Task 2.3 Carbon isotope fractionation
Laboratory studies.
The objective of this task has been to develop, calibrate, and apply carbon
isotope fractionation (e p) as a diagnostic tool with which to
characterise the nutritional status of model phytoplankton species with respect
to CO2. To achieve this objective we
examined the influence of various environmental and cell physiological factors
on stable carbon isotope fractionation in marine phytoplankton. These include
effects of i) CO2 concentration, [CO2aq], ii) growth rate, iii) light/dark cycles,
iv) photon flux density, v) cell size, vi) nutrient availability, and vii)
differences in carbon acquisition mechanisms (see Burkhardt et al. 1999, in
press; Kukert and Riebesell 1998; Riebesell et al. subm. a; b). Summarising the
results of this work leads to the following conclusions:
1. Carbon isotope fractionation is species-specific. In particular, factors
such as cell size, cell geometry, the mechanism of inorganic carbon
acquisition, the type of carboxylating enzyme (RUBISCO type I or II,
beta-carboxylases) significantly affect the isotopic signal incorporated during
organic matter production (Burkhardt et al. in press). Based on these (and
other) findings it has become evident that any application of isotope
fractionation in the field has to account for possible signals in the carbon
isotope composition d13C which are due to species-specific effects.
In general, this makes the use of the isotopic signal in bulk organic matter
rather impractical and highlights the need to focus on species- or
group-specific biomarkers (e.g. alkenones). The relationship of the isotopic signal
in alkenones relative to that in total organic matter was investigated by
Riebesell et al. (subm. b). The study shows that alkenone d13C
closely follows isotope fractionation responses of the whole cell organic
matter. However, the offset between d13C in alkenones and in bulk organic matter can
differ depending on growth conditions.
2. Carbon isotope fractionation is dependent on the light climate
experienced by the algal cell. Photon flux density (Riebesell in prep.) and the
light/dark cycle (Burkhardt et al. in press) strongly influence active
transport of inorganic carbon into the cell and, in consequence, isotope
fractionation by the cell. Specifically, when considering the ranges typically
occurring in the natural environment, the effect of differences in light
conditions can be several-fold stronger than that of changes in [CO2aq] (Burkhardt et al. in press b, Riebesell in
prep.). Thus, interpretation of carbon isotope data requires knowledge of the
light climate prevailing during photosynthetic carbon fixation.
3. Carbon isotope fractionation is affected by the nutritional status of the
algal cells. For a given growth rate and [CO2aq],
ep differs depending
on the growth limiting factor (Kukert and Riebesell 1998; Riebesell et al. subm.
a). Specifically, strong differences in isotope fractionation were obtained in
light- versus nitrate-limited cells under identical growth rates and CO2 concentrations. Based on these findings a
conceptual model was developed (Riebesell et al. subm. a) in which isotope
fractionation is largely a function of energy (light) supply over nutrient
availability (via energy controlling transport processes and nutrients
controlling organic matter production). This hypothetical model needs rigorous
testing.
In summary, carbon isotope fractionation is a multi-factorial process. Based
on the findings made in the framework of the MERLIM project, the present
concept of isotope fractionation largely being a function of the ratio of
growth rate and CO2 concentration needs to
be modified. Although the potential influence of these two factors on isotope
fractionation is still supported by the results of this project, it has become
evident that other factors are equally important in determining the isotopic
signal. Their effects are quantitatively in the same range or even greater than
those of growth rate and [CO2aq]. Thus,
with respect to the objective of this project, i.e. to develop and calibrate
carbon isotope fractionation as a diagnostic tool with which to characterise
the CO2 nutritional status of model
phytoplankton species, the outcome of this work is negative. Under most
practical purposes, the complexity of the processes involved in carbon isotope
fractionation make it extremely difficult to use carbon isotope data as a
diagnostic tool to characterise the CO2
availability for marine phytoplankton.
Field studies.
The Centre des Faibles Radioactivités acted as a subcontractor to the AWI
(Bremerhaven). CFR was involved in all aspects relating to the development and
application of carbon isotopes as a diagnostic tool. In this context, CFR has
paid attention to
- the influence of
metabolic processes on carbon isotope fractionation in phytoplankton
during mineral carbon fixation and transport, and
- the relationship between
growth rate and carbon isotopic fractionation.
With the exception of one field study of phytoplankton isotopic
fractionation in a Norwegian fjord (Kukert and Riebesell 1998), research
activities of the AWI group have focused on the analysis of isotope
fractionation in monospecific laboratory cultures. In contrast, the approach
developed by the CFR group to investigate the correlation of ep with environmental
parameters is mainly based on field studies in the open ocean. If CO2 was the driving factor to determine the
isotopic signal in bulk organic matter, a close correlation between ep and [CO2aq] would be expected. Any heterogeneity of
factors such as species composition, light climate, or nutritional status (see
above), on the other hand, might cause considerable variation in stable carbon
isotope fractionation which cannot be accounted for by differences in [CO2aq].
During the last two years, we had the opportunity to participate in several
cruises in the Indian Ocean and notably in the Somalian upwelling area
(MOZAPHARE-PEGASOM cruise, July, 1996), in the south Atlantic Ocean ( NAUSICAA,
October, 1996), and in the Pacific Ocean (IPHIS, June 1997). Simultaneous
measurements of carbon dioxide partial pressure (pCO2), sea surface temperatures (SST) and salinities, carbon
isotopic composition of both phytoplankton and total dissolved mineral carbon,
chlorophyll and organic carbon concentration, allowed examination of the
relationship between [CO2aq], growth rate
(or biomass), and carbon isotope fractionation. The aim of these studies was to
provide a large data set that can be compared with results obtained in culture
experiments or derived from models giving a way to validate this estimates
and/or to extrapolate to the global ocean phytoplanton populations.
From this data set (>200) it appears that the measurements of [CO2aq] (calculated from pCO2) in the global ocean surface waters are close to the values
corresponding to equilibrium between surface sea water and and atmosphere
expected in the upwelling areas. The large data set covering wide oceanic
areas, different seasons and ecosystems can account for the global scale
equilibrium hypothesis excluding systematic bias by sampling only high pCO2 areas in the ocean. The good agreement between
theoretical [CO2aq] and polynomial fit
between measured [CO2aq] and SST confirms
this hypothesis (Bentaleb and Fontugne 1996, Bentaleb et al in prep.). By
comparing to the theoretical equilibrium curve calculated for preindustrial
period, these results show the anthropogenic CO2
invasion of surface ocean. The d13C values show that phytoplankton recorded the
anthropogenic CO2 and present also a good
agreement with the theoretical curve as shown by the polynomial fit agreement
between d13C
and SST (Bentaleb and Fontugne 1996, Bentaleb et al. in prep.). However, a wide
scattering of d13C values is observed indicating that factors other than CO2 had a significant effect on stable carbon
isotope fractionation in natural phytoplankton of the open ocean. Fractionation
lower than predicted from the empirical relationship with CO2 is clearly illustrated in the upwelling areas.
In this area, CO2aq concentrations are
high and d13C
values appear to be independent of this parameter but covary with chlorophyll
or particulate organic carbon concentrations in surface water.
References:
Bentaleb, I. and M. Fontugne e (1996) Anthropogenic
CO2 invasion of the surface Ocean: its
influence on the organic carbon isotope composition of phytoplankton. C.R. Acad
Sci Paris, Ser.II, 322, 743-748.
Bentaleb,I. M. R. Fontugne, G. Haddad,, C.
Descolas-Gros, C. Riaux-Gobin Carbon isotopic composition of phytoplankton, a
sensitive indicator of anthropogenic CO2
invasion of the surface Ocean ? (in
prep).
Burkhardt,
S., Riebesell, U., Zondervan, I. (in press). Effects of growth rate, CO2 concentration, and cell size on the stable
carbon isotope fractionation in marine phytoplankton. Geochim. Cosmochim. Acta
63 (19/20).
Burkhardt,
S., Riebesell, U., Zondervan, I. (1999). Stable carbon isotope
fractionation by the marine phytoplankton in response to day-length, carbon
assimilation rate, and CO2 availability.
Mar. Ecol. Prog. Ser. 184: 31-41.
Kukert, H. and Riebesell, U. (1998). Phytoplankton
carbon isotope fractionation during a diatom spring bloom. Mar. Ecol. Prog.
Ser. 173: 127-137.
Riebesell, U., Burkhardt, S., Kroon, B. (submitted,
a) Carbon isotope fractionation by a marine diatom: dependence on the growth
rate limiting resource. Mar. Ecol. Progr. Ser.
Riebesell, U., Revill, A.T., Holdsworth, D.G., and Volkman,
J.K. (submitted, b) The effect of varying CO2
concentration on lipid composition and carbon isotope fractionation in
Emiliania huxleyi. Geochem. Cosmochim. Acta.
Riebesell, U. (in prep.). Differences in carbon
acquisition and isotope fractionation among marine diatoms. Limnol. Oceanogr.
Data
phaeodactylum.xls
phaeodactylum2.xls
gephyrocapsa.xls
skeletonema.xls
skeletonema2.xls
thallissiosira.xls
emiliania.xls
emiliania2.xls
