Biochemists build case for organic life beyond Earth

Meteorites from interplanetary space continue to yield chemicals essential to organic life. Biochemists have yet to find any sign of such life itself in these cosmic ''messengers.'' But, as the latest discovery illustrates, the building blocks of life arise naturally in places other than Earth.

Last week, at a meeting of the American Chemical Society, Cyril Ponnamperuma of the University of Maryland reported that his Laboratory of Chemical Evolution has identified the five chemical units of the genetic code in the Murchison meteorite which landed in Australia in 1969. Called adenine, guanine, cytosine, thymine, and uracil, these chemicals belong to a class known as bases.

The five bases are to the genetic code what dots and dashes are to Morse code. Incorporated in molecules of nucleic acid, such as DNA, sequences of these bases specify the structure, function, and development of living organisms.

Some of the bases had been found in the meteorite before. But not all of the identifications were certain. Finding all five of them in the meteorite implies ''that chemical evolution, the natural creation of life chemicals, is a very reasonable process,'' Dr. Ponnamperuma said.

Also, in an experiment simulating conditions thought to prevail in Earth's primitive atmosphere, the Maryland research team ran electric sparks through a mixture of methane, nitrogen, and water. Again, they found all five code-of-life bases. Such experiments have been done many times. This is the first occasion, however, in which all of the crucial bases have been produced.

The Murchison meteorite and some other meteorites have already yielded amino acids, the chemical building blocks of proteins. These acids take two basic forms known as left-handed and right-handed. While living organisms use only left-handed amino acids, meteorites seemed to contain 50-50 mixtures of both types. This had raised the question of why life processes prefer the left-handed variety when both types arise readily in nature.

Ponnamperuma reported that his team has found that proteins made from left-handed amino acids function better than right-handed proteins. This may explain why evolution has chosen the left-handed variety. Also, the Maryland biochemist said his team has found two amino acids linked together - a structure called a dimer - in the meteorite. This is an example of the kind of linkage that, in living organisms, produces the long chains of amino acids which constitute proteins.

Taken together, these new results reinforce the opinion widely held among life scientists that the chemical building blocks of organic life arise readily through simple chemical processes. ''They suggest,'' Ponnamperuma said, ''that life elsewhere in the universe is more likely. . . .''

Nevertheless, Ponnamperuma and most other scientists interested in this subject readily admit there is yet no direct evidence for such extraterrestrial life. Even the scanty indication of such life which a few scientists have claimed to have found is suspect.

For example, ''evidence'' cited by British astrophysicists Sir Fred Hoyle and Chandra Wickramasinghe of the University of Wales at Cardiff is not standing up under criticism. These two scientists have compared the infrared spectra of cosmic dust with laboratory samples and have claimed to have found the ''signature'' of bacteria. However, at a meeting of the British Association for the Advancement of Science last week, Harry Kroto of Sussex University disputed this identification.

Infrared (heat) radiation from an object is more intense at some wavelengths than at others. This wavelength pattern is called a spectrum. Kroto showed that, in comparing the spectra of cosmic dust with that of dried bacteria, Hoyle and Wickramasinghe neglected eight data points in the cosmic spectra for which there is no match in the bacterial spectra. When these neglected points are taken into account, there is no reason to believe that the infrared spectra of bacteria have been found among interstellar dust, according to Kroto.

Thus the tantalizing question as to whether or not organic life has arisen elsewhere in the universe remains open. But biochemists now have little doubt that the chemical constituents of such life arise readily in nature. Water's 'fingerprint'

Water added to fruit juices, as well as the regional origins of many fruit and vegetable foods, can be detected from what amounts to a characteristic ''fingerprint'' of water.

This is the ratio of two forms of the oxygen atom called isotopes 16 and 18. The ratio of the concentration of O-18 to that of O-16 in water is highly distinctive. In any one location, it depends on the complex history of the local water - what happened to it as it traveled with the winds, fell as precipitation , and passed through soil and vegetation.

In a paper in Nature, H. Forstel and H. Hutzen of West Germany's Julich Nuclear Research Center recently described their own extensive study of this isotope ratio. They find it to be so distinctive that they say it should be useful in such fields as plant research and food analysis.

They suggest that analysts should be able to determine the region of origin of fruits such as grapes and of the juices derived from them. In particular, they say, ''the differences between the (isotope ratio) . . . of tap water and fruit juice should be large enough to detect the addition of water. Greenhouse pollution

Operators of commercial greenhouses should beware of their heating systems. In protecting crops from the outside weather, they may be subjecting them to damaging pollution from heating fuel and from chemicals used in the greenhouse.

D.W. Hand of Britain's Glasshouse Crops Research Institute says fuels burned in greenhouses often release fumes harmful to the plants. Also, solvents used in paint, chemicals that preserve wood, and some preparations used with greenhouse plastic coverings emit plant-damaging chemicals. Hand told a session of the recent annual meeting of the British Association for the Advancement of Science that growers should be sure to use sulfur-free fuels and ventilate their greenhouses well.