Life in a Few Drops of Water: Diatoms

A few days ago, when we were out walking in Weardale, I collected a small sample of water from a pond in a disused quarry on the edge of the moors, then spent several hours that same evening exploring just a few drops of the pond water - maybe ten in total - under the microscope. The organisms pictures in this post are all diatoms - minute photosynthetic organisms encased in a shell of pure silica that often bears the most intricate pattern on its surface. This one, caught in the act of dividing, is Pinnularia.

 

Some are long and thin (this is Nitzchia sigmoidea), while.....

....others are joined together in chains.

Navicula? When diatoms die their silica shell remains, almost indestructible, and in that transparent state, with no contents, the full beauty of the sculpturing on their shells can be appreciated.

The pattern of fine ridges on Cymbella tests the ability of microscope lenses to resolve such fine detail.

Diatoms generally move in a gliding motion through the water but sometimes they can be attached to a substrate via mucilaginous stalks.....

... seen here at higher magnification.

Most of these organisms are less than one twentieth of a millimetre long.

Coming next: Desmids....mirror-images in miniature

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.

Bubbling stones

The pebbles in the pools left by the falling water level in the upper reaches of the River Wear were bubbling in the bright spring sunshine this morning. Scraping some of the thin film of slime from the stones onto a microscope slide revealed the cause – millions of minute, photosynthetic diatoms, producing oxygen bubbles. In amongst them I found several amoeba, like the one shown in the second-from-top photograph, grazing on these underwater meadows.