The influence of symbiont photosynthesis on the boron isotopic composition of foraminifera shells

Abstract

11 12Culture experiments were carried out with the planktonic foraminifer Orbulina universa under high and low light 13 levels in order to determine the influence of symbiont photosynthetic activity on the boron isotopic composition of 14 shell calcite. Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5x lower 15 compared to shells grown under high light (elevated photosynthetic rates). In terms of inferred pH, the lower boron 16 isotope values correspond to a reduction in pH of approximately 0.2 units. The boron isotopic composition of 17 Orbulina universa from plankton tows is similar to that of shells grown under low light conditions in the laboratory. 18 These data are consistent with reduced symbiont concentrations in recently secreted shells. In addition to laboratory 19 and field grown O. universa, we present the first data for a symbiont-barren foraminifer, Globigerina bulloides. Data 20 obtained for G. bulloides fall V1.4x below those of the field grown O. universa. Although the plankton tow results 21 are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters 22 responsible for species-specific vital effects. , 1996Spivack et al., 1993).

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral <i>Lophelia pertusa</i>

“…Moreover, this appears to be valid for both aragonite and calcite. A similar offset was also demonstrated for U / Ca ratios in calcitic planktonic Foraminifera (Russel et al, 2004) and for δ 11 B in Globigerina bulloides (no symbionts) and Orbulina universa (with symbionts; Hönisch et al, 2003).…”

Planktic foraminifers as recorders of seawater Ba/Ca

“…This is because symbiont photosynthetic activity and the associated CO 2 sequestration in planktic foraminifers are known to increase pH at the site of calcification, and this effect is larger at higher light intensities (Hönisch et al, 2003;Jørgensen et al, 1985;Rink et al, 1998). In contrast, pH in the microenvironment of symbiont-barren foraminifers such as G. bulloides is dominated by the CO 2 -producing processes of respiration and calcification, which lower pH at the site of calcification (Hönisch et al, 2003). Ba/Ca ratios of O. universa grown under high-light and low-light conditions agree at a 95% confidence level (0.67 ± 0.04 and 0.62 ± 0.05 μmol/mol, respectively), suggesting that variable light intensities and corresponding pH changes at the site of calcification do not significantly affect Ba incorporation.…”

Boron Isotopes in Foraminifera: Systematics, Biomineralisation, and CO2 Reconstruction

“…Respiration and calcification thus exert a greater net influence than photosynthesis at the habitat depth of O. universa, leading to an offset to lower d 11 B and pH. Indeed Hönisch et al (2003) note that their culture experiments that used low light levels lie closest to their O. universa data from plankton tows (though not core-tops).…”

“…The influence of these processes on the microenvironment of planktic foraminifera has been demonstrated using microelectrodes (Rink et al 1998;Köhler-Rink and Kühl 2000), which reveal pronounced gradients in pH and O 2 in the diffusive boundary layer surrounding planktic foraminifera. Culturing studies demonstrate that these processes are reflected in boron isotope composition (Hönisch et al 2003), with an increase in d 11 B of O. universa grown at increasing light levels, due to enhanced photosymbiont activity. These offsets have also been reproduced in a diffusion-reaction model of the foraminiferal microenvironment (Zeebe et al 2003; and see further discussion below).…”

“…These factors, along with limited depth and seasonally-resolved water-column pH data, may complicate proxy calibration in planktic foraminifera from the wild (e.g. core-tops, tows, sediment traps Hönisch et al 2003).…”