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Re: Actinic lights and algae problems
On Sat, 12 Feb 2000, James Purchase wrote:
> Diana Walstad's book "Ecology of the Planted Aquarium" certainly is full of
> information that I've never heard before....
I find it packed full of thought-provoking information.
> Frequently, people post about persistent problems with algae in their tanks,
> and quite often their list of "tank parameters" include high color
> temperature and often actinic lighting.
I'd be interested to hear from anyone who had algae problems with
high-Kelvin lights and solved the problem by making no change other than
replacing the lights.
> In Chapter X of her book, on "Algae Control", Diana discusses Iron as the
> limiting nutrient for algae. In oxygenated water, iron is generally found
> bound, either as an iron precipitate (FeOOH, FeCO3, etc.) or is is bound to
> dissolved organic carbon (DOC). Free iron (Fe++, Fe+++) are the only types
> of iron that algae can use. So far, so good.... nothing really new here...
>
> Diana goes on to explain that while rooted plants can get iron from the
> substrate, algae depends upon free iron in the water column. Again, nothing
> new.
I would have liked more discussion and documentation for the idea that
algae can't use organically complexed iron. I felt even stronger about
that when she also asserted that plants can't use organically complexed
iron; other sources claim that plants can and often do use organically
complexed iron.
Despite my reservations, I don't disagree with her conclusion. I find it
very consistent with the common observation from people using PMDD that
chelated iron over a given threshold (0.1 mg/l, I think it is) causes
algae problems. The onset of algae growth at that threshold could imply
that iron is growth-limiting at lower concentrations. However, iron is
needed by algae in only small quantities and the threshold concentration
is far too high for the chelated iron to be growth limiting. A possible
explanation for the seeming contradiction is that the algae can't use the
chelated iron -- they can only use free iron. The free iron might be
there in a very low concentration that is generally proportional to the
higher concentration of chelated iron.
> But she explains that a common process called "photoreduction of iron" is
> able to set the bound iron free:
>
> DOC-Fe+++ + light -> Fe++ + oxidized DOC
>
> Apparently, this reaction, which also takes place with manganese and copper,
> can be greatly accellerated by light.
I've read that this and other similar light-induced reactions with
dissolved organics play important roles in some natural ecosystems. The
original organics are typically stable and unreactive, but the
photo-oxidized molecules are labile and can be used as food by bacteria,
thus feeding an otherwise unproductive ecosystem.
> Now this is "slightly new", at least to me... but the kicker is yet to
> come...
>
> Diana cites research which indicated that UV and blue light induce the most
> photoreduction because wavelengths of light below around 500 nm are
> energetic enough to break the DOC-Fe+++ chemical bonds. She also explains
> that iron can bind to a variety of chemicals and different types of DOC, and
> that each separate "species" can vary in susceptibility to photoreduction.
I haven't read her source for this, but from her description I thought
that the control on the experiment might be inadequate for the conclusion
she drew. There might be reasonable alternatives and other factors
effecting the results.
> Now, to me at least, this IS new information, and another reason to stay
> away from 50/50 and actinic lights in freshwater plant tanks, especially
> those with older substrates and corresponding high levels of DOC in the
> water column.
I liked the emphasis that Ms. Walstad placed on dissolved organics in the
aquarium. DOC can potentially play several important roles in aquariums,
a couple of which she emphasized in her book. Unfortunately there's very
little technical information on DOC in aquariums, no convenient tests for
us to run and no body of knowledge that lets us understand the variability
or controls on the types, amounts or effects of DOC in the aquarium. Its
fairly easy (I know, because I've done it) to assert the importance of
constituents that aren't quantified or well-understood. It's a lot more
difficult to actually establish their importance.
> Many people (most of us I guess) use certain "aim points" for particular
> nutrients in the water column - we want iron, to be at one level, phosphate
> below another level, etc. Many of these "aim points" come from hydroponic
> studies. Diana reminds her readers that in tanks with enriched substrates
> (her's are soil based), iron should really come from the substrate, not the
> water - so our additions of chleated iron to the water column might be
> counter productive and based upon research which is not really applicable or
> appropriate for an aquarium.
I wonder what proportion of the APD readers use chelated iron in their
tanks. I'll guess that most of us do and that many of us use it without
algae problems that can be attributed to the iron.
> Comments or thoughts, anyone???
Ms. Walstad's point here is very interesting, but it's a point that I
think may be more relevant to her soil-based approach than it is to some
other successful methods. There are other places in her book where I
found her ideas to be very interesting and well thought out, but difficult
to factor into more general aquarium-keeping methods.
Roger Miller