19 April 2010

Why are there no single-celled terrestrial organisms? Part 1

In this post I discussed what its means for an animal to be terrestrial. All animals (Kingdom Animalia) are, by definition, multicellular, while the unicellular organisms belong to the Kingdom Protista. As far as I know there are no terrestrial protists.

In the present post I will give some reasons why I think there can be no terrestrial protists.

1. Unicellular protists tend to be very small for the obvious reason that they consist of but one cell. The ratio of the surface area to volume of an object increases as the object gets smaller. Therefore, a unicellular terrestrial organism would have a very large surface area to volume ratio and consequently great difficulty to prevent water loss thru its membranous boundary with the external environment. An impermeable coat could prevent water loss, but it would then be difficult for the protist to receive the required oxygen and food.

2. How would a unicellular terrestrial organism reproduce? Division or the production of any propagule would further decrease size and increase the surface area to volume ratio and the rate of water loss.

3. It would actually be quite difficult for a microscopic unicellular terrestrial organism to free itself from water droplets that it might come in contact with. Water tension would probably be an insurmountable force at such tiny dimensions.

Disclaimer: As I point out occasionally, many ideas that I post here for the whole world to read are often of the half-baked kind. The present ones may not be exceptions. If you intend to cite or steal them, do at your own risk.

Part 2


Gin said...

Of course, many unicellular "terrestial-aquatic" organisms (e.g., bacteria, yeast and other fungi) can form spores that protect them from dessication, heat, and bad chemicals to survive for years or millenia. You might not consider this living, but do we really know what goes on inside that capsule? :) Clearly they're not dead.



Of course, the spores & cysts are alive. It couldn't be otherwise, because death is irreversible. However, even though such resistant forms may survive in the absence of water, they don't become active again until they are immersed in water. Therefore, they are still aquatic organisms.