Understanding CO2/pH levels

[Manuel Arias]

The more o2 there is in the water the higher co2 concentration you can keep without fish issues.
There are many more things to consider as well but this statement is true. It doesnt mean there is no limit to how much co2 you can inject obviously.

Out of curiosity, which is the argument for this? I mean, if you can extend your point, as it is interesting, and I might be wrong. F

For many years in fish physiology has been stated that breath in fishes is not linked to the relative concentrations of O2 and CO2, unless CO2 concentrations in water are big enough to block the exchange of CO2 from the blood of the fishes with O2 in the water, or the O2 concentrations are not enough to cover the needs of the fishes. Meanwhile the CO2 levels in water are allowing the excretion of CO2 through the gills , the fishes have adaptations (until some point) to make available the oxygen in the water by increasing the ventilation rate and the cardiovascular activity to compensate the lower rate of exchange of CO2 due to the higher concentrations in the water. The intake of O2 is then driven by the Bohr-Haldane effect as result of the pH changes in blood due to this CO2 transfer. Meanwhile the O2 levels are suitable for the fishes needs (this level is independent of the CO2 levels in water), they have mechanisms to handle the higher levels of CO2 until the point in which the stress caused by the reduction of the gradient of concentrations overwhelms the capacity of the fish to compensate it by other means. As it is this gradient who controls the exchange rate of gases, does not matter if you increase the O2 concentrations in water, as then the fish will not be able to use that available oxygen because cannot remove the CO2 from the hemoglobin in the blood, and then suffocating.

The following figure explain this:



As you can see, it is the relative concentrations of CO2 between body and water what enables the breathing. If the CO2 levels in water are high enough, the gradient indicated in point (4) of the figure is much lower, difficulting the gas transfer. In fact, this gas transfer involving the change of the pH in the blood is what allows to the organism to grasp O2 from the water, via Bohr-Aldane effect, so regardless of the O2 concentration very high in the water, if the CO2 levels in the environment are high the mechanism is not possible.

It has been stated, however, that higher levels of O2 are positive for fishes, meanwhile the CO2 levels are on bay in specific ranges, but the reason for that relies in receptors of the gill´s cells that activate specific patterns of behaviour and physiologic reactions in the fishes. In other words: when the conditions are good the fish relax and suffer less stress. However, as mentioned, this only happen in specific ranges of relative concentrations of CO2 and O2 in water, and there is no general relationship between a better tolerance to CO2 by increasing O2 levels, as the mechanism allowing the breathing is the differences of concentration in CO2 between the blood and the water in a given temperature. In last years some people are publishing about the role of the pH of the environment in the mechanism of gas exchanges between gills and water, but they only point out in the direction that lower pH in the water can reduce the tolerance of fishes to CO2 levels, and not totally related to this discussion.

You can read abut the fishes adaptations and responses to high CO2 concentrations in water in the following paper, which also explains the role of the differences in the partial pressure of CO2 in environment and fishes and its effects:

http://onlinelibrary.wiley.com/doi/10.1029/2004JC002564/pdf

And a good reading about the Bohr-Haldane effect is found here, where you can se that the release of CO2 to the water is what triggers the O2 intake:

http://brauner-home.zoology.ubc.ca/...l-Interaction-between-O2-and-CO2-exchange.pdf

Well, now we smashed it with science. But I think we should move back to the topic of this thread, anyway.

 

[Jose]

Maybe more oxygen makes the whole process of CO2 transport/excretion more effective as compared to low o2.

 

[ceg4048]

Out of curiosity, which is the argument for this?

The argument is thus (it is clearly explained in your figure 5.8):

At low pH, Hemoglobin's affinity for Oxygen is poor and it's affinity for CO2 is high.
At high pH, Hemoglobin's affinity for Oxygen is high and it's affinity for CO2 is low.

This mechanism is by design. When the red blood cells are in close proximity to the tissues, the pH is low due to the tissues excretion of CO2. The Hemoglobin then drops it's payload of Oxygen and attracts the CO2 molecules. The blood cells then travel to the gills where the pH is normally higher they drop their payload of CO2 and attract the Oxygen molecules for the return trip to the tissues.

When the ambient water has a high partial pressure of CO2 it blocks the ability of the the bloodstream CO2 to diffuse through the gills to the water. This keeps the CO2 in bloodstream solution and causes a drop in blood pH. The cardovascular system combats the bloodstream acidity by releasing bicarbonate to buffer the acidity, however, the Hemoglobin's affinity never recovers to 100%. Typically, it can only achieve a maximum of about 96% of the pre-buffered affinity, and can be even lower depending on the pH and the buffering ability of the fish. Therefore, a higher Oxygen content in the water column can make up, to some extent, the loss of Hemoglobin-Oxygen affinity.

In our case, effective CO2 distribution and uptake by plants allows a higher rate of photosynthesis, which generates a higher Oxygen ejection into the water column. So it helps to some extent but as Jose mentions, cannot overcome all higher levels of CO2.

Cheers,

 

[Manuel Arias]

So it helps to some extent but as Jose mentions, cannot overcome all higher levels of CO2

Hi ceg4048,

This is the key mark: "At some extend", which is only valid to a reduced range of CO2 concentrations, as explained before, and also explained in one of the papers I put, sorry.

When the ambient water has a high partial pressure of CO2 it blocks the ability of the the bloodstream CO2 to diffuse through the gills to the water. This keeps the CO2 in bloodstream solution and causes a drop in blood pH. The cardovascular system combats the bloodstream acidity by releasing bicarbonate to buffer the acidity, however, the Hemoglobin's affinity never recovers to 100%. Typically, it can only achieve a maximum of about 96% of the pre-buffered affinity, and can be even lower depending on the pH and the buffering ability of the fish. Therefore, a higher Oxygen content in the water column can make up, to some extent, the loss of Hemoglobin-Oxygen affinity.

Here, you are just reasoning in the same way I did, but with one mistake: The mechanism I put in the figure is incomplete, and then, your conclusions are wrong. The complete process involves both Bohr and Haldane effect. As you say, the carboxilasa enzyma plays the role to compensate the CO2 productions by generating carbonates to regulate the pH in blood, so you are correct there. However you introduce the argument of O2 levels in water with no backup to it. It is true that hemoglobine has affinity to O2, which depends on the pH in the blood, so essentially, the organism plays with the pH in blood to ensure the transport of O2 to the cells and the release of CO2 to water, when that is possible. However, the hemoglobine is not available if it is not able to get rid of the CO2, thing that cannot happen, by law of gradients, if the CO2 levels in water are high enough, which is what controls really the capability of breath of the fish. The increment of the cardiovascular activity it in intended to compensate the reduction of this gradient: Given a rate of flow of CO2 between water and gills, the only way to increment this rate is by incrementing the flow of water through the gills, as well as the blood circulation, i.e. incrementing in the cardiovascular rhythm plus more movement of the gills, which is observed, and well explained in both papers I put.

Secondly, the mechanism itself is only possible by the changes of entropy in the molecules in the process. The fishes are able to pump the oxygen inside the organism by a mix of the O2 gradient plus using a proton pump that is fed... I will let you guess... by the additional proton that is released when diffusing the CO2 in the form of HCO3- to the water, which acidifies the pH of the blood, allowing the hemoglobine to be susceptible to oxidation. If this diffussion cannot happen, then there is no way to aborb oxygen to hemoglobin due to both factors: The lack of hemoglobine available for it, plus the interruption of the proton pump through the cell membranes to allow the "forced" diffussion of O2. Fishes have this mechanism because the concentrations of O2 in water are very low respect, for example, the atmosphere, so they link both systems in a way that improves the efficiency of the system.

So sorry, your argument is not valid in global context, and only valid at specific ranges of CO2, which are not the ones we usually keep in aquariums. Our tanks saturated in CO2 will not be compensated by adding more O2. The aireation helps to reduce the CO2 concentration in water, which is most likely the side effect that really helps, more than the increment of O2 in water.

I agree, as I already did, that additional O2 helps to fishe but not for a mere physiological reason: It is due to specific receptors in the gills that triggers a hormonal response of the fish to the environmental conditions. The fishes relax (or reduce their stress) only when both CO2 levels are easily reduced in blood and O2 is available enough, i.e. only when the levels of both gases are in a specific range of values, usually, the higher the O2, the lower the CO2. This response cannot be mistaken with the physiological mechanism of breathing, which is driven mainly by the capability of the fish to diffuse CO2 to water, i.e. linked to the concentration of CO2 in water, and not to the O2 levels in water, which only helps in given intervals of CO2 that are not the ones we usually keep in planted aquariums.

 

[Bacms]

Thank you Manuel and Clive for the clarification. I managed to get stable levels of CO2 at around 6.9 close to the substrate and constant during the photoperiod. What I did notice was that the TDS and KH oh the water was higher towards the end of the week which I first assumed was due to EI although the rising was pretty high. Well...turns out that the sand that came when I bought second hand seems to be coral and so it is buffering the pH drop. Don't know what to do about it at this point as I have tropica substrate underneath and only used the sand in the front where the HC is attached.

I did lost three ottos tonight don't know why and can't find the body of one of them which is probably realising ammonia. I am really down today because of all this

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[zozo]

Reading al this i come to think our fish could use some EPO and spinach...

 

[Manuel Arias]

Don't know what to do about it at this point as I have tropica substrate underneath and only used the sand in the front where the HC is attached.

Do you have lot of sand or just a small layer? Depending on that, and how deep goes in your substrate, then maybe you can suction it using a tube and "pumping" water at the same time. Its is likely the easier process to remove that sand on the top and replace it by an inert sand.

I did lost three ottos tonight don't know why

I am sorry hearing that, and hopefully the community can give you a solution. Optimal pH for Ottos is between 6 and 8. If during the night the pH is increasing for long time around 9, maybe is too agressive for those fishes... This will happen if you turn off the CO2 and you have a high kH and the CO2 is removed of the water by agitation and/or aireation. On the other hand, if you do not turn off the CO2 in the night, the problem can be that the pH goes too low for the kH values you have, saturating he water with CO2 and then suffocating the fishes. But this is just a guess from what you have mentioned here, as it might there be different causes. Note that a pH of 6.9 with a kH of 8 or higher it is enough to kill fishes for this reason. In general high kH and low pH values are a fatal combination.

 

[Bacms]

The problem with vacuum out the sand is that there is tropica substrate underneath so it can get quite messy. I have done a separation between the soil and the sand so there is about the centimetres at the front of the tank which goes all the way down to the bottom so quite a lot of it.

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[alto]

This discussion really isn't complete without some understanding of Fish Hemoglobins 😉

Braz J Med Biol Res, June 2007, Volume 40(6) 769-778 (Review)

Fish hemoglobins

P.C. de Souza and G.O. Bonilla-Rodriguez