Biology concepts – Protista, taxonomy, phylum, kingdom,
monophyletic, paraphyletic, cladistics, algae, diatom, dinoflagellate
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Euglena gracilis is an organism in the
Kingdom Protista. It has
one
long flagellar undulipodium, but it can also move by
amoeboid movement. It has chloroplasts and can do
photosynthesis,
but it also can eat other organisms. Is it any
wonder
that classifying protists is so hard?
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The best example of the inanity of classification is Kingdom Protista. The word means, “the
very first,” probably because it is supposed that these were the first
eukaryotes. How do we define the organisms of this Kingdom? The best we can
manage is to say that they are the eukaryotes that aren't animals, plants, or fungi. Really, is that the best we can
do?
In a perfect system, all the organisms of one kingdom would
be descended from a single common ancestor (be monophyletic, mono = one,
and phulon = tribe). But it don’t
work like that. And this is where Kingdom Protista serves as a good example.
There are protists that look a lot like animals, those that
resemble plants, and those that share features of fungi. No way did they all
come from a single ancestor. Protista is a paraphyletic
(para = near) kingdom, the group may exclude a member with a common ancestor.
As such, the protists are a catch-all, those that don’t fit in some other
kingdom. Protists are like pornography – hard to define, but you know it when
you see it.
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Classification
isn’t perfect, some groups come from
different
ancestors. A shows a group (in yellow) that
is
monophyletic, they all come from one ancestor.
The
paraphyletic group (B) shows that some groups
can’t
include all descendents of a common ancestor.
And
if a group is made from descendents of different
ancestors,
it is called polyphyletic (C).
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You might do it by common ancestor; let their genes do the
talking. We are learning more and more about who begot who – this is the study
of cladistics. But if you break down
protists into their clades – they don’t seem to make sense. Organisms that look
or act similar might be in different clades, with wildly different organisms
linked close together.
Alternatively, you might divide them up based on the characteristics, as
Linneaus did - the animal-like protists in one phylum, the plant-like
protozoans in another. But this may separate genetically related organisms into
very different phyla. Same problem. How about by the way they get around? Some
use flagella-like undulipodia, some use undulipodia called cilia, some use
cytoplasmic crawling called pseudopodia, and others are immotile. Again,
disparate organisms may be lumped together just based on their preferred mode
of travel.
The idea of the "phylum" is to place the organisms in
categories so that they are “more related to each other than they are to any
other group.” Wow, that sounds scientific. Related based on what? We just
discussed motility, genetics, and physical characteristics or behaviors.
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The
Kingdom Protista sits between the modern plants,
animals,
and fungi, and the ancient prokaryotes. As such,
they
end up being a catch all group. The right image
shows
how some people group the protists based on
undulipodia
characteristics, not ancestry.
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So we guess. And then we change things as new information becomes available. The work never ends, and the students never get to just memorize the categories.
As of today,
some scientists classify protists based on a combination of the characteristics
above. In the system I like best, there are 15 phyla, and we can roughly divide them as we show below. But
there are six different phyla just for the protists that perform
photosynthesis! The reason I like this system best -it roughly mimics the way they use undulipodia. And this is what we’re
interested in today.
Kingdom Protista contains the organisms that seem to have
made the most obvious uses of undulipodia. Eukaryotic flagella and cilia abound,
some protists have both, and some have them only part of the time. There are six
phylums of plant-like protists. Many
have flagella, none that I could find have cilia. Here are some examples:
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Pyrrophyta
organisms will bloom and then bioluminesce
in order
to scare predators away. Movement in the water
causes
vesicles in the dinoflagellate to rupture via action
potential
and release the reagents to make light. It’s exactly
the
same system that fireflies use.
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Phylum Euglenophyta
– This phylum includes the Euglena
gracilis organism shown in the animation at the beginning of the post.
These protists also have two flagella, but one of them is reduced and doesn’t
stick out. They have an eyespot, perhaps the genesis of our own eye. The
eyespot helps them to move away from strong light sources, sources that would
overheat them.
Euglena are common model organisms, on this world and in
(near) space. They traveled on the parabolic flights to have their flagellar
motions studied in zero gravity. The 2010 publication that resulted from the
experiments showed that the process of beating is regulated and physiologic, as
the change from hypergravity to microgravity stopped the flagellum from moving.
The opposite change in gravity reoriented the cells and they started swimming
to the bottom of their tank again.
The remaining phyla of plant-like protists can be included
in a supergroup called the Chromista
(colored organisms). In terms of
their undulipodia, the chromists tend
to have two flagella, one on each end. The forward flagellum is usually longer
and has lateral growths called mastigonemes. The best description for this type
of flagellum is that it looks like Christmas tinsel. The back flagellum is shorter and smooth.
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These
are the phyla of the Chromista; the colored protists.
Problem
is, not all of them are colored and some colored
protists
aren’t included in this group. Top right and bottom
left
are the chlorophyta, the green algae. These are the most
recognizable
algae. The bottom right is the diatoms, they have
the
most interesting shapes. Look them up.
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Green algae are the ones we recognize; they belong to the Chlorophyta phylum. These
are the ancestors of the land plants, and some have flagella in all stages,
while others only have flagellated gametes. We will see soon how some land plants still have flagellated
gametes.
Brown algae belong to the Phaeophyta phylum. They are exceptional amongst the
protists because every organism in
this phylum is multicellular. No brown algae live as individual cells. Kelp is
an example of brown algae. Kelp forests are multicellular example of brown algae
thalli, growing to 40-60 m (130-200 ft) in height! Kelp forests are some of the
most productive ecosystems on earth.
The gametes of the brown algae are flagellated like in most
of the other chromists. A 2014 study has started to look at the flagella of the
chromists, using brown algae as the model organism. The study found that the flagella have functions in
motility, signal transduction, and even metabolic activities. The two different types of flagella had
common proteins and proteins specific to each form, for a total of 495
different proteins associated with flagellar function and structure. For
instance, only the posterior flagellum has a protein that senses blue light,
and may be used for steering the organism.
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The
Rhodophyta are where we get food stuffs. On the left
represents
agar that can be used to make things that are like
Jello,
it fills the role of the gelatin. In the middle, agar is also
used
as a polysaccharide source of nutrition for growing
bacteria
in the lab. On the right, nori is a rhodophyta
seaweed
used in sushi.
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If you’ve eaten Japanese sushi rolls, then you’ve eaten red
algae in the nori that the rice and fish are wrapped in. Nori is made from
several species of red algae of the genus Porphyra.
Not a sushi fan? How about ice cream? Carageenans that make ice cream smooth
also come from red algae.
Ice cream is reason enough to love the red algae, but
there’s more. A 2014 study indicates that one compound found in the Porphyra is a strong antibiotic. Studies
of 1,8-dihydroxy-anthraquinone from this red algae genus can disrupt the cell
wall of Staphylococcus aureus. This
is hugely important, since many strains of S.
aureus (like MRSA and VRSA) are now resistant to most existing antibiotics.
Rhodophyta algae are red because although they use some
chlorophylls for photosynthesis, they also use phycoerythrins and phycocyanins.
Interestingly, these are the same pigments that are present in the
cyanobacteria. This suggests that there is an ancestral link. The link is
supported by one other factoid. Both cyanobacteria and red algae lack
undulipodia!
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The
seaweed Rhodophyta organisms often live in the tidal
pools.
The spongy material in the stalks and “leaves” is the
agar
and is related to the mucin product that attaches to the
male
gametes as they are released. I couldn’t find a picture of
the gametes with their mucin tails, so this will have to do.
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Whatever it is, the system seems to work. A 2014 study found
that fertilization success was dependent on male organism biomass, but neared
100% when there were relatively few male gametes present. This was hypothesized
to be possible because low tides in the tidal pools where the organisms live
greatly increase the chances of male/female interaction and fertilization.
Seaweed takes advantage of the moon’s effect on the tides to ensure reproductive
success – who needs flagella!?
So far we have met protists that use flagella at some point
in their life cycle (except for the red algae). Notice that none of them have used cilia. Next week, how about the
animal-like protists? I bet there are some
exceptions there as well.
For
more information or classroom activities, see:
Kingdom
Protista –
Euglena
–
Red
tide –
Pyrrophyta
–
Kelp
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