Some inconspicuous free swimming branching colonies of bi-flagellated freshwater protists 

by Wim van Egmond, The Netherlands


Pseudodendromonas in dark-field illumination. 16X obj.
This little article with its huge title is about several unrelated organisms. However, they all share one important characteristic, they are all inconspicuous free swimming branching colonies of BI-flagellated freshwater protists.

One of the thrills of the use of the microscope is that you discover hidden wonders of nature. When you start exploring the micro world everything you'll observe is new and unlike anything you know. Micro life is so unlike the world we see with the naked eye that it is as if you are observing another dimension.

After some years many of these wonders become quite familiar. But there will always appear new organisms to discover. Some of these hidden wonders remain illusive because they are tiny or hard to spot because of their transparency or because they don't resemble anything living.
One such almost invisible organism I found some time ago in a fen. I used a plankton net to collect the free swimming inhabitants of the fen. I always examine my plankton catch with a low power stereo microscope. In this sample I suddenly noticed some very delicate spherical structures of several hundreds of microns in diameter. It seemed like very thin bubbles of some sort of gelatinous material. Very gently I scooped one of the spheres with a pipette and placed it on a slide. I lowered the cover slip but supported it with dots of Vaseline. This should prevent damaging the organism. I transferred the slide to the compound microscope to examine it with high power. The spheres happened to be a remarkable colony of bi-flagellated protists. Tiny cells were at the end of long thin branching stalks. I tried to find out what kind of creature it was. In Patterson's 'Free-Living Freshwater Protozoa' I found an organism that resembled my catch. It was called Pseudodendromonas but according to the book the organism is a small branching colony. The colony I found was like a complete sphere.
I think the organism is Pseudodendromonas but it could also be Dendromonas which forms a similar branching colony but has flagella of different length. Pseudodendromonas has flagella of the same length.

I photographed the whole colony with dark field illumination. It was impossible to get a good image without flattening it a bit. For a close up (40X obj. I flattened the slide more and used Nomarski to enhance the contrast.

A much more common organism is Dinobryon. Although there are some solitary species most Dinobryon species form colonies. They belong to the Chrysophytes, small fresh water 'algae'. Within the cells of Dinobryon we see a flattened yellow brown chloroplast. A red eyespot is visible near the base of the flagella. So this indicates we are dealing with an organism that uses photosynthesis. Dinobryon has two flagella, a long one and a shorter one. They live in a vase-like tube (lorica). After cell division one of the cells moves towards the margin of the lorica. There it grows a new lorica. This way a colony develops. They are not dependent on this colony. When detached they can easily form a new colony. 

Dinobryon is quite common and can be found in masses during spring (April). It has a striking appearance but inconspicuous because it is tiny. For a close look you need to use the 40X objective. The image is cropped from a picture taken with the 40X lens.

Less easy to find is Rhipidodendron splendidum. The more inconspicuous the organism, the more prominent its name! It has long been regarded a Chrysophyte but according to Patterson it is not. A quick search on the Internet tells me it belongs to the Haptophyta. This means it is related to Phaeocystis, the organism that is the probable cause of the foam on the sea shores.

Image taken with a 10X obj.

The trouble with this organism is that the colony is easily mistaken for a piece of decaying matter. But if you examine it closer it appears to be delicate flat structures made by tiny flagellated protists. The branches are flat sheets formed by thin tubes of mucus. The BI-flagellated builders of the structures are sometimes visible, their flagella protruding from the tubes.

I must admit that I always get confused by all these little flagellated protists. Many unrelated groups look very similar and you need much more patience than I have to figure out what is what. But sometimes the structures they form make it a bit easier to identify them. 

Note: I call the organisms in this article 'protists' instead of 'protozoa'. 'Protozoa' is an informal name used for many unrelated unicellular organisms that don't use photosynthesis. They are heterotrophic (get their food from organic substances produced by other organisms) Algae is used for the more plant-like unicellular organisms. These are phototrophic (they use light as source of energy). So in this case you could call Pseudodendromonas and Rhipidodendron protozoa. Or is Rhipidodendronphototrophic? Now I am confused. I give up these tiny problematic organisms. Next time I'll write about something big, like aquatic insect larvae! 

As long as they don't bite!



Patterson D.J. - 'Free-Living Freshwater protozoa' (Manson Publishing)

A. Middelhoek - Microwereld (Thieme) 1944



All comments to the author Wim van Egmond are welcomed.

Visit Wim's home page for links to his many web pages on microscopy

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