Testing assumptions of nitrogen cycling between a temperate, model coral host and its facultative symbiont: symbiotic contributions to dissolved inorganic nitrogen assimilation

Coral symbioses are predicated on the need for mutual nutrient acquisition and translocation between partners. Carbon translocation is well-studied in this classic mutualism, while nitrogen (N) has received comparatively less attention. Quantifying the mechanisms and dynamics of N assimilation is critical to understanding the functional ecology of coral organisms. Given the importance of symbiosis to the coral holobiont, it is important to determine what role photosynthetic symbionts play in N acquisition. We used the facultatively symbiotic temperate coral Astrangia poculata and ^15N labeling to test the effects of symbiotic state and trophic status on N acquisition. We tracked assimilation of 2 forms of isotopically labeled dissolved inorganic N (DIN: ammonium, ^15NH_4+ and nitrate, ^15NO_3^-) by fed and starved colonies of both symbiotic and aposymbiotic A. poculata. Coral holobiont tissue was subsequently analyzed for δ^15N and changes in photosynthetic efficiency. Results suggest that corals acquired the most N from DIN via their symbiont Breviolum psygmophilum and that NH_4+ is more readily assimilated than NO_3^-. Photosynthetic efficiency increased with the addition of NH_4^+, but only for fed, symbiotic treatments. NO_3^- adversely affected photosynthetic efficiency among starved corals. Our results suggest that symbiosis is advantageous for DIN acquisition, that dysbiosis inhibits corals’ mixotrophic strategy of nutrient acquisition, and that either feeding or symbiosis alone does not fully provide the energetic advantage of both. This study lends support to the emerging hypothesis that symbionts are mutualists in optimal conditions but shift to a parasitic paradigm when resources or energy are scarce.Published versio

Field-linked States of Ultracold Polar Molecules

We explore the character of a novel set of ``field-linked'' states that were predicted in [A. V. Avdeenkov and J. L. Bohn, Phys. Rev. Lett. 90, 043006 (2003)]. These states exist at ultralow temperatures in the presence of an electrostatic field, and their properties are strongly dependent on the field's strength. We clarify the nature of these quasi-bound states by constructing their wave functions and determining their approximate quantum numbers. As the properties of field-linked states are strongly defined by anisotropic dipolar and Stark interactions, we construct adiabatic surfaces as functions of both the intermolecular distance and the angle that the intermolecular axis makes with the electric field. Within an adiabatic approximation we solve the 2-D Schrodinger equation to find bound states, whose energies correlate well with resonance features found in fully-converged multichannel scattering calculations

Deceleration and trapping of heavy diatomic molecules using a ring-decelerator

We present an analysis of the deceleration and trapping of heavy diatomic molecules in low-field seeking states by a moving electric potential. This moving potential is created by a 'ring-decelerator', which consists of a series of ring-shaped electrodes to which oscillating high voltages are applied. Particle trajectory simulations have been used to analyze the deceleration and trapping efficiency for a group of molecules that is of special interest for precision measurements of fundamental discrete symmetries. For the typical case of the SrF molecule in the (N,M) = (2, 0) state, the ring-decelerator is shown to outperform traditional and alternate-gradient Stark decelerators by at least an order of magnitude. If further cooled by a stage of laser cooling, the decelerated molecules allow for a sensitivity gain in a parity violation measurement, compared to a cryogenic molecular beam experiment, of almost two orders of magnitude

Testing the CAPM revisited

This paper re-examines the tests of the Sharpe-Lintner Capital Asset Pricing Model (CAPM). The null that the CAPM intercepts are zero is tested for ten size-based stock portfolios and for twenty five book-to-market sorted portfolios using five-year, ten-year and longer sub-periods during 1965-2004. The paper shows that the evidence for rejecting the CAPM on statistical grounds is weaker than the consensus view suggests, and highlights the pitfalls of testing multiple hypotheses with the conventional heteroskedasticity and autocorrelation robust (HAR) test with asymptotic P-values. The conventional test rejects the null for almost all sub-periods, which is consistent with the evidence in the literature. By contrast, the null is not rejected for most of the sub-periods by the new HAR tests developed by Kiefer et al. (2000), Kiefer and Vogelsang (2005), and Sun et al. (2008).CAPM Heteroskedasticity and autocorrelation robust tests Heteroskedasticity and autocorrelation consistent estimators Bartlett and Parzen kernels

CAPM Reconsidered: A Robust Finite Sample Evaluation

In this paper, the conventional test of the Sharpe-Lintner version of the Capital Asset Pricing Model (CAPM) are reconsidered. The CAPM is formulated as a Seemingly Unrelated Regression (SUR) system with an adding-up restriction. A statistical framework for conducting the conventional tests is provided. A Wald statistic that takes into account the adding-up restriction and that is robust to heteroskedasticity and/or autocorrelation of unknown form is used to test the CAPM. The robust Wald test with asymptotic critical values rejects the CAPM for most subsamples. Monte Carlo simulations are then performed to obtain the finite sample critical values. Our empirical results show that the CAPM is consistent with the data when the tests are based on finite sample critical values.CAPM; SUR system; Adding-Up; Finite sample distribution; Robust Wald test