Sand Algae

Some years ago, I encountered a number of pools with unusual deposits that showed up in spring after I cleaned the slime off the pool walls. Close examination revealed tiny granular deposits, that looked exactly like glued on sand particles. But, on testing with acid, it turned out that all these sand particles were not quartz but were actually calcium carbonate capsules. Even more interesting, macro-photography revealed that live algae remained INSIDE of these capsules.

Apparently, some species or morphs of algae . . . or perhaps algal communities, since all bio-films comprise multiple species, and usually multiple kingdoms . . . produce these capsules. It's not clear whether they do so as a means of protection, or as a byproduct of extracting carbon dioxide from the water.

Anyhow, are my photos, from 2001 are first, followed by two recent photos from an AG pool owned by PoolForum user reddolphin. Reddolphin was able to confirm composition of her scale, by collecting small bits of the deposits, and putting them in vinegar, where they fizzed and dissolved rapidly. See RedDolphin's thread here: http://www.poolforum.com/pf2/showthread.php/11918

Because the calcium of these capsules is thin, they can be dissolved by low alkalinity water with moderately low pH. Essentially, the process for lowering alkalinity, here, will also remove these deposits. However, high chlorine levels are needed to bleach out the algal stains

The original slime area, under which the sand algae lay.
(The photo is upside down, with waterline tiles at the bottom.)
https://lh4.googleusercontent.com/-n...gae_01_HHH.jpg

A partially cleaned area, covered by sheet-ed scale, rather than individual particles:
https://lh4.googleusercontent.com/-v...gae_03_MSS.jpg

Sheet-ed scale on steps:
https://lh4.googleusercontent.com/-d...gae_05_MSS.jpg

And, sand like deposits on an eyeball, with algae visible INSIDE of the capsules:
https://lh4.googleusercontent.com/-h...gae_09_HHH.jpg

Again, the sand-like capsules:
https://lh6.googleusercontent.com/-y...gae_03_HHH.jpg

A close-up of the capsules:
https://lh5.googleusercontent.com/-y..._HHH_close.jpg


From reddolphin's pool -- you can see the individual capsules adjacent to some larger more sheetlike areas.
The area to the left is wet; the white more distinct areas are dry:
https://lh4.googleusercontent.com/-o...MG_2544-ed.jpg

And, a slightly higher resolution shot from the same pool:
https://lh4.googleusercontent.com/-S...MG_2545-ed.jpg

Ben, that's what I thought might be happening, though not with any technical reason. (I'm thinking this is in response to the 'calcified' liner question)

I am wondering if this is a special type of algae or if the water balance is being pushed over the edge for scaling because of the algae giving off CO2 in their normal respiration and causing the the scale to encase the algae, which I suspect is the scenerio. I can see this happening if the pH is already high and the calcium is high. The localized increase in carbon dioxide in the water could be converted into bicarbonate under the right conditions and cause a localized scale deposit around the CO2 'bubble' and create what seems to be a 'grain of sand'. I have been searching and searching and cannot find any references to fresh water calciferous algae although it is very common in seawater aquariums and coral reefs.

Waterbear,

I thought the same thing, at the time. I even had created a web page for PoolSolutions to explain this mystery algae, anticipating that it would answer a lot of people's questions. Instead, I found nobody had questions, or even recognized it.

And what's weirder is, was seeing it a number of times in 2001 and 2002, but never since. So, I don't know.

Ben

because, as you said, they produce a thin capsule around the algae (which I asume is the CO2 bubble that algae can produce) and from looking at the pics where the capsules do look like bubbles made of scale I really do think this is something that happens under certain water balance conditions. It is reinfored by the fact that you saw it during a specific time period (and I will guess that the calcium levels in the local fill water or the alkainity or both were higher than normal at that time). Right now in my area calcium levels are much higher than normal because of our much lower than normal water table.

While I'm not the chemist y'all are, this sounds reasonable.

Evan, always good to 'see' you , my friend :)