The other is the direct production of ligands by living

o

The other is the direct production of ligands by living

organisms, probably mostly by prokaryotes. These sources of ligand are best described coupled to other processes that are present in the model (e.g. carbon remineralization and DOC production). The initial assumptions INCB018424 chemical structure made here are that the remineralization source of ligands is proportional to the remineralization of dead particulate organic carbon, with a constant ratio rL:C between the release of ligand and that of dissolved carbon, Srem = rL : CfT krem POC, where fT is the temperature dependence of detritus degradation, krem is the detritus degradation rate at reference temperature, and POC is the organic carbon in detritus. Ligand production by living organisms is described in the present model as proportional to the release of non-refractory dissolved organic carbon, again with a constant ligand:carbon ratio rL:DOC, i.e. SDOC = rL : DOCSDOC, where

SDOC is the source term for dissolved organic carbon from living organisms. Note that thus we do not make the production explicitly dependent on iron stress. In REcoM, however, DOC production is coupled to carbon overconsumption under nutrient stress ( Schartau et al., 2007), so one might argue that limitation is learn more taken into account indirectly. In PISCES this is not the case. Four loss processes for organic ligands are represented in the model. The first is bacterial degradation. While freshly produced siderophores Tangeritin are likely to be degraded quickly due to their small size and simple functional groups, the weaker ligands found in the deep ocean probably have a much longer degradation timescale as seen for DOC (Hansell et al., 2012). We attempt to take this continuum of ligands into account without explicitly resolving several distinct ligand pools by making the timescale of degradation τd a simple function of ligand concentration as equation(1) τd=max(τmin,τmaxexp(−aL))τd=maxτmin,τmaxexp−aLwhere L is the concentration of ligand and a is a scaling factor, that we set to 2 L nmol− 1. The total rate of degradation is then Rdeg = (fT/τ)L, where fT is the temperature

dependency of bacterial processes, which in our models is given by an Arrhenius function with a Q10 ≈ 2. The net result of Eq.  (1) is to make ligands at high concentrations degrade much faster than ligands at low concentration. The second loss process is photochemical degradation. Barbeau et al. (2003) have shown that some organic ligands are photoreactive, while others are not. In the model we parameterize the process simply as a degradation rate which is proportional to light, times the total ligand concentration Rphot = kphIL, where I is the downwelling irradiance. More complicated formulations are certainly conceivable, but are difficult to implement in a global model at this stage. The third process we include as a loss of ligands is uptake of organically complexed Fe by phytoplankton.

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