Download What is eutrophication?

Cyanobacteria
Cyanobacteria: a unique
group of bacteria that
undergoes photosynthesis
And the Environment
What is eutrophication? How is it caused and why is it a problem? Eutrophication can be defined as the input of excess nutrients into a water body. Healthy water systems have an adequate amount of nutrients to sustain aquatic life, but the addition of fertilizers, detergents, and sewage runoff can add nitrogen and phosphorus into the water. This process of nutrient over‐enrichment can stimulate the growth of problematic cyanobacteria, some of which produce harmful toxins. Eutrophication can also lead to loss of aesthetic and recreational qualities of a water system, depleted oxygen levels, and a foul taste and odor to the water. Are cyanobacteria algae, or bacteria? The word “algae” is often used as a generic term for aquatic, photosynthetic microorganisms. The photosynthetic physiology of cyanobacteria closely resembles that of algae, but their cellular composition is the same as other bacteria. While algae are eukaryotes, cyanobacteria are prokaryotes, with no membrane‐bound nucleus or specialized organelles. The three domains of life are shown above. Cyanobacteria are classified under the domain Bacteria. Are there any benefits to cyanobacteria? The picture above shows a bloom on Lake Taihu in China. This lake provides drinking water and crops to ~ 12 million people. A bloom is a highly concentrated assemblage of cyanobacterial cells that forms on the surface of the water. How dangerous are the toxins that cyanobacteria produce? Some, but not all, species of cyanobacteria have the ability to produce compounds that are toxic to humans. Certain environmental conditions, such as temperature, optimal levels of light, and high concentrations of nutrients are thought to control toxin production. This combination of factors enables cyanobacteria to rapidly reproduce, resulting in the formation of a cyanobacterial bloom. Blooms can lead to elevated levels of toxins, which can be harmful to animals and humans by causing damage to the brain, liver, and skin. Over a long period of time, exposure to some of these toxins may cause cancer and even death. The World Health Organization set a guideline which limits the level of safe drinking water to one microgram of toxin per liter of water (one microgram is 1/1,000,000 of a gram.) Much like plants, cyanobacteria perform photosynthesis and serve as primary producers in the food web. In fact, cyanobacteria are thought to be ancient oxygen‐producers that helped create the oxygen‐rich atmosphere that allowed plants and animals to evolve millions and millions of years ago. Certain cyanobacteria have specialized cells, called heterocysts, which perform nitrogen fixation. All living organisms, including animals, plants, and humans, require nitrogen to build proteins but cannot use nitrogen from the atmosphere. Heterocysts can take the form of nitrogen in the atmosphere and transform it into compounds that can be utilized by other organisms. For example, nitrogen‐fixing cyanobacteria are often used as biofertillizers in rice paddies (pictured below), as they enrich the soil with nitrogen and lead to higher yields of rice crops. DEPARTMENT of MICROBIOLOGY
Cyanobacteria
And the Environment
Cyanobacteria: a unique
group of bacteria that
undergoes photosynthesis
Does global warming affect the problem? Where are eutrophic lakes found? Cyanobacteria grow better at higher temperatures than other photosynthetic microorganisms, and this gives cyanobacteria a competitive advantage. Warmer waters also lead to stratification, which reduces the mixing that would otherwise take place among levels of a water system. The problem of eutrophication is evidenced in water systems throughout the world, including the Great Lakes of North America, as well as lakes in Africa and South America. The picture above is an example of stratified water. The upper, middle, and lower layers do not mix with each other. Cyanobacteria exploit stratified conditions by employing gas vesicles, which are specialized structures that act like small balloons and give cells buoyancy, allowing them to float. The picture to the right shows cyanobacteria that have been collected from lake water. Even after shaking up the bottle, the cyanobacteria float to the top of the bottle and settle onto the surface. Blooms of cyanobacteria form on the surface of the water and shade the non‐buoyant species below, allowing the cyanobacteria to outcompete similar organisms for light resources and CO₂. Consequently, cyanobacteria can upset the natural balance of organisms residing in a body of water. Why can’t the blooms just be scooped out of the water? Cyanobacteria can grow faster than they can be removed. Furthermore, it is often unknown how deep or concentrated a bloom is, making it nearly impossible to remove all of the cyanobacterial cells composing a bloom. This work has been sponsored in part by grants from the Division of Chemical, Bioengineering, Environmental and Transport Systems (CBET 0826838) and the Division of Integrative Organismal Systems (IOS 0841918) of the National Science Foundation . A bloom on Lake Erie A bloom on Beditti Lake in Argentina Is there any way to control these blooms? Reducing nitrogen and phosphorus released to water systems is the ideal approach in attempting to prevent eutrophication, but there are other methods to prevent cyanobacterial blooms have been suggested. Some experiments have shown viruses, fungi, protozoa, and even other bacteria to be effective in removing cyanobacteria from eutrophic waters by killing cyanobacterial cells. There have also been attempts to chemically control blooms by adding compounds such as copper sulfate to eutrophic water; however, studies have shown that this treatment method may actually cause toxins to be released into the water as the cyanobacterial cells die. So what can we do? Facilities that remove nutrients from sewage prevent a large amounts of nutrients from being dumped into water, providing “fuel” for harmful cyanobacteria to survive and reproduce. Additionally, reducing fertilizers, pesticides, and detergents that enter the water and contain nitrogen or phosphorus prevents eutrophication. The effects of global warming must also be considered as we to combat harmful blooms. Emission standards for factories and reducing greenhouse gas emissions are good steps towards sustaining healthy water systems and promoting a better environment to live in. Further Reading Guo, L. 2007. Doing Battle With the Green Monster of Lake Taihu. Sci‐
ence 317: 1166. Paerl, H.W. and J. Huisman. 2008. Blooms Like It Hot. Science 320: 57‐
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