Task 2.4 Model development carbon isotope fractionation
Models of carbon isotope fractionation (ep)
can be classified as kinematic or dynamic models. In kinematic models all
fluxes are prescribed. They have no predictive power but allow exploration of
the possible variations in isotope fractionation depending on prescribed
fluxes. The prototype of this kind of model has been proposed by Sharkey and
Berry (1985). On the other extreme end are dynamic models where fluxes are
driven by physico-chemical mechanisms (e.g. diffusion, reactions within the
carbonate system) or physiological constraints (e.g. rate of carboxylation,
rate of cellular carbon transport). The model of Rau et al. (1996) falls into
this category. It makes predictions for isotope fractionation depending on
environmental conditions (e.g. CO2
concentration, temperature) as well as physiological parameters (cell membrane
permeability for CO2, growth rate) and can
explain ep dependence on the cellular surface to
volume ratio. Between these two model types there are hybrid models in which
some fluxes are constrained whereas others are prescribed.
The ultimate goal, of course, is to have a dynamic model without any free
parameters. At present our understanding of carbon acquisition by marine
phytoplankton is insufficient, however, to develop such models. It is therefore
appropriate to explore the complex mechanisms associated with carbon isotope
fractionation with hybrid or kinematic models. Based on the extensive
experimental data set obtained during the MERLIM project (see section 2.3) it
became evident that compartmentalisation of the algal cell needs to be invoked
to explain the observed ep responses. We have therefore developed
a kinematic fractionation model (Wolf-Gladrow et al. in prep.) with one or two
inner compartments (representing e.g. the chloroplasts, the pyrenoids etc.).
Briefly, the model results show that consideration of inner compartments can
greatly affect the interpretation of carbon isotope fractionation data. For
instance, for a given ratio of carbon flux into the cell to CO2 leakage out of the cell, predictions of
isotope fractionation differ greatly with or without the inclusion of inner
compartments. These findings emphasise the need to consider cell internal
processes for the interpretation of carbon isotope fractionation in microalgae.
References:
Rau, G.A., Riebesell, U., Wolf-Gladrow, D.A.
(1996). A model of photosynthtetic carbon isotope fractionation by marine
phytoplankton based on diffusive molecular CO2 uptake. Mar. Ecol.
Prog. Ser. 133: 275-285.
Sharkey, T.D., Berry, J.A. (1985). Carbon isotope
fractionation of algae as influenced by an inducible CO2
concentrating mechanism. In: Lucas
WJ, Berry JA (eds.) Inorganic carbon uptake by aquatic photosynthetic
organisms. The American Society of Plant Physiologists, Rockville, MD, p.
389-401.
Wolf-Gladrow, D.A., Burkhardt, S., Riebesell, U.
(in prep.) Carbon isotope fractionation in microalgae: the effect of inner
compartments.
Data
cobalt.xls
iron.xls
iron2.xls
zinc.xls
numerics.xls
