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Pond Life

Microbiology is the study of organisms which are too small to see with the unaided eye. This includes unicellular prokaryotes (bacteria), but also a few of the smallest multicellular eukaryotes. Many of the multitude creatures which can be observed in a drop of water from a pond or an aquarium blur the boundary between microbiology and biology - you can see them, just, in the right lighting if your eyesight is good - but you can't see any detail without using a microscope.

So what sorts of organism live in a drop of water?

Here are just a few:

Amoebae:

While a few types of amoebae (such as Acanthamoeba and Naegleria)) are pathogens, Hartmannella is a harmless, free-living organism belonging to the so-called limax - "slug-like" - amoebae (watch the video to see why!). Older isolates of Hartmannella suspected of being opportunistic pathogens have now been reclassified as Acanthamoeba. It is capable of encysting, forming the round, resistant cells seen at the beginning of the video. After the amoeba hatches from the cyst it becomes mobile:

 

Ciliates:

Ciliates are single-celled protozoans which can be recognised by many hairlike cilia on their surface. The beating of these cilia are the means of propulsion for these highly motile cells. Most ciliates are quite small, e.g. Paramecium, a typical ciliate, is about 0.3 mm long. However, Stentor is a giant among ciliates. It expands and contracts its tubular body, but at extension, can be over 2 mm long - quite capable of eating smaller ciliates, flagelates and even small rotifers. Stentor moves by looping movements of its flexible body and generates strong currents using the cilia surrounding its mouth:

The long, angular objects being sucked into Stentor's mouth in the second part of the video are desmids, single-celled green algae common in freshwater. Yum yum, lunch!

 

Flagellates:

Naegleria gruberi is an amoeba-flagellate, so called because the organism can exist as an amoeba and also as swimming flagellates. The differentiation of Naegleria provides a valuable system for examining a number of interesting problems in eukaryote cell and developmental biology. Some of the advantages of the system arise from the ease and speed with which cells can be grown and induced to differentiate. Another important advantage is that the differentiation of amoebae into flagellates is relatively synchronous. This means that following the population of amoebae as they differentiate into flagellates is similar to following the changes in a single cell:

 

Rotifers:

Rotifers are incredibly variable in shape, ranging from worm-like animals to many very complex forms. There are over 2500 species known and they are common in aquatic habitats worldwide, mostly in freshwater but also to some extent in marine environments. All rotifers are characterized by a single, double or sometimes multiple crown-like "corona" of cilia which appear to rotate, which is how they derive their name, from Rota, Latin for wheel. These are multicellular organisms, and hence are members of the metazoa, rather than the unicellular protozoa. Most rotifers are less than 0.5 mm long, and feed by generating currents in the water using their beating cilia. The vortices formed can clearly be seen in this video of the rotifer Brachionus:

Rotifers attach themselves to a substrate by means of a stalk. Some are sessile (remain permanently attached and do not move), but other hop around - Brachionus is quite mobile and difficult to video! The two large dark disks attached to the base of the rotifer in this video are egg sacs.

 


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