If you head outside and you walk to a freshwater river, stream, or lake, you will probably find some rocks covered in what looks like a slimy, green film. While to some this may seem like a good time to pull out the hand sanitizer, that green film is possibly the most important thing you’ll find there. Indeed, this unusual substance, which is composed of diatoms, is a snapshot into the waterway’s history, going back thousands of years.

Seventy percent of the Earth is covered in water, but only half of one percent of the water on Earth is useable – the rest is saltwater, or frozen in glaciers, etc.1 Consequently, freshwater is one of our most precious, and finite natural resources, and it’s critical that we understand how our waterways work to ensure the future of water as we know it.

But studying waterways can be a complicated, expensive, and time-consuming process. To get a clear picture of how a river functions, for example, scientists have to collect a sample of the water, look at what types of organisms live in it (from fish to bacteria), analyze the soil in and around it, and so forth. Thankfully, diatoms, a type of single-celled algae that make up that green film, provides a Spark Notes version of a waterway’s characteristics and helps speed up the study of it. Scientists have used diatoms to study everything from ancient weather patterns to the effects of pollution and invasive species on the Earth.

Wait, The Green Slime is Made of What?

Diatoms are a specific type of algae, a microscopic organism that lives in water and makes its own food by turning light energy into sugar. They can be found virtually everywhere there is water. While some diatoms can also live suspended in the water, many grow attached to submerged surfaces and form the green films like the ones you see on rocks in a river. Diatoms, however, are not like most types of algae – they are composed of an unusual material, silica, the same molecule that is used to make glass. In fact, they live inside a glass casing called a frustule, which can be extremely ornate and even quite beautiful2.  

diatom protists what are diatoms

Diatoms are even used for elaborate artwork designs. Piece by artist Klaus Kemp, featured in an article on The Colossal3.

A Window into the Past

This glass frustule helps scientists understand the characteristics of a waterbody’s past, going back thousands of years. This unique quality stems from how the silica in their frustule does not break down like typical organic matter. As a result, we can look at what diatom frustules are present in different layers of sediment in a waterbody to understand that waterbody’s past. For example, by determining at what height in the water (top, middle, bottom) certain types of diatoms like to live in, we can see how high a lake might have been over long time periods. Scientists can look more closely at diatoms from different layers of sentiment and determine when there were droughts, heavy rains, or other possible weather events. All of this comes from studying the tiny diatom glass frustules we find in the dirt5.

Diatoms are Environmentally Picky

Not only do diatoms not break down, they are also extremely sensitive to the environmental conditions around them. We often find diatoms that survive only at specific pH levels or amounts of salt in water2, which means diatoms can be used as ecological indicators. More specifically, we can look at what types of diatoms are present in an environment, and accurately predict certain characteristics of the waterway in which they live4.

Diatoms also reproduce quickly, which helps scientists observe rapid changes in water quality just by looking at what diatoms are there. Combine these two factors, and we can map out much of what’s going on in a river at any given time, just by looking at a few snapshots.

From all the information diatoms can provide, scientists can better understand patterns in ecosystems and predict how water systems might change in the face of future threats like increased pollution and climate change. Equipped with this cheat sheet in our pockets, we will be better prepared for the test ahead.

Brittany Maule works as an Environmental Manager on protecting drinking water sources in the Office of Water Quality for the State of Indiana. You can read her blog Eco-Troublemakers to learn about invasive species and science communication, or you can connect with her on Twitter 


  1. USGS Water School. How much water is there on, in, and above the Earth? 2016. Web. http://water.usgs.gov/edu.
  2. Diatoms of the United States. What are Diatoms? 2016. Web. http://westerndiatoms.colorado.edu/about/what_are_diatoms.
  3. Jobson, Christopher. Contemporary Artistic Arrangements of Microscopic Diatoms by Klaus Kempby. 17 September 2014. Web. http://www.thisiscolossal.com/2014/09/diatomist/
  4. Stevenson, J. Refining diatom indicators for valued ecological attributes and development of water quality criteria. 2006. Advances in Phycological Studies: 365-383. Web.
  5. Wolin, J.A. and J.R. Stone. Diatoms as Indicators of Water-Level Change in Freshwater Lakes. , 2010. In: The Diatoms Applications to the Environmental and Earth Sciences, E.F. Stoermer and J.P. Smol (eds.), Cambridge University Press: 174-185.


  • Brittany Maule

    Brittany Maule received her master's degree in biology at Ball State University. Brittany writes her own blog, Eco-Troublemakers. You can follow her on Twitter @BrittanyMaule.

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On April 8th, 2024, a total solar eclipse will sweep across North America, from Mexico to the Maine-Canadian border. For those who experienced the spectacular solar eclipse of 2017, this one will be similar, crossing the United States from west to east and passing through or near several major metropolitan areas. And while its path is quite different this time, Carbondale, Illinois, a reasonable destination for Chicago-area residents, will once again be on the line of totality.    

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