Pronounced uptake and metabolism of organic substrates by diatoms revealed by pulse-labeling metabolomics

Diatoms contribute as a dominant group of microalgae to approximately 20 per cent of the global carbon fixation. In the plankton, these photosynthetic algae are exposed to a plethora of metabolites, especially when competing algae are lysed. It is well established that diatoms can take up specific metabolites, such as vitamins, amino acids as nitrogen source, or dimethylsulfoniopropoionate to compensate for changes in water salinity. It is, however, unclear to which extent diatoms take up other organic resources and if these are incorporated into the cells metabolism. Here, we ask about the general scope of uptake of metabolites from competitors. Using labeled metabolites released during lysis of algae grown under a 13CO2 atmosphere, we show that the cosmopolitan diatom Chaetoceros didymus takes up organic substrates with little bias and remarkable efficiency. The newly developed pulse label/ mass spectrometry metabolomics approach reveals that polarity and molecular weight has no detectable influence on uptake efficiency. We also reveal that the taken-up pool of metabolites is partly maintained unaltered within the cells but is also subject to catabolic and anabolic transformation. One of the most dominant phytoplankton groups is thus substantially competing with other heterotrophs for organic material, suggesting that the observed absorbotrophy may substantially impact organic material fluxes in the oceans. Our findings call for the refinement of our understanding of competition in the plankton.

Orientation selective grain sublimation–deposition in snow under temperature gradient metamorphism observed with diffraction contrast tomography

In this study on temperature gradient metamorphism in snow, we investigate the hypothesis that there exists a favourable crystalline orientation relative to the temperature gradient, giving rise to a faster formation of crystallographic facets. We applied in situ time-lapse diffraction contrast tomography on a snow sample with a density of 476 kg m−3 subject to a temperature gradient of 52 ∘Cm-1 at mean temperatures in the range between −4.1 and −2.1 ∘C for 3 d. The orientations of about 900 grains along with their microstructural evolution are followed over time. Faceted crystals appear during the evolution, and from the analysis of the material fluxes, we observe higher sublimation–deposition rates for grains with their c axis in the horizontal plane at the beginning of the metamorphism. This remains the case up to the end of the experiment for what concerns sublimation while the differences vanish for deposition. The latter observation is explained in terms of geometrical interactions between grains.

Active Sinking Particles: Sessile Suspension Feeders significantly alter the Flow and Transport to Sinking Aggregates

Sinking or sedimentation of biological aggregates plays a critical role in carbon sequestration in the ocean and in vertical material fluxes in waste-water treatment plants. In both these contexts, the sinking aggregates are "active", since they are hot-spots of biological life and are densely colonized by microorganisms including bacteria and sessile protists, some of which generate feeding currents. However, the effect of these feeding currents on the sinking rates, trajectories, and mass transfer to these "active sinking particles", has not previously been studied. Here we use a novel scale-free vertical-tracking microscope (a.k.a. Gravity Machine, Krishnamurthy et al. "Scale-free vertical tracking microscopy." Nature Methods (2020)) to follow model sinking aggregates (agar spheres) with attached protists (Vorticella convallaria), sinking over long distances while simultaneously measuring local flows. We find that activity generated by attached Vorticella cause substantial changes to the flow around aggregates in a dynamic manner and reshape mass transport boundary layers. Further, we find that activity-mediated local flows along with sinking significantly changes how aggregates interact with the water-column at larger scales by modifying the encounter and plume cross-sections and by inducing sustained aggregate rotations. In this way our work suggests an important role of biological activity in understanding the growth, degradation, composition and sinking speeds of aggregates with consequences for predicting vertical material fluxes in marine, freshwater and man-made environments.

A Monotonic Method of Split Particles

The problem of correct calculation of the motion of a multicomponent (multimaterial) medium is the most serious problem for Lagrangian–Eulerian and Eulerian techniques, especially in multicomponent cells in the vicinity of interfaces. There are two main approaches to solving the advection equation for a multicomponent medium. The first approach is based on the identification of interfaces and determining their position at each time step by the concentration field. In this case, the interface can be explicitly distinguished or reconstructed by the concentration field. The latter algorithm is the basis of widely used methods such as VOF. The second approach involves the use of the particle or marker method. In this case, the material fluxes of substances are determined by the particles with which certain masses of substances bind. Both approaches have their own advantages and drawbacks. The advantages of the particle method consist in the Lagrangian representation of particles and the possibility of” drawbacks. The main disadvantage of the particle method is the strong non-monotonicity of the solution caused by the discrete transfer of mass and mass-related quantities from cell to cell. This paper describes a particle method that is free of this drawback. Monotonization of the particle method is performed by spliting the particles so that the volume of matter flowing out of the cell corresponds to the volume calculated according to standard schemes of Lagrangian–Eulerian and Eulerian methods. In order not to generate an infinite chain of spliting, further split particles are re-united when certain conditions are met. The method is developed for modeling 2D and 3D gas-dynamic flows with accompanying processes, in which it is necessary to preserve the history of the process at Lagrangian points.

A hydraulic instability drives the cell death decision in the nematode germline

Oocytes are large cells that develop into an embryo upon fertilization1. As interconnected germ cells mature into oocytes, some of them grow—typically at the expense of others that undergo cell death2–4. We present evidence that in the nematode Caenorhabditis elegans, this cell-fate decision is mechanical and related to tissue hydraulics. An analysis of germ cell volumes and material fluxes identifies a hydraulic instability that amplifies volume differences and causes some germ cells to grow and others to shrink, a phenomenon that is related to the two-balloon instability5. Shrinking germ cells are extruded and they die, as we demonstrate by artificially reducing germ cell volumes via thermoviscous pumping6. Our work reveals a hydraulic symmetry-breaking transition central to the decision between life and death in the nematode germline.

Vegetation and Geomorphic Connectivity in Mountain Fluvial Systems

Rivers are complex biophysical systems, constantly adjusting to a suite of changing governing conditions, including vegetation cover within their basins. This review seeks to: (i) highlight the crucial role that vegetation’s influence on the efficiency of clastic material fluxes (geomorphic connectivity) plays in defining mountain fluvial landscape’s behavior; and (ii) identify key challenges which hinder progress in the understanding of this subject. To this end, a selective literature review is carried out to illustrate the pervasiveness of the plants’ effects on geomorphic fluxes within channel networks (longitudinal connectivity), as well as between channels and the broader landscape (lateral connectivity). Taken together, the reviewed evidence lends support to the thesis that vegetation-connectivity linkages play a central role in regulating geomorphic behavior of mountain fluvial systems. The manuscript is concluded by a brief discussion of the need for the integration of mechanistic research into the local feedbacks between plants and sediment fluxes with basin-scale research that considers emergent phenomena.

Orientation selective grain sublimation-deposition in snow under temperature gradient metamorphism observed with Diffraction Contrast Tomography

In this study on temperature gradient metamorphism in snow, we investigate the hypothesis that there exists a favorable crystalline orientation relative to the temperature gradient, giving rise to a faster formation of crystallographic facets. We applied in-situ time-lapse Diffraction Contrast Tomography on a snow sample with a density of 476 kg m−3 subject to a temperature gradient of 52 °C m−1 at mean temperatures in the range between −4.1 °C and −2.1 °C for three days. The orientations of about 900 grains along with their microstructural evolution are followed over time. Faceted crystals appear during the evolution and from the analysis of the material fluxes, we indeed observe higher sublimation-deposition rate for grains with their c-axis in the horizontal plane at the beginning of the metamorphism. This remains the case up to the end of the experiment for what concerns sublimation while the differences vanish for deposition. That latter observation is explained in terms of geometrical interactions between grains.

Recent Budget of Hydroclimatology and Hydrosedimentology of the Congo River in Central Africa

Although the Congo Basin is still one of the least studied river basins in the world, this paper attempts to provide a multidisciplinary but non-exhaustive synthesis on the general hydrology of the Congo River by highlighting some points of interest and some particular results obtained over a century of surveys and scientific studies. The Congo River is especially marked by its hydrological regularity only interrupted by the wet decade of 1960, which is its major anomaly over nearly 120 years of daily observations. Its interannual flow is 40,500 m3 s−1. This great flow regularity should not hide important spatial variations. As an example, we can cite the Ubangi basin, which is the most northern and the most affected by a reduction in flow, which has been a cause for concern since 1970 and constitutes a serious hindrance for river navigation. With regard to material fluxes, nearly 88 × 106 tonnes of material are exported annually from the Congo Basin to the Atlantic Ocean, composed of 33.6 × 106 tonnes of TSS, 38.1 × 106 tonnes of TDS and 16.2 × 106 tonnes of DOC. In this ancient flat basin, the absence of mountains chains and the extent of its coverage by dense rainforest explains that chemical weathering (10.6 t km−2 year−1 of TDS) slightly predominates physical erosion (9.3 t km−2 year−1 of TSS), followed by organic production (4.5 t km−2 year−1 of DOC). As the interannual mean discharges are similar, it can be assumed that these interannual averages of material fluxes, calculated over the longest period (2006–2017) of monthly monitoring of its sedimentology and bio-physical-chemistry, are therefore representative of the flow record available since 1902 (with the exception of the wet decade of 1960). Spatial heterogeneity within the Congo Basin has made it possible to establish an original hydrological classification of right bank tributaries, which takes into account vegetation cover and lithology to explain their hydrological regimes. Those of the Batéké plateau present a hydroclimatic paradox with hydrological regimes that are among the most stable on the planet, but also with some of the most pristine waters as a result of the intense drainage of an immense sandy-sandstone aquifer. This aquifer contributes to the regularity of the Congo River flows, as does the buffer role of the mysterious “Cuvette Centrale”. As the study of this last one sector can only be done indirectly, this paper presents its first hydrological regime calculated by inter-gauging station water balance. Without neglecting the indispensable in situ work, the contributions of remote sensing and numerical modelling should be increasingly used to try to circumvent the dramatic lack of field data that persists in this basin.