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1 Heavy Metals in the Marine Environment—An Overview
1.3.3 Iron (Fe)
Fe is the second most abundant metal and fourth most abundant element on the
earth’s crust (USEPA 1993; Jaishankar et al. 2014) and found in form of iron oxides
such as mineral hematite, magnetite, and taconite on the earth’s crust. Weathering
of rocks and soil, and atmospheric deposition contributes to Fe in the environment
naturally. Fe reaches the oceans mainly from rivers as suspended sediment, aeolian
dust transport, through hydrothermal activity and by recycling from shelf sediments
(Raiswell and Canfield 2012). The transport of aeolian dust over a timeframe of a
few days to weeks over large areas of the ocean surface become an efficient means of
distributing iron to iron-deficient regions (Emerson 2019). Anthropogenic emissions
of Fe include combustion of coal, petroleum, biofuel, fossil fuel and biomass (Guieu
et al. 2005; Takahashi et al. 2013; Wang et al. 2015).
In the photic zone, the sources of Fe mostly include wet and dry deposition
of atmospheric aerosols, vertical mixing and upwelling and biogenic recycling of
cellular iron in surface waters (Wells et al. 1995). Fe is an essential nutrient for the
growth of marine phytoplankton in surface waters (Cassar et al. 2007; Raiswell and
Canfield 2012; Mead et al. 2013) and thus contributes to ocean primary productivity. Fe modulates marine ecosystems, global carbon cycle and atmospheric carbon
dioxide uptake (Luo et al. 2008; Mahowald et al. 2009; Tagliabue et al. 2017). In
the seawater, Fe occurs in Fe
+3 and Fe
+2 oxidation states. Mahowald et al. (2010)
reported that changes in the soluble Fe input to the oceans could have an important
impact on oceanic carbon uptake and storage and hence indirectly affect climate.
Fe, along with P and other potential trace elements, is a (co) limiting nutrient of
nitrogen (N) fixation in the oligotrophic areas of the ocean (Falkowski 1997; Mills
et al. 2004) since nitrogen-fixing organisms (diazotrophs) tend to have higher Fe
requirements. The stimulation of these elements during phytoplankton growth and
primary production links to carbon sequestration and, consequently, the changing
climate throughout the geologic time (Martin 1990; Jickells et al. 2005). Moreover,
Bishop et al. (2002) and Boyd et al. (2007) reported that the amount of Fe in remote
oceans could increase the production of dimethyl sulfide (DMS) and/or organic
carbon from microorganisms in the ocean, which in turn affects the radiative forcing
in the atmosphere.
The mineral composition of dust is a key factor in the chemical forms of Fe,
and it determines the solubility and thus the bioavailability of Fe (Wang et al. 2015).
Cassar et al. (2007) summarized five sources of bioavailable Fe to the Southern Ocean
surface waters, i.e., melting of sea ice, the release of dissolved iron or resuspension
of sediments, upwelling supplies iron, vertical mixing supplies iron, and delivery of
soluble iron by aerosol deposition supplies.
1 Heavy Metals in the Marine Environment—An Overview
1.3.3 Iron (Fe)
Fe is the second most abundant metal and fourth most abundant element on the
earth’s crust (USEPA 1993; Jaishankar et al. 2014) and found in form of iron oxides
such as mineral hematite, magnetite, and taconite on the earth’s crust. Weathering
of rocks and soil, and atmospheric deposition contributes to Fe in the environment
naturally. Fe reaches the oceans mainly from rivers as suspended sediment, aeolian
dust transport, through hydrothermal activity and by recycling from shelf sediments
(Raiswell and Canfield 2012). The transport of aeolian dust over a timeframe of a
few days to weeks over large areas of the ocean surface become an efficient means of
distributing iron to iron-deficient regions (Emerson 2019). Anthropogenic emissions
of Fe include combustion of coal, petroleum, biofuel, fossil fuel and biomass (Guieu
et al. 2005; Takahashi et al. 2013; Wang et al. 2015).
In the photic zone, the sources of Fe mostly include wet and dry deposition
of atmospheric aerosols, vertical mixing and upwelling and biogenic recycling of
cellular iron in surface waters (Wells et al. 1995). Fe is an essential nutrient for the
growth of marine phytoplankton in surface waters (Cassar et al. 2007; Raiswell and
Canfield 2012; Mead et al. 2013) and thus contributes to ocean primary productivity. Fe modulates marine ecosystems, global carbon cycle and atmospheric carbon
dioxide uptake (Luo et al. 2008; Mahowald et al. 2009; Tagliabue et al. 2017). In
the seawater, Fe occurs in Fe
+3 and Fe
+2 oxidation states. Mahowald et al. (2010)
reported that changes in the soluble Fe input to the oceans could have an important
impact on oceanic carbon uptake and storage and hence indirectly affect climate.
Fe, along with P and other potential trace elements, is a (co) limiting nutrient of
nitrogen (N) fixation in the oligotrophic areas of the ocean (Falkowski 1997; Mills
et al. 2004) since nitrogen-fixing organisms (diazotrophs) tend to have higher Fe
requirements. The stimulation of these elements during phytoplankton growth and
primary production links to carbon sequestration and, consequently, the changing
climate throughout the geologic time (Martin 1990; Jickells et al. 2005). Moreover,
Bishop et al. (2002) and Boyd et al. (2007) reported that the amount of Fe in remote
oceans could increase the production of dimethyl sulfide (DMS) and/or organic
carbon from microorganisms in the ocean, which in turn affects the radiative forcing
in the atmosphere.
The mineral composition of dust is a key factor in the chemical forms of Fe,
and it determines the solubility and thus the bioavailability of Fe (Wang et al. 2015).
Cassar et al. (2007) summarized five sources of bioavailable Fe to the Southern Ocean
surface waters, i.e., melting of sea ice, the release of dissolved iron or resuspension
of sediments, upwelling supplies iron, vertical mixing supplies iron, and delivery of
soluble iron by aerosol deposition supplies.