Just continuing on from the "Calcium Levels" thread about nitrate and zooxanthellae.

So nitrate feeds the zooxanthellae increasing the density in the coral, making it appear more brown.

Very few soft corals produce the fluorescing proteins that hard corals do, that is why they are often brown. So high nitrate levels increases the density of zooxanthellae algae which in turn provide more food for the coral. With normal nitrate levels, the coral itself is able to regulate the nutrients that the coral gets, and therefore it density. But if the nitrate levels in the water column get too high, the zooxanthellae population increases, providing more and more food to the coral, and it will die if it can't get rid of it's waste fast enough.

Here is an article which touches on this topic, from the perspective of bleaching, but still gives a good idea about what role zooxanthellae algae plays in corals:

http://www.advancedaquarist.com/issues/feb2002/cw.htm

The colouration in corals comes from proteins:

Yellow, Black, Blue comes from melanins.

Yellow, Orange, Red comes from carotenoids.

White from astaxanthins.

There are also many other proteins and pigment which give these colours and others.

Layton

YEEEEAH an interesting topic

good to know where the colours come from.