The Sensitivity of Pore Density in Tests of Bolivina spissa to ambient Oxygen and/or Nitrate concentration in Sediment Bottom Water

The density and size of pores in the test surface are important aspects in the morphology of foraminifera. These pores are supposed to control the uptake of oxygen (O2) and to the release of carbon dioxide (CO2) as a waste product of respiration (Leutenegger and Hansen, 1979). Hence it seems obvious to test certain characteristics like pore-density and porosity as an indicator for O2 depletion in the water column and different redox-conditions, respectively. Here we present a first quantification of the relationship between bottom-water-oxygen-concentration ([O2]BW) and the pore-density of tests from Bolivina spissa collected off the Peruvian continental margin. For the experiment the pores of 216 specimens from 7 locations with different redox-conditions were counted. We found an inverse non-linear relation between the pore density (PD), water depth and the oxygen concentration of the adjacent water masses respectively (R2 = 0.55, N = 7; PD(P/μm2) = 0.0066.[O2]BW - 0.74). At higher oxygen-concentrations the pore-density reach an equilibrium value where the pore density is independent of [O2]BW. Surprisingly, a comparison of the pore-density to the bottom-water-nitrate-concentration ([NO3-]BW) suggests that the pore-density of B.spissa propably is even more sensitive to [NO3-]BW (negative linear correlation, R2 = 0.96, N = 7; PD(P/μm2) = -0.0003.[NO3-]BW + 0.0155). The observation that certain foraminifera may switch to nitrate respiration during oxygen depletion in the water column was recently documented to occur in different benthic foraminiferal species (Risgaard-Petersen et al., 2006). Thus the PD-[NO3-]BW-correlation suggests that the pores have a physiological function for intracellular nitrate-uptake. Future studies will test the application of the pore-density in tests of Bolivina spissa as a possible proxy for [NO3-]BW. References: Leutenegger, S., Hansen, H.J., 1979. Mar. Biol., 54:11-16. Risgaard-Petersen, N. et al., 2006, Nature 443 (7107), 93-96.