The appearance of dark, stubborn spots on a pool’s surfaces, commonly called “black algae,” is one of the most challenging issues pool owners face. This persistent problem is notoriously difficult to eradicate because it involves a highly resistant microorganism and a complex protective mechanism. Understanding the true nature of this organism and the specific conditions that allow it to thrive is the first step in addressing its presence.
Defining the Pathogen: Cyanobacteria
The organism responsible for “black algae” is not true algae but a type of photosynthetic bacteria known as cyanobacteria. These microbes are among the oldest life forms on Earth and are highly adapted to survive in various environments. Cyanobacteria are technically blue-green in color due to the presence of chlorophyll, which they use for photosynthesis.
The distinctive black or dark blue-green appearance is caused by the bacteria producing a dark pigment that acts like an internal sunscreen. This pigment shields the organism from intense ultraviolet (UV) light and helps protect it from chemical disinfectants like chlorine. Unlike true algae, which can be killed relatively easily, the black spots you see are dense, multi-layered colonies that have anchored themselves firmly to the pool surface.
Failure Points in Water Management
A black algae outbreak begins with the introduction of cyanobacteria spores into a compromised environment. These spores can enter a pool on contaminated swimwear, pool cleaning equipment used in natural water bodies, or simply by being carried through the air by wind or rain. Once introduced, the spores will only begin to colonize if certain environmental conditions are met.
A primary failure point involves low or inconsistent levels of free chlorine, the pool’s main sanitizer. Cyanobacteria can survive in chlorine concentrations that would easily kill most other contaminants, meaning a slightly below-standard chlorine level or a high pH that reduces chlorine effectiveness provides them a window to establish a foothold. The presence of high nutrient levels, such as nitrates and phosphates, fuel rapid growth and colonization.
Poor circulation and filtration create sheltered environments where the bacteria can settle without disturbance. Areas with minimal water flow, often called “dead spots,” include steps, ladder treads, corners, and areas behind skimmers or lights. These stagnant zones allow the organisms to attach to surfaces without being consistently exposed to circulating sanitizer.
The condition of the pool’s surface also plays a significant role in colonization. Cyanobacteria thrive on rough, porous surfaces like aging plaster, concrete, or deteriorated grout lines. These imperfections create microscopic crevices where the bacteria can embed themselves deeply, making them difficult to reach and physically remove with routine brushing. This physical shelter is a prerequisite for deploying its defense mechanism.
The Role of Biofilm in Persistence
Once the cyanobacteria have successfully colonized a suitable surface, they secrete a complex biological structure known as a biofilm. This biofilm provides a physical and chemical shield for the colony beneath, making elimination challenging. The material is a self-secreted, sticky matrix composed primarily of extracellular polymeric substances (EPS), which are complex molecules like polysaccharides, proteins, and lipids.
This slime-like matrix forms a dense, multi-layered cap over the colony, effectively sealing the bacteria off from the surrounding water. The biofilm’s structure physically impedes the penetration of chemical disinfectants, including high concentrations of chlorine. Chlorine molecules cannot diffuse through the thick polysaccharide layer quickly enough to kill the organisms embedded deep within the colony.
The bacteria within the colony are further protected by the biofilm’s ability to maintain a localized, favorable microenvironment. This defense allows the cyanobacteria to survive even temporary improvements in water chemistry that might otherwise kill them. The persistence of black algae results from this highly evolved, localized protective mechanism that shields the core colony from sanitation efforts.