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Diatoms are a large group of single-celled algae that are encased in a silica cell wall. They are a very diverse group and are known for their interesting physical structure.

Definition of Algae Groups

Algae are photosynthetic organisms, or living things that create energy from sunlight. This is a very diverse group, and includes single-celled organisms, all the way up to huge multi-celled masses. One major group of the algae are the diatoms. Diatoms are characterized by being unicellular with a silica-based cell wall called a frustule.

Diatomaceous Earth

Read any article about how to clean your home naturally, and you’re sure to run across something called diatomaceous earth. This is a miracle substance, it seems! It can be used to clean, disinfect, get rid of pests, and even used by companies to filter everything from pool water to beer. What a diverse substance! But what is it? Where does it come from? Well, diatomaceous earth is a powdery substance made from the fossilized remains of a single-celled algae known as a diatom.

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What Are Diatoms?

Diatoms are biological producers, meaning they produce energy in the form of sugar from sunlight. This gets energy into our world so the rest of us can eat it. Thanks, diatoms!Diatoms exist as single cells, although some of them flock together in colonies that create some really pretty forms. You can find diatoms in colonies that are shaped like filaments or ribbons, in zigzag shapes, in fan shapes, or even in the shape of stars.

Each individual cell is covered in a hardened frustule made from silica. This frustule gives each cell a particular shape:Aren’t these beautiful? And these are just a few of the thousands of different forms that diatoms can take. Nature is amazing! Diatoms first showed up in the fossil record right around the Jurassic period, so dinosaurs and diatoms hung out together long before we were around to study them.


A Widespread Group

There are over 100,000 species of diatoms that we know about on earth, and these species can be found wherever there is water, from oceans, to lakes, to even damp soils and puddles in your backyard.Most diatoms are microscopic, so we can’t see them with the naked eye. There are a few species that reach up to two millimeters long, though, but these are rare. For the most part, you don’t know that diatoms are there, although they are extremely important in the world. A recent study estimated that 45% of the primary production in the oceans was accomplished by diatoms.

What a busy organism!

Beautiful Forms

The diatom is most known for its silica shell, or frustule. The frustule is broken up into two halves, which overlap each other, kind of like the two halves of a petri dish. This shell formation allows the diatom to divide easily into daughter cells. Each daughter cell keeps one-half of the frustule, and then adds on more silica to form a full cell wall. After death, the frustule persists in the environment, which is where we get diatomaceous earth.

The physical form of the diatom is how scientists classify these organisms. Most scientists break up diatoms into two major groups: Centrales, the centric diatoms, and Pennales, the pennate diatoms.The centric diatoms are radially symmetrical, with parts radiating out from a central point. The pennate diatoms exhibit bilateral symmetry, meaning the left and right halves are mirror images of each other. As we study diatoms in more depth and learn more about their evolution and DNA, we will be revising this basic understanding of their classification. It is even possible that we haven’t discovered all of the species out there, and there may be one or two examples that don’t fall into these categories. Isn’t biology a wonderfully exciting science?

General Ecology

Diatoms live in water, or in very moist environments.

They are also subject to changing weather patterns, just like all living things on earth. In general, diatoms exhibit a boom and bust life cycle. When conditions are favorable, the diatoms reproduce very, very quickly, often out-competing most other life forms in the environment. This is the boom (also known as a bloom of diatoms) portion of their cycle. This part of the cycle tends to happen when nutrients and sunlight are in great abundance, such as during the spring and early summer months.As the population of diatoms grows, however, the cells begin using up all of the resources. Once the resources are gone, the diatoms start sinking to the bottom of the water, and form a type of hardy spore that can withstand unfavorable conditions.

This is the bust period of the cycle. When conditions get better, the cycle continues again. In ocean habitats, diatoms follow a yearly boom and bust cycle, proliferating during spring and summer, and sinking to the bottom of the ocean during fall and winter.

Some areas experience a boom of diatoms during autumn if it is warm, and there are lots of nutrients left in the water.The types of species that make up the community of diatoms is unique to a particular area. In other words, the exact species of diatoms you will find in your backyard pond is very different from that found out at the beach, or in the stream in your local park. This difference in species make up is helpful to crime scene investigators.

Diatoms Can Help Police

There have been many cases in which diatoms have helped the police determine where a crime was committed.

Because different species of diatoms live in different areas, it is possible to use diatoms as evidence. This is especially common in people who drown in water. By looking at the types of diatoms in someone’s lungs, crime scene investigators are able to determine which body of water the person died in. You’d be surprised how many people think they can hide an accident!

Lesson Summary

Diatoms are single-celled algae that form a silica-based cell wall. They are broken up into two major groups, the centric diatoms and the pennate diatoms, based on the form of their frustule.

Diatoms live in all aquatic environments, including the ocean, freshwater lakes, and moist soils.

Highlighted Terminology

Terms Meanings
Algae photosynthetic organisms
Diatoms characterized by being unicellular with a silica-based cell wall
Frustule silica-based cell wall
Diatomaceous earth a powdery substance made from the fossilized remains of a single-celled algae known as a diatom
Producers create energy in the form of sugar from sunlight
Radially symmetrical parts radiating out from a central point
Bilateral symmetry the left and right halves are mirror images of each other
Boom and bust the boom life cycle happens when nutrients and sunlight are in great abundance; during the bust period of the cycle, the resources are gone and the diatoms sink to the bottom of the water, forming a hardy spore that can withstand unfavorable conditions

Learning Outcomes

Study all of the above information on diatoms, then:

  • Define diatoms and recognize its relationship to the algae and frustule
  • Specify where diatoms may be located
  • Recall the importance of these organisms in the areas of ecology and policing

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