What Makes Protists So Distinctive in the Microbial World?

[TECHXHEAD]

Protists are generally defined by what they are not than by what they ARE. They are not monophyletic clades, they do not belong in other eukaryotic kingdoms, they contain much more diversity.

They can exist as algae, protozoan, absorptive, forms. As well as photoautotrophs, chemoheterotrophs, and mixotrophs. They are extremely diverse in terms of form, nutrition, and asexual reproduction.

So what is their distinguishing trait? What differentiates them and why are they defined in this matter? It is somewhat vague that they are described by what they are not than by what they are.

 

[jim mcnamara]

You missed the point, I believe. They are a classification of convenience. Everybody knows they do not fit together any which way. When I used to teach this I got questions like this. There is no solid defensible postion other than to say Ernst Haeckel created the protista and now we put clearly unrelated oddballs in there. It is a taxonomic trashbin.

Otherwise, some species/groups in there would be in their own kingdom all by themselves. Would you like to learn that there are ~67 kingdoms and here are there names? I do not see what purpose that would serve. As long as you mention that Protista are truly a dumping ground because we cannot deal with them, what harm?

My personal take is that many are sole survivors of unique phyla the evolved in pre-Cambrian times. We lost all of their antecendents 500 MYA. Blame it on 'white earth'. :)

 

[Spirochete]

I'd like to hijack this thread with a related question:

I also recently learned about protists being the "default" organisms in my General Bio 2 class last semester. We aso learned very specific definitions for an animal and a plant. For example: An animal is a sexual organism that reproduces with numerous small motile, sperm and larger, less numerous non motile eggs. Animals also develop from zygotes which divide to form a blastula which develops into a gastrula.

But this semester in general microbiology my professor referred to algae as plants and amoebas as animals, throwing all my painstakingly learned definitions out the window! I briefly asked him to clarify this after class and he reaffirmed what he said in lecture. This guy has a phd and is head graduate student advisor.

Could anybody offer any clarification on this?

 

[jim mcnamara]

You know in Math there are axioms - statements you assume to be true, and that you choose as a starting point to prove theorems.

In Biology there are almost-axioms are called definitions. These are largely observationally derived. Math axioms are not derived by observation AFAIK. Definitions like these are part of so-called alpha science - the science of describing everything out there so we can make sense of it all.

While there are excellent definitions of 'species', 'taxonomy', 'plant', and 'animal', not everyone uses exactly the same one. When that happens you get slightly different results downstream.

Why is this? Simply because for every definition there exists at least one or more species that break any definition of a phylum or a species you choose. These species that don't follow the axioms we used to decide. So you pick a definition that works for you.

There is no perfect definition. Some species break all rules. Example: switchgrass (Panicum virgatum). It ain't a species, at least not by any definition of a species I have seen. But we still call it a species. Because that is how we deal with everything else.
Square peg in round hole problem. This happens a lot in Biology.

Your prof is using the animals='eurokaryotes that do not do photsynthesis' definition. And
plants='eukaryotes that do pefrform photosynthesis''

This is common in Science, and as long as you are upfront about it nobody will complain - unless of course you violate your own definitions.

And the things you cited - amoebas and single cell algae like Chlorella really mess up our nicely man-man contrived taxonomic cubby-hole system. So we either call them plant and animal (old-fashioned) or protists (new-fangled). You pick.