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Thread: What is a Protist?

  1. #1 What is a Protist? 
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    I am trying to write an essay (for my own benefit, not for homework) on some groups of micro-organisms. One of those groups is the Protists. Whilst information about specific protists is quite freely available, I am struggling to come up with a definition of what a protist is.

    For my purposes a definition would have to do two things: Include all those things that are protists, and exclude all those things that are not protists.

    What I have so far is as follows: Some are unicellular, some are multi-cellular but without specialised tissues but beyond that the definition of a protist is not clear. So I am going to take the path of least resistance and avoid any attempt to define a protist other than to say that it is probably but not necessarily single celled, it is almost certainly a eukaryote without specialised tissues and that it is not an animal, plant, or fungus. The group includes algae, amoeba, diatoms, kelp, plankton, protozoa, and slime moulds.

    Is it possible for me to improve on this?

    I am also a little confused as to whether chromists, halophiles and methanogens are considered protists.


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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Methanogens are archaea not protist, chromist are protist. Halophile is a descriptive term, any organism that can live in high salt is a halophile, most of these are archaea but there are some protist and bacteria that can be halophiles.

    The idea of a kingdom Protista has become antiquated, most of the organisms that are classified as protist have very little to do with each other.

    Generally the protist are usually classified as the protozoa, protophyta, and slime molds. The best definition of protista is a single celled eukaryote that lives individually or in a colony without tissues. This definition overlaps with fungi, because single celled yeast are sometimes classified as protist,


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    Quote Originally Posted by i_feel_tiredsleepy
    Methanogens are archaea not protist, chromist are protist. Halophile is a descriptive term, any organism that can live in high salt is a halophile, most of these are archaea but there are some protist and bacteria that can be halophiles.
    Thank you, that clears up a lot of confusion in one fell swoop. Would it be expecting too much to ask if there is a definition of "high" salt? Suppose we say that the ocean is an "average" salt environment, then would say, twice that be considered "high"?

    Quote Originally Posted by i_feel_tiredsleepy
    The best definition of protista is a single celled eukaryote that lives individually or in a colony without tissues.
    Since chromists are now protists, does that definition actually cover the 50 metre-long giant kelp? I would find it hard to view that as a "colony without tissues". Mind you, that would have to depend on your definition of "tissue".
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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Quote Originally Posted by numbers
    Thank you, that clears up a lot of confusion in one fell swoop. Would it be expecting too much to ask if there is a definition of "high" salt? Suppose we say that the ocean is an "average" salt environment, then would say, twice that be considered "high"?
    Wiki says it's defined as 5 times the concentration of the ocean. I'm not at home at the moment so I don't have access to a text book to back that up.

    Quote Originally Posted by numbers
    Since chromists are now protists, does that definition actually cover the 50 metre-long giant kelp? I would find it hard to view that as a "colony without tissues". Mind you, that would have to depend on your definition of "tissue".
    That is indeed a difficult thing to define, but that is the major problem with taxonomy, life doesn't exist in fixed categories there are always going to be intermediates and borders where the definitions can be blurred.
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    Quote Originally Posted by i_feel_tiredsleepy
    ...life doesn't exist in fixed categories...
    Thank you. That is probably the best answer I could have hoped for, and I will do well to remember that as I proceed with the rest of my essay.

    Thank you very much.
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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    No problem, I checked an old text of mine, and it defined a halophile as an organism capable of thriving in 2 or more M NaCl
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    Quote Originally Posted by numbers
    Quote Originally Posted by i_feel_tiredsleepy
    ...life doesn't exist in fixed categories...
    Thank you. That is probably the best answer I could have hoped for, and I will do well to remember that as I proceed with the rest of my essay.

    Thank you very much.
    That's a very good way to think, Numbers. Nice one.

    The one category that does work (as far as we know, even though no model of criteria will be perfect) is the one of descent (or the cladistic one) that is now the primary model in taxonomy.

    Unfortunately for the purposes of your essay, in cladistic terms the notion of a 'protist' is ill-formed - there is no single clade that includes all so-called protists and yet excludes all non so-called protists.

    As a result, the category of protists is a bit like the category of 'fish' - so ill-defined that it begins to lose scientific utility.

    One, slightly better defined 'group' (albeit not a clade), could be that of unicellular eukaryotes: amoebas, paramaeciums and so on. They tend to overlap quite a lot with the old protist category, but bear in mind that they are not well-defined, since multi-cellular eukaryotes (and in particular those described as mega-eukaryotes) have arisen from various branches of the eukaryote clade - so we humans are more closely related to some single-celled organisms than we are to, since you brought it up, kelp, or even plants and fungi.

    To that extent you've set yourself a tricky task, but all the best with it.

    cheer

    shanks
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