Due to its high reactivity, phosphorus occurs in nature only in the form of
phosphorus compounds, to the greater extent
in the minerals of Earth crust. According to its average molar concentration of 1,6.10-6
mol/l, phosphorus is the sixteenth most abundant element in seawater. Expressed in mass units its
average concentration is 4,96.10-5 g/l (50 μg/l).
Phosphorus - (Phosphorum) - P
Due to its high
reactivity, phosphorus occurs in nature only in the form of phosphorus
compounds, to the greater extent in the minerals of Earth crust.
According to its average molar concentration of 1,6.10-6 mol/l, phosphorus
is the sixteenth most abundant element in seawater. Expressed in mass
units its average concentration is 4,96.10-5 g/l (50 µg/l).
Phosphorus occurs in seawater in organic particulate form, as well
as in dissolved form (inorganic and organic phosphorus). Organic phosphorus
is the product of animal excretion and of the disintegration processes
of deceased organisms. At the surface waters it represents a significant
- yet unstable - fraction of the total dissolved phosphorus in seawater.
The phosphorus cycle in seawater has still not been fully described and
understood. Inorganic dissolved phosphorus occurs in seawater mainly
in the form of H3PO4 (phosphoric
acid) dissociation products.
Influence on seawater chemistry
The dissociation of phosphoric acid leads to the formation of the
following phosphate anion forms:
H3PO4 -> H+ + H2PO4-
-> H+ + HPO42-
HPO42- -> H+ + PO43-
Average representation of individual phosphate anion forms in seawater
as a function of the pH value.
Phosphoric acid is present as H2PO4- at
ca 85% at pH 8,1 in seawater - and at 15% as PO43-.
From the reefkeepers point of view and for the purpose of this article
of major importance is the low pH value range, in which calcium reactors
operate – with lower pH values the PO43- form
disappears and the HPO42- and H2 fractions
become more prevalent. The occurrence rate of H2PO4- /
PO43- in seawater is influenced - among other parameters - mainly by the pH value.
It is very important to know that calcium (Ca) and magnesium (Mg) have
the ability to form ion pairs with the different phosphate ion forms
with varying bond force. The occurrence of the different phosphate forms
in seawater at 25°C and at a pH between 8,1 and 6,0 looks as follows:
In ion pair with:
In ion pair with:
Each ion pair type has different bond force (due to a varying association
constant) and may influence certain chemical and biological parameters
of calcium and magnesium. At pH 8,1 magnesium and calcium bind significant
quantities of inorganic phosphorus in ion pairs, and thus phosphorus
appears in its entirely dissociated form at only ca 45%. However when the
pH values sink, the number of ion pairs also sinks and free inorganic phosphorus
then occurs in free - entirely dissociated form - at 70% at a pH of around
Even though - as already mentioned - all aspects of the phosphorus occurrence
in seawater in its different forms and concentrations are not fully known
and understood, one should bear in mind that different combinations of
its absolute concentration and pH values can greatly influence the phosphate
reactivity and the bioavailability of calcium and magnesium. As a consequence
and under the certain conditions the influence of phosphorus on marine
organisms can be devastating.
Different forms of the phosphoric acid anions have the ability to form
differently strong ion pairs with the calcium and magnesium cations, possessing
Phosphorus is one of the key biogenic elements and is present in
all living organisms. Among others it occurs in the DNA and RNA molecules,
cellular energy suppliers (ADP, ATP) and certain fats (phospholipids).
As far as the marine organisms are concerned, phosphorus is a key nutrient,
which enters the food chain through sea autotrophic organisms - organisms,
which are able to metabolize inorganic substances into organic ones.
These are primarily photosynthesizing phytoplankton - algae (including
Zooxanthellae), cyanobacteria and certain protozoa - for which phosphorus
is the primary nutrient.
Nevertheless, phosphorus is also the limiting nutrient. All living organisms
need phosphorus, but the latter occurs in seawater in relatively small
concentrations. The available phosphorus quantities are therefore a limiting
growth factor for the living matter in a given water volume.
The life in general is a subject to the presence
and to the constant supply of the phosphorus.
Phosphorus and Reefkeeping
From the above mentioned facts it could be deduced, that if the available
phosphorus in seawater is lower than the demand for it - which holds true
-, then an increased phosphorus concentration in seawater should be advantageous
for marine organisms and should improve the situation - which does not
hold true. In this case it is necessary to differentiate between primary
phosphorus consumers – as is phytoplankton – and other living organisms,
as corals for instance. For phytoplankton an increased phosphorus concentration
would be definitely advantageous, while this would have a negative effect
on corals, because they are adapted to natural - and therefore a low -
phosphorus concentration. This leads to the following apparent paradox
It is necessary to guarantee the living organisms in the aquarium a constant
phosphorus supply; however the phosphorus concentration in seawater must
be kept at low, in essence trace concentrations.
This paradox is not easy to solve. This is, among other factors, due
to the fact that the character of the common phosphorus test-kits does
not allow precise determination of its concentration in the region around
50 µg/l. The only solution is to abide by the following rules:
Remove the phosphates from the tank, but not eliminate them
Ensure a phosphorus supply with the controlled application of the
right nutrition, containing:
particulate organic matter (POM)
as dissolved organic matter (DOM)
The combination of high phosphates concentration
and low pH value can have a very negative effect on the seawater chemistry
in the reef tank. It is necessary to keep the phosphates concentration
at trace levels (undetectable with common tests), while at the same time
to maintain a constant phosphorus supply to the tank in particulate and
dissolved form using the appropriate food preparation.
Occurrence in seawater:
Organic matter; different phosphate anion forms
50 µg/l; 0 when using common tests
All life parameters of organisms
All living organisms without exception
CS Reefkeeping Concept
An Introduction to the Chemistry of the Sea, Michael E. Q. Pilson, Pearson Education Chemical Oceanography, Second Edition, Frank J. Millero, CRC Press
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