I found an interesting news article, via the National Geographic website. Scientists at the Japan Agency for Marine-Earth Science and Technology wondered what would happen to bacteria when exposed many thousands of times the force of gravity. Human beings can tolerate up to 5 times the force of gravity before blacking out, and fighter pilots can tolerate up to 9 times gravity through training, acclimatization, and with the aid of special suits to prevent blood pooling in the extremities. Most animals will have similar tolerances, due to the constraints of the skeletal and circulatory systems. Eukaryotic cells can be centrifuged at several hundred times the force of gravity, but do begin to lose viability if the force becomes too great.
When several species of bacteria were put into an apparatus called an ultracentrifuge, they continued to grow and divide with little ill effect. One common soil bacterium, Paracoccus denitrificans, along with the intestinal bacterium Escherichia coli, were able to survive and thrive when exposed to 400,000 times the force of gravity. The cells would clump together under these conditions, but continued to grow.
It turns out that the prokaryotic cell structure is what enables these organisms to survive, whereas the eukaryotic cell would turn to mush under these conditions. One of the hallmarks of the eukaryotic cell is it’s compartmentalized structure, with organelles that carry out specialized functions for the whole cells. As the force of gravity increases, the organelles will sediment or clump to one side inside the cell, which prevents them from functioning normally. The ability of the whole cell to grow is negatively impacted. Prokaryotic cells, lacking organelles, do not experience this effect and can therefore continue to grow. The study does suggest, however, that some bacterial cells are better able to survive this treatment than others. I was surprised to find that the two champions of the study, E. coli and P. denitrificans, are both Gram negative bacteria. E. coli, in particular with it’s rod-like shape and relatively thin layer of peptidoglycan, is less adept at surviving physical stresses than Gram positives.
The take home message of this study, like many other topics from this course, is that the range of environments where we could look for life continues to grow as we learn more about how cells grow. Scientists estimate that the force of gravity on brown dwarf stars and massive planets such as Jupiter range from 10 to 100 times g. Our search for life in the Universe has focused on places with possibly Earth-like environments. It is obvious that our definitions of habitable environments needs to be expanded.