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More on Filter less Plant Tanks
During a recent message to Dave G., I commented on the practice of running a
planted aquarium without filtration under varying bio-load conditions. My
feeling is that it is certainly possible but depends upon what your consider
an average bioload (number and size of fish and amount and frequency of
feeding). For an inexperienced aquarist, the inclusion of a supplemental
biological filter is, in my opinion, a good form of insurance.
A lot would depend upon which is better at scavenging ammonium ions from the
water column - plants or bacteria. I don't know the answer to that one, and
would appreciate comments from anyone who does.
I just came across an article written by Diana Walstad and published in the
April 1994 issue of FAMA (Aquatic Plants Prefer Ammonium Over Nitrates). For
the article, Diana conducted an extensive literature search and what she
found might shed some light on filter less aquariums.
Aquatic plants prefer to absorb ammonia (NH3) or ammonium ions (NH4+) over
nitrate (NO3-), due in part to the fact that the interior of the cell is
negatively charged. The small NH3 molecule, carrying no electrical charge,
can freely diffuse across the cell membrane. The positively charged NH4+ ion
is also able to enter a cell's interior easily. To use nitrate, NO3, the
plant is required to expend considerable energy in a two stage process
during which they convert the NO3 first of all to nitrite (NO2-) and then
ammonium (NH4+). You will notice that this is a reversal of the "Nitrogen
Cycle" conversion undertaken by a bacterial filter.
If ammonium is available, neither aquatic plants nor algae will use nitrate,
due to the vast difference in energy required - it is just not economical
for the plant to do so. This is true for the vast majority of aquatic
plants - there are only a few species where the reverse is true (water
hyacinth and several species from the Netherlands).
While ammonium (NH4+) is non-toxic, NH3 is certainly not - although it is
far less toxic to plants than it is to fish. In an aquarium situation, the
potential toxicity of NH3 to plants would probably not inhibit the uptake of
NH3 by the plants. Diana also noted that aquatic plants prefer leaf over
root uptake of ammonium. If ammonium is present in the water column, root
uptake of the ion is inhibited by upwards of 77%; however root uptake does
not inhibit leaf uptake of ammonium.
Diana concluded by stating that in a heavily planted aquarium, in the
absence of a separate filter, a moderate overload of nitrogen would not lead
to NH3 toxicity to fish but rather a slow accumulation of nitrate NO3.
This last bit I don't understand totally - if all of the nitrogen input is
in the form of NH3 or NH4+, and is used by the plants for growth, there
should be no build-up of nitrate. Plants use the ammonia, they don't just
convert it and then excrete it, as bacteria do. However, I guess that if
part of the nitrogen input was in the form of NO2- or NO3 or if there was an
excess of any species over what the capacity for the plant biomass could
actually use, then the remainder would be available for use by bacteria
(NH3->NO2->NO3).
Finally, any of the nitrogen input which was already in the form of nitrate
would be ignored by the plants in the presence of either NH3 or NH4+. So I
guess even a filter less tank would benefit from water exchange (to export
the nitrate) or de-nitrification (in the substrate or in a separate
bio-filter???). Comments, anyone?
Do any of the botanist's on the list know which is better at scavenging
ammonia and/or ammonium - plants or bacteria?
James Purchase
Toronto