Extraterrestrial microbiology.

I N T H E E F F O R T of devising ways and means of studying the existence of living organisms on other planets, attention must be focussed on microorganisms. This is not entirely due to professi,onal prejudice of microbiology, but is the consequence of accepted realization that microorganisms are responsible for the major amount of turnover of matter on earth and that life of the higher plants and animals is inconceivable in the absence o,f microorganisms. TO cite only a few major examples, it must be remembered that 80 per cent of all photosynthesis, that is, 80 per cent of the total amount of oxygen released into the atmosphere is carried out by oceanic microorganisms, and 90 per cent of the total carbon dioxide released into the atmosphere is brought forth by the actions of microorganisms. Without microorganisms the nitrogen content of the soil would not be replenished, nor would sulfur be available in a biologically useful form. It is apparent then that, even in the absence of higher plants and animals, the basic ecology of the earth would not be changed. Since it is therefore possible