MAP Technical Reports Series No. 106 UNEP

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mineralization occurs essentially in two steps: first according to eq.(2b) in which
ammonium is the end product; ammonium is then oxidized to nitrate over two follow-up
reactions (cf. below).
If mineralization occurs mainly in the euphotic zone, one can speak of recycling, meaning
that the mineral components released are newly available for re-incorporation into new biomass.
b) Anaerobic mineralization, instead, occurs under anoxic conditions, or at very low oxygen
concentrations, mostly in deep waters. In this case, there exists no simple inversion
corresponding to eqs.(2). The end products of mineralization under severe anoxic
conditions are but only in part identical with the original mineral components, which may
result in vital nutrient losses to the system.
Among the various possibilities there are two variants of particular interest to the theme,
the first one when nitrates are still available, the second when nitrates are absent. Nitrates,
though being weaker oxidants than oxygen, are next in the redox-chain. One of the possible
reaction patterns is described with eq.(3a).
a) Nitrates available:
C 106N 16H 181O 44(PO 4)(SH) + 93 NO 3 - ==> Eq.(3a)
106 HCO 3 - + 16 NH4 + + HPO4 2- + SH - + 46.5 [N2] gas + 5H 20
b) Nitrates absent, sulphates available:
C 106N 16H 180O 44(PO 4)(SH) + 58 SO 4 2- ==> Eq.(3b)
106 HCO 3 - + 16 NH4 + + HPO4 2- + 59 [SH2] gas + 24 OH -
The meaning of eqs.(3a) and (3b) are that nitrogen is likely lost in the mineralization
process as nitrogen gas, and if nitrates are used up, sulphates are reduced to hydrogen
sulphide.
c) Under slightly less reducing conditions the following reaction may also take place
C 106N 16H 180O 44(PO 4)(SH) + 66 H 2O + 34 OH - ==> Eq.(3c)
16 NH 4 + + HPO4 2- + 48 HCO3 - + 58 [CH4] gas + [SH 2] gas
Accordingly, carbon would be lost in the form of methane, yet only as long as hydrogen
sulphide does not inactivate the microbiologically mediated process.
The processes described by eq.(3a) take place primarily in boundary layers between oxic
and anoxic waters, and those described by eq.(3b) and eq.(3c) near and in anaerobic
sediments. Depending on the particular circumstances under which they occur, these
processes may also take place in parallel. Also, other metabolic pathways may be followed,
such as processes of fermentation involving as intermediary products alcoholic compounds
and/or organic acids.