CLASS NOTES FOR SEMESTER –IV STUDENTS, Date- 6.5.2020

Dr.Harekrishna Jana

Assistant Professor

Dept. Of Microbiology

Raja N.L.Khan Women’sCollege

B.Sc (HONOURS) MICROBIOLOGY (CBCS STRUCTURE)
C-9: ENVIRONMENTAL MICROBIOLOGY (THEORY)

SEMESTER –IV

Unit 3 Biogeochemical Cycling No. of Hours: 12
Elemental cycles: Iron and manganese

Q. What is Iron cycle? Write the importance of Iron Cycle.

Ans.

Defination:

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.

Importance:

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.

Fig-1

Fig-2

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.

Fig-1

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.

Fig-1

Fig.2

Description:

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.

Reference

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.