Yes, I doubt the same bacteria are present in aquaria, but there are so many bacteria. I'm sure there are marine bacteria capable of metabolizing citric acid.
The paper I attached above showed citric acid ultimately raised the pH of the system. Initially, the pH was lowered as you would expect, but then the pH increased dramatically as the citrate was metabolized. As you explained to me yesterday in another post, metabolism of organics will decrease alkalinity due to the production of protons. So, I was confused why the experimental conditions showed pH actually increases. I hypothesize that the formation of polyphosphate is likely responsible as this consumes monoprotonated phosphate to add to the growing polyphosphate chain with concomitant formation of one molecule of water.
Yes, that would work sort of like a CaCO3/CO2 reactor plus organic carbon dosing.
reefs.com
Consumption of protons raises alkalinity. Thus, if bacteria are consuming protons to form polyphosphate, I think it has the potential to raise alkalinity. While pH is a poor proxy for alkalinity as you note, increasing pH is consistent with an increase in alkalinity. The bacteria used in the reactor were polyphosphate-accumulating (POA) bacteria, which are used to remove phosphate from wastewater. Many bacteria will produce high-energy polyphosphate as an energy store, but POA bacteria appear to do it even better. The general concept of removing phosphate while raising alkalinity seems plausible. Of course, the bacteria must be removed from the system through efficient protein skimming otherwise they will simply use the stored energy from polyphosphate and liberate protons when the carbon source is removed, ultimately decreasing alkalinity (I agree that you cannot escape the fact that metabolizing a carbon source generates CO2, reducing equivalents and lots of protons). I furthermore agree with you that the POA bacteria in this freshwater system are unlikely to exist in marine environments, but there are a lot of different bacteria and I have no doubt POA-marine bacteria exist. The exciting thing I extrapolated from the study was that citric acid, as key intermediate of the citric acid cycle, may perturb the normal metabolism favoring formation of polyphosphate as an energy store. All carbon sources are not the same and can have different consequences on metabolism. This is all highly speculative, and I think potentially interesting and worthwhile to explore.That paper is very complicated to interpret, but I'm not sure it says anything unexpected.
I'll note first that it is not seawater, and uses nutrients far from reef tank conditions (>20 ppm phosphate, for example).
Most importantly, it does not mention alkalinity, and pH is not a useful surrogate measure for alkalinity if CO2 values are changing. One can lower the pH of seawater to pH 5 with CO2, and that has no impact on alkalinity. Similarly, removing CO2 from seawater will raise the pH but have no impact on alkalinity (at least until something like magnesium hydroxide precipitates).
I discuss that (The Principle of Conservation of Alkalinity) here:
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.
In the paper you posted, they note that
"The introduction of a citric acid solution to the system contributed to a decrease in wastewaters pH value from pH 7.60 to pH 5.5 (Table 3). The consumption of citric acid by microorganisms and CO2expelling as a result of aeration was accompanied by an increasing pH value, which in the treated wastewaters accounted for pH 8.24–8.37 in Variant 1, for pH 8.11–8.36 in Variant 2, and for pH 8.25–8.32 in Variant 3 (Table 3)."
Thus, I do not see clear evidence for an alkalinity "change".
But I think there is likely an alkalinity increase due to the potential for denitrification, which they say they are trying to minimize, but give no data on the extent of it happening. Denitrification raises alkalinity.
