[Tripod1404]

I nature corals grow in very low soluble nitrate and phosphate levels (almost in trace amounts) and the alkalinity of natural sea water is around 7.5dKH.

In a tank if you try to replicate the low nutrient levels (through carbon dosing and subsequent consumption of nitrate and phosphate), you need to lower alk as well.

The reason for this is not very clear. But from a very simplistic point of view. corals need Ca and Alk to built their skeletons. More alk allows more skeleton to be built. But this also requires the coral soft tissue to grow larger and produce more calcium carbonate. So in higher alk coral cells themselves need more phosphate and nitrate to grow and built CaCO3 skeleton, but they also need N and P to feed their symbiotic algae. And in lower alk, the case is the opposite.

Phosphate is also known to be a substrate that protects corals from bleaching. Now its important to consider that whitening happens first around the tips, than the parts of the coral that receive more light and eventually the whole coral will turn pale.

Tips of the corals normally receive the most light and they are the parts that grow the most. So I think what happening is a nutrient imbalance between the symbiotic algae and the coral cells. Corals and zooxanthellae form a synbiotic relationship where corals provide the nitrate and phosphate and in turn algae share some of the carbon dioxide it fixes into organic compounds with the coral. When you have high alk but low N and P, it creates a large nutrient imbalance around the tips. When N and P drops but the alk is high, both zooxanthellae and the coral cells require N and P. Since there is not enough N and P for the both, coral expels some of the zooxanthellae to reduce the demand, hence they turn pale. This is followed by the paling of the parts of the coral that receive more light. In these tissues, coral growth is slow (but there still is growth as branches get thicker over time), but since they get good light algae still need N and P. If N and P are low, coral expels them to reduce the population. Otherwise they will drain the coral cell of N and P. The last parts to get pale are the shaded regions since the N and P demand of the algae living there are the lowest.

Now the actual tip burning (tissue necrosis) happens becouse of rapid N and P drops. In this case N and P become limiting so fast that coral doesn't have time to expel the zooxanthellae. THis cause an extreme N and P starvation. This cause generation of reactive oxygen (ROS) species that kill the cells.

So why does ROS form? It is because the mechanism how photosynthesis operate. This is true for zooxanthellae, cynaboacteria or higher plants. Photosynthesis is simply a process that cannot be turned off. Higher plants have the most complex regulation over this (and regulation corals have is no where near to what higher plants have) and they try to control it by moving their leaves or moving their chloroplast, etc to achieve shading, but even then they have a tolerance limit. Chlorophyll will simply continue to harvest light energy, if there is not enough N and P to produce the enzymes (require N) and ATP (require P) to harvest and distribute this energy, that energy will be dissipated by transferring it to oxygen and water molecules. This forms the highly reactive oxygen intermediates that simply burn the cell from inside out. And this results in tissue necrosis.

So long story short; low N and P require alk to be low as well, because high alk put more demand on N and P from the corals as well. When there isnt enough N and P for both the coral and the zooxanthellae. Zooxanthellae gets kicked out.