Algae colony?

[Goofy]

Hi,

I have recently viewed some water samples out of rotting red onion tissue. It is as usual - full of ciliates, tons of bacteria but also... lots of these round tiny (~15um in diameter) things:
https://photos.app.goo.gl/tgwWe7FSYBZQuvM98

What are they? Most seem dead, very dark, solidified inside. But on rare occasion they are semi transparent and bursting with inside movement like the one in example. Is this possibly some algae colony?

[Goofy]

I think I might have found the answer.
My best bet right now is Haematococcus pluvialis. This would explain different stages at which I observed them.
This is another example photo I took, among some small ciliates:

2019-12-11_0005_60X (1).jpg (101.2 KiB) Viewed 3524 times

And these are different stages found online:

fpls-07-00531-g001.jpg (50.15 KiB) Viewed 3524 times

[mrsonchus]

Most interesting post!
I really like the video - looks great quality to me, and has an 'unrushed' style to my eyes, which gives time for scrutiny whilst letting it run-on.
Interesting to see your proposed ID and the stages present/spotted.

I'm a 'plant botherer' but always appreciate any interesting adventure such as this one - please keep exploring and keeping us updated.

[Bruce Taylor]

They don't look algal. You seem to have Colpoda in that culture, and I suspect these are their resting and/or reproductive cysts. Colpodids form resting cysts to survive periods of drought, but also encyst for the purpose of division (palintomy).