Reassessing Foraminiferal Stable Isotope Geochemistry
507
scavenging of light respired CO 2 can explain the
observed difference between the HL and dark results and the trend noticed in the constant pH experiments. In addition, effective scavenging oflight
respired CO 2 could also explain the very negative
value of the symbiont organic matter (-24 %0) even
though HCO;is the main carbon source.
The proposed mechanism of CO 2 scavenging
can operate only if metabolic CO 2 diffuses freely
through the membrane of the host and is not stored
in an internal inorganic carbon pool. The pulsechase experiments demonstrate that such a pool is
not present in 0. universa (Fig. 6).
Proposed Fractionation Mechanisms
Based on our experiments we propose that 3
fractionation mechanisms can be recognized:
I) pH or [COn dependent kinetic fractionation
of both carbon and oxygen stable isotopes.
2) Incorporation of respired CO 2 depleted in l3C.
In symbiont bearing species in the light.
3) Enrichment of the calcifying environment with
l3C (compared to the dark) due to scavenging of
light respired CO 2 (RUBISCO related effects are
probably of minor importance).
Light/dark Shift of Shell Stable Isotopes
In all experiments (Calk" C~C02 and CpH),
the 0 18 0 value of shells grown in the light is
depleted compared to shells kept in the dark. Such
a decrease in shell 0 18 0 with increasing irradiance
has been documented before for G. sacculifer
and 0. universa (Spero and Lea 1993). Because
metabolic processes do not directly affect the
0 18 0 signal of the shell, we argue that an increase
of pH or [COn in the calcifying environment, either due to bulk carbonate chemistry changes or
induced by photosynthesis, depletes shells in 180
via mechanism 1.
The enrichment of Ol3C in the light observed in
all experiments (Calk" C~C02 and CpH) must be
explained in terms of competing processes. First
is the depletion of shell Ol3C kinetically driven by
the photosynthetically raised pH (mechanism 1).
Secondly, Zeebe et al. (1999) have demonstrated
that more respired CO 2 gets incorporated into the
shell at higher pH. Hence, because the Ol3C of respired CO 2 is significantly depleted compared to
bulk CO 2 , the fractional change of metabolic versus bulk CO 2 incorporation into the shell will reinforce the depletion. In addition, a higher light- compared to dark-respiration rate of the symbionts
(Weger et al. 1989) may deplete the calcifying
environment even more under HL. The enrichment
of shell Ol3C is dictated by very effective scavenging oflight respired CO 2 (mechanism 3). A decline
in the scavenging efficiency with increasing
[C0 2 aq] could explain the trend in the constant pH
experiments (more respired CO 2 escapes photosynthetic fixation and gets incorporated into the
shell).
The question may be raised why scavenging
of respired CO 2 dominates the impact of kinetic
fractionation (mechanism 1) on Ol3C signal but not
on the 0 18 0 signal? The reason is twofold. First,
theoretical considerations require that the impact
of kinetic fractionation on l3C/12C is half that of
18 0/ 16 0 (McCrea 1950). Second, the relative depletion of respired CO 2 with respect to l3C is large
whereas the depletion in 180 may be lost quickly
due to the large water reservoir compared to ~C02
and the fast exchange reactions.
Decoupling of the 813C/[COJ} Relationship
at High [COpq} in the Light
A change in the scavenging efficiency may also
explain the "plateau" or the decrease in Ol3C towards lower [COn in the constant alkalinity experiments (Fig. 3a). At higher [C0 2 aq] (i.e. lower
[Cot]), scavenging of metabolic CO 2 may become less effective and consequently an increasing portion oflight respired CO 2 is incorporated into
the shell. Again, the opposing affect of RUBIS CO
at high [C0 2 aq], i.e. the enrichment of the ambient ~C02 with l3C apparently plays a subordinate
role. As noted before is the photosynthetic
assimilation of carbon strongly CO 2 limited (W olfGladrow et al. 1999). Apparently, even at the maximum PC0 2 reached in the constant alkalinity experiments (ca. 1900 Itatm.) the symbionts are still
CO 2 limited. In seperate experiments (Spero, Bijma
and Lea unpubl results), PC0 2 was varied between
1400 Itatm and more than 8000 Itatm. The results
507
scavenging of light respired CO 2 can explain the
observed difference between the HL and dark results and the trend noticed in the constant pH experiments. In addition, effective scavenging oflight
respired CO 2 could also explain the very negative
value of the symbiont organic matter (-24 %0) even
though HCO;is the main carbon source.
The proposed mechanism of CO 2 scavenging
can operate only if metabolic CO 2 diffuses freely
through the membrane of the host and is not stored
in an internal inorganic carbon pool. The pulsechase experiments demonstrate that such a pool is
not present in 0. universa (Fig. 6).
Proposed Fractionation Mechanisms
Based on our experiments we propose that 3
fractionation mechanisms can be recognized:
I) pH or [COn dependent kinetic fractionation
of both carbon and oxygen stable isotopes.
2) Incorporation of respired CO 2 depleted in l3C.
In symbiont bearing species in the light.
3) Enrichment of the calcifying environment with
l3C (compared to the dark) due to scavenging of
light respired CO 2 (RUBISCO related effects are
probably of minor importance).
Light/dark Shift of Shell Stable Isotopes
In all experiments (Calk" C~C02 and CpH),
the 0 18 0 value of shells grown in the light is
depleted compared to shells kept in the dark. Such
a decrease in shell 0 18 0 with increasing irradiance
has been documented before for G. sacculifer
and 0. universa (Spero and Lea 1993). Because
metabolic processes do not directly affect the
0 18 0 signal of the shell, we argue that an increase
of pH or [COn in the calcifying environment, either due to bulk carbonate chemistry changes or
induced by photosynthesis, depletes shells in 180
via mechanism 1.
The enrichment of Ol3C in the light observed in
all experiments (Calk" C~C02 and CpH) must be
explained in terms of competing processes. First
is the depletion of shell Ol3C kinetically driven by
the photosynthetically raised pH (mechanism 1).
Secondly, Zeebe et al. (1999) have demonstrated
that more respired CO 2 gets incorporated into the
shell at higher pH. Hence, because the Ol3C of respired CO 2 is significantly depleted compared to
bulk CO 2 , the fractional change of metabolic versus bulk CO 2 incorporation into the shell will reinforce the depletion. In addition, a higher light- compared to dark-respiration rate of the symbionts
(Weger et al. 1989) may deplete the calcifying
environment even more under HL. The enrichment
of shell Ol3C is dictated by very effective scavenging oflight respired CO 2 (mechanism 3). A decline
in the scavenging efficiency with increasing
[C0 2 aq] could explain the trend in the constant pH
experiments (more respired CO 2 escapes photosynthetic fixation and gets incorporated into the
shell).
The question may be raised why scavenging
of respired CO 2 dominates the impact of kinetic
fractionation (mechanism 1) on Ol3C signal but not
on the 0 18 0 signal? The reason is twofold. First,
theoretical considerations require that the impact
of kinetic fractionation on l3C/12C is half that of
18 0/ 16 0 (McCrea 1950). Second, the relative depletion of respired CO 2 with respect to l3C is large
whereas the depletion in 180 may be lost quickly
due to the large water reservoir compared to ~C02
and the fast exchange reactions.
Decoupling of the 813C/[COJ} Relationship
at High [COpq} in the Light
A change in the scavenging efficiency may also
explain the "plateau" or the decrease in Ol3C towards lower [COn in the constant alkalinity experiments (Fig. 3a). At higher [C0 2 aq] (i.e. lower
[Cot]), scavenging of metabolic CO 2 may become less effective and consequently an increasing portion oflight respired CO 2 is incorporated into
the shell. Again, the opposing affect of RUBIS CO
at high [C0 2 aq], i.e. the enrichment of the ambient ~C02 with l3C apparently plays a subordinate
role. As noted before is the photosynthetic
assimilation of carbon strongly CO 2 limited (W olfGladrow et al. 1999). Apparently, even at the maximum PC0 2 reached in the constant alkalinity experiments (ca. 1900 Itatm.) the symbionts are still
CO 2 limited. In seperate experiments (Spero, Bijma
and Lea unpubl results), PC0 2 was varied between
1400 Itatm and more than 8000 Itatm. The results