Environmental Engineering Reference
In-Depth Information
This effect is the opposite as the previous one, and is most likely accounted for by
the low production of nutrients from low contents of DOM and POM.
On the other hand, a decrease in PSII efficiency with changes in cellular physi-
ology of microalgae can result into nutrient (and mostly nitrogen) stress, ultimately
followed by a cell stress (Parkhill et al.
2001
; Babin et al.
1996
; Cullen et al.
1992
;
Geider et al.
1993
; Graziano et al.
1996
). These studies thus show that a decrease
of photosynthetic efficiency is caused by nutrient stress. Nitrogen stress is found to
reduce the maximum quantum yield of carbon fixation (Babin et al.
1996
). The mech-
anism behind the N-containing (NO
2
−
−
and NO
3
) nutrient stress is presumably that
•
−
−
the strong oxidizing agent HO
, could
react with the functional groups bound to PSII and can damage the cells. The result
is a decline of the overall photosynthetic efficiency that suppresses the primary pro-
duction. In addition, the synergic effect of UV radiation due to depletion of the stra-
trospheric ozone layer in combination with N-containing nutrient stress can generate
extremely high contents of HO
, photogenerated from both NO
2
and NO
3
•
, which can kill aquatic microorganisms. Note that in
Antarctic seawater during an ozone hole event, the production of HO
•
is enhanced by
at least 20 %. Such enhancement would mostly come from nitrate photolysis and to
a minor extent from DOM photoinduced reactions (see also chapter
“
Photoinduced
Generation of Hydroxyl Radical in Natural Waters
”) (Qian et al.
2001
).
Nutrients Effects on Higher Plants
Plant growth is enhanced at 200 mg l
−
1
N (as NH
4
NO
3
) in cvs. (cultivars)
'Licord' and 'Okapi', but it is reduced when the N concentration increases up to
300 mg l
−
1
(Bybordi
2010
). Nitrate reductase (NR), a substrate inducible enzyme,
is slightly inhibited by salinity in tomato roots, while leaf NR is decreased sharply
(Cramer and Lips
1995
). In the leaves of tomatoes and cucumbers, NR activity
can increase with exogenous nitrate concentration (Martinez and Cerda
1989
). NR
is decreased in leaves under salinization, which can subsequently decrease NO
3
−
uptake by plants under salt stress (Bybordi
2010
; Cramer and Lips
1995
; Lacuesta
et al.
1990
; Abd-El Baki et al.
2000
). The decreased of nitrate is accompanied by
a high Cl
−
uptake and low rate of xylem exudation in high osmotic conditions, by
either NaCl or other nutrients (Parida et al.
2004
; Tabatabaie et al.
2004
). Reduced
nitrate uptake or translocation can lead to lower NO
3
−
concentration in leaves,
which can consequently decrease the NR activity under saline conditions.
Several factors (e.g. salinity) can modify the uptake of some nutrients (e.g. Fe,
Mn, Cu, Zn, K, etc.), and either increase or decrease their contents in various parts
of most plants (Vıllora et al.
2000
; Turhan and Eris
2005
; Wang and Han
2007
;
Achakzai et al.
2010
; Tunçtürk et al.
2011
).
5.7 Effects of Trace Elements on Aquatic Microorganisms
Aquatic organisms that carry out photosynthesis are largely affected by trace ele-
ments, and PSII is thought to be the primary and most sensitive site of inhibition
