In the last entry, I raised a puzzle. Why is it that Tetrahymena cells survive and reproduce when a fresh culture contains at least 1000 cells/ml, but die when the culture only contains 100 cells/ml?

The clue was found in the following sentence:

All you need is some media, which would be a solution that contains all the ingredients needed for cell growth, and some cells, obtained from another previous culture.

Note the highlighted portion. When you start up a new culture of cells, you not only introduce the cells into the new media, but also some of the old surrounding media.

What would thus explain the puzzle is this: Tetrahymena secrete some “growth factor” into their surrounding environment, where cells stimulate each other (or themselves) to survive and/or divide. When you transfer one cell into 1 microliter, enough of the growth factor is transferred with it so the cell survives and divides. But if the same cell is put into 10 microliters, the greater volume dilutes the growth factor enough such that it is now insufficient to stimulate survival. The cell dies in a sea of food.

This, by itself, is interesting because it shows that single-celled organisms display a level of inter-dependence that would foreshadow the evolution of multicellularity. In fact, there are many examples of this. Among bacteria, there is a phenomenon known as quorum sensing , where bacteria secrete hormone-like molecules that allow them to coordinate and function as a population.

But as you might guess, this story gets more interesting than this.