Life in a few Drops of Water: Desmids

This is the second in the series of images of some of the organisms found in just a few drops of water collected from a pond in a disused quarry on the edge of the moors in Weardale.

This is  a desmid - probably a species of Cosmarium. Desmids are typically constricted in the centre of the cell to form two mirror-image halves.

These are single-celled, photosynthetic algae that often have a patterned cell wall that's ...

.... most clearly visible after the cell has died and lost its chlorophyll.

This appears to be one half of a desmid that has broken at the bridge joining the two halves (known as the isthmus), revealing the fractured hole.

Coming next: Dinoflagellates

Pond scum

Three weeks of warm weather had left my pond covered with large slimy masses of 'blanket weed' or 'pond scum', the filamentous green algae that tend to plague ponds that have too much nitrogen in the water. When I'd fished most of it out I took a look at a few filaments under the microscope and - like so many living organisms - it revealed structures of great beauty when it was magnified a few hundred times. Inside each cell in the filament the chloroplasts were arranged like strings of green pearls. Various filamentous algae have chloroplasts in different conformations and the most familiar is the spiral chloroplast in Spirogyra..... but this is a different genus.....

The series of fine rings that you can see around the bottom of the upper cell on the left here, just above its junction with the cell below it, identify this alga as a species of Oedogonium. A ring forms each time a cell of this genus divides, so this cell appears to have divided three times.

In amongst the algal filaments there were also desmids - this crescent moon-shaped example is Closterium. The clear areas at the tips of the 'moon' are vacuoles, that contain insoluble crystals of calcium sulphate - a diagnostic feature of this genus.

The most interesting alga in my pond, however, was this one - Coleochaete. It may look like just a pad of simple cells (with some of them apparently dead) but this is an organism of great evolutionary significance. Modern molecular biological studies, and comparative investigations into the ways in which cells divide in this species and in land plants, indicate that Coleochaete shares a common ancestor with present day land plants - mosses, liverworts, ferns, conifers and flowering plants. At some point - maybe half a billion years ago - algae like this, perhaps living in a warm pool of nutrient-rich water like my garden pond, started to colonise the mud and begin the long series of evolutionary changes that led to the development of today's terrestrial vegetation.

A discovery like this makes the chore of cleaning out the garden pond a whole lot more interesting.......... 

Swallowing a Brick

Microscopic photosynthetic organisms called desmids and diatoms live in vast numbers in the surface plankton of lakes and oceans, where they are responsible for absorbing about 20% of the atmosphere's carbon dioxide and releasing oxygen, all the while acting as the foundation of the food chains for most aquatic animals. The top image here is of a desmid called Euastrum. Desmids are beautifully shaped, bilaterally symmetrical organisms with a distinct waist that divides them into two mirror-image halves. The next two images down show a diatom called Tabellaria. Diatoms are like microscopic pill boxes made of silica, and when the time comes to divide the top half forms a new bottom and the bottom half produces a new top and then – hey presto! – two perfect copies of the original. Tabellaria forms zig-zag chains of cells, joined at their corners. The glassy cases of diatoms are decorated with the most beautifully sculptured patterns, that are best appreciated at high magnification under an electron microscope. You can see some examples and read more about them at
http://www.ucl.ac.uk/GeolSci/micropal/diatom.html

The fourth image down shows another diatom, this time brick-shaped, and the final image shows a minute protozoan that has somehow managed to ingest one of these – you can just about make out the diatom’s outline inside the protozoan cell, which has a fringe of beating cilia like a monk’s tonsure which it uses for propulsion. Ingesting something this size is quite a feat, roughly equivalent to you or I swallowing a brick, and the diatom’s silica case is just about as digestible as a brick, so once the contents have been digested the diatom shell will be expelled. Diatom’s silica shells are virtually indestructible so layers of diatoms that lived in oceans millions of years ago and were digested like this one form fossil deposits, identifiable by their exquisitely preserved surface patterns. These deposits, known as diatomaceous earths, have been mined and used as an abrasive for polishing surfaces and also in filtration systems. The number of diatoms that can live in the ocean’s surface waters is ultimately limited by a shortage of essential iron that they require for growth and development. One proposal for tackling global warming has been to seed the ocean with iron, precipitating vast blooms of diatoms that will remove carbon dioxide from the atmosphere; desperate measures for desperate times, and very controversial too – see http://news.bbc.co.uk/1/hi/sci/tech/7856144.stm

The desmid and diatoms here came from a small boggy pool in Teesdale, in the North Pennines. All of the individual objects in the pictures are less than one tenth of a millimetre in diameter.