Higher than normal alkalinity increases coral growth in two species of coral?

[HighlandReefer]

This is the link to an article entitled, "Bicarbonate Stimulation of calcification and photosynthesis in two hermatypic corals": http://www3.interscience.wiley.com/c...93461/PDFSTART

Perhaps Randy can shed some light to exactly what they are stating in this article for me.

[bertoni]

They showed that adding bicarbonate increased the rate of calcification and photosynthesis, with different limits of the maximum useful level of augmentation. Acropora responded to higher levels than did Porites. Calcification rates went up even at night. I can't convert from mM to meq/L off-hand, but the upper limits were fairly high.

[MCsaxmaster]

They worked at constant pHnbs (=8.2) and made up artificial sea water. The total CO2 (TCO2) was set at 0.5, 1, 2, 4, 6, 8 mM for the various treatments. TCO2 is the sum of the concentration of all 4 DIC species:

[TCO2] = [CO2] + [H2CO3] + [HCO3-] + [CO3=]

Carbonate alkalinity is made up of HCO3- and CO32=. Bicarbonate contributes 1 equivalents of alkalinity, since it can take up 1 proton, while carbonate contributes 2 eq since it can take up 2 protons:

CA = [HCO3-] + 2[CO3=]

For a simplified system we can approximate the total alkalinity by assuming the carbonate system provides all of the alkalinity (it's ~97% in NSW, so it's close enough for our purposes).

From the pH and TCO2 we can easily calculate the concentration of each species and the alkalinity, though that probably isn't useful here (so take my word for it ). From this information, the calculated alkalinity is 1.15x the TCO2 at this pH (not true if we change pH even slightly):

For example, the TCO2 in a treatment is 2 mM, so the alkalinity is 1.15x2 = 2.3 meq/l (normal sea water, in other words). Hence, they used treatments at constant pH of 8.2 with alkalinity ranging from 0.575 - 9.2 meq/l, or 1.6 - 25.8 dKH.

For both Porites porites and Acropora sp. they saw increasing rates of photosynthesis as well as light and dark calcification with increasing TCO2/alkalinity, most likely attributable to an increase in bicarbonate conc. in both cases. Other work has shown a similar trend. Unpublished data from my MS show the same, and more strongly suggest that the calcification response is related to [HCO3-].

For the two corals they used, the Porites porites experienced increasing calcification to about 6 mM TCO2 (6.9 meq/l, 19.3 dKH alk). The Acropora sp. showed a linear increase in calcification with increasing TCO2/alk up to at least 8 mM TCO2 in the light whereas dark calcification became HCO3- saturated somewhere between 4 mM and 6 mM TCO2 for that coral. I found somewhat lower levels of light calcification HCO3- saturation with Madracis mirabilis (~4 meq/l alk).

So, the short version: corals have higher rates of calcification and photosynthesis with higher alkalinity at normal pH (likely due to higher [HCO3-]). How much higher and how much HCO3- it takes to saturate them depends on the species and conditions in culture.

Chris

[HighlandReefer]

"1.6 - 25.8 dKh", I guess it could be dangerous to increase alkalinity in our systems to see if other corals will respond the same way?

I was thinking about bumping my alkalinity up from 10.6 kH (windows open with fan in window) to maybe 14 kH, since I have CO2 issues in my house. I have heard other reefers keep their's at 14 kH. I believe at 14 kH, it may bump my pH from 8.1-8.2 to 8.2-8.3? I thought this might help when the windows go closed for the winter, where my pH dives down as low as 7.7. It would be nice if I could even go higher with the alkalinity to maintain a pH of about 8.2.

[Randy Holmes-Farley]

So, the short version: corals have higher rates of calcification and photosynthesis with higher alkalinity at normal pH (likely due to higher [HCO3-]). How much higher and how much HCO3- it takes to saturate them depends on the species and conditions in culture.


Hence my idea for coral farmers to really boost alkalinity and possibly calcium to unusually high levels, while adding something to the water (like polyacrylic acid) that prevents abiotic precipitation of calcium carbonate.

[HighlandReefer]

Could this also help reefers with CO2 problems by allowing us to keep our alkalinity higher? Should we add polyacrylic acid in the mix also? Seems like good news.

[MCsaxmaster]

Originally posted by Randy Holmes-Farley
So, the short version: corals have higher rates of calcification and photosynthesis with higher alkalinity at normal pH (likely due to higher [HCO3-]). How much higher and how much HCO3- it takes to saturate them depends on the species and conditions in culture.


Hence my idea for coral farmers to really boost alkalinity and possibly calcium to unusually high levels, while adding something to the water (like polyacrylic acid) that prevents abiotic precipitation of calcium carbonate.

Randy,

What kind of molecular weight were you thinking? I'm thinking the bigger the better. Also, while recent evidence seems to suggest that coral skeletons really are pretty well sealed off from the external environment by the tissues, I'd still want to carefully rule out the possibility that this was getting in the calcifying space and thereby screwing up calcification. This might be really worth trying though

Chris

[Randy Holmes-Farley]

Yes, that is a concern. The ones I tried were on the order of several thousand to several tens of thousands, but I have no idea if they can get into the calcifying space of typical corals or not (I'd guess not, but you never know). The other obvious concern is elimination from the water column, by skimming, binding to everything in sight, etc.

But I was able to show the first step of stabilizing very high supersaturations that without it added ended up with crystals forming in the test tubes.

[MCsaxmaster]

Hmmm, interesting. What sort of calcium, alkalinity, and pH were you working with, out of curiosity? Presumably this was in sea water, yes?

Chris

[Randy Holmes-Farley]

It was my reef tank water, but there are quite a few literature studies with "seawater". I boosted the supersaturation a lot using the alkalinity part of B-ionic. I don't recall the exact values offhand, but I think I went up to 10 meq/L or so and the pH was, of course, high too.

I saw precipitation of both magnesium hydroxide (determined by reacidifying to pH 8.2 and seeing it dissolve) as well as slower precipitation of calcium carbonate (which did not redissolve on acidification).