Environmental Engineering Reference
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(Hirose 2004 ). In addition, autochthonous DOM originating from phytoplankton
or algal biomass may contain amino and sulfidic functional groups in its molecular
structure, which may form complexes with trace metals in water (Xue and Sigg
1993 ; Xue et al. 1995 ).
Fe uptake by phytoplankton is significantly enhanced in the presence of humic
substances (Provasoli 1963 ; Prakash et al. 1973 ), which is presumably caused by
improved metal chelation in aqueous solution (Anderson and Morel 1982 ). Under
low-Fe conditions, Fe allocation in the diatoms Thalassiosira weissflogii and
Thalassiosira oceanica is localized in photosynthetic light-harvesting and elec-
tron-transport proteins (Strzepek and Harrison 2004 ). Increased iron quotas and
lowered iron-use efficiencies are often observed in phytoplankton, in response to
decreased light levels (Hopkinson and Barbeau 2008 ; Strzepek and Harrison 2004 ;
Sunda and Huntsman 1997 ). Iron requirements by phytoplankton increase as avail-
able light for photosynthesis decreases, which can lead to the hypothesis that phyto-
plankton may be colimited by iron and light in low-light environments (Sunda and
Huntsman 1997 ). In an iron-light colimited state growth and photosynthesis are
ultimately limited by light processing, whilst production of photosynthetic proteins
able to harvest and process light is constrained by iron availability (Hopkinson and
Barbeau 2008 ). Iron- light colimitation may occur in low-iron regions with deep
mixed layers, such as the Southern Ocean, or even in macronutrient-limited and
stratified waters, near the base of the euphotic zone (Sunda and Huntsman 1997 ).
An iron-light colimitation is observed during winter in the subarctic North Pacific.
Here a deep mixed layer (80 m), low incident irradiance, and lack of available iron
are all combined to limit photosynthesis, which maintains low phytoplankton bio-
mass (Maldonado et al. 1999 ). Iron can limit growth in an area with a relatively
shallow (40 m) mixed layer in the Subantarctic Front. However light, in conjunc-
tion with iron, can control growth in an area with deeper (90 m) mixed layers
(Boyd et al. 2001 ). Iron-light colimitation should also be a factor influencing phy-
toplankton growth during the North Atlantic spring bloom (Moore et al. 2006 ).
Availability of iron alone has also been implicated as an important factor in the
bloom of some harmful algal species (Bruland et al. 2001 ; Maldonado et al. 2002 ),
whilst an increase in the toxicity of Microcystis aeruginosa has been observed when
iron is limited (Luka ˇ and Aegerter 1993 ). Iron deficiency can affect the electron
transfer rate in Pisum sativum chloroplasts (Muthuchelian et al. 2001 ), and stable
organic Fe(III) complexes (FeL) photolytically produce dissolved inorganic iron at a
higher extent than thermal decomposition and cell-surface reduction of FeL. Such a
process can facilitate phytoplankton uptake of iron in the ocean (Fan 2008 ). On the
other hand, during nighttime the reactive oxygen species (H 2 O 2 and O 2
) produced
by reductases on cell surfaces react with FeL, producing Fe(II). Such a process
slows down the oxidation of Fe(II) and the subsequent formation of FeL, thereby
maintaining significant levels of bio-available Fe (Fan 2008 ).
A significant effect of toxic metals on photosynthesis is observed, and the rel-
evant photosynthetic efficiency can be either enhanced or suppressed in natural
waters (Zhang et al. 2010 ; Burda et al. 2003 ; Koukal et al. 2003 ; Sigfridsson et al.
2004 ; Berden-Zrimec et al. 2007 ; Pan et al. 2009 ; Mayer et al. 1997 ; Horton and
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