Vaour hosts a billion visitors in its pond

The good people of Vaour got together to save the pond at the Cite du Lac from trees that had fallen into it and the blanket of weed that had begun to choke the water. I wrote a little about their heroic efforts in another post (https://petermobbs.com/eco-2/). My guess is that when they were carrying out the work, they were thinking of the importance of ponds for insects, amphibia, mammals and birds. Indeed, ponds are important for many animals including those mammals that come to them to quench their thirst. However, as my catch-phrase says, ‘the closer you look, the more you see’ and this is particularly true of ponds and lakes.

Recently, I set up a microscope with a camera to enable me to ‘look closer’ and I decided to turn the power of microscopy to examining the minute creatures that live in the waters of the pond at Vaour’s Cite du Lac. For me, the art of microscopy and macrophotography is an escape from the madness of the human macrocosm into the microcosm where a million (a severe under-estimate!) fascinating creatures lurk unseen. As far as I am concerned, the microcosm begins where my macro lenses are no longer of use – a world where things are measured in 1/1000ths of milimetre (microns) and the creatures and plants of interest are less than about a milimetre in size. Though I had my suspicions that the the waters of my chosen pond would be teaming with life, I was not prepared for what I found. The video below shows a tiny volume of water from the pond – significantly less than 1/10th of a millilitre (100 micro-litres). The water swarms with motile and sessile single-celled organisms – protozoans, rotifers, gastrotrichs, algae, diatoms, bacteria and many more. These creature sit near the bottom of the food chain and water that is rich in their numbers and variety, though it may look brown and uninviting, is an essential food source for many larger animals. It is at this level of the animal and plant kingdom that the distinction between the two breaks down; things that are filled with chlorophyll or other photosynthetic pigments, zip around powered by their cilia; almost invisible whip-like hairs that twist to corkscrew them through the water. Unicellular organisms step outside our ideas of what is possible without being multi-cellular; how large a single cell can be, specializations that we usually attribute to things that have a brain and gut, legs and other organs.

A tiny volume within a tiny drop of pond-water. Only when you look under a microscope do you really come to understand just how full of life it is.

So, just how many creatures are there in the glorious soup of life at the Cite du Lac? I am going to do an impossible sum. It should be taken with a pinch of salt (a handful or a lorry load!) – to say it is a ball park figure is to lend it a credence that it does not deserve but then again, you have to start somewhere! I placed a single drop of pond water on a microscope slide. To be fair, this was taken from the benthos. However, I let this settle before I took my tiny sample from the clearer water. At a magnification of 100X (10X eyepiece and 10X objective combined), I gave up counting the things that moved when I reached 100. My 10X lens allows me to view an area about 1.5mm square and the drop under a glass cover-slip is 22mm square. So, my microscope allows me to view about a 1/215th of the area at one time. Thus it would appear to mean that one drop of water contains about 21,000 creatures. So, let’s go truly wild and estimate the number of motile creatures in the entire pond – I mean, why not? I am going to say the pond is 20 x 10 metres and on average 0.25 metres deep ie it holds about 50 cubic metres of water. I won’t spell out the arithmetic here but this means the pond may contain as many as 1,000,000,000 (1 billion) motile organisms not including the ones I couldn’t be bothered to count (lots!), the things that were too small, and those that did not move.

Since I plan in the future to write about some of the denizens of the pond, here I will confine myself to a very few. While I am a biologist by training, I am not an expert on microscopic life forms but I am hopeful I can say something meaningful about one of two of the creatures under my microscope. Let’s start with something that sits still – spirogyra. I admit that I am including it here because it is very easy to take nice photographs of it. Spirogyres, is a genus of green, filamentous algae of the family Zygnemataceae. There are about 300 species in this genus and I have no idea which species is in my photograph. They love poorly mineralized water that is clear, calm, sunny but always cool in spring. Their filaments are unbranched, transparent and covered with a sticky mucilaginous coating that gives them a slimy feel. They have an interesting form of reproduction – neighboring filaments act as female and others as male. Where they touch the cells involved develop tubes that extend between them and eventually the cytoplasm of each cell forms a sphere – one is motile (in effect a sperm) and makes its way through the tube to the female sphere. The motile spheres of the male filament (equivalent to sperm), after having made its way through the tube to fuse with the sphere of a female cell in the other filament, results in a spore that can resist poor conditions such as the pond drying up and then germinate when better conditions prevail.

A photograph that is the result of ‘Z-stacking’ 280 images. Each filament is about 50 microns in width. The image has been produced using ‘Rheinberg illumination’ in which the scene is lit with cone of blue light surrounded by a hollow cone of yellow light. Each of the dots within the cells is a chloroplast. You can clearly see the cellulose walls that divide the filaments into individual cells.
Stacked from 296 images. A closer look at the above. The images are created by using a 4k video camera to focus through the filed of view. The in-focus regions are then combined using a computer program.

The picture below is of Stentor sp. About 20 species of Stentor have been described but I cannot say exactly which species I have found at the Cite du Lac. Stentor is a single cell that can grow to 2 or even 3 millimetres in length. It is trumpet shaped and is sometimes referred to as the ‘trumpet animicule’. The mouth around the bell of the trumpet is surrounded by cilia that beat to create a current that brings food to its ‘mouth’. However, Stentor is not always trumpet-shaped. It only adopts this form from time-to-time. I was simply amazed to see just how quickly it could contract to form a sphere or globule. The movement probably represents some kind of escape behaviour. It can at will, also detach from the detritus or plant to which it is usually attached and use its cilia to swim away. Its cytoplasm can be seen to contain multiple nuclei – one large nucleus and many smaller micro-nuclei. Another prominent feature is the presence of a large vacuole that Stentor uses to store water. Because the cytoplasm contains various salts, water enters the cell and this must be partitioned and ejected to prevent swelling and the fatal dilution of the cell’s biochemistry. Stentor reproduces by dividing in two  but they can if cut into pieces, also regrow to form new individuals. The green colouration seen in my picture and movie is the result of symbiosis between Stentor and algae. I am frankly amazed by the biology of Stentor – particularly its rapid changes in form. I had not imagined that any single-celled animal could do anything that quickly.

A still image of Stentor in its trumpet form. This individual is about 1mm in length. The cilia that it uses to waft food particles into its mouth are clearly visible around the edge of the ‘bell’ as is the vacuole that the cell uses in osmoregulation. The cell is packed with symbiotic algae. The illumination here is ‘darkfield’ – a hollow cone of light, dark at its centre is focused on the object of interest.

https://www.youtube.com/watch?v=95t86PUUEVo

The link above is to a video I made showing Stentor making rapid shape changes – I don’t know why it will not show as an image – click on it to see Stentor in action on YouTube.

One last video showing some tiny ‘worms’ that originally, because of their size, I had thought must be multicellular. However, as I looked more closely, I could see their bodies are powered along by cilia and I couldn’t make out out any individual cells. Despite being 3 to 5 millimetres long, these are unicellular organisms that I believe to be Spirostomum sp. The genus lives in the benthos where the conditions often can become hypoxic. However, due to a unique metabolic pathway they are able to survive without oxygen. They are very sensitive to pollution and their presence in the water of the pond at Vaour indicates that it is free from toxic metals etc.

I found dozens of other species in the waters of ‘our pond’. I found myself fascinated by some of the diatoms of which more another time. For now, I cannot help from telling you an amazing fact about them. Perhaps you knew that they are in part made of glass (silicon) but I certainly did not know that through photosynthesis, they produce between 20% and 40% of the oxygen we breathe! Unfortunately, I need to perfect some quite challenging chemistry and photography before I can show their true beauty – I am working on it!

Translate »