New bacteria find yields clues to the evolution of photosynthesis
It was only a flubbed classroom experiment. But the grungy mess Prof. Howard Gest almost threw out contained one of the most remarkable bacteria yet discovered.
It may be a direct descendant of one of the earliest photosynthetic organisms. Thus it may bring new understanding of this basic biological process.
As the Indiana University microbiologist explains it, someone erred in preparing an undergraduate experiment with photosynthetic bacteria extracted from soil in front of the biology building. Gest and his assistant, Jeffrey L. Favinger, would have dumped the flask, had they not noticed something odd growing near the bottom. They investigated and isolated the ''new'' bacterium.
What makes the organism unusual is its extreme sensitivity to oxygen and its unique form of chlorophyll. Gest says the microbe was hard to isolate because, at first, they didn't know that oxygen strongly inhibits its growth. Once they learned this and took steps to protect the bacterium from oxygen, it flourished.
Further study has also shown that the bacterium's chlorophyll, unlike the usual form, lacks an oxygen atom in its molecular structure where such an atom would be expected. This, plus its sensitivity to oxygen, suggests the bacterium has preserved an early form of photosynthesis.
The process would have developed before Earth's atmosphere contained oxygen, as was the case during our planet's first 2 billion years. That would have been before green plants evolved the kind of photosynthesis that today releases oxygen to the air. ''It would be hard to say we are right at the beginning of the evolution of photosynthesis,'' Gest says. But, he adds, ''This may well be a descendant of an organism which was at an early stage of that evolution.''
Paleobiologists generally believe that photosynthesis did arise first in bacteria, enabling these simple organisms to make their own food with the aid of sunlight. Photosynthetic bacteria today can also ''fix'' their own nitrogen fertilizer from the nitrogen in the air, something many green plants cannot do. Microbiologists are thus studying such bacteria hopefully, as they look for ways to transfer this capability to crop plants such as corn or wheat.
Meanwhile, Heliobacterium chlorum (sun-green bacterium), as the Indiana researchers call their find, has been made available to other researchers through the American Type Culture Collection at Rockville, Md.
As an organism, Heliobacterium chlorum would not be a duplicate of the early photosynthetic bacterium. It is not a living fossil. But, Gest says, it may well preserve some of the ancient processes in its biochemistry. Its unusual chlorophyll, in particular, may be a chemical fossil that can help scientists study the early evolution of photosynthesis.
This accidental discovery emphasizes how little scientists still know about soil bacteria. Gest says there are probably many more photosynthetic bacteria yet to be found.
Then there is the puzzle of how an oxygen-avoiding bacterium such as this can thrive in the soil and take nitrogen out of the air. Somehow the soil protects these organisms from the bad effects of oxygen. But just how is a mystery.
Meanwhile, what of the student who messed up the experiment? Will he or she be given a good grade after all? ''Actually, we don't know which student it was, '' says Gest. He adds, ''Those things (profitable lab accidents) happen more often than laymen realize. You just have to keep your eyes open.''