Raise your hand if you know what a diatom is. Don’t feel bad if you didn’t raise your hand; you are in the majority of people around you. Maybe you recall your 7th grade biology teacher mentioning them, but you never quite understood what they were. I say it’s high time we understood them a bit better given that these mighty diatoms are responsible for producing about 50% of the oxygen we breathe through that amazing process — photosynthesis. Yes, something that is microscopic and virtually unknown by most people is responsible for a critical element, O2, that is vital to sustaining life on earth.
- Micron: Also known as a micrometer — a unit of length equal to one millionth of a meter.
- Organelles: Specialized structures that perform various jobs inside cells. Literally “little organs”; just as familiar organs (heart, lungs, kidneys, etc.) serve specific functions to keep an organism alive, organelles serve specific functions to keep a cell alive.
- ATP: Adenosine Triphosphate — an organic compound that provides energy to drive many processes in living cells. It is found in all known forms of life. The human body recycles its own body weight equivalent in ATP every day.
+: Nicotinamide adenine dinucleotide phosphate — a cofactor used in anabolic reactions (like the Calvin Cycle). NADPH is the reduced form of NADP+
- Stroma: Tissue that serves a structural or connective role in a cell.
- Plankton: Includes a diverse collection of organisms found in water or air that cannot propel themselves against a current. Some examples include bacteria, algae, protozoa, plant spores, pollen. Most are microscopic however some are quite large, including jellyfish. They are a crucial food source for many small and large aquatic organisms.
What are Diatoms?
Diatoms are single-celled algae found in the oceans, waterways, and soils of the world. They are the only organism on the planet with cell walls composed of transparent, opaline silica. They are quite beautiful and unique when viewed under the microscope displaying an amazing kaleidoscope of shapes. I’ve added a few pictures below. You can also check out some of the references below for good pictures. Their size ranges from 2 – 500 microns with the largest being about the width of a human hair. They constitute about half the organic matter found in the ocean. There are an estimated 20,000 – 2,000,000 different species of diatoms with more being discovered every year. Various species have developed structural adaptations to be able to move about or attach themselves to rocks or other organisms. This may allow them to stay afloat or resist wave action as needed depending on their environment. Diatom species are particular about the quality of water they live in.
What is Photosynthesis?
Like plants, diatoms and other algae use sunlight to transform water and carbon dioxide into oxygen and simple carbohydrates during the process known as photosynthesis — didn’t your 7th grade teacher also mention something about photosynthesis during biology class? Considering that photosynthesis is essential for the existence of all life on earth, it seems important to have a basic understanding of that process. Here’s my very simplified explanation of how photosynthesis works.
The photosynthesis process takes place in cell organelles called chloroplasts. Chloroplasts contain a green-colored pigment call chlorophyll — chlorophyll is responsible for the green coloration in plant leaves. Phototsynthesis occurs in two stages: a light dependent reaction and a light independent reaction (the Calvin Cycle). The light dependent reaction occurs in the thylakoid cells where energy from sunlight is converted to ATP and NADPH which is then used to power the Calvin Cycle. During the light reaction, the hydrogen from water is used and oxygen is produced. The light independent reaction, or Calvin Cycle, is also referred to as the carbon-fixing reaction. This reaction occurs in the stroma of the chloroplast. Water and carbon dioxide, along with the ATP and NADPH are converted to sugar (glucose) molecules that feed the plant.
Diatoms in the Great Lakes
Diatoms comprise the bottom rung of an aquatic food web. Zooplankton (small protozoans that feed on other plankton) feed on algae, smaller fish feed on zooplankton, bigger fish feed on smaller fish, and on up the food chain. Diatoms are busy photosynthesizing year round, even in lakes covered by both ice and snow. Diatoms need just the right balance of depth and sunlight to do their thing. If they sink too deep they don’t get enough sunlight, and if they are higher in the water column they can get burned. Snow may be protective against too much sunlight. Without diatoms to support zooplankton during the winter months, the lakes productivity for the rest of the year suffers.
Researchers have found that, over the past 115 years, individual diatoms are getting smaller and this decrease in size seems related to climate change. As the lakes become warmer, the bigger diatoms sink and are unable to harvest adequate sunlight to photosynthesize. The trend is toward smaller diatoms and fewer of them. Additionally, invasive species of mussels that have been introduced into the Great Lakes have caused the numbers of diatoms to plummet; mussels can filter the amount of water in Lake Michigan (removing plankton, including diatoms) in about a week or less. In Lake Erie, diatom numbers have plunged 90% in the last 35 years. A loss of this magnitude in a keystone species should be alarming to everyone, but again, think about how many people even know what a diatom is.
How do Diatoms Reproduce?
I’m suspecting you remember more about human reproduction from your 7th grade Biology teacher than diatom reproduction, but hang in here…this is fascinating!
Diatoms reproduce by both an asexual and a sexual process. The asexual process is primary and occurs by binary fission to produce two new diatoms with identical genes. You can see from the diagram below that the frustule splits to form two daughter cells; one with the larger half of the frustule (the epitheca) and one with the smaller half (the hypotheca). The diatom that receives the hypotheca remains smaller than the parent. With continued asexual reproduction, the average cell size of the diatom population decreases.
In order to restore the diatom population to it’s original cell size, sexual reproduction occurs through meiosis. A special structure, called an auxospore, is formed. This is a unique type of cell that possesses silica bands rather than a rigid silica cell wall. This unique cell allows the cell to expand to it’s maximum size. Once an auxospore divides by cell division, it produces a normal diatom cell which then continues to get smaller with each asexual cell division.
Diatoms in the Fossil Record
The silica cell walls of diatoms are inorganic, so they do not decompose. These structures are found in the fossil record back as far as the early Jurassic (~185 million years ago). It has been suggested that the evolutionary ability of these organisms to produce a resting stage (the Auxospore) along with the ability to photosynthesize had an adaptive advantage over other organisms during intense climatic, tectonic, and geochemical changes that led to a mass extinction period close to the Permian-Triassic boundary (~251 million years ago). After the mass extinction event, many niches (habitats) in the aquatic realms opened up and diatoms appear to have diverged at this time and evolved to develop silicic cell walls. Thus, they are found in greater abundance in the fossil record since this time. The fossil record shows diatom diversity to be very sensitive to global temperature. Warmer oceans, particularly warmer polar regions, have in the past been shown to have substantially lower diatom diversity. Thus, future warmer oceans could, in theory, result in a significant loss of diatom diversity although it is unclear how quickly this change would happen.
MAKING THE CONNECTION:
I hope I have led you to a greater understanding and appreciation of what diatoms are and the important role they play in sustaining life on earth. By studying the fossil record, we know diatoms have been with us for millions of years and have evolved over time as climactic and geochemistry conditions changed. We also know that in order for organisms to adapt to changing conditions (evolve), changes need to occur relatively slowly. We can observe today how local conditions in lakes and oceans are affecting diatom populations. We can also acknowledge that there is a lot more to learn about how diatoms adapt, and how quickly, to changing conditions. One thing seems clear — we should be showing more gratitude and respect for these amazing organisms. So the next time you take a big gulp of air (like now) remember to give thanks to the mighty diatoms who work tirelessly to keep us supplied with oxygen!