How Phosphate and nitrate cause SPS bleaching?

[Marco.Polo]

Hi!

I have tried to understand the reason SPS bleach at high phosphate and nitrate levels.
i found a reasonable explanation that claim that at high level the coral will expel the zooxanthallae .

My questions is why?
I saw people claim that
"This browning is caused by their zooxanthallae in the corals tissue absorbing nitrate. "

What that mean ? the zooxanthallae take nitrate and how its actually affect the SPS coral?

 

[Allmost]

browning ? or bleaching ?

2 different terms.

 

[Ssteve]

As shown in the "guess my phosphate levels" thread, I wouldnt state they cause bleaching as a fact. I have pretty high phosphates in my tank recently and had several new frags color up really well in a very short period of time. I've also had nitrates over 20 and colors never suffered.

 

[yuckywacky]

Both i would say

 

[Marco.Polo]

Browning i mean also what cause them bleach.