Filamentous Cyanobacteria: Roles, Risks, and Uses

Cyanobacteria, also known as blue-green algae, are ancient photosynthetic organisms found in nearly every environment on Earth. As prokaryotes, their cells lack a nucleus. Filamentous cyanobacteria are a specific group distinguished by forming long, chain-like structures of individual cells, which allows for complex interactions. These organisms inhabit a vast range of ecosystems, from freshwater lakes to terrestrial soils, giving them a multifaceted impact on global ecology.

Defining Characteristics and Cellular Structures

Filamentous cyanobacteria arrange their cells into long chains called trichomes, which can be straight, coiled, or branched. These trichomes are often encased within a gelatinous sheath. This colonial structure is an early form of multicellularity, enabling coordinated responses and the development of specialized cells.

Specialized cells perform distinct functions within the filament. Heterocysts are dedicated to nitrogen fixation, converting atmospheric nitrogen into usable ammonia. They have thick walls to create the oxygen-free internal environment required for this process. Under harsh conditions, some cells become akinetes, which are spore-like resting cells with thickened walls and food reserves that enable survival through periods of cold or darkness.

Dispersal is facilitated by hormogonia, short, motile filaments that break away to establish new colonies. Many free-floating species also have gas vesicles, which are intracellular compartments used to regulate buoyancy for optimal positioning in the water. Their blue-green color comes from photosynthetic pigments, including chlorophyll a and accessory pigments called phycobiliproteins.

Ecological Roles and Environmental Significance

As primary producers, filamentous cyanobacteria form the foundation of many food webs by converting sunlight into organic matter. Their ability to perform nitrogen fixation is another important ecological contribution. By converting atmospheric nitrogen into usable forms, they enrich nutrient-poor environments and make nitrogen accessible to other organisms like plants.

On land, filamentous cyanobacteria are components of biological soil crusts (biocrusts), especially in arid regions. The net-like structure of their filaments binds soil particles, reducing erosion from wind and water. These crusts also improve soil fertility and water retention, creating a better environment for plant growth.

Filamentous cyanobacteria engage in symbiotic relationships with other organisms. They are the photosynthetic partners for fungi in many lichens and form relationships with plants like the aquatic fern Azolla and cycads. These partnerships provide the host with nutrients, such as fixed nitrogen, in exchange for a protected environment.

Filamentous Cyanobacteria in Blooms and Toxin Production

Some filamentous cyanobacteria form dense growths known as cyanobacterial harmful algal blooms (CyanoHABs), which appear as scums on freshwater and brackish water. Bloom formation is driven by a combination of factors, including excess nutrients from runoff, warm water temperatures, calm conditions, and high light availability.

Genera known for producing blooms include Dolichospermum, Aphanizomenon, Cylindrospermopsis, Planktothrix, and Lyngbya. A major concern is their capacity to produce cyanotoxins. These toxins are contained within the cells and released into the water when the cells die and rupture, often at the end of a bloom.

Cyanotoxins fall into several categories based on their effects.

• Hepatotoxins, such as microcystins, cause severe liver damage.
• Neurotoxins, including anatoxin-a, affect the nervous system and can lead to paralysis.
• Cylindrospermopsin can damage the liver and kidneys.
• Dermatotoxins, such as lyngbyatoxin, cause skin rashes and irritation.

These toxins pose a risk to human and animal health through contaminated water and can also cause taste and odor problems in drinking water supplies.

Current and Potential Biotechnological Applications

The metabolic capabilities of filamentous cyanobacteria offer various biotechnological uses. Their ability to fix nitrogen has been harnessed in agriculture for centuries, especially in rice paddies where they act as biofertilizers. This practice enriches the soil, reducing the need for synthetic fertilizers.

Research is exploring their potential as a source for renewable biofuels. They grow rapidly and produce large amounts of biomass that can be converted into fuels like biodiesel and bioethanol. A benefit of this approach is that cyanobacterial cultivation does not require arable land needed for food crops.

Filamentous cyanobacteria are a source of many natural products with commercial and scientific applications.

• Phycobiliproteins are used as natural colorants and as fluorescent markers in research.
• Exopolysaccharides and unique enzymes have various industrial applications.
• Some species are used in wastewater treatment to remove excess nutrients.
• They are a source of novel bioactive compounds for potential new pharmaceuticals.