aerobic bacteria

Lecture 27:MIC – Role of Aerobic and Anaerobic Microorganisms NPTEL Web Course

1 Course Title: Advances in Corrosion Engineering

Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore

Lecture 27

MIC – Role of Aerobic and Anaerobic Microorganisms

Keywords: Aerobic, Anaerobic, Sulfate-Reducing-Bacteria

Corrosion initiated and accelerated by microorganisms (metals and alloys)

MIC is also referred to as Biocorrosion

Microbial corrosion

Microbially-induced corrosion

Biofouling – all types of biological attachment and growth on metal and nonmetal

surfaces in contact with natural waters (fresh or sea water).

Micro and macro-fouling refer to deposits through the growth of microbes and other

higher organisms.

Biodeterioration generally refers to deterioration of nonmetallic materials or

degradation brought about by microbes.

Microorganisms associated with MIC are generally characterized by a number of

features such as:

Small size (few micrometers)

Ubiquitous and omnipotent

Sessile or motile (active or sedentary)

Ability to attach to substrates and grow colonies.

Extremophiles (tolerant to wide range of metal concentrations, acidity,

temperature, pressure, oxygen and lack of oxygen)

Existence of consortia and mutualism

Rapid reproduction.

Generate organic and inorganic acids, alkalis, chelating agents and

extracellular polymeric substances such as proteins and polysaccharides.

Can oxidize or reduce metals and ions.




Most of the microorganisms involved in MIC are chemolithotrophs and can be

aerobic – anaerobic, mesophilic-thermophilic, autotrophs- heterotrophs, acidophilic-

neutrophilic and many are slime formers.

Chemotrophs get energy from chemical sources unlike photosynthetic organisms.

Microorganisms involved in MIC can be generally classified as

a) Sulfur-sulfide oxidizing bacteria.

b) Manganese oxidizers / reducers

c) Iron-oxidizing /reducing bacteria.

d) Sulfate-reducing bacteria (SRB).

e) Bacteria secreting exopolymers / slime and organic acids.

Sulfate-reducing bacteria (SRB) are a group of diverse anaerobes which bring about

dissimilatory sulphate reduction to sulfides. Although they are considered as strict

anaerobes, some genera can tolerate oxygen, hydrogen serving as electron donor.

Oil, gas and shipping industries are seriously affected by SRB activities (soil and

water) due to H2S generation. Common SRB include Desulfovibrio, Desulfobacter

and Desulfotomaculum. SRB are capable of growing in soil, fresh water and sea-

water environments and also in stagnant areas.

They oxidize organic substances to organic acids or CO2, by reduction of sulfate to

sulfide through anaerobic respiration.

Tolerate pH ranges 5-9.5.

Black deposits of precipitated sulfides and odour of H2S emanation are characteristic

of SRB growth

Environmental growth conditions and metabolic features of some corrosion –

causing bacteria are illustrated in Table 27.1.

Growth conditions and corrosion aspects of some heterotrophs are illustrated in

Table 27.2.

Characteristics of some sulfate reducing bacteria relevant to MIC are given in Table

27.3.




Table 27.1 Environmental and metabolic aspects of MIC – causing bacteria

Bacteria

pH

Temp

(0C)

Oxygen

Corrosion

Characteristics

Desulfovibrio

D. desulfuricans

4-8

10-40

Anaerobic

Iron and

steel,

stainless

steels,

aluminum

zinc, copper

alloys

Utilize hydrogen in

reducing SO4-- to S

--

and H2S promote

formation of sulfide

films.

Desulfotomaculum

D .nignificans

6-8

10-40

Anaerobic

Iron and

steel

stainless

steels

Reduce SO4-- to S

--

and H2S (spore

formers).

Desulfomonas

…..

10-40

Anaerobic

Iron and

steel

Reduce SO4--

to S--

and H2S.

Acidithiobacillus

thiooxidans

0.5-8

25-40

Aerobic

Iron and

steel copper

alloys,

concrete

Oxidizes sulfur and

sulfides to form

H2SO4, damages

protective coatings.

Acidithiobacillus

ferrooxidans

1-7

25-40

Aerobic

Iron and

steel

Oxidizes ferrous to

ferric




Table 27.2 MIC – causing heterotrophs

Organism

pH

Temp 0C

Oxygen

Affected

metals

Action

Gallionella

7-10

20-40

Aerobic

Iron and

steel

Oxidizes ferrous

and manganous-

tubercle

formation

Sphaerotilus.

7-10

20-40

Aerobic

Iron and

steel

Oxidizes ferrous

and manganous -

tubercle

formation

S.natans

…..

…..

…..

Aluminium

alloys

Pseudomonas.

4-9

20-40

Aerobic

Iron and

steel

Some strains can

reduce Fe+++

to

Fe++

P.aeruginosa

4-3

20-40

Aerobic

Aluminium

alloys

Cladosparium resinae

(fungi)

3-7

10-45

Aerobic

Aluminium

alloys

Produces organic

acids.




Table27.3 Important characteristics of some sulfate – reducing bacteria

Desulfovibrio

Single flagellum. Do not form spores. Hydrogenase present.

Dv. desulfuricans

Dv. vulgaris

Dv. Salexigens

Curved rods (vibrios); sometimes spirilloid, occasionally straight.

Typical size 3-5 µm / 0.5-1 µm single flagellum.

Dv. Gigas Large curved rods or spirilla, 5-10 µm / 1.2 – 1.5 µm.

Desulfotomaculum Peritrichous flagella

Dt .nigrificans Hydrogenase activity variable. Growth on pyruvate in sulphate-

free media.

Dt. Orientis Fat curved rods, 5 µm x 1.5 µm. Hydrogenase absent.

aerobic bacteria

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