CLASS NOTES FOR SEMESTER –IV STUDENTS, Date- 6.5.2020
Dept. Of Microbiology
Raja N.L.Khan Women’sCollege
B.Sc (HONOURS) MICROBIOLOGY (CBCS STRUCTURE)
C-9: ENVIRONMENTAL MICROBIOLOGY (THEORY)
Unit 3 Biogeochemical Cycling No. of Hours: 12
Elemental cycles: Iron and manganese
Q. What is Iron cycle? Write the importance of Iron Cycle.
The iron cycle (Fe), also known as the Ferrous Wheel, is the biogeochemical cycle
of iron through the atmosphere, hydrosphere, biosphere and lithosphere. While Fe
is highly abundant in the Earth’s crust, it is less common in oxygenated surface
waters. Iron is a key micronutrient in primary productivity, and a limiting nutrient
for the growth of plants and animal.
1. Iron (Fe) follows a geochemical cycle like many other nutrients. Iron is
typically released into the soil or into the ocean through the weathering of rocks or
through volcanic eruptions. Iron is required to produce chlorophyl, and plants
require sufficient iron to perform photosynthesis.
2. Iron is the most abundant element on Eaith and the most frequently utilized
transition metal in the biosphere. It is a component of many ceIlular compounds
and is involved in numerous physiological functions. Hence, iron is an essential
micronutrient for all eukaryotes and the majority of prokaryotes. Prokaryotes that
need iron for biosynthesis require micromolar concentrations, levels that are often
not available in neutral pH oxic environments. Therefore, prokaryotes have evolved
specific acquisition molecules, called siderophores, to increase iron bioavailability.
Q. Draw the Iron Cycle and briefly describe the Iron Cycle.
Description of Iron Cycle:
On our planet, iron Is ubiquitousi n the hydrosphere,li thosphere,b iospherea nd
atmosphere, either as particulate ferric [Fe(ill)] or ferrous [Fe(ll)] iron-bearing
minerals or as dissolved ions. Redox transformations of iron, as weIl as dissolution
and precipitation and thus mobilization and redistribution, are caused by chemical
and to–a significant extent by microbial processes (Fig. 1). Microorganisms
catalyze the oxidation of Fe(ll) under oxic or anoxic conditions as weIl as the
reduction of Fe(ill) in anoxic habitats. Microbially infiuenced transformations of
iron are orten much raster than the respective chemical reactions. They take place in
most soils and sediments, both in freshwater and marine environments,-and play an
important role in other (bio ) geochemical cycles, in particular in the carbon cycie.
Iron exists in a range of oxidation states from -2 to +7; however, on Earth it is
predominantly in its +2 or +3 redox state and is a primary redox-active metal on
Earth. The cycling of iron between its +2 and +3 oxidation states is referred to as
the iron cycle. This process can be entirely abiotic or facilitated by microorganisms,
especially iron-oxidizing bacteria. The abiotic processes include the rusting of
iron-bearing metals, where Fe2+ is abiotically oxidized to Fe3+ in the presence of
oxygen, and the reduction of Fe3+ to Fe2+ by iron-sulfide minerals. The biological
cycling of Fe2+ is done by iron oxidizing and reducing microbes.
Iron is an essential micronutrient for almost every life form. It is a key component
of hemoglobin, important to nitrogen fixation as part of the Nitrogenase enzyme
family, and as part of the iron-sulfur core of ferredoxin it facilitates electron
transport in chloroplasts, eukaryotic mitochondria, and bacteria. Due to the high
reactivity of Fe2+ with oxygen and low solubility of Fe3+, iron is a limiting nutrient
in most regions of the world.
The ferrous form of iron, Fe2+, is dominant in the Earth’s mantle, core, or deep
crust. The ferric form, Fe3+, is more stable in the presence of oxygen gas. Dust is a
key component in the Earth’s iron cycle. Chemical and biological weathering break
down iron-bearing minerals, releasing the nutrient into the atmosphere. Changes in
hydrological cycle and vegetative cover impact these patterns and have a large
impact on global dust production, with dust deposition estimates ranging between
1000 and 2000 Tg/year. Aeolian dust is a critical part of the iron cycle by
transporting iron particulates from the Earth’s land via the atmosphere to the ocean.
Q. Draw and describe the Manganese cycle.
Microorganisms have long been known to mediate manganese (Mn) oxidation in a
variety of environments, including caves (fig-1). Microbial Mn oxide minerals are
typically dark brown to black in color, nm-scale, and poorly crystalline, with
birnessite (layer) or todorokite (tunnel) crystal structures. Both bacteria and fungi
produce Mn oxide minerals, although the exact mechanism for Mn oxidation
remains elusive. Chemolithoautotrophic Mn oxidation is highly unlikely to be
carried out with the enzymes currently known, although indirect oxidation of Mn
during heterotrophic growth or reproduction has been observed in both bacteria and
fungi. Differences in nutrient availability in caves influence not only the microbial
community structure associated with ferromanganese deposits, but also Mn cycling
and microbial functions.
Q.3. Draw and describe the couple iron-Manganese cycle.
Mn can precipitate at high pH, lowering Mn availability so deficiencies are most
likely to occur in high pH soils (calcareous soils or over-limed soils).Manganese is
most available at soil pH levels of 5 to 6.5. The transformations of iron and
manganese in nature and the relationship between the cycling of these and other
biologically active elements occurs. Both the oxidation and the reduction of iron
and manganese in natural environments is, to a large extent, promoted by microbial
catalysis, but abiotic transformations are also important and may compete with the
biological processes. In the Earth’s crust, iron and manganese are mainly found as
minor components of rock-forming silicate minerals such as olivine, pyroxenes, and
amphiboles. Iron has a high abundance of 4.3% by mass in the continental crust. At
a 50-fold lower crustal abundance than iron, manganese is the second most
abundant redox-active metal. There are many similarities between iron and
manganese in terms of both geochemistry and microbiology. Microbes play an
important role in the oxidation of reduced iron and manganese. Dissimilatory iron-
and manganese-reducing microorganisms catalyze the reduction of Fe (III) to Fe
(II), and of Mn (III) or Mn (IV) to Mn(II). The microbial manganese and iron
reduction also occur in aquatic environments. Three basic conditions—absence of
oxygen and presence of electron donors and oxidized manganese or iron in an
appropriate form—are required to fulfill for microbial iron or manganese reduction
to thrive in normal aquatic environments of near neutral pH. The first is a direct
quantification of changes in Fe(III) or Fe(II) pools during sediment incubations.
The second approach determines rates of dissimilatory iron and manganese
reduction by comparing the depth distribution of total carbon oxidation, based on
production of dissolved inorganic carbon, to measured rates of sulfate reduction.
1. Thamdrup 2000; Straub et al. 2001; Comell and Schwertmann 2003.
2. Krishnamurthy, Aparna; Moore, J. Keith; Mahowald, Natalie; Luo, Chao; Doney, Scott
C.; Lindsay, Keith; Zender, Charles S. (2009). “Impactsof increasing anthropogenic
soluble iron and nitrogen deposition on ocean biogeochemistry.”Global Biogeochemical
Cycles. 23 (3). doi:10.1029/2008GB003440. ISSN 1944-9224.
3. Fortin D, Langley S. Formation and occurrence of biogenic iron-rich minerals. Earth Sci
Rev,2005, 72, 1–19.