Light and freshwater discharge drive the biogeochemistry and microbial ecology in a sub-Arctic fjord over the Polar night

The polar night has recently received increased attention as a surprisingly active biological season. Yet, polar night microbial ecology is a vastly understudied field. To identify the physical and biogeochemical parameters driving microbial activity over the dark season, we studied a sub-Arctic fjord system in northern Norway from autumn to early spring with detailed monthly sampling. We focused on the impact of mixing, terrestrial organic matter input and light on microbial ecosystem dynamics. Our study highlights strong differences in the key drivers between spring, autumn, and winter. The spring bloom started in March in a fully mixed water column, opposing the traditional critical depth hypothesis. Incident solar radiation was the key driver maximum Chlorophyll was reached in April. The onset of the autumn phytoplankton bloom was controlled by vertical mixing, causing nutrient upwelling and dilution of zooplankton grazers, which had their highest biomass during this time. According to the dilution-recoupling hypothesis grazer dilution reduced grazing stress and allowed the fall bloom formation. Mixing at that time was initiated by strong winds and reduced stratification as a consequence of freezing temperatures and lower freshwater runoff. During the light-limited polar night, the primary production was extremely low but bacteria continued growing on decaying algae, their exudates and also allochthonous organic matter. A melting event in January could have increased input of organic matter from land, supporting a mid-winter bacterial bloom. In conclusion, polar night biogeochemistry and microbial ecology was not only driven by light availability, but strongly affected by variability in reshwater discharge and allochthonous carbon input. With climate change freshwater discharge will increase in the Arctic, which will likely increase importance of the dynamics described in this study

Light and freshwater discharge drive the biogeochemistry and microbial ecology in a sub-Arctic fjord over the Polar night

The polar night has recently received increased attention as a surprisingly active biological season. Yet, polar night microbial ecology is a vastly understudied field. To identify the physical and biogeochemical parameters driving microbial activity over the dark season, we studied a sub-Arctic fjord system in northern Norway from autumn to early spring with detailed monthly sampling. We focused on the impact of mixing, terrestrial organic matter input and light on microbial ecosystem dynamics. Our study highlights strong differences in the key drivers between spring, autumn, and winter. The spring bloom started in March in a fully mixed water column, opposing the traditional critical depth hypothesis. Incident solar radiation was the key driver maximum Chlorophyll was reached in April. The onset of the autumn phytoplankton bloom was controlled by vertical mixing, causing nutrient upwelling and dilution of zooplankton grazers, which had their highest biomass during this time. According to the dilution-recoupling hypothesis grazer dilution reduced grazing stress and allowed the fall bloom formation. Mixing at that time was initiated by strong winds and reduced stratification as a consequence of freezing temperatures and lower freshwater runoff. During the light-limited polar night, the primary production was extremely low but bacteria continued growing on decaying algae, their exudates and also allochthonous organic matter. A melting event in January could have increased input of organic matter from land, supporting a mid-winter bacterial bloom. In conclusion, polar night biogeochemistry and microbial ecology was not only driven by light availability, but strongly affected by variability in reshwater discharge and allochthonous carbon input. With climate change freshwater discharge will increase in the Arctic, which will likely increase importance of the dynamics described in this study

Dedicated controller design for a dual-stage opto-mechatronic system

To improve the resolution of the Very Large Telescope Interferometer (VLTI) a two-stage mechanical system, a so called Differential Delay Line (DDL), is developed jointly by the EPFL and the Observatory of Geneva. The system is designed to reach nanometer accuracy at high bandwidth over large displacements. The coarse stage features a permanent magnet (PM) stepper motor driving a lead screw connected to a double parallelogram flexure with notch-hinges (blade) guiding system, and the fine stage features a stacked piezoelectric device, combine to one single measurable output. This paper compares different control approaches for the DDL with their respective advantages and disadvantages. The developed control methods are based on modern linear and nonlinear control theory. The performance of the control schemes is illustrated via simulation and measurement on the available prototype. The new developed methods are compared to the currently implemented decoupled SISO design which features a direct-coil controller for the coarse stage and a simple PID-controller for the fine one

Sugars dominate the seagrass rhizosphere

Seagrasses are among the most efficient sinks of carbon dioxide on Earth. While carbon sequestration in terrestrial plants is linked to the microorganisms living in their soils, the interactions of seagrasses with their rhizospheres are poorly understood. Here, we show that the seagrass, Posidonia oceanica excretes sugars, mainly sucrose, into its rhizosphere. These sugars accumulate to mu M concentrations-nearly 80 times higher than previously observed in marine environments. This finding is unexpected as sugars are readily consumed by microorganisms. Our experiments indicated that under low oxygen conditions, phenolic compounds from P. oceanica inhibited microbial consumption of sucrose. Analyses of the rhizosphere community revealed that many microbes had the genes for degrading sucrose but these were only expressed by a few taxa that also expressed genes for degrading phenolics. Given that we observed high sucrose concentrations underneath three other species of marine plants, we predict that the presence of plant-produced phenolics under low oxygen conditions allows the accumulation of labile molecules across aquatic rhizospheres. Seagrass meadows are important carbon sinks. Here, the authors show that organic carbon in the form of simple sugars can accumulate at high concentrations in seagrass rhizospheres because plant phenolic compounds inhibit their consumption by microorganisms

The ESPRI project: astrometric exoplanet search with PRIMA I. Instrument description and performance of first light observations

The ESPRI project relies on the astrometric capabilities offered by the PRIMA facility of the Very Large Telescope Interferometer for the discovery and study of planetary systems. Our survey consists of obtaining high-precision astrometry for a large sample of stars over several years and to detect their barycentric motions due to orbiting planets. We present the operation principle, the instrument's implementation, and the results of a first series of test observations. A comprehensive overview of the instrument infrastructure is given and the observation strategy for dual-field relative astrometry is presented. The differential delay lines, a key component of the PRIMA facility which was delivered by the ESPRI consortium, are described and their performance within the facility is discussed. Observations of bright visual binaries are used to test the observation procedures and to establish the instrument's astrometric precision and accuracy. The data reduction strategy for astrometry and the necessary corrections to the raw data are presented. Adaptive optics observations with NACO are used as an independent verification of PRIMA astrometric observations. The PRIMA facility was used to carry out tests of astrometric observations. The astrometric performance in terms of precision is limited by the atmospheric turbulence at a level close to the theoretical expectations and a precision of 30 micro-arcseconds was achieved. In contrast, the astrometric accuracy is insufficient for the goals of the ESPRI project and is currently limited by systematic errors that originate in the part of the interferometer beamtrain which is not monitored by the internal metrology system. Our observations led to the definition of corrective actions required to make the facility ready for carrying out the ESPRI search for extrasolar planets.Comment: 32 pages, 39 figures, Accepted for publication in Astronomy and Astrophysic

Metabolomic analysis of symbiosis between chemoautotrophic bacteria and marine invertebrates

Olavius algarvensis is an oligochaete characterized by the absence of digestive and excretory systems. It harbors a thick layer of extracellular symbionts below its cuticle. It is assumed that the worm gains most of its nutrition though those bacteria. The study of its metabolomic pro le re- vealed another of its characteristics. Olavius algarvensis has -sitosterol as main sterol which is unusual for an animal , most of them having cholesterol as main sterol. This nding arises the question of the origin of this phytosterol in the gutless worm. Due to its lack of digestive sys- tem and to the absence of sterols in bacteria, Olavius algarvensis can not gain phytosterol through its diet. Furthermore only plants are able of -sitosterol synthesis. It was hypothezised that Olavius algarvensis takes up -sitosterol from its environment.Olavius algarvensis lives in the vicin- ity of the seagrassPosidonia oceanica which happens to have -sitosterol as main sterol. It it not the only metabolites that can be nd in both systems, sucrose and myo-inositol are sugars found in high abundance in both organisms. To test this hypothesis, the sterols content of Olavius al- garvensis habitat was investigated. Sterols were extracted from sea water, pore water and sediment sampled along a transect going from a seagrass meadow to plain sand, away from direct plant in uence. Two sterols were detected in Olavius algarvensis environement, cholesterol and -sitosterol. The highest concentration were measured inside the seagrass meadows. Pore water was richer in sterols than the overlaying seawater by a factor 1000. Concentration in the pore water were ranging from 1 to 17 M. The next question to arise was is Olavius algarvensis able to take up sterol from its environement. To answer this question, worms were incubated with di erent sources of sterol for up to 9 weeks, time points being collected every 2 weeks. The worms were incubated with pure cholesterol, labeled deuterium -sitosterol-d7 or seagrass leaves. Their metablomic pro les were then analysed. The worms did not take up the labeled -sitosterol- d7 in detectable amounts. The ratio between cholesterol and -sitosterol was analyzed as well. Some trends were visible but should be interpreted with caution due the high variability within an incubation and the rela- tively low replication (n=3). Metabolomic analyis of gutless oligochaetes and seagrasses from Belize revealed that Olavius algarvensis from Elba are not the only gutless oligochaetes having -sitosterol as main sterol. Literature research revealed that other organisms as phyto-parasitic ne- matodes and some insects also have a phytosterol as main sterol. This suggest that this observation is more widespread and therefore of possible importance

On Achieving Periodic Joint-Motion for Redundant Robots

The consequence of the loss of involutivity of a specific set of vector fields on the periodicity of the joint motion is examined for redundant robots. An output task, defined as a one dimensional periodic closed curve embedded in a two dimensional working surface, is realized through the computation of joint velocities in the configuration space. Depending on the manner in which the joint velocity is computed from the end-effector velocity, the resulting joint motion can become unpredictable and of a chaotical nature, even though the end-effector movement is periodic and predictable. The paper proposes an improvement over classical pseudo-inverse computation of the joint motion by first suitably selecting two involutive vector fields (used as a basis for parameterization) in the tangent bundle of the output manifold. It also presents a sufficient condition for the periodicity of all the joint configuration based on the involutivity of two vector fields in the tangent bundle of the joint space. The results are illustrated on a five-link rotary redundant robot (5R robot)

Strategy for the Control of a Dual-stage Nano-positioning System with a Single Metrology

A double-stage feedback control structure for a double-stage mechanical system, with a single optical metrology is developed to reach the nanometer accuracy at high bandwidth over large displacements. A piezoelectric stack actuator is used for fine positioning, while a permanent magnet (PM) stepper motor handles the coarse positioning. Two different control approaches are compared to drive the PM stepper motor, while a classical PID controller is designed to drive the piezoelectric actuator. Since only a single measurement device is used, the references for both control loops (fine and coarse) must be suitably obtained. An adequate control structure including a partial observer is designed so as to take into account the influence of the fine actuator on the position estimation of the coarse actuator. The complete control mechanism and strategy ensure the tracking of the real reference with sufficient accuracy and bandwidth

Hands-on Introduction to Automatic Control with focus on Teleoperation and Real-Time Interaction

Introducing automatic control has always been challenging. As a technology, control is generally hidden by the enclosing application. As a methodology, control is often seen as a theoretical subject. Therefore, to grasp the interest of both practitioners enrolled in continuing education programs and students enrolled in introductory control courses, novel applications should be exploited. This paper presents an approach to demonstrate basic control concepts using a micropositioning experiment. It also shows that the importance of real-time interaction and teleoperation features in today control solutions can be implicitly underlined by providing a remote access to the experiment

Real-time compensation of hysteresis in piezoelectric-stack actuator tracking a stochastic reference

This paper presents a convenient way to invert the classical Preisach model to compensate the hysteresis of a piezoelectric stack actuator in real-time. The advantage of the proposed method lies in the possibility to track a stochastic signal and compensate the hysteresis in real-time. Experimental results show a reduction of the RMS tracking error by 67 % to 90 % by using the compensation algorithm designed