Carbon in the form of CO2, HCO3

- and CO3-2, are

oxidized forms of C, and tend to be the only forms present where O2 is plentiful.

In anoxic environments methanogens (Archaea) convert organic C and CO2 into methane (CH4).

Methane is a gas and can bubble out of the water or it can be oxidized to CO2 by methylotrophic bacteria.

Carbon in anaerobic aquatic environments

http://faculty.plattsburgh.edu/jose.deondarza/images/Organisms/methanogen.jpg

Methanogens are not true bacteria, they belong to the Archaea

Most methanogens can grow on CO2 and H2 as their sole energy source: Chemoautotrophs —chemical bond energy is their energy source

they utilize CO2 as their C source

C-transformations in aerobic and anaerobic environments

•Under anaerobic conditions organic molecules break down to methane instead of CO2—This process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria.

•They utilize the energy released from 2H2 + Organic C (CH2O)→CH4 + H20 to build their biomass.

-4

+4

0

Oxidation state

Where do we find methanogens?

We keep track of the e- transfer using Oxidation numbers (Ox#)

For each e- transferred the Ox# changes by 1

2H2 + O2 2H2O0 0 +1 -2

Some rules for Oxidation numbers1. In free elements Ox# =02. For ions with one atom Ox# = charge. eg H+ Ox# of H+ = 13. Ox# of O in most compounds is -2, 4. Ox# of H in most compounds is +1,5. For a complex ion like SO4

-2 , the net Ox# = charge (Thus S=+6)

How to assign Oxidation numbers

The Cycling of Nitrogen

N is an important nutrient that frequently limits primary productivity in aquatic ecosystems

It is rare in the earth’s crust, but makes up 79% of the atmosphere (N2)(oxidation state =0)

Most algae and plants require NO3¯(+5) (NO2 ¯) (+3) or NH3 (NH4

+) (-3)to synthesize amino acids to make proteins

N-fixing microorganisms can take up N2 and convert it to NH3

N2 + 3H2 → 2NH3

Many plants have N-fixing mutualists (eg Azolla)

Denitrifying bacteria can convert NO3¯ back to N2

Azolla, an aquatic fern used in rice culture

•The leaves of this aquatic fern have cavities that harbour filamentous cyanobacteriaAnabaena azollae

•The large cells (heterocysts) are specialized for N-fixation

•Traditional rice farming in many countries involve planting Azolla to build up N concentrations in rice paddy.

The Nitrogen cycle involves many different oxidation states, and the redox processes are facilitated by plants and wide variety of bacteria

-3

+3

+5

0

+1

Nitrite

Photoautotrophs (PA) Chemoautotrophs(CA)

CH

Chemoheterotrophs (CH)

PA

CH

-2

+4

+6

0

Photoautotrophs (PA) Chemoautotrophs(CA)

CH

Chemoheterotrophs (CH)

PA

Streams draining mine tailings are extremely acidic—the effect of Thiobacillus oxidizing pyrites and iron.

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Thiobacillus ferrooxidans oxidizes both the iron, Fe(+2) to Fe(+3)and the sulphur in the pyrites, S(-1) to S(+6) using molecular oxygen.

This reaction splits water to produce a great deal of acid.

How do you suggest that mine tailings should be stored?

Desulfovibrio : Sulfate reducing bacteriacommonly found in anaerobic aquatic environments with high levels of organic material, such as mud in lakes and ponds.•have metal reductases which can precipitate metal sulfides from the water—•bioremediation potentials for toxic radionuclides such as uranium by a reductive bioaccumulation process.

Sulfate reduction can absorb H+ and counteract acid rainThey also contribute to methylation of Mercury

Ene

rgy

Sou

rce

Carbon Source

Light

Chemical

CO2 Organic

Photo-autotroph

Photo-heterotroph

Chemo-autotroph

Chemo-heterotroph

Four nutritional categoriesNutrition and Metabolic Diversity