Dave Evans recently wrote th following comments regarding the Discover article:
>But the
>author mentions the plant PRODUCES acid (protons, H+) to break
>down stuff it catches and wonders how the mosquitoe larva can
>in the plants. Now if a plant is growing in a bog, pH 5.5-4.5,
>what is so amazing about it having low pH, wouldn't the protons
>just get through by osmosis? Correspondingly, what is so amazing
>about the mosquitoes and midges living in this acid environment
>when all the water around is acid anyway? They were probably
>adapted to living in bogs before there were Purple Pitchers
>(A.K.A. S.purpurea) to tenement them in.
For a number of years I have been studying the ecology of the
community of insects and microbes that live in S. purpurea pitchers, so
I'll throw in my $.02 worth.
The pH actually varies considerably over the course of the 30 days
or so when the pitcher is actively attracting and capturing prey. It can
range from around just below 5 to over 8, depending on the age of the
pitcher and how much prey is being digested. Fluid pH tends to start low
(rainwater), go up as prey is captured and ammonia is released through
microbial degradation, and then rapidly drop and remain low. Although
there has been speculation on whether the plant itself controls the drop in
pH, I'll have to do some checking to see if this has been confirmed. The
early rise in pH is due primarily to microbial activity, however. H+ ions
would not likely be transported through the waxy cuticle of the plant into
the pitcher fluid.
Wyeomyia smithii, the pitcher plant mosquito, is a member of a
primarily container-breeding group, and probably switched from treeholes or
some other small plant held water (phytotelm) to the pitchers. Therefore,
I don't think it was "preadapted" to life at low pH by virtue of being a
bog species. The larvae do quite well at a variety of pH values. The
midge, Metriocnemus knabi, also can be found doing just fine at all pH
values. Since the eggs of both species are laid almost as soon as the
pitcher opens even before there is water present, the larvae are exposed to
a constantly changing environment.
Quite a number of protozoa, algae, rotifers, and other aquatic creatures
live out their lives in the pitchers, and the pitcher itself to me seems a
fairly hospitable place, not much more "nasty" than any water-filled
container. Since the plant doesn't play an active role in killing and
digesting it's prey (once lured into the pitcher they simply drown), I
don't find the existence of these critters at all surprising. In fact, the
plant has a couple of traits that make me think that perhaps there was a
considerable amount of adaptation on its part to attract and keep its
tenants (hope that doesn't sound too teleological ;-)). For example, S.
purpurea has chloroplasts in the epidermal cells of the inner side of the
pitcher. Carbon dioxide is absorbed from the pitcher fluid and oxygen is
secreted into the fluid thus making things more comfy for its guests. The
plants appear to utilize nutrients from the prey more efficiently, and to
grow much better if the midge and mosquito larvae are present. In
addition, the plant produces an attractant for the female mosquito (and
perhaps the midge) that is chemically different from the nectar used to
lure prey.
I think that sometimes the journalists get a little carried away with the
idea of "wow - a carnivorous plant, must be really nasty inside those
traps." For some species this is no doubt true, but for S. purpurea, it
just "ain't necessarily so."
Looking forward to reading the article,
Liane
Liane Cochran-Stafira
Dept. of Ecology and Evolution
The University of Chicago
1101 East 57th Street
Chicago, Illinois 60637-5415
phone: 312-702-1930
e-mail: lcochran@midway.uchicago.edu