Parasitism and Symbiosis: isotope effects in mistletoe and foraminifera )

Carbon isotope composition, gas exchange and heterotrophy in Australian mistletoes Marshall, Ehleringer, Schulze, and Farquhar,

δ 13 C of plant tissue related to photosynthetic gas exchange by this equation: δ 13 C plant =δ 13 C air -a-(b-a) (c i /c a ) δ 13 C plant = carbon isotope of tissue δ 13 C air = atmospheric CO 2 (-8‰) C i =intercellular CO 2 concentration C a =atmospheric CO 2 concentration (look familiar?)

It would make sense for parasite 13 C to be intermediate between host plant carbon (lower c i ) and mistletoe photosynthate. δ 13 C parasite = (A[δ 13 C air -a-(b-a)(c i /c a )]+Ec x δ 13 C)/A+Ec x A= rate of photosynthesis E= rate of transpiration C x = [carbon] in xylem sap δ 13 C= isotopic ratio of xylem carbon (=host tissue) Due to leakiness, mistletoes expected to have lighter δ 13 C, but remain heavy if taking carbon from host.

Xylem carbon concentration was estimated from measured photosynthetic rate (A), and transpiration rate (E) C x = (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 c i /c a ; causing isotopic difference Host Average δ 13 C= ‰ Parasite Average δ 13 C= ‰ ~2X difference in A/E (Marshall et al., 1994)

Measured δ 13 C plotted against δ 13 C predicted with basic equation: δ 13 C plant =δ 13 C air -a-(b-a) (c i /c a ) “calculated mistletoe” includes contribution of host xylem carbon

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 δ 13 C= ‰ Parasite Average δ 13 C= ‰

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 NH 4+ PO 4 3-

δ 13 C: As irradiance (and algal activity) increases, δ 13 C ratios increase δ 18 O: As irradiance (and algal activity) increases, δ 18 O ratios decrease

Test size is dependant on light levels Ca HCO 3 - -> CO 2 (to photosynthesis) +CaCO 3 (calcite) + H 2 O

Largest individual tests (>750microns) give most accurate isotopic ratios for intercore comparison of δ 13 C, because all organisms grew under similar, P max conditions. Medium sized tests were calcified under wide range of sub-P max conditions, and will yield variable δ 13 C values. Small tests 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 Goldilocks and the three Globigerinoides

Punchline: Symbionts on spines or within rhizopodial web preferentially uptake 12 C, creating a microenvironment enriched in 13 C that surrounds the test calcifying environment. (large kinetic fractionation associated with ribulose biphosphate carboxylase enzyme) (Spero, 1998)

Mechanism of δ 18 O 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 CO 2 + H 2 O H 2 CO 3 and rapid exchange of CO 2 across membranes This does not allow for complete equilibration between cellular and seawater carbon reservoirs So the resulting calcite is depleted in δ 18 O as compared to seawater values

Gametogenic calcite δ 18 O effects (Spero, 1998) Pre-gametogenicRelease of gametes Gametogenic calcite δ 18 O values enriched relative to seawater, about 2‰