http://mmcconeghy.com/arizona/images/mistletoe.jpg
(Spero, 1998)
Parasitism and Symbiosis:
isotope effects in mistletoe
and foraminifera
Carbon isotope composition, gas exchange and
heterotrophy in Australian mistletoes
Marshall, Ehleringer, Schulze, and Farquhar, 1994
• δ13C of plant tissue related to
photosynthetic gas exchange by this
equation:
• δ13Cplant=δ13Cair-a-(b-a) (ci/ca)
•
•
•
•
δ13Cplant= carbon isotope of tissue
δ13Cair= atmospheric CO2 (-8‰)
Ci=intercellular CO2 concentration
Ca=atmospheric CO2 concentration
(look familiar?)
It would make sense for parasite 13C to be
intermediate between host plant carbon (lower
ci) and mistletoe photosynthate.
• δ13Cparasite=
(A[δ13Cair-a-(b-a)(ci/ca)]+Ecx δ13C)/A+Ecx
•
•
•
•
A= rate of photosynthesis
E= rate of transpiration
Cx= [carbon] in xylem sap
δ13C= isotopic ratio of xylem carbon (=host tissue)
Due to leakiness, mistletoes expected to have
lighter δ13C, but remain heavy if taking carbon
from host.
• Xylem carbon concentration was
estimated from measured photosynthetic
rate (A), and transpiration rate (E)
• Cx= (AH)/E(1-H)
• H is a measure of proportional
heterotrophy
• Large differences in the A/E ratio between host and
parasite should be associated with similar differences
in ci/ca; causing isotopic difference
~2X
difference
in A/E
Host Average
δ13C=-26.81+0.25‰
Parasite
Average
δ13C=-28.67+0.23‰
(Marshall et al., 1994)
Measured δ13C plotted against δ13C predicted with
basic equation: δ13Cplant=δ13Cair-a-(b-a) (ci/ca)
“calculated
mistletoe”
includes
contribution of
host xylem
carbon
(Marshall et al., 1994)
Carbon and nitrogen isotope ratios,
nitrogen content and heterotrophy in
New Zealand mistletoes
Bannister and Strong, 2000
• Smaller difference in isotopic ratios of host
organisms and parasites
Host Average δ13C=-28.89+-0.12‰
Parasite Average δ13C=-29.51+-0.10‰
Australia
New Zealand
•Arid, water limited
•Mistletoe forced to maintain low
water potential and high
transpiration rate to take water
from stressed host
Temperate climate drives similar
physiology in host and mistletoe
leading to similar isotopic ratios…
Intraspecific stable isotope variability in the
planktic foraminifera Globigerinoides sacculifer:
Results from laboratory experiments
Spero and Lea, 1993
(Spero, 1998)
Benefits of symbiosis in foraminifera
•Energy from photosynthesis
•Ability to assimilate dissolved inorganic
nutrients
•Metabolite removal NH4+ PO43-
(Spero, 1998)
δ13C:
As irradiance (and algal activity) increases, δ13C ratios increase
δ18O:
As irradiance (and algal activity) increases, δ18O ratios decrease
Test size is dependant on light levels
Ca2+ + 2HCO3- -> CO2 (to photosynthesis)
+CaCO3 (calcite) + H2O
Goldilocks and the three Globigerinoides
Largest individual tests (>750microns) give most accurate isotopic ratios for
intercore comparison of δ13C , because all organisms grew under similar,
Pmax conditions.
Medium sized tests were calcified under wide range of sub-Pmax conditions,
and will yield variable δ13C values.
Small tests belonged to forams living in the mixed layer/thermocline
boundary where there is low light and heterogeneous δ13C conditions.
δ13C=1.5‰ variation from light level changes
http://webpersonal.uma.es/~FSL/imag/Gnoides.sacculifer.jpg
Punchline:
Symbionts on spines or within rhizopodial web preferentially
uptake 12C, creating a microenvironment enriched in 13C that
surrounds the test calcifying environment. (large kinetic fractionation
associated with ribulose biphosphate carboxylase enzyme)
(Spero, 1998)
Mechanism of δ18O depletion with increased symbiont
Photosynthesis: a possible explanation
•Rate of calcite precipitation is positively correlated with light
intensity
•High calcification rates imply rapid reaction cycles of CO2 +
H2O <-> H2CO3 and rapid exchange of CO2 across
membranes
•This does not allow for complete equilibration between
cellular and seawater carbon reservoirs
•So the resulting calcite is depleted in δ18O as compared to
seawater values
Gametogenic calcite δ18O effects
Pre-gametogenic
Release of gametes
δ18O
values
enriched
relative to
seawater,
about 2‰
Gametogenic calcite
(Spero, 1998)