The Marine Nitrogen Cycle

REDOX reactions
1. Loss of electrons is oxidation
2. Gain of Electrons is reduction
3. Ar ed = A ox + e-
  1. B ox + e- = B red
4. Ecell = Eright - E left
  1. More negative on top going backwards to write equation for electrode potentials
    1. Determines feasibility
5. Electron Donor: H2
  1. Electron Acceptor: O2
Marine Nitrogen Cycle and Oxidation States
1. NH4+ = charge of 1+. Hydrogen is 1+, N will be -3 charge to match 1+ charge.
2. The Nitrogen Cycle: the series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms, including nitrogen fixation (The process of converting N2 into biologically available nitrogen) and decomposition.
  1. Nitrogen: Nitrogen is one of the primary nutrients critical for the survival of all living organisms. Although nitrogen is very abundant in the atmosphere, it is largely inaccessible in this form to most organisms.
Why the Nitrogen Cycle
1. Efficiency of biological pump in C-export to deep ocean depends on primary production which in turn is often regulated by th availiablity of nitrate
  1. Most fixed N occurs as nitrate (NO03-) in the oceans, but mostly locked in deep ocean. Fixed Nitrogen = Reactive N = Bioavailable N)
Redfield Ratio (C:N:P = 106:15:1)
1. N*=(NO3-)-16(PO4 3-) + 2.9 umol kg-1 = overall budget ratio
  1. Relationship used to find deviation type and identify process of N.
    1. Positive Deviation (N*>0) = N is in excess, N gain process e.g. N2 fixation
      1. N2 Fixation
        N2+8H+ + 8e- + 16ATP = 2NH3 + H2 + 16ADP + 16Pi
        N2 unusable for most organisms due to high activation energy to break its triple bond.
        Mediated by enzyme nitrogenase, which needs Fe and often Mo (Molybdenum)
        Occurs in oligotrophic regions e.g. subtroical gyres (whirl), and upwelling regions
        Maybe limited by Fe or P or both (nutrient competition)
        Bloom forming cyanobacteria Trichodesmium and unicelluar cyanobacteria being discovered. Including Heterotrophic and symbiotic bacteria
    2. Negative Deviation (N*<0) = N deficit, N-loss process e.g. denitrification, anammox, N2O production
      1. Denitrification
        A step wise process: NO3- = NO2- = NO = N2O = N2
        Major remineralisation process in Oceans as NO3- is thermodynamically the most favourable electron acceptor after O2
        Denitrifying organisms are diverse, usually facultative (capable of but not restricted to a particular function or mode of life.) anaerobic microogranisms (bacteria, archaea) but also foraminifera.
        Only known since 1995
        Autotrophic (Plant) denitrification (e.g. coupled with sulphide oxidation) found to be important in sulphidic water coloumn.
      2. Anammox
        NH4+ + NO2- = N2 + 2H2O
        Performed by a group of chemolithoautotrophic bacteria that belong to planctomycetales
        Marine sediment evidence 2002, now global in suboxic water and sediments with distribution of marine anammox bacteria
      3. Nitrification
        Nitrification is the process that converts ammonia to nitrite and then to nitrate. Mostly carried out by yprokaryotes. Two steps of nitrification that are carried out by distinct types of microorganisms.
        The second step in nitrification is the oxidation of nitrite (NO2-) to nitrate (NO3-). This step is carried out by a completely separate group of prokaryotes, known as nitrite-oxidizing Bacteria.
        First step is the oxidation of ammonia to nitrite, carried out by microbes known as ammonia-oxidizers. Aerobic ammonia oxidizers convert ammonia to nitrite via the intermediate hydroxylamine, a process that requires two different enzymes, ammonia monooxygenase and hydroxylamine oxidoreductase.
    3. N loss in Indian and Pacific Ocean, Gain in Atlantic Ocean
Anthropogenic (human activity) impact
1. Haber Bosch, fossil fuel consumption, nitrogen-based fertilizers, N fixation cultivation ,etc
  1. nearly 80% of the nitrogen found in human tissues originated from the Haber-Bosch process
2. Human activity with N is increasing with 4x increases added to atmosphere depostion.
  1. Terrestrial ecosystems, the addition of nitrogen can lead to nutrient imbalance in trees, changes in forest health, and declines in biodiversity.
  2. Nearshore marine systems, increases in nitrogen can often lead to anoxia (no oxygen) or hypoxia (low oxygen), altered biodiversity, changes in food-web structure, and general habitat degradation. One common consequence of increased nitrogen is an increase in harmful algal blooms . Toxic blooms of certain types of dinoflagellates have been associated with high fish and shellfish mortality in some areas. Also increased acidity in freshwater systems.
    1. Eutrophication of water - leads to blue baby syndrome East Anglia, and links to cancer
see http://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632 for more detail
Summary: REDOX. Nitrogen has numerous oxidation states and gaseous forms lead to Nitrogen cycle. Nitrogen enters ocean via atmosphere, rivers and nitrogen fixation. Nitrogen lost by denitrification and anammox, and some as N2O via denitrification and nitrification. Anthropogenic impact has significantly perturbed the marine N-cycle

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The Marine Nitrogen Cycle

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