Environmental Engineering Reference
In-Depth Information
lasting more than two decades revealed a striking
change in the composition of the phytoplankton
community, as diatoms decreased and flagellates
increased more than tenfold because of fourfold
increase in the ratio of available N and P to silica.
Further, eutrophication also results in higher lev-
els of dissolved organic matter (DOM) and affects
the availability of biologically useable forms of
iron and other essential metals. Since iron must
be in soluble form to be bioavailable and higher
DOM leads to higher soluble iron, this leads to
changes in the species composition of the phy-
toplankton community and poses enormous con-
sequences for animal grazers and predators. The
degradation or complete loss of seagrass beds in
an aquatic ecosystem is another consequence of
nutrient eutrophication, because plant growth in
these beds is often light-limited and lower light
availability by nutrient-stimulated growth of
phytoplankton and algal bloom. It brings marked
changes in the associated animal life. Coral reefs,
one of the most diverse ecosystems in the world
and sensitive to nutrient pollution, are found in
naturally nutrient-poor surface waters in the trop-
ics and subtropics; however, a high nutrient level
is generally detrimental to reef health and lead
to shifts away from corals towards algal turfs or
seaweeds that overgrow or cover the reefs (Bo-
mans et al.
2005
).
constituent heavy metals in agricultural soil and
may also be hazardous for human health. Several
heavy metals that are present as contaminants
include cadmium (Cd), arsenic (As), chromium
(Cr), lead (Pb), mercury (Hg), nickel (Ni) and
vanadium (V). However, the metal that is of the
most concern is Cd because of its maximum bio-
availability on acid soils, while the rest are not
as readily absorbed by plants on P-fertilised soils
(Mortvedt and Beaton
1995
; Bomans et al.
2005
).
7.4
Phytases: An Introduction
to Their Use in Phosporous
Pollution Management
Considering the aforementioned consequences of
P pollution and eutrophication, several alterna-
tive approaches have been suggested for reduc-
tion of phytic acid levels in animal feed and are
categorised as chemical methods (e.g. extraction
and precipitation), feed processing and enzymat-
ic method (phytase application). Chemical meth-
ods affect the nutritional quality of the products
and are generally expensive (Pandey et al.
2001
);
however, phytase application for P management
is considered as the most promising alternative.
Numerous animal trials have shown that add-
ing phytase to feed at 500-1000 phytase units/
kg may replace inorganic P supplements for pigs
and poultry and reduce their P excretion by ap-
proximately 50 % (Lei et al.
1993
; Augspurger
et al.
2003
).
Phytases are acid phosphohydrolases that ca-
talyse the hydrolysis of phosphate from phytic
acid to inorganic phosphate and
myo
-inositol
phosphate derivatives (Roopesh et al.
2006
).
Phytases can be classified into three classes de-
pending on the position of the first dephosphory-
lation of phytate, namely, 3-phytases, 4/6-phy-
tases and 5-phytases. Within each class, there are
not only structural differences but also different
mechanisms for the hydrolysis of phytic acid.
The 3-phytase (
myo
inositol hexakisphosphate-
3-phosphohydrolase, E.C.3.1.38) removes the
phosphate from the 3-position of phytate and is
found typically in microorganisms. The 4/6-phy-
tase (
myo-
inositol-hexakisphosphate-4/6-phos-
7.3.3
Heavy Metal Pollution
Phosphorus is a major limiting factor to crop pro-
ductivity on many types of soils, including acidic
and infertile soils. Therefore, they require appli-
cations of fertilisers for better productivity in the
form of organic and inorganic P fertilisers. Dur-
ing the production process of inorganic P fertilis-
ers from phosphate rock (PR), varying amounts
of heavy metals (minor constituents in the PR
ores) are transferred to P fertilisers. These heavy
metals may accumulate in the soil with repeated
fertiliser applications. The main organic fertilis-
ers, animal manure and sewage sludge (biosol-
ids) are also applied for better crop production
and may also contain heavy metal contaminants.
Their repeated application will cause pollution of
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