Acknowledgments: Members of the Green Ocean Project http:lgmacweb'env'uea'ac'ukgreen_oceanindex'shtm

1
Functional representation of N2 fixation in a
Dynamic Green Ocean Model Nicholas Stephens14,
Corinne Le Quéré23, Erik T. Buitenhuis2 1Max-Planc
k-Institut für Biogeochemie, Hans-Knöll-Str. 10,
07745 Jena, Germany 2School of Environmental
Sciences, University of East Anglia, Norwich, NR4
7TJ, UK 3British Antarctic Survey, High Cross,
Madingley Road, Cambridge, CB3 OET, UK 4email for
correspondence, stephens_at_bgc.mpg.de
4. Physiological models to describe Trichodesmium
spp.
A number of physiology based mathematical models
describing the inter-relationships of the
inorganic N assimilatory pathways in N2-fixing
cyanobacteria have been developed (Fig. 4). These
models, based on biochemical processes, describe
the N assimilatory processes in terms of N and C,
providing the functional description (Fig. 5)
from which the Plankton Functional Type has been
developed in the Ocean model described in the
panel below.

• 1. Introduction
• Generally N2 fixation is poorly represented in
  ocean circulation models, being described in
  terms of P rather than N. From a physiological
  perspective this approach is limited
• In addition to simulating the biogeochemical
  interactions of N2-fixing cyanobacteria and the
  other plankton represented in the model, the
  ability to describe associated fluxes of N and C
  is possible

2. The problem When included in ocean
biogeochemical models, N2 fixation is represented
in terms of phosphorus (P). Primarily a result of
the Redfield ratio observed (NO3PO416), it is
often assumed that this ratio extends to the
biology. The ability of certain cyanobacteria to
fix N2 has led to the additional assumption that
nitrogen (N) does not limit growth.
From a physiological perspective,
describing N2 fixation in terms of P appears
limited. In recent studies, increased P
concentrations led to an increased availability
of organic matter for degradative processes
(Thingstad et al., 2005). Describing increased N2
fixation and denitrification, gains and losses of
DIN respectively, by the same variable (P)
question such assumptions (Fig. 1). Experimental
results from cultures supported by different N
sources (NH4, NO3-, N2) show that regulation
between the N assimilatory pathways can be
achieved by consideration of the cellular N
status (Fig. 2). This is not the case in absence
of such information. The suggestion that
N2-fixing cyanobacteria such as Trichodesmium
can vertically migrate to overcome P limitation,
the lack in ability to represent culture based P
results and the potential ability to accumulate
polyphosphate storage granules present a case for
representation and investigation in an ocean
circulation model.
Figure 4 Key aspects from the model describing
Trichodesmium physiology to be included in the
Ocean Model. These include a) preference for N
source, b) rates of N2 fixation activity, c)
steady state transitions between N2 fixation and
substrate concentrations of NH4 and NO3- and d)
change in rates of growth associated with the
transition between N2 fixation and DIN source
5. PlankTOM5.0 Model description
Figure 1 NP has been suggested to be able to
distinguish between N2-fixing oligotrophic
conditions (NPgt16) and eutrophic conditions
(NPlt16) with high primary productivity (Arrigo,
2005). Physiologically these conditions are
described in terms of cellular NC
Figure 2 NC for Gloeothece cultures grown on
different N sources (Stephens, unpublished)
3. N2 fixation in the marine environment The N
cycle is primarily mediated by biological
reductive and oxidative processes (Fig. 3). In
particular the reduction of N2 to NH4, although
confined to a number of prokaryotic species, can
result in significant fluxes of N and C into the
marine environment. The inclusion of components
to describe growth supported by N2-fixation leads
to better representation in regulating the total
amount of N in the ocean and the relationship to
potential losses of N through denitrification
type process (reducing fixed N into N2). In
particular N2-fixers contribute to open ocean
primary productivity, although questions exist
concerning the magnitude of C export.
Figure 5 Relationship between the original 5
Plankton Functional Types (PFT) of the
PlankTOM5.0 model and the additional tracer
included (highlighted in yellow)
Recent advances in C based biogeochemical
modelling have led to the development of the
PlankTOM5.0 model as part of the Dynamic Green
Ocean Project (see the website referenced below
for more information). The PlankTOM5.0 model
includes five different PFTs (Fig. 5) with
additional components describing the actions of
N2-fixing phototrophs and bacterial processes
representing denitrification. The PFTs and those
nutrients deemed necessary to describe the N
cycle are represented as tracers contained in a
physical OPA/ORCA model. At present, this model
has the ability to simulate a number of global
marine processes with a significant, although not
conclusive degree of success. Until recently,
components describing so-called N2 fixation have
been included as an offset to denitrification and
sedimentation to conserve mass. Presently, these
are the primary losses of nitrogen with N being
described in terms of, and hence coupled to, P
within the model. Present activities include the
addition of a tracer (DIN) uncoupling N and P
(Fig. 6), and an additional PFT to facilitate the
representation of N2 fixation, and the
development of denitrification describing
processes.
Figure 3 N based reactions associated with the
marine environment
Acknowledgments Members of the Green Ocean
Project (http//lgmacweb.env.uea.ac.uk/green_ocean
/index.shtml), all fundamental to the development
of PlankTOM5.0 and subsequent versions of the
model. This research is funded through a Marie
Curie Fellowship through the Greencycles project,
based at the Max Plank Institute for
Biogeochemistry, Jena.

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