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In addition to good information supplied by Dave from Manitoba,
Canada on Isoetes the following reference may be of interest:
Keeley, J.E. 1983. Crassulacean acid metabolism in the seasonally submerged
aquatic Isoetes howellii. Oecologia 58:57-62.
In that and other papers it was determined that Isoetes
and some other aquatic species, such as Crassula aquatica,
are able to utilized CO2 at night (Crassulacean Acid Metabolism
= CAM). The reason for this night-time up-take of CO2 appears
to be an adaptation to low CO2 in the water during the day
time in the vernal pools where these species grow. CO2 in
the water column that formed from night-time respiration is
basically all used by mid-morning. The pH variation in
the vernal pools ranges from a pH of 6 at night to a pH of 8.3
by mid-day. It is interesting that the CAM physiology only
occurs in submerged leaves and not emersed leaves.
The significance to aquatic plant enthusiasts is that Isoetes
might be a good choice for low CO2 conditions, but they probably require a
high light levels. Also, the species growing in
shallow vernal pools such as Isoetes howellii may not be the
best choice for the aquarium. Species found in deeper ponds
that spend a majority or all of their life-cycle submerged may
be better adapted to aquaria. Christel Kasselmann in her book
Aquarien-pflanzen mentions Isoetes velata var. sicula. Kasselman
notes that species can tolerate acidic or alkaline conditions.
Darin Gasperson of Aquatic Greenhouse includes an "Isoetes" on
his plant list. No doubt others have species that can do well
In California we have six native species. Three species,
I.howellii, I.nuttallii, and I.orcuttii, grow in vernal pools
and are submerged for several months then grow out of water
once thepools dry-up for another month or two before going
dormant. The other three speces, I.echinsopora, I.bolanderi,
and I. occidentalis grow in deeper ponds and are evergreen.
The later three could be suitable for aquaria and perhaps some California
aquarists have tried them and could comment on their culure.
As Dave noted the Isoetes at his university also uses CO2 from
the substrate which is a source of CO2 for other aquatic plants,
but not restricted to plants that have CAM physiology. For CO2
to be generated in the substrate it requires bacteria to be established in
an medium with organic matter. Organic matter is
needed in the substrate as a carbon source for the bacteria. Unfortunately,
having too much organic matter in the substrate
can lead to the problem that Karen Randall mentioned where potted plants in
one of her aquaria developed severe anaerobic conditions. The result of too
much organic matter can lead to H2S (hydrogen sulfide) production which is
toxic to fish if the concentrations
get too high. The H2S also binds to Iron (Fe+2) to form insoluble
FeS (Iron sulfide) which removes iron from the system.
The bottom line is not to have too much organic matter in the aquarium
substrate. As many people have mentioned previously we
do not want to duplicate the marshes and wetlands found in nature where
organic matter can be up to 40 percent or more of the soil substrate. May
be Neil Frank and Karen Randall can shed light on
the amount of organic matter in the substrate in their aquaria.
The fact that some aquatic plants can take advantage of CO2 from
the substrate does not mean that it a requirement. If fish,
shrimp, snails, other organisms, and the smell of H2S (the rotten
egg smell) are not an issue then using substrates with higher
organic matter can be used to grow aquatic plants. I have grown
many wetland and aquatic plants in organic mucks that they grow
in in the wetland habitats, but I don't keep fish in them or put
them in the house.