Parasitism and Symbiosis Biao Zhu Environmental Studies, UC Santa Cruz Parasitism mistletoes and host trees 13 C Symbiosis corals and zooxanthellae 13 C Symbiosis foraminifera and algae 13 C

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 c x ) H1: leaf 13 C of mistletoes will differ from predicted values based on gas-exchange only H2: Mistletoes A + C transport via xylem sap = Host A

A δ 13 C predicted + (E c x ) δ 13 C x = δ 13 C measured (A + E c x ) A – mistletoes photosynthesis rate δ 13 C predicted – mistletoes δ 13 C value predicted based on eqn 1 E – mistletoes transpiration rate C x – C concentration in the xylem sap δ 13 C measured – mistletoes δ 13 C value measured x

H = (E c x ) / (A + E c x ) H – proportional heterotrophy, proportion of C from host (E c x ) to total C gain of mistletoes (A + E c x ) A – mistletoes photosynthesis rate (measured) E – mistletoes transpiration rate (measured) C x – C concentration in the xylem sap (calculated/estimated)

Host δ 13 C = ± 0.25‰ Parasite δ 13 C = ± 0.23‰

H1: leaf 13 C of mistletoes will differ from predicted values based on gas-exchange only.

H2: Mistletoes A + C transport via xylem sap = Host A  E c x :15% of total C  c x : 10 ± 2 mM  Equivalent/correlated shoot growth

As depth increases Both zooxanthellae and coral tissue δ 13 C values decrease The difference in δ 13 C values between zooxanthellae and coral animal tissue increases

The higher A, the higher zooxanthellae δ 13 C value

CO 2 (aq) + H 2 O CO 2 (g) CO 2 (aq) Dissolution (Henry’s law, T dependent) H 2 CO 3 H + + HCO 3 - Equilibrium ε HCO 3/ CO 2 = 25 o C CO 2 balance in the ocean water Ocean water pH = 8.2

 “depletion-diffusion” hypothesis  Shallow water, high A, CO 2 met depleted and HCO 3 - limited by diffusion --> CO 2 limitation --> low fractionation --> high δ 13 C value of zooxanthellae; coral animal tissure 13 C slightly lower (why?)  Deep water, low A, no CO2 limitation --> high fractionation --> low δ 13 C value of zooxanthellae; coral animal tissue δ 13 C much lower due to allochthonous C sources (e.g. 13 C-depleted oceanic POC/DOC)

In the foraminifera fossil record, larger shell size -- higher δ 13 C value. Why? Symbiotic algae on spines or within rhizopodial web preferentially uptake 12 C (large kinetic fractionation associated with rubisco), creating a microenvironment enriched in 13 C that surrounds the shell calcifying environment. Hypothesis: higher light/irradiance (shallow water) -> higher symbiotic algae photosynthesis -> more 13 C- enriched environment -> higher foraminifera δ 13 C value and larger foraminifera shell size (co-variation)

Largest individual shells (>750 μm) give most accurate isotopic ratios for intercore comparison of δ 13 C, because all organisms grew under similar, P max (high light, shallow water) conditions. Medium sized shells were calcified under wide range of sub-P max conditions, and will yield variable δ 13 C values. Small shells belonged to forams living in the mixed layer/thermocline boundary where there is low light and heterogeneous δ 13 C conditions. δ 13 C = 1.5‰ variation from light level changes Should we just use fossil records of non-symbiotic zooplanktons (no potential contamination of δ 13 C value by symbionts) to infer ocean water CO 2 or HCO 3 - δ 13 C value and climate? Paleoceanographic implications of δ 13 C value of G. sacculifera

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