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Dissolved organic substances.
Besides dissolved gases and dissolved minerals,
the third component dissolved in all waters,
both natural and captive ones, comprises
various forms of dissolved organic carbon
(DOC). DOC comprises all the soluble carbon-based
molecules that have been assembled by living
organisms.
The biological cycling of nitrogen, phosphorus,
etc. are sketched for you in the "Nutrient
Cycles" pages. Carbon is also recycled.
More than just a nutrient, carbon is the
basic building block of all life. The vehicle
for carbon cycling is dissolved organic carbon
(DOC). Some organic carbon molecules in the
aquarium water are quite transient, such
as the biodegradable amino acids, polypeptides
and proteins, sugars and carbohydrates. Other
forms of DOC are the more resistant and permanent
humic substances.
DOC may originate on land, in leaf litter
and other organic debris, and then get washed
into the water. Or it may originate in the
water, in algae and plants, or in bacteria
or planktonic animals. In its dissolved organic
forms, carbon is constantly being decomposed,
cycling down until it is finally completely
oxidized to carbon dioxide. Carbon dioxide
and carbonate minerals form the carbon cycle's
inorganic phase. (Similarly, the phosphate
cycle has its own inorganic phase, orthophosphate,
and the nitrogen cycle has an inorganic phase
in water, ammonia/ammonium.)
Once carbon has been oxidized to CO2, it is ready to be taken up by plants and
algae through photosynthesis, to be built
into organic carbon once more. In the broadest,
simplest sense, that's the carbon cycle in
fresh water.
DOC can be separated into two groups: the
humic substances and every other kind of
dissolved organic carbon.
Non-Humic Dissolved
Organic Carbon. Not all organic carbon is built into the
large molecules you might think of when you
think of "organic" molecules: simple
amino acids are also part of the DOC loading
in water. Other examples of non-humic DOC
are substances like carbohydrates (simple
sugars and starches), proteins and the peptides
they are made from, fats and waxes (lipids),
resins, pigments, and other low molecular
weight substances.
In general the non-humic
organic substances
are more easily assimilated
by micro-organisms.
Down at the base
of the trophic web, these
forms of DOC constitute
a fundamental food
source for bacteria
and fungi, and so they
exhibit rapid flux
rates. In other words,
they don't last long
in the aquarium before
they are assimilated
by some organism. You
don't need to worry
about these organics
building up in the
aquarium.
Of the various forms that dissolved organic
carbon may take, the humic substances are the only ones likely to build up as
metabolic end products in a dynamically balanced
natural planted aquarium.
The humic substances constitute the bulk
of organic matter in natural and captive
freshwater systems, partly because they don't
break down so easily. Humic substances may
account for as much as 95% of the total dissolved
matter in blackwater systems, where the concentration
of humins is greater than all the inorganic
ions in the water put together. Humins are
described as acidic, dark colored, partially
aromatic, chemically complex substances with
high molecular weights.
Humic DOC participates in many aspects of
water chemistry. It is often underestimated
as a source of acids, in the acid-base buffering
of freshwaters, particularly in soft water.
DOC chelates trace metals, binding them in
water-soluble forms. Its golden to brown
color may even affect the transmission of
light.
The rest of this
folder concerns humic substances.
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