plants and animals (including man) are quite sensitive to, or incapable of, surviving severe chemical and physical changes or extremes of environment, a large number of micro-organisms are highly adapted and flourish in environments usually considered lethal. Certain chemoautotrophic bacteria require high concentrations of ammonia, methane, or other chemicals to grow. Anaerobic bacteria grow only in the absence of oxygen.