Swimming Pool “Black Algae”: Why the Name Is Wrong and the Science Matters

Few problems frustrate swimming pool professionals more than so-called black algae.

The name alone suggests something mysterious and difficult to eliminate. Pool operators encounter dark colonies attached to plaster surfaces, treat them aggressively with chlorine and algaecides, and often watch them return.

For decades, the pool industry has referred to these organisms as algae. The term appears in service manuals, retail training, and product labels.

But when examined through the lens of microbiology, the name “black algae” is scientifically inaccurate.

What pool professionals call black algae is not algae at all.

In most documented cases, the organisms responsible for these colonies belong to a group of photosynthetic bacteria known as cyanobacteria.

Understanding that distinction explains why these colonies behave differently from common swimming pool algae and why they are so difficult to eliminate once established.

Misidentifying the organism leads to misidentifying the solution.

Cyanobacteria: Ancient Photosynthetic Bacteria

Cyanobacteria are among the oldest known life forms on Earth.

Fossil evidence indicates that cyanobacteria have existed for more than 3.5 billion years, making them one of the earliest organisms capable of photosynthesis. Research published by the University of California Museum of Paleontology and numerous microbiology studies identifies cyanobacteria as the organisms largely responsible for producing the oxygen that eventually accumulated in Earth’s atmosphere.

Unlike algae, which are eukaryotic organisms with membrane-bound nuclei, cyanobacteria are prokaryotic cells. Prokaryotes lack a defined nucleus and many of the cellular organelles found in eukaryotic organisms.

This fundamental biological difference places cyanobacteria in the bacterial domain, not within the Protista or plant kingdoms where algae reside.

In microbiological classification, cyanobacteria are often referred to as blue-green algae, a historical name that has contributed significantly to confusion within aquatic management industries.

Despite the name, cyanobacteria are bacteria.

Cyanobacteria in Aquatic Environments

Cyanobacteria are widely distributed throughout freshwater, marine environments, and soil ecosystems. Their ability to perform oxygenic photosynthesis allows them to thrive in environments containing sunlight, carbon dioxide, and water.

Many species also possess the ability to fix atmospheric nitrogen, allowing them to survive in environments where nitrogen nutrients are limited.

Research published in aquatic ecology journals and documented in environmental monitoring studies conducted by the United States Environmental Protection Agency identifies numerous cyanobacteria genera capable of forming dense microbial mats.

These include:

• Nostoc

• Oscillatoria

• Microcoleus

These genera are commonly observed in freshwater ecosystems and have been documented in biofilm communities attached to submerged surfaces.

The ability of some cyanobacteria to fix nitrogen provides a competitive advantage in nutrient-limited environments. While many algae rely on dissolved nitrogen sources such as nitrates or ammonia, nitrogen-fixing cyanobacteria can produce usable nitrogen internally.

This metabolic capability helps explain why these organisms sometimes persist even when nutrient levels in the water are relatively low.

Biofilm Formation: The Real Challenge

One of the defining characteristics of cyanobacteria is their ability to form biofilms.

A biofilm is a structured microbial community embedded within a protective matrix of extracellular polymers. This matrix, often described as mucilage, acts as a protective barrier between the microorganisms and the surrounding environment.

Research in microbiology and environmental engineering has shown that biofilms can significantly increase microbial resistance to disinfectants.

Within a biofilm, multiple layers of microorganisms may coexist. Outer layers absorb or neutralize disinfectants before they reach cells located deeper within the structure.

This protective structure explains why colonies of cyanobacteria often appear as dark, raised patches attached to plaster surfaces.

The colony itself is not penetrating the plaster.

Rather, it is anchored to the surface through a microbial biofilm matrix.

The persistent belief that black algae “grows roots into plaster” is therefore another misconception.

Cyanobacteria attach to surfaces through adhesive biofilm structures, not through root-like structures.

Field Sampling and Microscopic Identification

In 2018, samples of suspected “black algae” were collected from residential swimming pools experiencing persistent dark microbial colonies.

Samples were preserved using Lugol’s iodine solution, a common fixative used in microbiological preservation of aquatic microorganisms.

Microscopic analysis was conducted in the Phycology Laboratory at the University of Florida’s Center for Aquatic and Invasive Plants.

The samples were examined under light microscopy by researchers specializing in algal physiology and aquatic microbial ecology.

The dominant organisms identified within the samples were filamentous cyanobacteria forming tightly bound microbial mats.

The genera identified included:

• Nostoc

• Microcoleus

• Oscillatoria

These organisms were embedded within a mucilaginous biofilm structure, consistent with the typical morphology of cyanobacterial mat communities documented in aquatic ecosystems.

The presence of Nostoc was particularly notable because several species within this genus possess the ability to perform nitrogen fixation, allowing them to produce biologically usable nitrogen compounds even when external nutrients are limited.

This capability may help explain why some cyanobacterial colonies persist in environments where nutrient control strategies have been implemented.

Cyanobacteria and Toxin Production

Certain cyanobacteria species are known to produce cyanotoxins, which can be harmful to humans and animals.

Studies published in environmental toxicology literature have documented toxin production in some species of Oscillatoria and Nostoc.

However, toxin production varies widely among species and strains.

The presence of toxin-producing cyanobacteria in swimming pools has not been widely studied, and determining toxin production would require species-level identification and laboratory toxin analysis.

Environmental monitoring programs conducted by the EPA and WHO focus primarily on cyanobacteria in natural water bodies rather than swimming pools.

Nevertheless, the possibility of toxin production reinforces the importance of effective microbial control in recreational water environments.

Why Cyanobacteria Resist Traditional Treatments

Cyanobacteria possess several biological features that increase their resistance to chemical treatments commonly used in swimming pools.

First, the extracellular polymer matrix of biofilms reduces the penetration of disinfectants.

Second, layered colonies allow outer cells to absorb oxidative damage before inner cells are exposed.

Third, nitrogen fixation allows some species to survive in environments with reduced nutrient availability.

These characteristics collectively contribute to the persistence of cyanobacterial colonies once they become established on pool surfaces.

Metals and the Oligodynamic Effect

Certain metal ions exhibit antimicrobial activity through a phenomenon known as the oligodynamic effect.

The oligodynamic effect refers to the ability of small concentrations of metal ions to disrupt microbial cell membranes, enzyme systems, and metabolic processes.

Research in microbiology has documented antimicrobial activity for metals such as:

• copper

• silver

Copper ions can interfere with photosynthetic electron transport systems and damage cellular membranes.

Silver ions are known to interact with microbial proteins and enzymes, disrupting metabolic pathways essential for survival.

Because cyanobacteria are photosynthetic organisms, disrupting photosynthesis can inhibit growth and reproduction.

However, the use of metal ions in swimming pools must be carefully managed due to the potential for metal staining and water discoloration.

Disinfection and Advanced Oxidation

Cyanobacteria are susceptible to several oxidizing agents used in water treatment, including:

• chlorine

• ozone

• permanganate

• chlorine dioxide

Ultraviolet radiation has also been shown to inactivate cyanobacteria in water treatment systems.

However, the effectiveness of these treatments depends heavily on contact time, disinfectant concentration, temperature, and pH.

Studies conducted in drinking water treatment systems often examine individual cyanobacterial cells suspended in water rather than biofilm-protected colonies attached to surfaces.

As a result, treatments that are effective against free-floating microorganisms may be less effective against established biofilm colonies.

Why Accurate Identification Matters

Misidentifying cyanobacteria as algae leads to ineffective treatment strategies.

While algae are eukaryotic organisms, cyanobacteria are bacteria with different metabolic processes, cellular structures, and environmental adaptations.

Understanding the microbiology behind these organisms allows pool professionals to approach treatment more strategically.

For aquatic facility operators and service technicians, this type of scientific understanding is increasingly emphasized in Certified Pool Operator (CPO) training, where water chemistry, microbiology, and disinfection principles intersect.

Accurate identification of microorganisms in recreational water environments is essential for maintaining safe and properly sanitized pools.

The Takeaway

What the swimming pool industry calls black algae is, in most documented cases, a colony of cyanobacteria embedded within a microbial biofilm.

These organisms are not algae.

They are ancient photosynthetic bacteria capable of forming dense colonies that resist chemical treatment through biofilm protection and adaptive metabolism.

Understanding this distinction changes the way pool professionals approach treatment, prevention, and sanitation.

The problem is not algae.

It is cyanobacteria forming a biofilm community on submerged surfaces.

And like many microbial problems in water treatment, the solution begins with understanding the biology behind the organism.

References

Phlips, E. J. (University of Florida). Research in algal physiology and cyanobacteria ecology.

Wojtowicz, J. A. Studies on aquatic microbial communities and water chemistry.

United States Environmental Protection Agency (EPA). Cyanobacteria and Cyanotoxins in Freshwater Systems.

University of California Museum of Paleontology. Cyanobacteria and the Evolution of Earth’s Atmosphere.

World Health Organization (WHO). Toxic Cyanobacteria in Water.

Marks, H. C., Wyss, O., & Strandskov, F. B. (1945). Studies on the Mode of Action of Compounds Containing Available Chlorine.

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