The term “black algae” is a misleading common name for a variety of dark-colored microbial growths found in damp environments, particularly aquatic systems and exteriors of homes. These growths are not true algae, but rather highly resilient bacteria or fungi that appear dark due to protective pigments. Determining the potential harm these organisms pose to health or property depends entirely on the specific organism and its location.
Identifying the Organisms Called “Black Algae”
The dark spots commonly identified as “black algae” in swimming pools are most often a form of cyanobacteria, which are photosynthetic bacteria also known as blue-green algae. These organisms appear black or dark blue-green because they produce a protective outer layer that shields them from ultraviolet light and chemical sanitizers. Unlike free-floating green algae, this pool-based cyanobacteria establishes deep, root-like filaments in the porous surfaces of plaster, concrete, and grout.
On exterior surfaces like roofs and siding, the black streaks are caused by a completely different organism, the cyanobacterium Gloeocapsa magma. This airborne bacterium thrives in warm, humid climates and forms dark-pigmented colonies to protect itself from the sun. In natural bodies of water, such as lakes and slow-moving rivers, dark blooms are often caused by various species of cyanobacteria, including genera like Microcystis and Anabaena, which are known to form harmful algal blooms (HABs).
Health Risks from Ingestion and Contact
The most significant health threat from organisms identified as “black algae” comes from bloom-forming cyanobacteria in natural water sources. These organisms can produce potent chemical compounds called cyanotoxins, which are classified based on their target organ, such as microcystins and anatoxins. Microcystins are hepatotoxins that primarily target the liver, while anatoxins are neurotoxins that can cause rapid neurological distress. Exposure can occur through ingesting contaminated water, inhaling aerosolized toxins near a bloom, or direct skin contact.
Acute symptoms following exposure to these cyanotoxins often include gastrointestinal issues like nausea, vomiting, and diarrhea, along with headaches and fever. In pets, particularly dogs that drink or swim in contaminated water, ingestion of these highly concentrated toxins can lead to severe illness or death.
In contrast, cyanobacteria found in properly maintained swimming pools pose a lower direct toxicological health risk. While the pool organism itself is not known to produce cyanotoxins, its presence signals a breakdown in sanitation and water quality. This poor condition can allow other, more dangerous waterborne pathogens to thrive, potentially causing skin irritations or minor illnesses upon contact or accidental ingestion. The roof-based Gloeocapsa magma is considered harmless to human and animal health.
Structural Damage and Removal Difficulty
The presence of “black algae” on both pools and homes causes significant physical damage and increases maintenance costs due to the difficulty of removal. In swimming pools, the cyanobacteria penetrate the microscopic pores of concrete and plaster, making them extremely resistant to standard chemical treatments. The organism forms a robust, multi-layered protective film that shields the inner cells from chlorine and algaecides.
Removing these deep-seated spots requires aggressive mechanical action, such as scrubbing with a stainless steel brush, to break the protective layer before applying specialized, concentrated chemicals. If the infestation is severe or recurring, the cyanobacteria can leave permanent stains, eventually requiring costly pool resurfacing or an acid wash to fully eradicate the growth.
On asphalt shingle roofs, the Gloeocapsa magma cyanobacterium causes damage by feeding on the calcium carbonate filler material within the shingles. This biological action leads to the premature loss of protective granules and overall degradation of the roofing material. The dark streaks also increase the roof’s heat absorption, which can raise attic temperatures and reduce the energy efficiency of the home. The sustained moisture retention by the cyanobacteria further contributes to the degradation of the shingles over time.