Parasitism and Symbiosis

Biao ZhuEnvironmental Studies, UC Santa Cruz

Parasitism

mistletoes and host trees 13C

Symbiosis

corals and zooxanthellae 13C

Symbiosis

foraminifera and algae 13C

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 (aq) + H2O

CO2 (g)

CO2 (aq)Dissolution(Henry’s law,T dependent)

H2CO3 H+ + HCO3-

EquilibriumεHCO3/CO2 = +9‰ @ 25oC

CO2 balance in the ocean water

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 13C-enriched environment -> higher foraminifera δ13C value and larger foraminifera shell size (co-variation)

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?

Paleoceanographic implications of δ13C value of G. sacculifera

Thanks!