Parasitism
Symbiosis
Symbiosis
mistletoes and host trees
13C
corals and zooxanthellae
13C
foraminifera and algae
13C
Parasitism and Symbiosis
Biao Zhu
Environmental Studies, UC Santa Cruz
Mistletoes
• Photosynthetic (C source 1)
• Acquire host resources (C & Nutrients) via xylem and/or
phloem (C source 2)
• Why low A/E (high E)? – obtain host C via xylem sap (E cx)
• H1: leaf 13C of mistletoes will differ from predicted values
based on gas-exchange only
• H2: Mistletoes A + C transport via xylem sap = Host A
A δ13Cpredicted + (E cx) δ13Cx = δ13Cmeasured (A + E cx)
A – mistletoes photosynthesis rate
δ13Cpredicted – mistletoes δ13C value predicted based on eqn 1
E – mistletoes transpiration rate
Cx – C concentration in the xylem sap
δ13Cmeasured – mistletoes δ13C value measured
x
H = (E cx) / (A + E cx)
H – proportional heterotrophy, proportion of C from
host (E cx) to total C gain of mistletoes (A + E cx)
A – mistletoes photosynthesis rate (measured)
E – mistletoes transpiration rate (measured)
Cx – C concentration in the xylem sap (calculated/estimated)
Host δ13C = -26.81 ± 0.25‰
Parasite δ13C = -28.67 ± 0.23‰
H1: leaf 13C of mistletoes will differ from predicted values
based on gas-exchange only.
H2: Mistletoes A + C transport via xylem sap = Host A
 E cx:15% of total C
 cx: 10 ± 2 mM
 Equivalent/correlated
shoot growth
As depth increases
•
Both zooxanthellae and coral
tissue δ13C values decrease
•
The difference in δ13C values
between zooxanthellae and
coral animal tissue increases
The higher A, the higher zooxanthellae δ13C value
CO2 (g)
CO2 (aq)
Dissolution
(Henry’s law,
T dependent)
CO2 (aq) + H2O
CO2 balance in the ocean water
Equilibrium
εHCO3/CO2 = +9‰ @ 25oC
H2CO3
H+ + HCO3-
Ocean water
pH = 8.2
“depletion-diffusion” hypothesis
Shallow water, high A, CO2met
depleted and HCO3- limited by
diffusion --> CO2 limitation --> low
fractionation --> high δ13C value of
zooxanthellae; coral animal
tissure13C slightly lower (why?)
Deep water, low A, no CO2
limitation --> high fractionation -->
low δ13C value of zooxanthellae;
coral animal tissue δ13C much
lower due to allochthonous C
sources (e.g. 13C-depleted oceanic
POC/DOC)
In the foraminifera fossil record, larger
shell size -- higher δ13C value. Why?
Symbiotic algae on spines or within
rhizopodial web preferentially uptake 12C
(large kinetic fractionation associated
with rubisco), creating a
microenvironment enriched in 13C that
surrounds the shell calcifying
environment.
Hypothesis: higher light/irradiance
(shallow water) -> higher symbiotic
algae photosynthesis -> more 13Cenriched environment -> higher
foraminifera δ13C value and larger
foraminifera shell size (co-variation)
Paleoceanographic implications of δ13C value of G. sacculifera
Largest individual shells (>750 μm) give most accurate
isotopic ratios for intercore comparison of δ13C, because
all organisms grew under similar, Pmax (high light, shallow
water) conditions.
Medium sized shells were calcified under wide range of
sub-Pmax conditions, and will yield variable δ13C values.
Small shells 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
Should we just use fossil records of non-symbiotic
zooplanktons (no potential contamination of δ13C value
by symbionts) to infer ocean water CO2 or HCO3- δ13C
value and climate?
Thanks!