LEC.173 Biogeochemical cycles

Question 1

How is the nitrogen cycle coupled with the carbon cycle?

Answer 1

Through organic matter and biological processes

Question 2

Earth’s disequilibrium is biogenic, what is this mainly caused by?

Answer 2

The coexistence of N2 O2 and H2O(l) instead of more stable nitrate.

Question 3

Nitrogen species: name, and oxidation state for NO3- NO2 and NO2-

Answer 3

NO3- nitrate, +5
NO2 nitrogen dioxide, +4
NO2-, +3

Question 4

Nitrogen species: name, and oxidation state for NO N2O N2 and NH3/R-NH2

Answer 4

NO nitric oxide, +2
N2O nitrous oxide, +1
N2 nitrogen, 0
NH3/R-NH2 ammonia/amino, -3

Question 5

NO and NO2 are important air pollutants, which one is a free radical?

Answer 5

Nitric oxide

Question 6

Global nitrogen reservoirs?

Answer 6

Fixation, terrestrial biomass, atmosphere, river runoff, denitrification, soil, sediments, marine biomass, ocean, sedimentation

Question 7

Global nitrogen fluxes?

Answer 7

Natural terrestrial, natural oceanic, leguminous crops, chemical fertiliser, combustion, biomass burning, weathering

Question 8

4 nitrogen fixation reactions to convert N2 (inert) to reactive forms of N?

Answer 8

Anaerobic microbial reduction to NH3
Reduction to NH3 by Haber-Bosch process
Oxidation to NO by lightning
Oxidation to NO by anthropogenic, high T-P combustion processes

Question 9

Anaerobic microbial reduction to NH3?

Answer 9

2N2 + 6H2O -(nitrogenase)> 4NH3 + 3O2
- done by various bacteria
- dominant fixation process over geological time

Question 10

Reduction to NH3 by Haber-Bosch process?

Answer 10

N2 + 3H2 -(high T-P and metal catalyst)> 2NH3
- explosives and fertilisers
- helps to feed an increasing global population
- large amount of N lost to the environment via inefficient use

Question 11

Oxidation to NO by lightning?

Answer 11

N2 + O2 + energy -> 2NO (nitric oxide)
- <5% of fixation

Question 12

Oxidation to NO by anthropogenic, high T-P combustion processes?

Answer 12

N2 + O2 + fossil-fuel energy -> 2NO
- bi product of combustion processes

Question 13

Ammonification? what to what

Answer 13

Organic nitrogen -> NH4+

Question 14

Nitrification? what to what

Answer 14

NH3/NH4 -> NO2- -> NO3-

Question 15

Denitrification? what to what

Answer 15

NO3- -> N2

Question 16

What forms of N can be taken up by plant roots?

Answer 16

Nitrates and ammonium

Question 17

In aerobic soil is nitrate or ammonium the dominant form of N found?

Answer 17

Nitrate

Question 18

Assimilation by autotrophs, bacteria, archaea ?

Answer 18

• ammonia assimilation
• assimilatory nitrate reduction i.e. NO3- absorbed and convert to amino-N
• e.g. algae in oceans
• ammonia uptake is important in flood fields e.g. rice production

Question 19

Ammonification (mineralisation)?

Answer 19

• biological conversion of organic-N to ammonia (i.e. inorganic reactive N) under anaerobic conditions
• part of energy production system of these organisms
• e.g. heterotrophic bacteria

Question 20

Nitrification?

Answer 20

• microbial step-wise conversion of Nh3/NH4+ to NO3- under aerobic conditions
• part of the energy production system of some bacteria and archaea
• nitrosomonas NH3 to NO2- - very efficient process in soils

Question 21

Denitrification?

Answer 21

• microbial step-wise anaerobic respiration processes, i.e. energy yielding

Question 22

Why do mercury levels get much quicker change than e.g. nitrogen?

Answer 22

Big human health impact of mercury, so quicker reaction than to things that have environmental impacts.

Question 23

Photosynthesis eqn is actually very simplified, whats two steps that give a more detailed description of photosynthesis?

Answer 23

step 1 - a photolytic reaction involving capture of photons by chlorophyll and resulting oxidation of O in water

H2O(-II) + hn -> 2H+ + 2e- + ½O(0)2

step 2 – series of reactions, e.g. Calvin cycle - can be regarded as reduction of C by the electrons released

C(IV)O2 + 4H+ + 4e- -> C(O)H2O

Question 23

Photosynthesis eqn?

Answer 23

6CO2 + 6H2O + hv = C6H12O6 + 6O2

Question 24

From a chemical perspective whats a more meaningful representation of photosynthesis?

Answer 24

6C(IV)O2 + 12H2O(-II) -> C(0)6H12O6 + 6O(0)2 + 6H2O

Question 25

What are the main environmentally important biochemical reactions associated with creation and destruction of OM?

Answer 25

photosynthesis and respiration

Question 26

Autotroph?

Answer 26

Primary producer Utilises inorganic C sources such as CO2 and HCO3-

Question 27

Heterotroph?

Answer 27

Secondary producer Utilises complex C sources - C from pre-formed organic compounds - diverse range of organisms

Question 28

Oxygenic photosynthesis?

Answer 28

CO2 + H2O + hv → Ch2O + O2 - dominant source of biomass today and much of geological time - e.g. plants, algae, cyanobacteria - not first to evolve, end product - light limitation - generates O2

Question 29

Anoxygenic photosynthesis

Answer 29

6CO2 + 12H2S (source of electrons) + hv → C6H12O6 +12S +6H2O - globally very minor source of biomass today - e.g. purple sulphur bacteria - evolved before oxygenic photosysnethis

Question 30

Chemoautotroph?

Answer 30

- inorganic C source, independent of light - energy from oxidising reduced inorganic compounds - productivity independent of light - important in soils, sediments, hydrothermal vents - mainly bacteria and archaea (single cells, no nucleus - prokaryotes) e.g. nitrification

Question 31

Photoheterotrophs?

Answer 31

energy from sunlight, e.g. purple non-sulfur bacteria (cannot use CO2 as sole carbon source)

Question 32

Chemoheterotrophs

Answer 32

energy from oxidation of pre-existing reduced compounds – normally organic compounds (organotrophs). But can be inorganic compounds (lithotrophs) - many bacterial examples. (do not use CO2 e.g. cyanobacteria N2 fixing producing NH3)

Question 33

Aerobic respiration?

Answer 33

CH2O + O2 → CO2 + H2O - O2 is external or terminal electron acceptor (oxidising agent) - preferred energy yielding reaction, as maximum energy yield per mole of glucose - e.g. animals, plants, algae, bacteria, protozoa, fungi - Gibbs = -2880kJ per mole C6H12O6

Question 34

Anaerobic respiration?

Answer 34

- use various terminal electron acceptos, other than O2 - occupy hypoxic/suboxic (<~0.1 mg/L or 3 M O2) and anoxic environments - e.g. some bacteria, archaea, protozoa e.g. denitrification is a type of anaerobic respiration

Question 35

Fermentation?

Answer 35

- anaerobic, energy yielding rxns, involves redox changes to C in large organic molecules but with no external oxidising agent. - e.g. bacteria and fungi (yeast) - limited environmental importance

Question 36

Methanogenesis?

Answer 36

(previously regarded as a form of fermentation) e.g. acetic acid - small entitle molecules, methane produced (important GHG), archaea e.g. in digestive tract of cows

Question 37

Weathering: dissolution?

Answer 37

- water as a medium to break a mineral down into its constituent elements - water not active in the reaction

Question 38

Weathering: hydrolysis?

Answer 38

- Rxn with water, generally leading to an excess of H+ or OH-

Question 39

Weathering: acid-base rxns?

Answer 39

Faster than dissolution and hydrolysis, and generally more important. Weathering as a global acid-base rxn.

Question 40

Weathering: redox rxns?

Answer 40

Oxidation fog minerals by O2 OILRIG

Question 41

Weathering: example of dissolution?

Answer 41

CaCO3, one of the fastest weathering rxns CaCO3(s) -> Ca2+(aq) + CO32-(aq) Apatite dissolution (most common P mineral)

Question 42

Weathering: example of hydrolysis?

Answer 42

Alumino-silicate weathering

Question 43

Weathering: example of acid-base rxns?

Answer 43

CaCO3 + H2CO3 -> Ca2+ + 2HCO3-

Question 44

Globally significant effects of weathering?

Answer 44

- atmospheric CO2 concs - soil development - water chemistry - economic mineral deposits