When we are talking about pH we are normal not aware that the pH scale is logarithmic. It means that the hydrogen concentration (H+) (actually the hydronium concentration (H
3O+) ) at pH 7 have 10 times more H ions (H
3O ions) compared with pH 8. If we compare pH 8.3 and pH 8.0 - PH 8 have nearly 2 times more hydrogen ions (hydronium ions) compared with pH 8.3.
@Randy Holmes-Farley statement below maybe not - in the first thoughts - mean so much if the pH increase "only" goes from 8 to 8.3
It might be because it is much easier to pump out the H+ needed for calcification (for two reasons, to convert bicarbonate into carbonate, and to otherwise raise the pH in the region of the coral where calcium carbonate deposition is actually taking place).
But - at the second thought - it cost energy if you have to pump against a gradient - energy that can be used for other purposes. The "contra pressure" will nearly double if you go from 8.3 to 8.0. Compare with the energy input you have to do if you lift 20 kg stone compared with one at 10 kg.
Next step - in this model HCO
3 is used as an inorganic carbons source for both photosynthesis and calcifiering. It cost energy (to convert HCO
3 to CO
2 (photosynthesis) and to convert HCO
3 to CO
3 (calcification)) - Further on - the process will work into an acidification of the tank water but will be contradicted (total or partly) of a tendency for CO
3 to be converted to HCO
3 if the pH go down - buffering capacity)
When I read
@Randy Holmes-Farley ´s statement and the
article I linked to in post #36 I feel rather sure of what process is the major pathway for at least the calcification process. But - and a large but - there is on very obvious fact that every reefer know that bother me.
Why do we see a pH rise when the photosynthesis work fully out if HCO3 is the major pathway for both processes?
If the calcification process use CO
3 we would have a tendency of pH decrease. The consumption of pure CO
3 will force the buffer system to convert HCO
3 into CO
3 - liberating one hydrogen (hydronium) ion into the water. This is not my problem
As I see it - if our organisms major pathway is to use HCO
3 as inorganic C source both for photosynthesis and calcification - this process is at least pH neutral - this is my real problem - because in an experimental way - the photosynthesis process drive up the pH rather much.
Lets go back to the photosynthesis - this process (on molecular level) need CO
2 as inorganic carbon source. CO
2 is a gas and gases can diffuse in or out of many cells and tissues. It a passive transport driven by gradients - not any energy costly process against gradients.
In my world - if CO
2 is present - the most cost effective process for the whole process of photosynthesis would be to use available CO
2 in first hand and transfer to a more energy costly process when the CO
2 get sparse. If the organism use CO
2 already present in the water - the pH will rise in a closed system. My explanation to all this is that the photosynthesis of the corals use of CO
2 as first choice is in line with the natural princip of always use the most cost effective method of energy use. When CO
2 decrease in our aquarium the natural equilibrium point of CO
2 between the air and water tend to give an injection of CO
2 into the water but this is normally a rather slow process compared with the photosynthetic process consumption of CO
2 in a closed system.
This means that the fast rise in pH during photosynthesis can be caused of CO
2 uptake into the photosynthesis process and everything we see experimental make sense. If there is no CO
2 uptake - I can't explain the pH rise. I have also noted that there seem to be a maximal pH during day - and many times the same in spite of the night pH.
Where do I stand now? From what i know now - I lean to believe that there is a "sweet point” there the energy demand for using HCO
3 in calcification even out with the energy gain into using CO
2 (instead of using HCO
3) for photosynthesis. This “sweet point” will be expressed as a certain pH. Exactly which – no idea but I have a feeling that it could be somewhere around 8.15. This means that – in this imagined point – the energy balance is optimal. It does not mean that it is optimal for the organism itself – but for the energy balance in order to maximize input relative to output. It gets maximal energy for other tasks. Growth of soft tissue as an example. Another way to maximize the metabolism of the animal is to rise the temperature. In the last few years there have been many experiments both in vivo and vitro looking at the relationship between rising temperatures and declining pH for corals health. All studies I have seen indicate such a relationship and that 1 + 1 will be higher than 2 in such cases. That this probably is not true for 100 % of corals species (or individual clones) and it is another case and have nothing with the big picture to do. As a friend use to say when some not expected and odd things happen in biological systems – there is idiots everywhere.
However – for many years I have been thinking of to do a system where pH should be fixed at (as an example) 8.15 and see if I could get an optimal growth of my corals. A little bit in line with the first post in this thread – what´s happen if we try to have the pH stabile instead of KH. During photosynthesis I need to add CO
2 and during dark respiration – I need to take out CO
2.
@Randy Holmes-Farley - My question is if you can supplement your excellent table with the amount of CO
2 in the different pH and 8 dKH in alkalinity.
I have no idea if my thoughts are crazy or not but look forward to if others can confirm or reject the ideas. As I mentioned before – we get some unexpected results with S.hystrix grown in different pH. We did construct that system in a way there the pH was constant and not jump up and down along a hysteresis curve. The pH variation was in the plus – minus 0.01 variation. During photosynthesis - it was lowered with CO
2. During dark metabolism - it was raised with help of aeration and CO
2 free air
Sorry for long post but this is probably not main street thoughts - I need to explain my self
Sincerely Lasse
