233
16 Task of Mineral Nutrients in Eutrophication
directly by the inhibition of phosphate uptake and indirectly
by reducing the permeability of cell membranes (Nalewajko
and Olaveson 1994). Hall et al. (1989) also found greater
Cu toxicity in P-limited cultures of Chlorella vulgaris. They
concluded that P-limited cells were more sensitive to Cu,
owing to impaired metal exclusion/elimination mechanisms
produced by P-limitation. The pH of the media has been
considered an important factor influencing the toxicity of
Cu on periphyton (Guasch et al. 2002) and on algal cultures
(Stadorub et al. 1987). These authors showed an increase in
Cu toxicity when pH was reduced. This can be explained by
Cu speciation, because the amount of Cu
2+
(which is considered the most available form of Cu for algae) increases at
lower pH (Guasch et al. 2002). Cu toxicity was progressive
by indiscriminately damaging the algae at or protruding biofilm surface to deeper biofilm layers; thus, the magnitude of
the effect depended on the initial biomass in the short term
(Guasch et al. 2004).
16.4.12 Sources and Functions of Iron
Iron (Fe) is available to plants as iron ions (Fe
2+
or Fe
3+
).
Fe is essential in the heme enzyme system in plant metabolism (photosynthesis and respiration). The enzymes involved
include catalase, peroxidase, cytochrome oxidase, and other
cytochromes. It is part of protein ferredoxin and is required
in nitrate and sulfate reductions. Fe is also essential in the
synthesis and maintenance of chlorophyll in plants and is
strongly associated with protein metabolism (Uchida 2000;
Marschner 2002).
16.4.13 Role of Iron in Eutrophication
Besides nitrogen and phosphorus, iron should also be considered important regarding the regulation of eutrophication
in lakes. The fluctuation in iron concentration may have an
important influence on the metabolic activity of the algal
cells, and the spectral characteristics of algae could reflect
the physiological features of algae. Studies of algal spectral
properties under different iron supply would be meaningful for determining the algal bloom and for developing the
remote sensing warning system of lake eutrophication. Iron
reduction leads to the release of both Fe- and Fe-bound P,
and sulfate reduction forms toxic H 2 S gas, which is capable to reduce the Fe oxides efficiently, leading to blocking
of Fe cycling. Iron reacts with sulfide to produce the virtually insoluble precipitate of ferrous sulfide (FeS). It was
postulated that iron might function as an efficient scavenger
of sulfide, which would prevent it from reaching the toxic
concentrations. In contrast, oxygen production by the microphytobenthos may result in iron oxidation. Ferric iron (Fe
3+
)
is known to form an insoluble complex with phosphate.
Thus, iron may also play a role in regulating the availability of phosphate for algal growth. However, the mechanisms
of iron immobilization and mobilization are not precisely
known. The bulk of iron is present as ferrous sulfide which is
virtually insoluble. Benthic microorganisms such as cyanobacteria may also bind with and accumulate iron (Stal 1994).
Thus, immobilization of iron may be associated with microbial activities such as (i) sulfate- and sulfur reduction, which
produce the sulfide that will precipitate as FeS, (ii) oxygenic
photosynthesis by microphytobenthos that produce the oxygen that will oxidize iron and form insoluble iron hydroxides
either chemically or biologically, (iii) binding to extracellular polysaccharide sheaths of benthic microorganisms (e.g.,
Cyanobacteria; Decho 1990). Mobilization and liberation of
the iron from the sediments is, therefore, possible when the
activities described above cease. When acid volatile sulfide
is subsequently oxidized (chemically and/or biologically)
iron will be liberated. Ferrous iron (Fe
2+
) (when not precipitated as metal sulfide) is very soluble and this might explain
the mobilization and dissolution of iron from the sediment,
which would be most pronounced in the surface layer. Oxygen produced by the Cyanobacteria may precipitate the ferric
iron and, therefore, these organisms have also been shown to
accumulate iron in their extracellular polysaccharide sheath
(Stal 1994). Ferrous iron reacts instantaneously with oxygen.
Ferrous iron has also been reported as a possible electron
donor for cyanobacterial photosynthesis (Cohen 1989).
16.5 Impact of Eutrophication on Aquatic Life
Phosphates and nitrates occur in small amounts in all aquatic
environments. They are required to maintain the growth and
metabolism of aquatic plants and animals. However, excessive amounts of these minerals can prove to be quite harmful
for aquatic life. From different sources, dissolved minerals
and nutrients flow into streams, lakes, and other water bodies. A good portion of these dissolved minerals consists of
phosphates and nitrates. Levels of phosphates and nitrates,
which are intolerable to aquatic organisms, result in depletion
of dissolved oxygen levels by causing algal blooms. Inflow
of high amounts of phosphates and nitrates to water bodies (eutrophication), is the main cause in the destruction of
lake ecosystems around the world (Khan and Ansari 2005).
Several scientists have studied mineral levels in different
water bodies, concluding that the levels of phosphates and
nitrates adversely affect the overall health of the water and
its inhabitants (Yanamadala 2005). Excessive accumulation
of nutrients (specifically those of phosphates and nitrates)
cause luxuriant growth of algae and bacteria in water bodies. It also causes a burst of growth in other aquatic plants
and phytoplankton, forming a layer of green slime across the
16 Task of Mineral Nutrients in Eutrophication
directly by the inhibition of phosphate uptake and indirectly
by reducing the permeability of cell membranes (Nalewajko
and Olaveson 1994). Hall et al. (1989) also found greater
Cu toxicity in P-limited cultures of Chlorella vulgaris. They
concluded that P-limited cells were more sensitive to Cu,
owing to impaired metal exclusion/elimination mechanisms
produced by P-limitation. The pH of the media has been
considered an important factor influencing the toxicity of
Cu on periphyton (Guasch et al. 2002) and on algal cultures
(Stadorub et al. 1987). These authors showed an increase in
Cu toxicity when pH was reduced. This can be explained by
Cu speciation, because the amount of Cu
2+
(which is considered the most available form of Cu for algae) increases at
lower pH (Guasch et al. 2002). Cu toxicity was progressive
by indiscriminately damaging the algae at or protruding biofilm surface to deeper biofilm layers; thus, the magnitude of
the effect depended on the initial biomass in the short term
(Guasch et al. 2004).
16.4.12 Sources and Functions of Iron
Iron (Fe) is available to plants as iron ions (Fe
2+
or Fe
3+
).
Fe is essential in the heme enzyme system in plant metabolism (photosynthesis and respiration). The enzymes involved
include catalase, peroxidase, cytochrome oxidase, and other
cytochromes. It is part of protein ferredoxin and is required
in nitrate and sulfate reductions. Fe is also essential in the
synthesis and maintenance of chlorophyll in plants and is
strongly associated with protein metabolism (Uchida 2000;
Marschner 2002).
16.4.13 Role of Iron in Eutrophication
Besides nitrogen and phosphorus, iron should also be considered important regarding the regulation of eutrophication
in lakes. The fluctuation in iron concentration may have an
important influence on the metabolic activity of the algal
cells, and the spectral characteristics of algae could reflect
the physiological features of algae. Studies of algal spectral
properties under different iron supply would be meaningful for determining the algal bloom and for developing the
remote sensing warning system of lake eutrophication. Iron
reduction leads to the release of both Fe- and Fe-bound P,
and sulfate reduction forms toxic H 2 S gas, which is capable to reduce the Fe oxides efficiently, leading to blocking
of Fe cycling. Iron reacts with sulfide to produce the virtually insoluble precipitate of ferrous sulfide (FeS). It was
postulated that iron might function as an efficient scavenger
of sulfide, which would prevent it from reaching the toxic
concentrations. In contrast, oxygen production by the microphytobenthos may result in iron oxidation. Ferric iron (Fe
3+
)
is known to form an insoluble complex with phosphate.
Thus, iron may also play a role in regulating the availability of phosphate for algal growth. However, the mechanisms
of iron immobilization and mobilization are not precisely
known. The bulk of iron is present as ferrous sulfide which is
virtually insoluble. Benthic microorganisms such as cyanobacteria may also bind with and accumulate iron (Stal 1994).
Thus, immobilization of iron may be associated with microbial activities such as (i) sulfate- and sulfur reduction, which
produce the sulfide that will precipitate as FeS, (ii) oxygenic
photosynthesis by microphytobenthos that produce the oxygen that will oxidize iron and form insoluble iron hydroxides
either chemically or biologically, (iii) binding to extracellular polysaccharide sheaths of benthic microorganisms (e.g.,
Cyanobacteria; Decho 1990). Mobilization and liberation of
the iron from the sediments is, therefore, possible when the
activities described above cease. When acid volatile sulfide
is subsequently oxidized (chemically and/or biologically)
iron will be liberated. Ferrous iron (Fe
2+
) (when not precipitated as metal sulfide) is very soluble and this might explain
the mobilization and dissolution of iron from the sediment,
which would be most pronounced in the surface layer. Oxygen produced by the Cyanobacteria may precipitate the ferric
iron and, therefore, these organisms have also been shown to
accumulate iron in their extracellular polysaccharide sheath
(Stal 1994). Ferrous iron reacts instantaneously with oxygen.
Ferrous iron has also been reported as a possible electron
donor for cyanobacterial photosynthesis (Cohen 1989).
16.5 Impact of Eutrophication on Aquatic Life
Phosphates and nitrates occur in small amounts in all aquatic
environments. They are required to maintain the growth and
metabolism of aquatic plants and animals. However, excessive amounts of these minerals can prove to be quite harmful
for aquatic life. From different sources, dissolved minerals
and nutrients flow into streams, lakes, and other water bodies. A good portion of these dissolved minerals consists of
phosphates and nitrates. Levels of phosphates and nitrates,
which are intolerable to aquatic organisms, result in depletion
of dissolved oxygen levels by causing algal blooms. Inflow
of high amounts of phosphates and nitrates to water bodies (eutrophication), is the main cause in the destruction of
lake ecosystems around the world (Khan and Ansari 2005).
Several scientists have studied mineral levels in different
water bodies, concluding that the levels of phosphates and
nitrates adversely affect the overall health of the water and
its inhabitants (Yanamadala 2005). Excessive accumulation
of nutrients (specifically those of phosphates and nitrates)
cause luxuriant growth of algae and bacteria in water bodies. It also causes a burst of growth in other aquatic plants
and phytoplankton, forming a layer of green slime across the