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Re: [APD] plant/algae competition

Ann wrote:

> Tom, you once (or several times) have mentioned that plants "leak"
> nutrients.

I hate to butt in, but this is one of my pet topics, so I will anyway.

> Is this the mechanism for algae growth on leaves, (and when nutrients are
> leaked in quantity, even growth on other surfaces)?

I have an article coauthored by one of the most respected limnologists around
in which the authors suggest not only that this is a mechanism for algae
growth on leaves, but that it is an evolved-in defense mechanism.  The plants
"intentionally" promote growth of algae on their leaves so that herbivores are
more likely to eat the algae and less likely to eat the leaves.

I won't comment on whether that makes a lot of sense, but that is what they said.

> Is it possible that when something is in short supply, the other (now)
> excess nutrients leak out, attracting algae as scavengers to absorb that
> leakage?

I talked to Claus Christensen about this at the AGA conference in Houston.  He
agreed that plants will leak inorganic nutrients into the water.  I don't
recall whether he also agreed that the leakage was accelerated by a nutrient
limitation.  It makes sense that it would be.

I'm not sure what significance this would have.  If those nutrients are excess
to the plants then they are probably already present in the water at
concentrations that are more than sufficient for algae growth. 

Now extending that thought a little ...

According to at least one authoritative source some -- perhaps most -- algae
are capable of autotrophic behavior.  They can use sugars and other fixed
carbon sources from their environment as direct replacements for the sugars
they would fix by their own photosynthesis.

That made me wonder if plants might leak simple sugars with that leakage
increasing under at least some conditions of nutrient limitation.  I asked
around at the local university and a biology professor there told me that
algae are known to lose quite a bit of fixed carbon when they are nutrient
stressed, but he wasn't sure that plants did.  More recently I read that
nutrient-stressed algae can lose as much as 50% of the carbon they fix.  They
are a major source of dissolved organics.

If plants show the same or similar behavior then they could leak organics into
the water that some algae -- acting autotrophically -- might use.  If so then
we would have not only an explanation for why algae problems crop up when
plants are nutrient-stressed, but also a part of the answer to why such a
small number of algaes (out of the thousands out there) seem to reach nuisance
levels in mature planted aquariums.

This is all a rather complicated speculation that needs quite a bit of
experimental work.  Just the same, the concept is to me an appealing
explanation for some of the odd plant-algae relationships we see in aquariums.

Roger Miller
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