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Re: an algae experiment


On Wed, 13 Jun 2001, Christopher Newell wrote:

> That was an interesting post a couple of days back.
> I've been trying to digest it.  I'm out of my element
> with most of this discussion, but wanted to comment.

Thanks for taking the time.  I hoped to get enough feedback to see if I
needed to change my thinking.

> In a discussion with friends, it was pointed out to me
> that plants (which includes algae) can make use of
> sugars, amino acids, etc. - it is the basis of tissue
> culture.

The ability of plants (and algae) to take simple carbohydrates out of
solution is also the basis for Seachem's Excel product.  While I have
found plenty of references that indicate *some* algae will use sugars that
they get out of the water I haven't found any author willing to say that
they *all* will.  I expect conciderable variability.

As a note, I'm using the term "sugar" to refer to most simple
carboyhydrates.

> You are referring to a nutrient stressed situation.
> Im assuming that you are referring to high light tank
> with co2 supplementation and without adequate nutrient
> supplementation, because most planted fish tanks
> have an abundance of nutrients.  The low tech planted
> fish tank is where most folks experience algae
> problems.  But again, you are talking about a
> nutrient stressed situation.

Specifically I'm referring to planted tanks low in nitrogen and/or
phosphorus.  This could be high or low in other factors, though the
problem is likely to be more evident in tanks where there is a high rate
of photosynthesis.

> You mention that the plants might be leaking sugars
> into the water, which makes this sugar food source
> available for algae to use.  This is difficult for me
> to follow, but certainly, Im not saying that you are
> wrong.

As near as I can tell from the literature it is well-known that aquatic
plants do leak organics into the water.  There may be an evolutionary
advantage to that behavior.

> During lean times when plants are nutrient
> stressed you conjecture that our plants begin to make
> sugars as terrestrial plants do.  I would think that
> the plant is making sugar for its own use.  The plant
> controls the water flow into and out of the tissue.
> If the sugars leak then wouldnt it be in miniscule
> quantities?

All plants (aquatic or terrestrial) and algae make sugars any time the
light is bright enough.  Simple carbohydrates are the initial product of
photosynthesis.  The plant normally builds all of its components out of
those simple sugars.  Phosphorus is necessary in the intermediate steps of
carbohydrate synthesis and for the plants to make proteins and other
biochemicals they need other nutrients as well.  That's were nitrogen,
more phosphorus and most of those other essential elements fall in.

If nitrogen or phosphorus is deficient then the plant can't make proteins
and other classes of biochemicals and the excess simple sugars build up.
Some of the sugars can be converted to woody material and some can be
stored, but not all of it.  In terrestrial plants the excess sugars build
up; I think that after some point aquatic plants tend to lose the sugars
rather than retain them.  Terrestrial plants might leak them too if they
could, but they since they aren't emersed in water it isn't so easy for
them to lose the sugar.

I have never read of a mechanism by which a plant could effectively slow
down the rate of photosynthesis, so the simple sugars are produced
regardless of whether the plant needs them, can use them, or has any way
to store them.  Even detached leaves will continue to photosynthesize.  At
some point the plant must start leaking those sugars back into the water.
The leakage rate would not be miniscule; the plant could potentially leak
all of the sugars that it synthesizes.

I think I've seen that happen in phosphate-deficient vals.  Given CO2 and
light they bubbled and bubbled, which is direct evidence for
photosynthesis, but despite the evident photosynthesis there was no
evident growth for weeks.  Where did they put the products of their
photosynthesis?  Vals don't have noticable storage organs and there was no
anthocyanin evident in the leaves to indicate sugar buildup in the leaves.
I concluded that the plants leaked almost everything they
photosynthesized.  There were concurrent algae problems.

> The sugars need to be leaked by the
> plant and absorbed by opportunistic algae.  This is a
> byproduct of a byproduct, as it were.  Would this
> effect be measurable in an aquarium?  I would think
> that the bacteria bed would make use of the sugar
> before the algae would.

The "leaked" sugars could represent a big part -- even all -- of the
plants primary production, so this is a potentially major process, not
simply a byproduct of a byproduct.

The opportunistic algae are another matter entirely.  Algae are highly
evolved specialists.  The algae that live in our tanks probably have some
specific adaptation that allows them to thrive under aquarium conditions.
Out of all the thousands of algae species out there, there are only a few
that thrive to nuisance levels in our tanks.  I suspect that the ability
to use dissolved sugars is one of the factors that determines which algae
species thrive in our tanks.  So the algae aren't merely opportunists,
they are specialists at exploiting the conditions we provide them.

You are right that bacteria should be able to out compete algae for the
dissolved sugars, and in a natural environment that is probably what
happens.  At least that is what I was told by a biology professor at the
local University who has a special interest in organic compounds in
natural systems.

We aren't dealing with natural conditions.  Instead I'm talkin about
pathologic conditions brought on by nutrient stress in a mostly closed
environment.  I think the availability of those sugars would be so high
that competition would be irrelevant.  The sugar would be available to
both algae and bacteria.

This theory isn't really a "revolution" in thinking.  Instead it's just a
different way of explaining a number of observations that are otherwise
difficult to understand; like how it is you can add nutrients and
discourage algae, which lots of us have found to work.


Roger Miller