Coral confusion

This seems like an important bit of research. Not all corals appear to stop calcifying with lower pH, and it would seem that the precise mechanisms for calcification are not as well understood as you might expect.

This paper reports that it would appear that what is crucial for good coral calcification (at least in one species) is bicarbonate concentration:

The corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH. Corals calcified at normal or elevated rates under low pH (7.6 to 7.8) when the seawater bicarbonate concentrations were above 1800 μM. Conversely, corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered. These results demonstrate that coral responses to ocean acidification are more diverse than currently thought, and question the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification.

What is not explained is whether lower ocean pH has any consistent effect on bicarbonate concentration in shallow waters. It would appear from here that dissolved CO2 result in increased bicarbonate. However, I'm sure there have been lab based coral studies where they bubbled CO2 through the water and the coral calcified less. So what was the reason for that?

So: this may (or may not) represent relatively good news for the future of coral. But, let's wait for more news and analysis.