Most obligate aquatic plants (e.g. Egeria, Cabomba, Myriophyllum, Vallisneria etc) can use bicarbonate as a carbon source when grown under ambient CO2 concentrations
not all of those species can utilise bicarbonates.
I think you're right. I don't know where I got Cabomba from.
although they all stop doing this when there's ample CO2 available.
Not all plants stop, in fact some species (eg. vallisneria) cant survive without bicarbonates, carbonates and calcium as this would mean they cannot utilise 'them' properly. If you read up on polarisation of the leaves you will see why. It is here in this tropica page - box 2.
http
/www.tropica.com/article.asp?type=aquaristic&id=835
was this what you was reffering to in your last post?
Yeah. In the box you mention, they state that both forms of bicarbonate use (polarization of leaves, and bicarbonate uptake via ion channels) stop when CO2 level becomes non-limiting. (
"Regardless of the model in use, bicarbonate uptake is an energy consuming process and thus, even good bicarbonate users do not produce the necessary enzymes unless needed. Thus, in environments with high CO2, these bicarbonate users cannot use bicarbonate without going though a period of time with low CO2 during which the necessary enzymes are produced.")
Not sure what you mean by surviving without, but Vallisneria certainly doesn't require a
high amount of carbonate hardness, if you provide it with co2 either in the water column or in the substrate (which is supposedly the case in Walstad-type tanks). I have spiralis growing like a weed at a KH of 2 - 3.
The decrease in bicarbonate brings the KH down.
The amount of HCO3 used isn't alot, and has a negligible affect on KH. The energy and time required to complete the process of uptaking HCO3 and converting it etc makes the process slow.
Is it negligible? I don't know. We know that a densely planted tank can deplete a significant amount of nitrate/nitrite/ammonia easily. Yet plants are composed of 20+ times more carbon than nitrogen, so it seems they must bind carbon at a roughly similar ratio, right? If they can't get enough CO2 from the water column and substrate to match the light level in the tank, they'll take bicarbonate (assuming the species can do so) no matter how slow or inefficient it is. With the polarized-leaf variety, you'll sometimes see either a dusty carbonate precipitate or an encrusted deposit on the top of the leaves as a result of this (the high pH on that side of the leaf precipitates calcium carbonate out of the water).
In the days of yore, before CO2 injection became common, people did seem to be concerned about the fairly rapid depletion of KH in planted tanks, which seems unlikely to be attributable to nitric acid buildup, since in a planted tank you generally need to add nitrate rather than worry about it accumulating. Granted, they used high lights and no added CO2, which does result in wacky conditions.
Only a few plants such as vallisneria and a few myriophyllum species are adapted for maxiumum efficiency in utilising HCO3. Other plants which can use HCO3 have to polarize the leaves whereas the species listed above dont have to do this. So unless you have a tank full of plants which are 'good' at utilising HCO3, then you will still probably only notice a change of about 1dKH providing it is a CO2 limiting tank.
Agreed on the maximum efficiency. That's why I said "some plants" in my original post. Vallisneria is pretty common in the hobby though, given that many hobbyists find them easy to grow in low tech tanks, and Myriophyllum isn't all that exotic either, even if they're not as commonly sold as before.
I think you're probably right that in most tanks, under typical aquarium conditions, this isn't a very significant factor. But this is the Scientific section after all, so it's good to go into some detail.
