Filamentous cyanobacteria are photosynthetic bacteria that form long, thread-like structures. Often called “blue-green algae,” they are prokaryotes, meaning their cells lack a nucleus and other membrane-bound organelles. These ancient organisms are major producers of oxygen and are also responsible for forming widespread, sometimes toxic, growths in water bodies.
Biological Structure and Function
Filamentous cyanobacteria are organized into long chains of cells called trichomes, formed when individual cells remain connected after division. This filament is often encased in a protective, mucilaginous layer called a sheath that buffers against environmental stress and aids in movement. This simple multicellular arrangement has been successful for over two billion years.
Within these filaments, some cyanobacteria develop specialized cells for specific jobs. One such cell is the heterocyst, a thick-walled cell dedicated to nitrogen fixation. This process converts atmospheric nitrogen gas into ammonia, a form of nitrogen that other organisms can absorb. The heterocyst’s thick wall limits oxygen entry, protecting the oxygen-sensitive enzymes required for this process.
Another specialized cell is the akinete, a dormant survival cell. Akinetes are larger than regular vegetative cells and have thick envelopes packed with food reserves. They form in response to unfavorable conditions like darkness or nutrient limitation, allowing the organism to endure harsh periods and germinate when conditions improve.
Cyanobacteria derive their energy from oxygenic photosynthesis, the same process used by plants. They capture light using chlorophyll a and unique accessory pigments called phycobilins. The combination of these pigments, particularly the blue-colored phycocyanin, gives them their characteristic blue-green coloration and fuels their activity.
Ecological Roles and Habitats
Filamentous cyanobacteria are found in nearly every illuminated environment on Earth. Their habitats range from freshwater lakes and rivers to the open ocean, damp soils, and extreme locations like desert crusts and polar regions. This adaptability makes them a foundational component of many ecosystems.
In these environments, they serve as primary producers at the base of the food web. Through photosynthesis, they create organic matter that becomes a food source for small aquatic animals and protozoa. This role is especially important in nutrient-poor waters where other photosynthetic organisms may be scarce.
The nitrogen fixation performed by species with heterocysts naturally fertilizes ecosystems. This enrichment is beneficial in open oceans and pristine lakes where nitrogen is a limiting nutrient. They also form symbiotic relationships, such as with fungi to create lichens, providing nutrients in exchange for protection.
Harmful Algal Blooms and Toxin Production
Filamentous cyanobacteria can cause harm when they multiply uncontrollably, forming dense harmful algal blooms (HABs). These events are triggered by a combination of factors, primarily eutrophication—nutrient pollution from agricultural runoff and wastewater. Calm, stagnant water and warm temperatures further encourage their rapid growth.
The consequences of these blooms are significant. Dense mats of cyanobacteria block sunlight, killing aquatic plants below. When the bloom dies, its decomposition by bacteria consumes dissolved oxygen, leading to low-oxygen conditions that can cause large-scale fish kills.
A direct threat comes from the production of compounds called cyanotoxins. These toxins include hepatotoxins like microcystins, which target the liver, and neurotoxins like anatoxins, which affect the nervous system. Ingesting contaminated water poses a risk to wildlife, livestock, pets, and human health.
Human and Industrial Applications
The biological capabilities of filamentous cyanobacteria are being explored for several applications, including biofuel production. Because they grow quickly and can be cultivated on non-arable land with non-potable water, they are a potential feedstock for sustainable fuels like biodiesel and bioethanol.
Their ability to fix atmospheric nitrogen makes them useful in agriculture as biofertilizers. When species like Anabaena are grown in rice paddies, they can reduce the need for synthetic nitrogen fertilizers. This practice lowers farming costs and mitigates environmental damage from fertilizer runoff.
Cyanobacteria are also useful for bioremediation, the use of biological systems to clean up pollution. Certain strains can absorb or break down contaminants in wastewater, including heavy metals and organic pollutants. Using them in controlled ponds or bioreactors offers an eco-friendly method for treating effluent.
Some filamentous cyanobacteria are cultivated for their nutritional value. Arthrospira, known by its trade name Spirulina, is grown commercially in large, open ponds. It is harvested, dried, and sold as a high-protein food supplement rich in vitamins, minerals, and antioxidants.