Microelectrode arrays and the use of PEG-functionalized diblock copolymer coatings

PEG-modified diblock copolymer surfaces have been examined for their compatibility with microelectrode array based analytical methods. The use of PEG-modified polymer surfaces on the arrays was initially problematic because the redox couples used in the experiments were adsorbed by the polymer. This led the current measured by cyclic voltammetry for the redox couple to be unstable and increase with time. However, two key findings allow the experiments to be successful. First, after multiple cyclic voltammograms the current associated with the redox couple does stabilize so that a good baseline current can be established. Second, the rate at which the current stabilizes is consistent every time a particular coated array is used. Hence, multiple analytical experiments can be conducted on an array coated with a PEG-modified diblock copolymer and the data obtained is comparable as long as the data for each experiment is collected at a consistent time point

Intraguild predation enables coexistence of competing phytoplankton in a well-mixed water column

Author Posting. © Ecological Society of America, 2019. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecology, (2019): e02874, doi: 10.1002/ecy.2874.Resource competition theory predicts that when two species compete for a single, finite resource, the better competitor should exclude the other. However, in some cases, weaker competitors can persist through intraguild predation, that is, by eating their stronger competitor. Mixotrophs, species that meet their carbon demand by combining photosynthesis and phagotrophic heterotrophy, may function as intraguild predators when they consume the phototrophs with which they compete for light. Thus, theory predicts that mixotrophy may allow for coexistence of two species on a single limiting resource. We tested this prediction by developing a new mathematical model for a unicellular mixotroph and phytoplankter that compete for light, and comparing the model's predictions with a laboratory experimental system. We find that, like other intraguild predators, mixotrophs can persist when an ecosystem is sufficiently productive (i.e., the supply of the limiting resource, light, is relatively high), or when species interactions are strong (i.e., attack rates and conversion efficiencies are high). Both our mathematical and laboratory models show that, depending upon the environment and species traits, a variety of equilibrium outcomes, ranging from competitive exclusion to coexistence, are possible.HVM and MGN designed the model. HVM and MDJ designed the experimental test system. HVM performed the model analysis, conducted the experiments, and analyzed the data. All authors wrote the paper. We thank Susanne Wilken for generously providing axenic CCMP 2951 and 1393 cultures for our use. R. Germain, S. Louca, G. Owens, N. Sharp, P. Thompson, and J. Yoder provided valuable feedback on figure design. We also thank J. Bronstein, S. Diehl, J. Huisman, C. Klausmeier, and four anonymous reviewers for comments on earlier versions of this manuscript. HVM was supported by a United States National Science Foundation Postdoctoral Research Fellowship in Biology (Grant DBI‐1401332) and a University of British Columbia Biodiversity Research Centre Postdoctoral Fellowship. This material is based upon work supported by the National Science Foundation under Grants OCE‐1655686 and OCE‐1436169, by a grant from the Simons Foundation/SFARI (561126, HMS), and by the Woods Hole Oceanographic Institution's Investment in Science Program. Research was also sponsored by the U.S. Army Research Office and was accomplished under Cooperative Agreement Number W911NF‐19‐2‐0026 for the Institute for Collaborative Biotechnologies

Acquired phototrophy stabilizes coexistence and shapes intrinsic dynamics of an intraguild predator and its prey

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Ecology Letters 19 (2016): 393-402, doi:10.1111/ele.12572.In marine ecosystems, acquired phototrophs|organisms that obtain their photo- synthetic ability by hosting endosymbionts or stealing plastids from their prey|are omnipresent. Such taxa function as intraguild predators yet depend on their prey to periodically obtain chloroplasts. We present new theory for the effects of acquired phototrophy on community dynamics by analyzing a mathematical model of this predator-prey interaction and experimentally verifying its predictions with a lab- oratory model system. We show that acquired phototrophy stabilizes coexistence, but that the nature of this coexistence exhibits a `paradox of enrichment:' as light increases, the coexistence between the acquired phototroph and its prey transitions from a stable equilibrium to boom-bust cycles whose amplitude increases with light availability. In contrast, heterotrophs and mixotrophic acquired phototrophs (that obtain <30% of their carbon from photosynthesis) do not exhibit such cycles. This prediction matches eld observations, in which only strict (>95% of carbon from photosynthesis) acquired phototrophs form blooms.HVM is supported by a United States National Science Foundation Postdoctoral Research Fellowship in Biology (Grant No. DBI-1401332). EP is supported by the Academy of Finland through research grant 276268. MDJ acknowledges NSF Grant No. IOS-1354773. MGN acknowledges support provided by the Independent Research and Development Program at the Woods Hole Oceanographic Institution.2017-02-0

Vortex ring refraction at large Froude numbers

We have experimentally studied the impact of an initially planar axisymmetric vortex ring, incident at an oblique angle, upon a gravity-induced interface separating two fluids of differing densities. After impact, the vortex ring was found to exhibit a variety of subsequent trajectories, which we organize according to both the incidence angle, $\theta_i$, and the interface strength, defined as the ratio of the Atwood and Froude numbers, $A/F$. For grazing incidence angles ($\theta_i \gtrsim 70$ deg.) vortices either penetrate or reflect from the interface, depending on whether the interface is weak or strong. In some cases, reflected vortices execute damped oscillations before finally disintegrating. For smaller incidence angles ($\theta_i \lesssim 70$ deg.) vortices penetrate the interface. When there is a strong interface, these vortices are observed to curve back up toward the interface. When there is a weak interface, these vortices are observed to refract downward, away from the interface. The critical interface strength below which vortex ring refraction is observed is given by $\log_{10}{(A/F)}= -2.38 \pm 0.05$.Comment: 26 pages, 11 figures; Submitted to Physical Review

Light-dependent grazing can drive formation and deepening of deep chlorophyll maxima

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Moeller, H. V., Laufkötter, C., Sweeney, E. M., & Johnson, M. D. Light-dependent grazing can drive formation and deepening of deep chlorophyll maxima. Nature Communications, 10(1), (2019):1978, doi:10.1038/s41467-019-09591-2.Deep Chlorophyll Maxima (DCMs) are subsurface peaks in chlorophyll-a concentration that may coincide with peaks in phytoplankton abundance and primary productivity. Work on the mechanisms underlying DCM formation has historically focused on phytoplankton physiology (e.g., photoacclimation) and behavior (e.g., taxis). While these mechanisms can drive DCM formation, they do not account for top-down controls such as predation by grazers. Here, we propose a new mechanism for DCM formation: Light-dependent grazing by microzooplankton reduces phytoplankton biomass near the surface but allows accumulation at depth. Using mathematical models informed by grazing studies, we demonstrate that light-dependent grazing is sufficient to drive DCM formation. Further, when acting in concert with other mechanisms, light-dependent grazing deepens the DCM, improving the fit of a global model with observational data. Our findings thus reveal another mechanism by which microzooplankton may regulate primary production, and impact our understanding of biogeochemical cycling at and above the DCM.We thank the Sea Education Association and the students and crew of SEA Cruise S272 for collecting and sharing CTD cast data from the South Pacific. We also thank M. Lepori-Bui for assistance in assembling grazing data, A. Mignot for sharing global DCM estimates, J.G. John for providing the COBALT control simulations, E.B. Olson, M.G. Neubert, C.A. Stock, and J.P. Dunne for advice on model formulation, and B.E. Harden for valuable discussion. We thank members of the UCSB EEMB Department for helpful feedback on earlier versions of this manuscript. H.V.M. gratefully acknowledges an NSF Postdoctoral Fellowship (DBI-1401332) and a UBC Biodiversity Center Postdoctoral Fellowship

The Influence Of Particle Size And Frictional/Cohesional Shear Strength Components On UK Salt Marsh Substrate Stability

Salt marshes and tidal flats contribute valuable ecosystem services, by providing habitats, storing pollutants and reducing flood and erosion risk in the coastal hinterland. However, salt marsh areal extent is decreasing both globally and regionally (e.g. in Northwest Europe). While we know that salt marshes are retreating, this could be occurring due to biological, geochemical and geotechnical properties of the marsh and tidal flat, and/or due to changes in hydrodynamic forcing. Until now, very few studies have assessed how substrate geotechnical properties influence both the erosion processes and the erodibility of the marsh edge and tidal flat surface. Here, we compare frictional and cohesional strength components at two hydrodynamically-similar but sedimentologically-different salt marshes and tidal flats in the UK. As such, we assess how sediment composition and behavior may influence marsh resistance to hydrodynamic forcing.This work was funded by a NERC PhD studentship (LCAG/329; 2016-2020), and a Collaborative Award in Science and Engineering with the British Geological Survey (LCAG/352)