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And the Biofilm Thickens: Investigating the True Nature of Swimming Pool “Black Algae”

For several years, I have incorporated discussions about swimming pool algae into my Certified Pool Operator (CPO) training programs and other aquatic facility education courses. While preparing educational materials for my students, I initially set out to compile existing research on what pool service professionals commonly refer to as “black algae.”

In the context of this discussion, understanding Swimming Pool “Black Algae” is crucial for effective treatment and prevention.

What I quickly discovered was that very little peer-reviewed research exists specifically addressing black algae growth in chlorinated swimming pools.

The nature of Swimming Pool “Black Algae” can lead to misunderstandings among pool owners.

Most scientific literature focuses on cyanobacteria in natural freshwater systems such as lakes and reservoirs. These organisms are widely studied because some species can produce cyanotoxins, compounds that pose environmental and public health concerns.

Swimming Pool “Black Algae” is often mistaken for other types of algae found in pools.

However, relatively few studies examine cyanobacterial biofilms in disinfected recreational water systems, particularly residential and commercial swimming pools.

Identifying Swimming Pool “Black Algae” correctly is essential for proper maintenance.

Because of this lack of literature, we decided to investigate the phenomenon directly through field sampling and laboratory analysis.

Our research shows that Swimming Pool “Black Algae” can be more complicated than previously thought.

Our initial findings suggested something important:

What pool professionals commonly call black algae may not be algae at all.

This new understanding of Swimming Pool “Black Algae” could change how we treat it.

Instead, the growth appears to be a biofilm dominated by filamentous cyanobacteria, organisms historically referred to as “blue-green algae.” Modern microbiology recognizes cyanobacteria as photosynthetic bacteria rather than true algae.

This interpretation aligns with the broader microbiological understanding of biofilm communities in aquatic systems.

Understanding the microbiological aspects of Swimming Pool “Black Algae” is vital for pool maintenance.

Cyanobacteria and Biofilm Formation

Cyanobacteria are among the oldest known photosynthetic organisms on Earth and play significant roles in aquatic ecosystems. According to research published in journals such as Water Research and Environmental Microbiology, cyanobacteria frequently grow within biofilm structures composed of extracellular polymeric substances (EPS).

The presence of Swimming Pool “Black Algae” can significantly impact water quality.

These biofilms form protective matrices that allow microorganisms to adhere to surfaces and resist environmental stresses.

In swimming pools, surfaces such as plaster, tile grout, and rough concrete can provide suitable substrates for microbial attachment. Once established, cyanobacterial colonies may form dark, spot-like growths that pool technicians often identify as black algae.

Swimming Pool “Black Algae” requires a targeted treatment approach to manage effectively.

Laboratory analysis of samples collected from swimming pools during our early investigation identified cyanobacteria belonging to the genus Nostoc.

Species within the Nostoc genus are widely documented in microbiological literature and are known to form gelatinous colonies surrounded by exopolysaccharide matrices. These matrices are typical of microbial biofilms and contribute to their persistence on submerged surfaces.

Addressing issues related to Swimming Pool “Black Algae” is essential for pool operators.

Laboratory Testing vs Field Conditions

During conversations with manufacturers involved in algaecide development, I also learned that many product efficacy tests are conducted using laboratory-cultured algae species rather than organisms collected directly from swimming pool environments.

Research about Swimming Pool “Black Algae” will help develop better cleaning products.

This practice is not unusual in chemical testing. Controlled laboratory cultures allow researchers to standardize experimental conditions. However, laboratory organisms may not perfectly represent the complex microbial communities present in real pool biofilms.

This distinction is important because biofilms can behave very differently from free-floating microorganisms.

Understanding how Swimming Pool “Black Algae” interacts with disinfectants is crucial for treatment.

Research from the Center for Biofilm Engineering at Montana State University, one of the leading institutions studying microbial biofilms, has demonstrated that organisms embedded within biofilms often show increased resistance to chemical disinfectants and antimicrobial treatments.

For this reason, field studies using real pool samples can provide valuable information about how treatments perform under actual operating conditions.

Studies on Swimming Pool “Black Algae” can lead to improved sanitation techniques.

Investigating the Potential for Cyanotoxins

After identifying cyanobacteria in pool biofilms, another question emerged.

Investigating Swimming Pool “Black Algae” is vital for ensuring safe swimming conditions.

Some cyanobacteria species are known to produce toxins in natural water bodies. These compounds, collectively known as cyanotoxins, include substances such as microcystins, anatoxins, and cylindrospermopsins.

The presence of toxin-producing cyanobacteria in recreational water is a subject of ongoing scientific study.

Discovering new treatments for Swimming Pool “Black Algae” is an important area of research.

Given this background, we considered whether cyanobacteria found in pool biofilms might also produce toxins.

At the same time, it is important to recognize that swimming pools operate under continuous disinfection conditions. Chlorine concentrations in properly maintained pools are typically maintained at 1–3 mg/L (ppm) or higher, depending on regulatory standards.

Efforts to combat Swimming Pool “Black Algae” involve understanding its biology.

Numerous studies in water treatment science show that chlorine is capable of oxidizing and degrading many cyanotoxins, including microcystins, under appropriate conditions.

Based on these findings, our working hypothesis was that normal disinfectant levels in properly maintained swimming pools would likely degrade cyanotoxins if they were present. Nevertheless, laboratory confirmation was necessary.

We need to understand Swimming Pool “Black Algae” to develop effective management strategies.

Field Sampling in Atlantic Beach, Florida

To continue our investigation, we sought additional samples from swimming pools exhibiting black algae growth.

Sampling for Swimming Pool “Black Algae” will help assess its prevalence in different locations.

A public pool facility in Atlantic Beach, Florida provided an opportunity to collect specimens prior to treatment. Pool professional Lisa Arquilla Dodge of Pool Care Pros assisted with access to the site.

At the time of collection I was teaching out of state, so Dr. Fred Singleton, Senior Scientist of Microbiology at Earth Science Laboratories, conducted the field sampling.

Identifying factors associated with Swimming Pool “Black Algae” can enhance treatment approaches.

Dr. Singleton holds advanced degrees in microbiology and ensured proper specimen handling during collection, transport, and laboratory preparation.

The samples were subsequently submitted to GreenWater Laboratories, a laboratory recognized for its work in cyanobacteria and cyanotoxin analysis.

Further testing of Swimming Pool “Black Algae” can lead to breakthroughs in pool care.

Laboratory Identification of the Atlantic Beach Samples

Microscopic examination of the collected material revealed that the dominant organism present in the Atlantic Beach biofilm was a filamentous cyanobacterium belonging to the genus Leptolyngbya.

As we continue our work on Swimming Pool “Black Algae”, new insights will emerge.

Leptolyngbya species are widely documented in aquatic microbiology literature. Unlike some other cyanobacteria, these organisms are not commonly associated with cyanotoxin production.

Because toxin-producing species were not identified in the samples, further toxin analysis was not pursued for that particular specimen set.

The ongoing study of Swimming Pool “Black Algae” is crucial for public health.

However, the results provided another important insight.

The microbial composition of swimming pool biofilms can vary significantly between locations, even within relatively short geographic distances.

Awareness of Swimming Pool “Black Algae” can empower pool operators to better manage it.

Geographic Variability in Biofilm Communities

During the same time period, I attended a lecture at the World Aquatic Health Conference presented by Dr. Darla Goeres of the Montana State University Center for Biofilm Engineering.

This systematized approach to Swimming Pool “Black Algae” can foster better maintenance practices.

Dr. Goeres discussed how microbial biofilm communities often vary significantly depending on environmental conditions and geographic location.

Our findings appeared consistent with that principle.

Addressing Swimming Pool “Black Algae” will make pools safer for all users.

Previous samples collected from another Florida pool contained different cyanobacterial genera, including organisms resembling Nostoc, Microcoleus, and Oscillatoria.

In contrast, the Atlantic Beach specimens were dominated by Leptolyngbya.

Future research on Swimming Pool “Black Algae” holds promise for innovation in pool care.

Even though the two sampling sites were less than 100 miles apart, the microbial composition of the biofilms differed substantially.

The Collective Cyanobacteria Community

Recognizing the impact of Swimming Pool “Black Algae” can guide effective treatment protocols.

Microscopic analysis of the Atlantic Beach biomass also revealed the presence of several other microorganisms within the biofilm matrix.

These included:

The dynamics of Swimming Pool “Black Algae” reveal the complexity of biofilm management.

• Aulacoseira ambigua and Aulacoseira herzogii (diatoms)

• Staurastrum species (green algae)

  Understanding the lifecycle of Swimming Pool “Black Algae” can inform maintenance strategies.

• an unidentified dinoflagellate

• small chlorophyte unicellular algae

  Collective research on Swimming Pool “Black Algae” enhances knowledge across the industry.

These findings suggest that what pool professionals identify visually as black algae may actually represent a complex microbial biofilm community, with cyanobacteria serving as the dominant structural organism.

The extracellular polymeric substances produced by cyanobacteria appear to provide a matrix that allows other microorganisms to coexist within the biofilm.

Ultimately, the goal is to effectively manage Swimming Pool “Black Algae” for healthier swimming environments.

For this reason, we began referring to the growth informally as a Collective Cyanobacteria Community, reflecting the diverse biological composition of the biomass.

Investigating the True Nature of Swimming Pool “Black Algae”

Because the Atlantic Beach samples did not contain cyanobacteria known to produce toxins, additional sampling efforts continued in search of colonies containing toxin-associated genera.

Further investigation may provide additional insights into:

• geographic variability of pool biofilms

• cyanobacterial species present in swimming pools

• interactions between biofilms and disinfectants

• effective treatment strategies for persistent surface growth

Dr. Darla Goeres summarized the broader significance of this work well when she commented on the findings:

“Rudy, your research points to the importance of conducting a larger study where a number of pools are sampled to look for trends and how and if the trends correlate to certain pool parameters… It also demonstrates the importance of validating the efficacy of a product in field trials.”

Her observation reflects an important principle in applied microbiology.

Real-world systems—especially something as complex as a swimming pool—often behave differently than laboratory models.

Similar Article Swimming Pool Black Algae

Recognition from Industry Microbiologists

The investigation into cyanobacterial biofilms in swimming pools also drew attention from microbiologists working within the water treatment and pool chemical industries.

One of those voices was Roy Vore, who at the time served as a microbiologist at BioLab, one of the major manufacturers of swimming pool treatment chemicals in North America. During discussions about the findings surrounding cyanobacteria in swimming pool biofilms, Vore offered a candid assessment of the work being conducted.

“You probably know more about black algae than anyone else on the planet, Rudy.”
— Roy Vore, Microbiologist, BioLab

While informal in nature, the comment reflects a broader reality within the swimming pool industry: very little dedicated microbiological research has historically focused on the organisms responsible for swimming pool “black algae” growth in chlorinated swimming pools.

Most academic research on cyanobacteria has focused on freshwater ecosystems, harmful algal blooms (HABs), and drinking water reservoirs, where cyanotoxin production poses environmental and public health risks. Comparatively few studies examine cyanobacterial biofilms in disinfected recreational water systems, leaving a gap between academic microbiology and the practical challenges faced by pool operators.

For pool professionals, this gap reinforces the importance of field observation, laboratory collaboration, and applied research when addressing persistent microbial growth in aquatic facilities.

Why This Matters for Pool Operators and Aquatic Facility Professionals

Understanding the microbiology behind swimming pool biofilms is more than an academic exercise. For professionals responsible for maintaining recreational water quality, it directly affects how problems are diagnosed and treated.

Biofilm-forming microorganisms—including cyanobacteria (Swimming Pool “Black Algae” ) —can exhibit increased resistance to chemical treatments because the extracellular polymeric matrix protects the organisms embedded within the colony. Research published by the Montana State University Center for Biofilm Engineering has demonstrated that microorganisms growing in biofilms often behave very differently from planktonic cells exposed directly to disinfectants.

For Certified Pool Operators (CPOs) and aquatic facility managers, recognizing the difference between:

• simple algae growth

• cyanobacterial biofilms

• and multi-species microbial communities

can influence treatment strategies, sanitation practices, and long-term pool maintenance protocols.

Take a Specialized Ondemand Algae Certification Class

Acknowledgments

This work would not have been possible without the collaboration of several individuals and organizations who contributed expertise and resources:

• Lisa Arquilla Dodge – Pool Care Pros, Jacksonville Florida

• Dr. Fred Singleton – Earth Science Laboratories

• Mark Aubel and the team at GreenWater Laboratories

• Earth Science Laboratories

Their assistance enabled continued exploration of the microbiology behind one of the most persistent challenges in swimming pool maintenance.

Related Research Topics for Aquatic Professionals

Cyanobacteria biofilms in swimming pools
Swimming pool microbiology and biofilm formation
Black algae treatment strategies for pool operators
Water chemistry management in commercial pools
Certified Pool Operator training and aquatic facility education

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