Article by Hannah Weiss
Have you ever seen a greenish liquid floating on top of a body of water? At first glance, it may not seem like much.
The organism, known as cyanobacteria, is actually a key component of many ecological processes and a vital part of life on Earth. However, too much cyanobacteria can be a bad thing. Cyanobacteria can cause ecological harm by outcompeting other organisms, release toxic compounds, and cause unpleasant tastes and flavors in fish.
To figure out how to mitigate some of these harmful impacts, we first have to learn about how cyanobacteria function.
What are cyanobacteria?
Cyanobacteria are microscopic organisms that live in water. They are bluish green in color, hence the name “cyan”. Cyanobacteria are commonly referred to as blue-green algae. In reality, however, they’re a type of bacteria.
Unlike most bacteria, which are single-celled, some types of cyanobacteria are multicellular. This allows them to spread much faster than other types of bacteria. Blue-green algae is also photosynthetic, meaning that it consumes carbon dioxide and produces oxygen.
Blue-green algae plays many important ecological roles, both past and present. It was key to the formation of the Earth’s oxygen atmosphere about 2.4 billion years ago. It’s also important for plant growth because it is one of the only organisms that can convert nitrogen into a form that plants can use.
However, cyanobacteria can also be harmful. When they build up on the surface of a body of water, the dense blooms prevent light from reaching below the water’s surface, making it difficult for anything else to grow. In addition, cyanobacteria can release toxic compounds that are harmful to humans and animals.
What is known about the compounds that cause off-flavors in catfish?
Cyanobacteria naturally release taste and odor compounds into the water, which then impact the taste and smell of the fish living in that body of water these taste and odor compounds are non-toxic. However, they are highly unpleasant, and they pose major problems for the catfish industry.
The main compounds that affect taste and smell are geosmin and 2-methylisoborneol (2-MIB). Cyanobacteria are the major contributors of these two compounds in aquatic ecosystems.
Geosmin is a neutral colorless oil that produces an earthy flavor. In fact, that’s where it got its name – the word “ge” comes from the Greek word for earth and “osme” means odor. In fish, geosmin can be detected at ranges from less than 1 to 10 micrograms per litre. Since farm-raised catfish have a relatively mild flavor, geosmin can be detected at low concentrations.
2-MIB, the other major taste and odor compound, imparts a musty flavor and odor. It is a water-soluble compound that is crystalline white solid at room temperature. In catfish, the average consumer can detect 2-MIB at 0.7 micrograms per litre.
How do these compounds affect the taste of catfish?
Unpleasant flavors mainly occur when the odor-causing compounds are absorbed across the catfish’s gill membranes. The compounds may also be absorbed in the stomach lining and intestines if the fish swallows contaminated water.
After entering the bloodstream, the compounds become concentrated in edible tissues – namely, fatty skin tissue and visceral fat. When a consumer bites into a catfish filet, the unpleasant flavors are noticeable.
Why do cyanobacteria affect catfish ponds in particular?
Cyanobacteria are most commonly found in shallow, slow moving, or still water. They can multiply quickly in warm, nutrient rich environments, creating blooms that spread across the surface of the water.
Catfish require warm water, and they are raised in ponds — the perfect conditions for cyanobacteria to flourish. Cyanobacteria are especially prevalent in summertime catfish ponds, and they can bloom rapidly in these conditions.
What’s the solution?
Odor-causing compounds are one of many problems posed by blue-green algae, but it’s important to remember that the bacteria play an important role in many ecological processes.
The solution is not to eliminate blue-green algae altogether, but rather to control it and limit its spread. At the same time, scientists are trying to find ways to limit some of its unpleasant effects.
For example, the United States Department of Agriculture – Agricultural Research Service is currently sponsoring a crowdsourcing challenge to find ways to eliminate unpleasant flavors in catfish contaminated by compounds found in blue-green algae. To find out how you can get involved, visit the HeroX challenge page.
