Re: How does a Protist catch the bacteria on which it eats ?

Amanda:
   This is a great question.  Not many people are interested in these 
smallest of protists; (they tend to measure only 10 um or less, less than 
one-tenth the width of one of your hairs) yet they do some wild things.  
For example, a little flagellate called Epipyxis that lives in lakes and 
ponds uses its two hairy flagellum like chop sticks, grabbing bacteria and 
stuffing them in a tiny mouth.  The mouth itself only opens on command, 
when an internal microtubule attached to the base of the flagellum slides 
against another microtubule and forces a loop of the microtubule to push 
against the side of the cell, causing a pocket (mouth) to open up.  When 
the bacteria is placed into the pocket, the microtuble tightens, forcing 
the mouth to snap shut. (A picture of this would be more helpful; I could 
mail you one if you send your home address to: djacobson@whitworth.edu)

(Microtubules are the bones of the protist, except they are more flexible 
than your bones.)

In the ocean, flagellates called choanoflagellates have a single flagellum 
(which pushes water past the cell from the rear). The one flagellum is 
surrounded by a collar-like ring of tiny finger-like projections called 
microvilli.  The microvilli are sticky, like velcro, so when a bacteria 
bumps into them it sticks; then the microvilli swells up and swallows the 
bacteria (kind of like an amoeba's pseudopod).  Incidently, the 
choanoflagellates protect themselves by building a cage of silica rods that 
looks very much like a barrel-shaped play-ground jungle gym (pretty clever 
for a single cell!)

Some of the most common bacteria-eating protists are ciliates and amoeba 
that live in the soil, puddles, ponds and lakes.  Ciliates usually have a 
distinct funnel-shaped mouth surrounded by rapidly waving cilia that push 
water (and bacteria) into their mouths.  Other ciliates have a flat 
underside so they can rub closely against rocks or algae and bite off 
bacteria attached to these surfaces.  Some even have teeth!  The teeth are 
stiff rods made of microtubules, and can number from 2 to a dozen or more, 
arranged in a cylinder.  These "teeth" can protrude from the mouth and 
pluck up bacteria.

Amoebas are less fancy... they just crawl along, surrounding and engulfing 
bacteria with their oozing pseudopodia.

I hope this helps... feel free to send my an email if you would like some 
illustrations (I wrote and illustrated a short textbook, not yet published, 
that explains the feeding behavior of these protists)

Cheers,
Dean Jacobson, protist watcher