I previously corresponded with a science teacher in India, who wrote me with some questions about Volvox. After our initial exchange, my correspondent wrote

Can you please name if there is any unicellular colonial microorganism found?

I asked for clarification and received this reply:

I read about colonial organisms being unicellular and multicellular. Few people think Volvox as colonial organism which is unicellular while Phylum Bryozoa has colonial organisms which are multicellular. The confusion started here. What are colonial microorganisms really? If they are unicellular and multicellular why are they called as colonial then? Bacteria being unicellular which form colonies thought Can bacteria be called as colonial organism? I tried to look for the same but I have not found something solid which says bacteria can be called as colonial organisms. I want to explain colonial organisms to children and don’t want to provide wrong information.

Can you please help in understanding do colonial unicellular microorganism exist? I asked one of the microbiologist I know in here she is also not clear with the concept or probably I might have read something wrong. Need guidance.

As before, if you have anything to add to my reply (or to argue against it), feel free to leave a comment:

Hi Subhashini,

Okay, that makes sense. I think most people regard colonial microbes as intermediate between unicellular and multicellular, with cellular differentiation as the dividing line between colonial and multicellular. For example, these are the definitions applied in the attached paper. By that view, colonial microbes are those with more than one cell, but in which all cells are essentially identical. Tetrabaena would then be a good example of a colonial microbe.

Of course, nothing in biology is quite so simple; in reality there is a whole spectrum with different degrees of complexity and integration between unicellular and multicellular, as the volvocine algae show.

Whether or not bacteria can be considered colonial organisms is a difficult question, one that biologists and philosophers have debated (for example, see https://freethoughtblogs.com/fierceroller/?p=819). It certainly seems to me that some bacteria can be colonial, and some even multicellular (for example, filamentous cyanobacteria with heterocysts).

Take care,

Matt