Algae are microscopic aquatic plant-like organisms that do not have roots, stems and leaves like higher plants. However, they do require the same type of nutrients to grow, and they utilize sunlight to carry out photosynthesis.
While algae can be quite a nuisance in swimming pools, they are
actually the most important photosynthesizing organisms on earth.
They capture more of the sun’s energy and produce more oxygen
(a biproduct of photosynthesis) than all other plants combined. In
addition, many species of animals depend on algae as a food
Algae size can vary greatly — they can grow in a number of
different habitats, endure a wide range of temperatures, and live
everywhere from hot springs to deep within polar ice. Microscopic
algae, called phytoplankton, are typically found in lakes and
oceans. The largest forms of algae are seaweeds that can grow up to
300 feet and stretch from the bottom of the ocean to the surface of
the water. Although most algae grow in water, they can also grow on
soil, trees, under or inside porous rocks, and even on some
Types of algae in swimming pools
Green algae are the most common, with more than 7,000 different
species in existence. Approximately 10 percent of these are marine
species (found in the ocean) with the remainder being found in
lakes, rivers, ponds and swimming pools. Green algae are named for
the chlorophyll (the molecule that captures light energy to carry
out photosynthesis) in the cell which gives them their green color.
Green algae can be free-floating or surface clinging, and can be
found in all types of pools. They are typically the easiest to
treat, however, some species may be more difficult to manage than
Mustard algae have been described as “adapted green
algae.” Mustard algae also contain chlorophyll, but the green
color is masked by the presence of beta-carotene. Carotenoids (such
as beta-carotene) are found in many photosynthetic organisms. These
compounds are responsible for color (like the orange color of
carrots), which is why mustard algae are yellow rather than
Carotenoids are anti-oxidants (or reducing agents). They protect
against damage cause by oxidation. Mustard algae can use these
compounds as a defense mechanism to help them survive in a
chlorinated environment. Because carotenoids protect against
oxidation, chlorine (as an oxidizer) may have little effect on
algae once this defense mechanism has been activated within the
So-called “black algae” are actually not algae at all
— they’re aquatic photosynthetic bacteria called
cyanobacteria. They are single-celled but grow in large colonies.
While black algae contain chlorophyll, they also have compounds
called phycobilins, which mask the green color of chlorophyll.
There are two types of phycobilins: phycocyanin, a blue pigment,
which gives the cyanobacteria their name; and phycoerythrin, a red
pigment, which exists in red or pink algae (often found around
sinks or drains). Cyanobacteria are very important organisms that
assist in the growth of many types of plants. They are one of very
few organisms that can convert inert atmospheric nitrogen into a
form that plants can use, such as ammonia or nitrate.
These bacteria can form a “sticky layer” on the outside
of each cell, which protects them from the surrounding environment.
The process of photosynthesis carried out by the bacteria depletes
carbon dioxide in the surrounding water. A decrease in carbon
dioxide concentration causes the precipitation of calcium carbonate
in that area. So the calcium carbonate, along with any other
sediment that may precipitate, becomes trapped within the sticky
layer. The bacteria will then grow through and over the sediment as
they continue to photosynthesize and grow. This process will occur
over and over again, forming more layers and making the black algae
more difficult to treat. Brushing is extremely important when
treating black algae because that protective layer has to be broken
in order for the sanitizer or algaecide to come in contact with the
Phosphate and algae growth
Phosphorous plays an important role in the photosynthesis and
respiration of many different types of plant life. However, the
optimum phosphorous requirement varies widely between algae
species. Because there are more than 7,000 species of green algae,
it would be difficult to predict the minimum phosphorous level
required for a specific type of algae that may exist in a
In addition, it has been determined that most algae have the
ability to store phosphorous within their cells. This is known as
“luxury phosphorous uptake,” and it means that algae
can continue to grow even in a phosphorous-deficient environment.
Without the presence of orthophosphate, many species of algae can
convert other phosphorous-containing compounds into a usable form
because of enzymes that exist at the cell surface (also known as
phosphomonoesterases or phosphatases). This is just another defense
mechanism that algae have to help them survive.
The growth of algae will be affected by the presence of sanitizer
and/or algaecide. For example, hypochlorous acid will enter the
cell wall of bacteria or algae and disrupt metabolic activity. With
metabolic activity being disrupted, growth cannot continue, even in
nutrient-rich surroundings. It is important to choose products for
algae treatment or prevention that have been registered by the U.S.
Environmental Protection Agency. EPA registration is a rigorous
process that requires the submission of supporting data. Compounds
that currently carry EPA registration for algae treatment include
hypochlorous acid, hypobromous acid, ammonium chloride quats,
polyquats, copper and silver.
Quaternary ammonium compounds (quats) are positively charged and
are therefore attracted to the negative charge on the cell wall of
the algae. The quat’s wetting agent properties as well as
this charge attraction allow it to enter the cell wall which causes
the cell wall to break. The structure of the quat makes a
difference when it comes to the effectiveness of the product.
Quaternary ammonium compounds have a “chain” of carbons
on the molecule, and these carbon chains can vary in length.
Through research and testing, it has been determined that biocidal
activity peaks at a carbon chain length of 14.
Polymeric quat compounds work in a similar manner to ammonium
chloride quats (as discussed above). However, because they are much
larger molecules, they tend to work a little slower than ammonium
Copper is effective at killing algae because it disrupts enzymatic
activity within the cell. If the enzymes do not function properly,
the organism cannot survive.
Preventing algae growth
Ultimately, proper maintenance is the key to keeping algae growth
in check. The most important consideration is the presence of
sanitizer — or even better, sanitizer and algaecide.
Establish a maintenance system that includes maintaining
appropriate sanitizer levels, routine oxidation and application of
a preventative algaecide. Remember that algae treatments will be
much less effective if the pool is unable to maintain a sanitizer
But don’t forget the physical aspect involved in pool care
too. Proper circulation and filtration, as well as routine brushing
and vacuuming of the pool surface, are all very important. Because
some algae have a protective outer coating, brushing will help
break through that outer coating, allowing products to work more