Life in a Glass Box

One of the largest and most ecologically significant groups of organisms, diatoms are exquisite micro fossils found abundantly across the earth, and blessed with innumerable distinct characteristics, some of which scientists are still trying to unravel.

If you find yourself in a place that is wet and well lit, you are probably surrounded by thousands upon thousands of diatoms. What seems like a drop of mucky water ora string of sticky slime can transform into a world of frantically moving, transparent bodies tinged with color, when seen under the microscope. Magnify them further,and intricate patterns rivalling the world’s best sculptures emerge.

Diatoms are made of just a single cell, equipped with chloroplasts for photosynthesis. They contain the pigment chlorophyll, which gives them the green color seen in plant leaves; they also contain beta-carotene, the pigment that gives carrots their beautiful orange. This makes diatoms glow with a characteristic golden hue, which is what you see when you observe diatoms swimming in a drop of water on a slide under the microscope.

Under the electron microscope, which magnifies the tiny diatoms from less than a width of human hair (2 to 500 microns) to grey monotones of astonishing detail,you can see the sculptures. There are the cell walls called frustules, made of the elements silicon and oxygen,plastered together with water molecules. The frustule is made up of two shallow half cylinders that fit together like a small jewel box, encasing the live cell inside. Basically,diatoms live in glass boxes. The frustules are perforated by many small holes, which allow the diatom to exchange water, organic materials and nutrients with the outside world.

There are different estimates about the number of diatom species in the world; ranging from 20,000 to over 1-2 million, because scientists are still figuring out the basics of what a diatom species is. They are placed into two broad classes – “centric” diatoms, that are round in shape, like a clock, and “pennate” diatoms, which are elongated and spindle shaped. Pennate diatoms have along slit, through which some species secrete slime to move over a surface, like miniature slugs.

Karthick Bala subramanian, a scientist with the Agharkar Research Institute in Pune, has been working on Indian diatoms for the last eight years. While starting his research, he painstakingly put together a list of publications that describe diatoms from the Indian subcontinent. About 15,000 individual records yielded 6,800 diatoms, including marine, freshwater and estuarine forms.

“In the last 20 years, about 30 new species have been described from the Indian subcontinent. I have described 19 of them, and the others have been described by foreigners,” Bala subramanian reports. Through the aid of a grant, Bala subramanian and a group of others set off to explore the biodiversity of diatoms in the Western Ghats.A team of six stream researchers started from near Pune,and traveled all the way down the Ghats to Ponmudi near Thiruvananthapuram in Kerala, over four weeks in October-November 2013.

Dr Rex Lowe, a Professor Emeritus from Bowling Green State University, Ohio, United States, was part of the trip. Apart from getting his taste buds tingled by biryani and dosas, he was held in spell by the cultural and social diversity of India, and the diatoms of course.“I had read about the Western Ghats, a very old and extremely bio diverse area where many new species had been discovered. Why not diatoms? I am most anxious to begin examining our diatom collections from the Western Ghats,” he wrote in an email. Lowe had reason to be excited. During the trip, the team made slides to take a casual look at the diatoms once in a while. What they realized was that about 60-70% of Indian diatoms would probably fall under new species.

What diatoms do

In large water bodies like a big lake or the oceans, diatoms undergo a boom and bust cycle. When the upper layers of the water body are awash with nutrients and light, for instance during spring, the number of diatoms increases exponentially. With time, they use up the nutrients and sink down to the bottom. Vertical mixing of the waters carries the settled diatoms back to the upper layers,setting the scene for the next diatom burst.

Due to their sheer abundance in the world’s oceans,especially in nutrient rich areas, it is estimated that diatoms account for as much as 20% of photosynthetic fixation of carbon – which is more than all the huge trees in the tropical rain forests around the world put together.Diatom taxonomist Patrick Kociolek expresses a parallel for the amount of oxygen diatoms generate during photosynthesis – “they give us every third breath”.

When a diatom divides to produce two daughter cells,each cell keeps one of the two halves and grows a smaller half within it. As this continues, daughter diatoms become smaller and smaller, until the right conditions materialize – the diatom then forms a special kind of cell called anauxospore, which takes the diatom back to its original size.

Collecting diatoms

Diatoms are found both as floating plankton, and attached to surfaces – to filamentous algae, on submerged plants, as a slippery brown coating on rock surfaces and submerged sticks. To get them out of hiding, the collectors fished the floating plankton out with a net, or scraped them off different surfaces with a toothbrush or a knife.

“We use the markings on the frustule to make species identifications,” said Bala subramanian. “We need to boil the diatoms with acid or bleach to remove organic matter and the cell content. We then mount this on the slide. A special chemical is necessary to see the frustule, because- it’s all glass. The slide, the cover slip, the frustule are all transparent, made of silica! We use a chemical which makes the frustule diffract light differently from the slide and cover slip.”

Diatoms as bio indicators

Diatom species are very particular about the chemistry of the water they live in. Each species has a preferred range for pH, salt and nutrient content, sediment load,elevation and most importantly, human disturbances in the form of pollutants. This sensitivity makes them excellent bio-indicators. In many countries in Europe and the Americas, diatoms have been used as bio-indicators for the last 12 years.

The Indian Pollution Control Board, on the other hand,only carries out physical and chemical monitoring as of now. “Their water quality parameters are directly adopted from other countries, without any adaptation to local Indian conditions, even important and obvious ones like the Monsoon,” said Bala subramanian, who has the ambitious idea of putting together a manual and website with indicator diatoms for the Pollution Control Board. But of course, his first task now is to study and classify the mountain of samples that has been gathered from last year’s exploration trip.

 

How cool are diatoms?

• Every third time you breathe in, that Oxygen is produced by diatoms.
• Diatoms play critical role in maintaining the aquatic food chain.
• Diatoms, scientists say can play a crucial role in reversing/arresting Global Warming as they sequester CO2 in huge quantities!
• Diatoms are increasingly being used to know how and to what extent we humans have modified our aquatic ecosystems and hold the key to our past.

 

Interesting facts:

• Though complex in appearance, diatoms are unicellular organisms.
• The range of diatoms shapes is as complex and diverse as snowflakes, and all diatoms are generally exhibit radial or bilateral symmetry.
• Diatomaceous earth, forms a chalky substance,is widely used as a cleaning agent, including being added to toothpaste. So next time you clean your teeth you may be using fossils millions of years old.

 

Diatom images are courtesy of Karthick Bala subramanian, Agharkar Research Institute, Pune, India and Gubbi Labs ;Jonathan Taylor, North West University, Potchefstroom,South Africa; Paul Hamilton, Canadian Museum of Nature, Ottawa. Canada.

 

This article was first published in the March 2014 edition of Saevus magazine