In that other thread I suggested this white deposit was due to the minerals in the water, and I believe I mentioned having read about it somewhere but couldn't lay my hands on it. Now that the proper term has been mentioned, I found it. Diana Walstad mentions biogenic decalcification in her book Ecology of the Planted Aquarium
First: bicarbonates, or perhaps more scientifically correct as hydrogen carbonate (HCO3-), is an intermediate form of carbonic acid. Bicarbonates are higher in hard alkaline water. Plants can obtain carbon from CO2 (carbon dioxide) or bicarbonates, though they often have a preference. Most of those in aquaria are soft water plants that prefer CO2 because it is less work for the plant to extract the carbon. Plants such as Vallisneria
(to name only a few) which naturally occur in hard water are better at using bicarbonates. Some, such as the mosses, the pygmy chain sword, Ceratopteris
, and several others cannot use bicarbonates at all. [There is a list if you're interested in Walstad, p. 97, with the reference to the scientific study that determined this.]
Interestingly, many of the amphibious plants [the bog or marsh plants like Echinodorus
, etc] cannot use bicarbonates well, and it is hypothesized that they use the aerial strategy (emersed leaves) to supplement their carbon uptake.
Plants use bicarbonates much less effectively than algae. This is probably part of the reason that algae tends to be worse in hard water compared to soft, all else being equal.
Now with the background understood, I will cite directly from Walstad to define biogenic decalcification since this is highly technical and beyond my comprehension:
Some bicarbonate users polarize their leaves during bicarbonate uptake. Polarized bicarbonate uptake has been described for Potamogeton lucens. The plant excretes H= (acid) on the leaf's underside to generate a pH of about 6. The acidity converts bicarbonate to CO2, which diffuses into the leaf to be used for photosynthesis. In order for the plant to maintain its internal charge balance, H= is taken up by the plant on the leaf surface resulting in a high, localized pH (about 10) and a high hydroxide (OH-) concentration.
The OH- combines with calcium bicarbonate [Ca(HCO3)2] in the water causing the precipitation of calcium carbonate (CaCO3) on the top of the leaf. In hard, alkaline water, this reaction, which is called "biogenic decalcification," may be so great that crusts of precipitated CaCO3 may weigh more than the underlying plant. I have seen CaCO3 deposited as small white "pimples" on the leaves of Egeria densa and Ludwigia repens when they were grown in hardwater under intense light.
Some aquatic plants (e.g., Myriophyllum spicaytum and Vallisneria spiralis) that use bicarbonates do not polarize their leaves during bicarbonate uptake.
There we are.