Feedback control architecture & the bacterial chemotaxis network

Bacteria move towards favourable and away from toxic environments by changing their swimming pattern. This response is regulated by the chemotaxis signalling pathway, which has an important feature: it uses feedback to ‘reset’ (adapt) the bacterial sensing ability, which allows the bacteria to sense a range of background environmental changes. The role of this feedback has been studied extensively in the simple chemotaxis pathway of Escherichia coli. However it has been recently found that the majority of bacteria have multiple chemotaxis homologues of the E. coli proteins, resulting in more complex pathways. In this paper we investigate the configuration and role of feedback in Rhodobacter sphaeroides, a bacterium containing multiple homologues of the chemotaxis proteins found in E. coli. Multiple proteins could produce different possible feedback configurations, each having different chemotactic performance qualities and levels of robustness to variations and uncertainties in biological parameters and to intracellular noise. We develop four models corresponding to different feedback configurations. Using a series of carefully designed experiments we discriminate between these models and invalidate three of them. When these models are examined in terms of robustness to noise and parametric uncertainties, we find that the non-invalidated model is superior to the others. Moreover, it has a ‘cascade control’ feedback architecture which is used extensively in engineering to improve system performance, including robustness. Given that the majority of bacteria are known to have multiple chemotaxis pathways, in this paper we show that some feedback architectures allow them to have better performance than others. In particular, cascade control may be an important feature in achieving robust functionality in more complex signalling pathways and in improving their performance

A global database of sea surface dimethylsulfide (DMS) measurements and a procedure to predict sea surface DMS as a function of latitude, longitude, and month

47 pages, 13 figures, 7 tablesA database of 15,617 point measurements of dimethylsulfide (DMS) in surface waters along with lesser amounts of data for aqueous and particulate dimethylsulfoniopropionate concentration, chlorophyll concentration, sea surface salinity and temperature, and wind speed has been assembled. The database was processed to create a series of climatological annual and monthly 1°x1°latitude-longitude squares of data. The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters. Instead, an annual map of sea surface DMS was produced using an algorithm similar to that employed by Conkright et al. [1994]. In this approach, a first-guess field of DMS sea surface concentration measurements is created and then a correction to this field is generated based on actual measurements. Monthly sea surface grids of DMS were obtained using a similar scheme, but the sparsity of DMS measurements made the method difficult to implement. A scheme was used which projected actual data into months of the year where no data were otherwise presen

Glacial Chronology over the Past 450 kyr Around the Margins of the Southern North Sea (UK and Netherlands) From Quartz Luminescence Dating

Terrestrial glacigenic deposits containing evidence for the repeated growth of former ice sheets are distributed widely in North America and continental Europe. The chronology of the younger component of these deposits during the Last Glacial Maximum (LGM) is relatively well established, but considerably less is known about the extent and timing of earlier periods of Pleistocene glaciation. This study focuses on the well preserved glacigenic sediment and landform assemblages that are exposed across lowland areas of eastern Britain and the northern Netherlands, which document the repeated advance of Pleistocene British/Fennoscandinavian ice sheets. Previously, it is unknown whether the deposits belong to a single glaciation equated with Marine Isotope Stage (MIS) 12, or represent deposition over a number of separate cold climate stages. We have collected > 80 samples from glaciofluvial and glaciolacustrine deposits in this region for luminescence (OSL) dating with the approach of obtaining 5-10 replicate dates per stratigraphical unit. The luminescence characteristics of the samples including thermal stability, dose response, ability to measure a known dose, and presence of desirable components separated using LM-OSL (linear modulation) indicate that equivalent doses can be reliably measured across the timescale of interest. Furthermore, low concentrations of radioactive isotopes in sediments of this region enable the traditionally accepted age limit of luminescence dating to be considerably extended. The luminescence ages confirm that the largest glaciation in the UK occurred during MIS 12 (427 ± 26 kyr). An additional tight cluster of ages at 166 ± 8 kyr also indicates that major ice advances occurred synchronously in the UK and Netherlands during MIS 6, although the dimensions of the ice sheet in the UK appears to be of a similar or lesser extent than the LGM. This work also justifies that coherent sets of luminescence ages can be obtained with an overall uncertainty of < 10%, sufficient to delineate glaciation events at the isotope stage level but precluding discrimination of sub-isotope stage advances

Electronic interactions in bridged bis(cluster) assemblies - a comparison of para-CB10H10C, para-C6H4 and C-4 bridges

The electrochemical response of bis-Co2C2(CO)(4)(mu-dppm) complexes featuring bridging para-CB10H10C (5) and para-C6H4 (6) moieties are similar, each exhibiting two oxidations separated by ca. 100 mV, and two reductions separated by 80 mV, evidencing a degree of "electronic communication". A computational study of these systems and of the butadiyndiyl-bridged species (7) reveals an increasing contribution from the bridge pi-orbitals in the frontier MO's of the monocations 5(+) < 6(+) < 7(+). Thus, while similar conclusions about electronic interactions between the cluster-based redox probes through the cluster or organic bridges may be drawn from electrochemical studies, the mechanism by which these effects transmitted is subtly different in each case

Estimating the motion of plant root cells from in vivo confocal laser scanning microscopy images

This is an author-created version of a paper to be published in the Springer journal Machine Vision and Applications. The published version will be available at www.springerlink.com) Images of cellular structures in growing plant roots acquired using confocal laser scanning microscopy (CLSM) have some unusual properties that make motion estimation challenging. These include multiple motions, non-Gaussian noise and large regions with little spatial structure. In this paper, a method for motion estimation is described that uses a robust multi-frame likelihood model and a technique for estimating uncertainty. An efficient region-based matching approach was used followed by a forward projection method. Over small timescales the dynamics are simple (approximately locally constant) and the change in appearance small. Therefore a constant local velocity model is used and the MAP estimate of the joint probability over a set of frames is recovered. Occurrences of multiple modes in the posterior are detected, and in the case of a single dominant mode, motion is inferred using Laplace’e method. The method was applied to several Arabidopsis thaliana root growth sequences with varying levels of success. In addition, comparative results are given for three alternative motion estimation approaches, the Kanade-Lucas-Tomasi tracker, Black and Anandan’s robust smoothing method, and Markov random field based methods.