Help needed: Kh drop, Ph stable

[h.buhamad]

Hello Everyone, I'm new to both the forum and the hobby.

I got myself an used aquarium (mature tank) but I'm a newbie(3 months only) to reef tanks.

I try to keep my Kh around 8.2 and and my Ph level around 8.4 , and i usually do water testing weekly. Upon this week's water check the Kh dropped to below 5 while ph is stable. What could have caused that Kh drop? Ps: i have a minor algae problem that I'm working on.

Thanks in advance

 

[Ron Reefman]

I'd worry less about why the dKH dropped and consider testing a bit more often and keeping it stable. The pH effect from keeping the dKH higher is VERY temporary. And the pH of 8.4 seems a bit high. Most stable and mature tanks run pH from 7.8 to 8.2. Frankly, I quit testing for pH many years ago. If your tank is healthy, pH will take care of itself.

 

[h.buhamad]

Thanks for your reply Ron.
I also don't usually check for pH as I've been told from other experienced reefers, but since i noticed the dKh drop i thought to check on it, and I'm glad pH is stable.
From my little internet research, I've noticed that whenever dKh drops pH follows. When only dKh drop, is that an indicator of any sort? As in: what could have caused dKh to drop while not a change of pH? Or is it expected for pH to stay stable until dKh hits zero then pH follows?
Hope you excuse my English as its my second language if i fall short there lol, thanks in advance

 

[Ron Reefman]

Thanks for your reply Ron.
I also don't usually check for pH as I've been told from other experienced reefers, but since i noticed the dKh drop i thought to check on it, and I'm glad pH is stable.
From my little internet research, I've noticed that whenever dKh drops pH follows. When only dKh drop, is that an indicator of any sort? As in: what could have caused dKh to drop while not a change of pH? Or is it expected for pH to stay stable until dKh hits zero then pH follows?
Hope you excuse my English as its my second language if i fall short there lol, thanks in advance

To be 100% honest here, I have no idea if pH follows dKH at all.

I can say that back when I did test for pH, it seemed very stable over time, even as my dKH went up and down by small amounts (+/- 1.5 dKH). Now that I run a 40g aio and dose manually every day (and only test once a week) I have not noticed any issues that I would attribute to pH levels.

I only test for SG, Ca, alk and on rare occasions for Mg and ammonia or nitrate. Since I don't have any fish, I can feed a lot less than tanks with fish. Now I do have other inverts like sea stars, shrimp and sea cucumbers, so I do need to feed, just not nearly as much. So my water levels are pretty easy to keep in line with where I want them.

BTW, your English is excellent. I know people who were born and raised here in the US who don't speak it (or write it) as well as you have!

 

[Randy Holmes-Farley]

pH is directly related to alkalinity, but pH is not suitable to know when you need alkalinity since pH changes also from changes in the CO2 level in the water.

 

[SantaMonica]

Rapid algae growth can consume alk, and can raise (or help keep from falling) the pH.

 

[Randy Holmes-Farley]

Rapid algae growth can consume alk, and can raise (or help keep from falling) the pH.

Unless it is a calcifying algae like halimeda, algae cannot consume alkalinity, whether it is fast growing or not.

Where would the alkalinity go?

 

[SantaMonica]

It happens when the carbon consumption of GHA is faster than CO2 can dissolve into the water. It's speed dependent. You see it when larger scrubbers with strong lighting are growing fast, and can reduce alk by one point per day or so.

 

[Randy Holmes-Farley]

It happens when the carbon consumption of GHA is faster than CO2 can dissolve into the water. It's speed dependent. You see it when larger scrubbers with strong lighting are growing fast, and can reduce alk by one point per day or so.

I’m not seeing any plausible mechanism for that idea. pH rises for the reason you note, but CO2 is not needed for alkalinity. As algae strip out CO2, pH rises but alk is stable, no matter the molecular level mechanisms the algae use to get CO2.

It is an axiom of chemical oceanography that neither addition nor removal of CO2 to any degree can add or deplete alkalinity directly.

Of course, as the pH rises, calcification can increase from both corals and abiotic precipitation of calcium carbonate, and that deposition of calcium carbonate can reduce alk.

 

[SantaMonica]

It has something to do with the carbonic acid that is made from CO2 going into the water. I just know it happens a lot and is repeatable.

 

[Randy Holmes-Farley]

It has something to do with the carbonic acid that is made from CO2 going into the water. I just know it happens a lot and is repeatable.

That is not a thing. Neither CO2 nor carbonic acid at any level impact alkalinity. I discuss it in the article below.

If it happens, it’s for the reason I gave about pH and its effect on calcification.

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

Alkalinity Facts​

There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow (“Aquatic Chemistry Concepts”, 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to “lower alkalinity” by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3–

(6) HCO3– ==> H+ + CO3—

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

 

[Randy Holmes-Farley]

It is true that many organisms, including some macroalgae, get CO2 from bicarbonate, and that may be where the misunderstanding arises. That process uses bicarbonate, but does not consume any alkalinity.

I discuss that in this article:

Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com


Obtaining Carbon Dioxide from Bicarbonate: Carbonic Anhydrase

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If an organism is to obtain carbon dioxide from bicarbonate, several potential processes are available, and different organisms take different approaches. In many cases, the exact mechanisms have not been established. It is much easier to show that bicarbonate is a source of carbon dioxide for marine organisms than to show exactly how they take it up. A bicarbonate ion, being charged and insoluble in organic phases, cannot readily diffuse across cell membranes, so other mechanisms are needed.

Such uncertainty of mechanism is the case for Ulva lactuca, for example. It has been shown to be able to photosynthesize when out of the water (say, exposed at low tide), taking up carbon dioxide directly, and also when in the water, taking up bicarbonate.10 But the exact mechanism of using bicarbonate to obtain carbon dioxide isn't known in this species.

One common way to use bicarbonate is for the cells exposed to the seawater to use extracellular carbonic anhydrase on their surfaces. As mentioned above, the enzyme carbonic anhydrase catalyzes the hydration and dehydration of carbon dioxide and carbonic acid, respectively. These organisms present this enzyme to the bicarbonate-rich seawater surrounding them. Because the bicarbonate is naturally in rapid equilibrium with carbonic acid, and the carbonic anhydrase keeps the carbonic acid in rapid equilibrium with unhydrated carbon dioxide, the bicarbonate is used as a ready pool to supply carbon dioxide to passively cross cell membranes and be taken up (shown schematically below).

The agarophyte Gracilaria lemaneiformis11 has been shown to take up carbon in this fashion. It has carbonic anhydrase both inside the organism and out. Inhibiting either of these types of carbonic anhydrase greatly decreases photosynthesis, but adding an anion transport inhibitor does not. Adding TRIS buffer to the extracellular fluid (seawater) also has no effect (the purpose of which is discussed in the following section relating to proton pumping as a possible mechanism).

Photosynthesis in this organism is greatly reduced as the pH is raised (73% reduction when going from pH 8.0 to 9.0), presumably because the bicarbonate's propensity to form carbonic acid is reduced at higher pH.

The brown alga, Hizikia fusiforme (Sargassaceae),12 from the South China Sea, has also been shown to exhibit carbonic anhydrase activity, both inside and out, and has been shown to be incapable of actively and directly transporting bicarbonate. Consequently, its carbon dioxide concentration likely operates by the mechanism shown above.

Two species of marine prymnesiophytes (Dicrateria inornataand Ochrosphaera neapolitana)13 have been shown, through the use of various carbonic anhydrase inhibitors, to use extracellular carbonic anhydrase to collect carbon dioxide from ambient bicarbonate. They also employ an energy dependent process for taking up carbon dioxide itself. Growth in high carbon dioxide environments represses the expression of carbonic anhydrase active in these species, but does not reduce the active uptake of carbon dioxide.




Obtaining Carbon Dioxide from Bicarbonate: Direct Uptake

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An alternative way to obtain carbon dioxide via seawater bicarbonate is to take up the bicarbonate through protein transport mechanisms across the cell membranes, and then once inside the cells where it is needed, carbonic anhydrase converts it into carbon dioxide and hydroxide ion. The hydroxide is then pumped out, or H+ is pumped in, to achieve pH balance.

​

Transporting ions across cell membranes using protein transporters is a widespread mechanism whereby organisms can get needed ions across a membrane through which they do not normally diffuse. Some of these are active transporters, using chemical energy to "pull" ions out of the extracellular fluid (our push them out, as necessary), and other transporters simply allow specific ions to pass though from high concentration on one side to lower concentration on the other side.

The marine red alga Gracilaria conferta has been shown to have an active bicarbonate uptake mechanism.14 Three marine bloom-forming (red tide) dinoflagellates, Prorocentrum minimum, Heterocapsa triquetra and Ceratium lineatum,15have been shown to take up bicarbonate directly. They show little carbonic anhydrase activity, yet bicarbonate accounts for approximately 80% of the carbon dioxide they use in photosynthesis. It is believed that these dinoflagellates are not carbon limited in photosynthesis due to their efficient direct bicarbonate uptake mechanisms.

The marine diatom Phaeodactylum tricornutum16 was found not only to have an active bicarbonate uptake mechanism, but the researchers further identified at least two different mechanisms. In particular, they showed that part of the uptake depended on the presence of extracellular potassium, and this part of the total carbon dioxide uptake was eliminated when potassium was missing from the medium. A second direct bicarbonate uptake mechanism was independent of potassium, indicating the presence of at least two different pathways for transporting bicarbonate into this organism.

Obtaining Carbon Dioxide from Bicarbonate: Proton Pumping

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Another way to obtain carbon dioxide via seawater bicarbonate is to pump H+ out of the cells into the extracellular fluid (seawater near the cells) or into a special cavity where bicarbonate is present.17 This low pH causes the bicarbonate to become protonated to become carbonic acid. The carbonic acid can then transform into carbon dioxide, and pass across the cell membranes.

​

The seagrass Zostera noltii Hornem18 has been shown, for example, to use proton pumping to gather bicarbonate in the form of carbonic acid from the water. It contains no extracellular carbonic anhydrase, but rather uses ATP (adenosine triphosphate, the fundamental currency of chemical energy in most organisms) to drive the export of H+. Evidence for this mechanism is found by adding a buffer to the seawater (TRIS) without changing the pH. This buffer keeps the pH near the cell surface constant, counteracting the beneficial effect of the proton pumping in lowering pH and converting bicarbonate into carbonic acid. The simple presence of a non-absorbed buffer in the water can decrease the rate of photosynthesis in this organism by almost 80%.

Interestingly, those seagrass specimens acclimated to high light (where high rates of photosynthesis and consequent uptake of bicarbonate would be highest) showed the greatest ability to actively take up bicarbonate. In high light experiments, these previously high light-acclimated specimens were shown to be only light limited, while the shade-acclimated organisms were both light and carbon limited when put into high light.18 Other seagrass species (e.g., Z. mulleriand Z. marina) have been shown to have external carbonic anhydrase, and so may have different uptake mechanisms.18

Photosynthesis of Macroalgae as a Function of pH

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One of the side effects of the necessity of taking up carbon dioxide to photosynthesize is that pH may affect the rate of photosynthesis, because the amount of carbon dioxide (as CO2or H2CO3) in the water varies with pH. Assuming constant carbonate alkalinity, the effect is quite strong. A drop of 0.3 pH units implies a doubling of the carbon dioxide concentration. A reef aquarium at pH 8.5, for example, has one fourth the carbon dioxide of a reef aquarium at pH 7.9, assuming the carbonate alkalinity is the same.

Aquarists may rightly wonder whether organisms are able to photosynthesize efficiently as the pH is raised. The answer is mixed. Some can and some cannot. Those organisms that rely solely on carbon dioxide may not. Those that rely on both carbon dioxide and bicarbonate have a better chance of retaining efficiency at higher pH because a much larger amount of bicarbonate is present, and it does not change as rapidly with pH over the range of interest to aquarists.

Table 1 shows the response of a variety of macroalgae in terms of their ability to photosynthesize at pH 8.1 and 8.7. In seawater with constant carbonate alkalinity, there is 20% as much carbon dioxide at pH 8.7 as at pH 8.1, so an organism relying on carbon dioxide alone might experience a large drop in photosynthetic rate over this range. Clearly, the response varies with species. Chaetomorpha aerea, in particular, may be of substantial interest to aquarists. It is not necessarily the exact species that many grow in refugia (which is unidentified as far as I can tell), but this species of Chaetomorpha shows a 25% drop in photosynthesis when exposed to the higher pH. That drop is not as large as some other species, but may still be important, and it is more than many other species of macroalgae.

Of course, the photosynthesis rate does not necessarily translate to growth rates. If other nutrients are limiting growth (nitrogen, phosphorus, iron, etc.), then it may not matter if the rate of photosynthesis is reduced at higher pH. But because these nutrients are often present in surplus in reef aquaria, it may well be that carbon uptake is limiting in some cases, and in those cases aquarists might benefit from ensuring that the pH is not too high.