Modelling the effect of vertical mixing on bottle incubations for determining in situ phytoplankton dynamics. II. Primary production

The estimation of in situ phytoplankton primary production is pivotal to many questions in biological oceanography and marine ecology both in a local and global context. Applications range from earth system modelling, the characterisation of aquatic ecosystem dynamics, or the local management of water quality. A common approach for estimating in situ primary production is to incubate natural phytoplankton assemblages in clear bottles at a range of fixed depths and to measure the uptake of carbon (14C) during the incubation period (typically 24 h). One of the main concerns with using fixed-depth bottle incubations is whether stranding samples at fixed depths biases the measured CO2 fixation relative to the 'true' in situ mixed conditions. Here we employ an individual based turbulence and photosynthesis model, which also accounts for photoacclimation and -inhibition, to examine whether the in vitro productivity estimates obtained from fixed-depth incubations are representative of the in situ productivity in a freely mixing water column. While previous work suggested that in vitro estimates could either over- or underestimate the in situ productivity, we show that the errors due to arresting the incubation bottles at fixed depths are indeed minimal. We present possible explanations for how previous authors could have arrived at contradictory results and discuss whether they might be artefacts related to the particular sampling protocol used. We discuss the errors associated with chlorophyll-based incubation methods for determining in situ phytoplankton growth rates in Ross et al. (2011; Mar Ecol Prog Ser 435:13-31). © Inter-Research 2011

Genetic requirement for Ras in the transformation of fibroblasts and hematopoietic cells by the Bcr-Abl oncogene.

To determine the functional importance of Ras in transformation by Abl oncogenes, we used a genetic approach to measure the effect of impaired Ras activity on the ability of Bcr-Abl or v-Abl to transform cells. Expression of the catalytic domain of the GTPase activating protein for Ras (Gap C terminus) impaired soft agar colony formation by fibroblasts expressing v-Abl or Bcr-Abl by 70-80%. To test Ras function in a model that more closely resembles clinical diseases involving Bcr-Abl, double gene retroviruses expressing Bcr-Abl paired with the Gap C terminus or dominant negative Ras were introduced into naive mouse bone marrow cells. Transformation by Bcr-Abl was completely blocked in both situations. Coexpression of normal c-H-Ras accelerated the transforming activity of Bcr-Abl. These findings show that Ras activation is essential for the leukemogenic activity of Abl oncogenes in two distinct model systems. The results genetically define a connection between the Bcr-Abl cytoplasmic tyrosine kinase and Ras and add to the accumulating evidence that deregulation of Ras is a central event in the genesis of a number of molecularly distinct forms of human myeloid leukemia

Tidal Excitation of Modes in Binary Systems with Applications to Binary Pulsars

We consider the tidal excitation of modes in a binary system of arbitrary eccentricity. For a circular orbit, the modes generally undergo forced oscillation with a period equal to the orbital period ($T$). For an eccentric orbit, the amplitude of each tidally excited mode can be written approximately as the sum of an oscillatory term that varies sinusoidally with the mode frequency and a `static' term that follows the time dependence of the tidal forcing function. The oscillatory term falls off exponentially with increasing \b (defined as the ratio of the periastron passage time to the mode period), whereas the `static' term is independent of \b. For small \b modes (\b \approx 1), the two terms are comparable, and the magnitude of the mode amplitude is nearly constant over the orbit. For large \b modes (\b \gta a few), the oscillatory term is very small compared to the `static' term, in which case the mode amplitude, like the tidal force, varies as the distance cubed. For main sequence stars, $p$, $f$, and low order $g$-modes generally have large \b and hence small amplitudes of oscillation. High overtone $g$-modes, however, have small overlap with the tidal forcing function. Thus, we expect an intermediate overtone $g$-mode with \b \sim 1 to have the largest oscillation amplitude. The dependence on mode damping and the stellar rotation rate is considered, as well as the effects of orbital evolution. We apply our work to the two binary pulsar system: PSR J0045-7319 and PSR B1259-63.Comment: 28 pages of uuencoded compressed postscript. 9 postscript figures available by anonymous ftp from ftp://brmha.mit.edu/ To be published in ApJ

Modelling the effect of vertical mixing on bottle incubations for determining in situ phytoplankton dynamics. I. Growth rates

Reliable estimates of in situ phytoplankton growth rates are central to understanding the dynamics of aquatic ecosystems. A common approach for estimating in situ growth rates is to incubate natural phytoplankton assemblages in clear bottles at fixed depths or irradiance levels and measure the change in chlorophyll a (Chl) over the incubation period (typically 24 h). Using a modelling approach, we investigate the accuracy of these Chl-based methods focussing on 2 aspects: (1) in a freely mixing surface layer, the cells are typically not in balanced growth, and with photoacclimation, changes in Chl may yield different growth rates than changes in carbon; and (2) the in vitro methods neglect any vertical movement due to turbulence and its effect on the cells' light history. The growth rates thus strongly depend on the incubation depth and are not necessarily representative of the depth-integrated in situ growth rate in the freely mixing surface layer. We employ an individual based turbulence and photosynthesis model, which also accounts for photoacclimation and photo - inhibition, to show that the in vitro Chl-based growth rate can differ both from its carbon-based in vitro equivalent and from the in situ value by up to 100%, depending on turbulence intensity, optical depth of the mixing layer, and incubation depth within the layer. We make recommendations for choosing the best depth for single-depth incubations. Furthermore we demonstrate that, if incubation bottles are being oscillated up and down through the water column, these systematic errors can be significantly reduced. In the present study, we focus on Chl-based methods only, while productivity measurements using carbon-based techniques (e.g. 14C) are discussed in Ross et al. (2011; Mar Ecol Prog Ser 435:33-45). © Inter-Research 2011

SPMHD simulations of Structure Formation

The intracluster medium of galaxy clusters is permeated by {\mu}G magnetic fields. Observations with current and future facilities have the potential to illuminate the role of these magnetic fields play in the astrophysical processes of galaxy clusters. To obtain a greater understanding of how the initial seed fields evolve to the magnetic fields in the intracluster medium requires magnetohydrodynamic simulations. We critically assess the current Smoothed Particle Magneto-Hydrodynamics (SPMHD) schemes, especially highlighting the impact of a hyperbolic divergence cleaning scheme and artificial resistivity switch on the magnetic field evolution in cosmological simulations of the formation of a galaxy cluster using the N-body/SPMHD code gcmhd++. The impact and performance of the cleaning scheme and two different schemes for the artificial resistivity switch is demonstrated via idealized test cases and cosmological simulations. We demonstrate that the hyperbolic divergence cleaning scheme is effective at suppressing the growth of the numerical divergence error of the magnetic field and should be applied to any SPMHD simulation. Although the artificial resistivity is important in the strong field regime, it can suppress the growth of the magnetic field in the weak field regime, such as galaxy clusters. With sufficient resolution, simulations with divergence cleaning can reproduce observed magnetic fields. We conclude that the cleaning scheme alone is sufficient for galaxy cluster simulations, but our results indicate that the SPMHD scheme must be carefully chosen depending on the regime of the magnetic field.Comment: 15 pages, 11 figures, published (MNRAS 476 2890