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an algae experiment


For a while now I've been chasing the idea that at least some algae
problems can be related to organic compounds in the water.  The general
thinking goes like this:

1) Some (but not necessarily all) algae can live in the dark by using
glucose and other soluble organics in the water.

2) Aquatic plants "leak" soluble organic material into the water. 
There's even some published speculation that this behavior evolved to
encourage attached algae communities (herbivores would then eat the
attached algae, and leave the plant alone).

3) Terrestrial plants, when deficient in nitrogen, phosphorus or sulfur
can photosynthesize, but fail to use the simple sugars produces by
photosynthesis; instead, the sugars accumulate in the plant and lead to
anthocyanin (reddish-purple pigment) in the leaves and stems.

4) Aquatic plants should behave under nutrient stress like terrestrial
plants, but I reason that rather than accumulating the sugars, they will
probably leak the sugars back into the water.  This leaking may help
explain why I don't usually see anthocyanin buildup in aquatic plants
that otherwise seem to be deficient in nitrogen or phosphorus.  It's
perhaps interesting but irrelevant that stressed algae leak large
amounts of soluble organics.

5) Algae are more efficient than plants at concentrating dissolved
nutrients, so in the case where both plants and algae depend on the same
nutrient supply, nutrient stress and loss of organics should effect
plants before it effects algae.

6) The sugars lost by a nutrient-stressed plant can be used by algae; it
will act as a suppliment for the algae and allow them to grow at rates
and under conditions (low light, in particular) where they wouldn't
normally thrive.

If the last item could be proven true, then we might be able explain
some of our toughest algae problems, and in particular might explain how
it is that we can make algae disappear by fertilizing an aquarium. 
Well-nourished plants stop losing sugars and without the extra subsidy
from the plants the algae no longer thrives and start to disappear.

So that's the theory.  Of the items above, 1-3 are supported in the
scientific literature.  There's a problem with applying item 1 (see
below), but otherwise I'll just regard 1-3 as facts.

Item 5 is also supported in the literature, but the case where both
plants and algae are dependent on the same nutrient supply is fairly
hypothetical.  It generally doesn't occur in nature where plants use a
supply of nutrients in the soils that isn't available to algae.  It
comes close to being true in aquariums where the principle nutrient
supply is added to the water column.

Item 4 is supported qualitatively by my own observations.  It could be
supported more quantitatively by monitoring DOC (Dissolved Organic
Carbon, or just adsorbance at 254 nm in a spectrophotometer) in a
planted aquarium.  If correct, then DOC in an aquarium should be low
when the plants are completely nourished and should be higher when the
plants are deficient in nitrogen or phosphorus.  A more rigorous support
would be possible by determining the actual sugars in solution, but that
would be more difficult.

Item 6 is the conclusion.  It could be supported by proving item 4 and
showing that algae growth rates and occurance correlate statistically
with the changes in DOC in an aquarium.  

One of the weaknesses of all this reasoning is that we don't know if our
common nuisance algae actually can use simple sugars; that's the problem
with appying item 1.  So last week I set out to see if some of our
common nuisance algaes can use dissolved sugars.  I only have a few in
my tanks, so that's what I used.

I set up two 1-quart bottles in a simple controlled experiment.  Each
bottle was covered in duct tape to exclude light.  One was filled with
dechlorinated tap water and the other was filled with dechorinated tap
water plus 1/24th of a 5 gram glucose tablet -- about 200 mg of
glucose.  The water in both containers was changed every two days.

I took three algae samples from my tanks and split them more-or-less
equally between the containers.  One algae sample was a green,
long-stranded hair algae that grows unattached in a sunlit bowl with my
emersed sword plant.  One algae sample was a short-stranded green hair
algae that grows in bright artificial light or dim light and forms
loosely attached, dense mats on the substrate and tufts in protected
areas on leaves and aquarium equipment.  The last sample was black brush
algae.  This is a red algae that grows under bright or dim light (but is
more characteristic of dim light) and is firmly attached.

I placed the bottles in a cabinet, put an air stone in each container
and bubbled air through them for a week.  I observed the contents of
each bottle every day.  Today (a week after starting the test) I dumped
the bottles to see what I had left.

There were several problems with the experiment.  Mostly, the agitation
caused by aeration made the hair algaes get matted and entangled. 
Future attempts need to use less agitation and keep the samples
separated.  Second, I didn't have any way to measure or weigh the
samples and hoped for clear-cut results such as the complete
disappearance of the failed samples.  That didn't happen, so any
conclusion depends on my recollection of the sample size and a
comparison of the samples.  Finally, the glucose concentration probably
needs to be smaller to more nearly represent conditions in an aquarium.

So, after it was all done, here's what I noticed and what it might mean.

I half-expected that one or both bottles would be taken over by blue
green algae.  That didn't happen, and I'm not sure why.  That may
indicate that whatever BGA accompanied my samples didn't use glucose.  I
find that a little surprising.

The long-stranded hair algae in both bottles was smaller than it started
out to be.  That could be just because it was matted and twisted by the
aeration, but my sense is that there was about a 50% loss of mass in
both bottles.  This would indicate no response to glucose.  This doesn't
surprise me much, as in my tanks the algae occurs only in bright light.

The short-stranded hair algae disappeared completely from the bottle
without glucose.  Most of the sample appeared to break up into loose
fibers after the first 24 hours of the test.  Some of the short-stranded
algae may still have been present but unobserved at the end of the
test.  If it was present then it was tangled with the long-stranded
algae.  The short-stranded hair algae was still present in the bottle
with glucose.  It was loosely attached to the bottom of the bottle and
appeared to have at least as much mass as it started with.  This
suggests that the short-stranded hair algae did use the glucose.

The black brush algae was still clearly identifiable in both bottles,
but in the bottle without glucose it appeared to lose about half of its
mass.  Some of that BBA sample could have become entangled with the long
hair algae.  The original mass appeared to be entirely intact in the
bottle with glucose, but it didn't appear to grow.  So BBA appeared to
use the glucose.

There are enough problems here that I can't draw any real conclusions.

Ideally, if better experiments in the future prove out the results of
this test then we will have not just an explanation for some of out
sticky algae problems, but a new approach (organics control) for
cleaning up and controlling nuisance algae.

Roger Miller