Originally Posted by 150EH
Chlamydomonas is probably the most studied algae on planet. More is known about it than any other. It's used as a model because it's easy to to cultivate and has many aspects to it's physiology.
GDA goes through it's daily cell cycle. Each adult cell will divide internally up to 3 times a day, producing 2, 4, or 8 zoospores. They break out and grow into adult cells. This is the algae's method of asexual reproduction and it repeats continously *under favourable conditions*.
Eventually the number of cells will increase and the glass will go green. During the (subjective) night, the cells produce a substance that is responsible for them sticking to the glass. Flow in the tank will push more cells onto/passed certain spots and they will build up.
How long it takes before you see this buildup depends on level of ammonium, intensity of light, presence of calcium, temperature (apparently 33 celsius will halt the cell cycle completely!), possibly flow and as always, other stuff! You can slow this process down by reducing light intensity and reducing ammonium but you can also halt it.
To do this you can provoke the adult cell it into it's sexual cycle. There it will eventually end up as spores, no longer dividing asexually and effectively frozen in time. To achieve this, you have to subject it to unfavourable conditions which is known to include removal of ammonium (but might include other sources of N also).
Now, the adult cell no longer divides into zoospores, but instead divides into gametes. These burst out, swim around and pair with each other, grow into a zygote and turn into a zygospore. You can tell when this has happened because the GDA will go a reddish colour due to oils and starch being stored in the spore.
This is a know method to get rid of GDA. Leave it for 3 weeks, let it go red, clean it out then it won't come back. This doesn't always work, possibly because it only works when GDA is in it's sexual cycle, ie in the absence of ammonium. If you have ammonium in your tank, you will be stuck in the perpetual cell cycle of asexual division.
It also uses phototaxis to swim toward the light in the day (to grow) and chemotaxis to swim toward a source of ammonium at night (to reproduce). Once it attaches to the glass it will divide. It does this at night to avoid UV damage during DNA replication - a feature that developed billions of years ago when the planet had little or no ozone and UV was far more dangerous.