Black Algae and Skin Infections


New member
May 19, 2019
Olympia, WA
Hello, I'm posting this as a dermatologist. I just saw a young girl with an eczema like rash on her thighs that she thinks got started from sitting on a cement barrier in a pool that has several areas that look like there is "black algae".

There are several other explanations for her rash, but I was curious whether anyone out there has seen these types of algae cause skin infections in humans? I'm doubtful that they can. However, I'm also pretty certain that if i swabbed the area and sent it to the microbiology lab, it is horribly unlikely that they'd be able to culture and grow any of these organisms, so infection may be challenging to prove.


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
If the pool had black algae in it, then the sanitation was compromised which means the water could have been host to any number of bacterial pathogens. Although I’m not an MD (I just play one on TV), You should probably treat with a broad spectrum antibiotic to cover all your bases. Lab confirmation would take too long and bacteria can be difficult to culture if you’re not sure what you’re looking for. Better to treat based on symptoms and follow up in a few days to ensure the patient is recovering.

And someone should tell the pool owner to clean the dang pool and get it properly sanitized😤
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Bronze Supporter
Nov 16, 2016
I work in a lab which identifies algae and cyanobacteria under the microscope. You don't need to culture the cyanobacteria. If it is visible to the naked eye there will be enough cells to see and identify under the microscope. Cyanobacteria are usually around 2-10microns in size which make them easy to see under the microscope without culture.


TFP Expert
Mar 2, 2011
sitting on a cement barrier in a pool that has several areas that look like there is "black algae".
The cement surface is rough and it will scratch the skin and allow the toxins from the bacteria to get below the surface and cause problems.

As noted, there will also be other bacteria in the water if the sanitation is not being maintained, which seems to be the case.
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