The plumber was just in my apartment to clear a clogged bathroom sink.
He pulled up the drain stopper and pointed out that it was all covered
with something he called "bacteria". It extended down to fill up the
trap. He snaked the pipe and let the debris fall into a bucket, which
he then dumped in the toilet, and pointed to a particularly large piece
that could have been the source of the problem. He says it is a common
problem in apartments.

I think it is the same as the kind of black or very dark green stuff I
sometimes see around the drain of the sink.

Anyway, I'd just like to know whether there is a kind of algae that does
this and whether in certain seasons it "blooms" and clogs up the pipes.

The plumber recommends draining a couple of tablespoons of bleach in a
sinkful of water once a week. I don't know how environmentally friendly
that is. If the algae only blooms seasonally, I might be able to limit
the bleach treatment only to the season when the algae is in bloom.

As long as it is living in my apartment, I figure I might as well study it.
I have a microscope, even though I'm not very skilled at using it. It can
magnify from between about 50X to about 675X. I don't know if there is any
chance of identifying the exact species of algae (if that is what it is).
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.