Annual Thermal Stress Increases a Soft Coral’s Susceptibility to Bleaching

© 2019 by the authors. Bioassay-guided fractionation of an EtOAc extract of the broth of the endophytic fungus Nemania sp. UM10M (Xylariaceae) isolated from a diseased Torreya taxifolia leaf afforded three known cytochalasins, 19,20-epoxycytochalasins C (1) and D (2), and 18-deoxy-19,20-epoxy-cytochalasin C (3). All three compounds showed potent in vitro antiplasmodial activity and phytotoxicity with no cytotoxicity to Vero cells. These compounds exhibited moderate to weak cytotoxicity to some of the cell lines of a panel of solid tumor (SK-MEL, KB, BT-549, and SK-OV-3) and kidney epithelial cells (LLC-PK11). Evaluation of in vivo antimalarial activity of 19,20-epoxycytochalasin C (1) in a mouse model at 100 mg/kg dose showed that this compound had weak suppressive antiplasmodial activity and was toxic to animals

Photoacclimatization by the coral Montastraea cavernosa in the mesophotic zone: light, food, and genetics

Most studies on coral reefs have focused on shallow reef (<30 m) systems due to the technical limitations of conducting scientific diving deeper than 30 m. Compared to their shallow-water counterparts, these mesophotic coral reefs (30–150 m) are understudied, which has slowed our broader understanding of the biodiversity, ecology, and connectivity of shallow and deep coral reef communities. We know that the light environment is an important component of the productivity, physiology, and ecology of corals, and it restricts the distribution of most species of coral to depths of 60 m or less. In the Bahamas, the coral Montastraea cavernosa has a wide depth distribution, and it is one of the most numerous corals at mesophotic depths. Using a range of optical, physiological, and biochemical approaches, the relative dependence on autotrophy vs. heterotrophy was assessed for this coral from 3 to 91 m. These measurements show that the quantum yield of PSII fluorescence increases significantly with depth for M. cavernosa while gross primary productivity decreases with depth. Both morphological and physiological photoacclimatization occurs to a depth of 91 m, and stable isotope data of the host tissues, symbionts, and skeleton reveal a marked decrease in productivity and a sharp transition to heterotrophy between 45 and 61 m. Below these depths, significant changes in the genetic composition of the zooxanthellae community, including genotypes not previously observed, occur and suggest that there is strong selection for zooxanthellae that are suited for survival in the light-limited environment where mesophotic M. cavernosa are occurring

Microbiome diversity and metabolic capacity determines the trophic ecology of the holobiont in Caribbean sponges

Sponges are increasingly recognized as an ecologically important taxon on coral reefs, representing significant biomass and biodiversity where sponges have replaced scleractinian corals. Most sponge species can be divided into two symbiotic states based on symbiont community structure and abundance (i.e., the microbiome), and are characterized as high microbial abundance (HMA) or low microbial abundance (LMA) sponges. Across the Caribbean, sponge species of the HMA or LMA symbiotic states differ in metabolic capacity, as well as their trophic ecology. A metagenetic analysis of symbiont 16 S rRNA and metagenomes showed that HMA sponge microbiomes are more functionally diverse than LMA microbiomes, offer greater metabolic functional capacity and redundancy, and encode for the biosynthesis of secondary metabolites. Stable isotope analyses showed that HMA and LMA sponges primarily consume dissolved organic matter (DOM) derived from external autotrophic sources, or live particulate organic matter (POM) in the form of bacterioplankton, respectively, resulting in a low degree of resource competition between these symbiont states. As many coral reefs have undergone phase shifts from coral- to macroalgal-dominated reefs, the role of DOM, and the potential for future declines in POM due to decreased picoplankton productivity, may result in an increased abundance of chemically defended HMA sponges on tropical coral reefs

An extended view of the Pisces Overdensity from the SCUSS survey

SCUSS is a u-band photometric survey covering about 4000 square degree of the South Galactic Cap, reaching depths of up to 23 mag. By extending around 1.5 mag deeper than SDSS single-epoch u data, SCUSS is able to probe much a larger volume of the outer halo, i.e. with SCUSS data blue horizontal branch (BHB) stars can trace the outer halo of the Milky Way as far as 100-150 kpc. Utilizing this advantage we combine SCUSS u band with SDSS DR9 gri photometric bands to identify BHB stars and explore halo substructures. We confirm the existence of the Pisces overdensity, which is a structure in the outer halo (at around 80 kpc) that was discovered using RR Lyrae stars. For the first time we are able to determine its spatial extent, finding that it appears to be part of a stream with a clear distance gradient. The stream, which is ~5 degrees wide and stretches along ~25 degrees, consists of 20-30 BHBs with a total significance of around 6sigma over the background. Assuming we have detected the entire stream and that the progenitor has fully disrupted, then the number of BHBs suggests the original system was similar to smaller classical or a larger ultra-faint dwarf galaxy. On the other hand, if the progenitor still exists, it can be hunted for by reconstructing its orbit from the distance gradient of the stream. This new picture of the Pisces overdensity sheds new light on the origin of this intriguing system.Comment: 8 pages, 4 figures, accepted by Ap

Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456

The principal aim of this project is to determine the structural parameters of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our work forms part of an ongoing programme to constrain the internal characteristics of hot B subdwarfs with the long-term goal of differentiating between the various formation scenarios proposed for these objects. First comparisons of asteroseismic values with evolutionary theory look promising, however it is clear that more targets are needed for meaningful statistics to be derived. The observational pulsation periods of PG 0911+456 were extracted from rapid time-series photometry using standard Fourier analysis techniques. Supplemented by spectroscopic estimates of the star's mean atmospheric parameters, they were used as a basis for the "forward modelling" approach in asteroseismology. The latter culminates in the identification of one or more "optimal" models that can accurately reproduce the observed period spectrum. This naturally leads to an identification of the oscillations detected in terms of degree l and radial order k, and infers the structural parameters of the target. From the photometry it was possible to extract 7 independent pulsation periods in the 150-200 s range with amplitudes between 0.05 and 0.8 % of the star's mean brightness. An asteroseismic search of parameter space identified several models that matched the observed properties of PG 0911+456 well, one of which was isolated as the "optimal" model on the basis of spectroscopic and mode identification considerations. All the observed pulsations are identified with low-order acoustic modes with degree indices l=0,1,2 and 4, and match the computed periods with a dispersion of only ~0.26 %.Comment: accepted for publication in A&A, 14 pages, 13 figure

Star Formation at z~6: i-dropouts in the ACS GTO fields

Using an i-z dropout criterion, we determine the space density of z~6 galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of 23 objects are found over 46 arcmin^2, or ~0.5 objects/arcmin^2 down to z~27.3 (6 sigma; all AB mag) (including one probable z~6 AGN). Combining deep ISAAC data for our RDCS1252-2927 field (J~25.7 and Ks~25.0 (5 sigma)) and NICMOS data for the HDF North (JH~27.3 (5 sigma)), we verify that these dropouts have flat spectral slopes. i-dropouts in our sample range in luminosity from ~1.5 L* (z~25.6) to ~0.3 L* (z~27.3) with the exception of one very bright candidate at z~24.2. The half-light radii vary from 0.09" to 0.29", or 0.5 kpc to 1.7 kpc. We derive the z~6 rest-frame UV luminosity density using three different procedures, each utilizing simulations based on a CDF South V dropout sample. First, we compare our findings with a no-evolution projection of this V-dropout sample. We find 23+/-25% more i-dropouts than we predict. Adopting previous results to z~5, this works out to a 20+/-29% drop in the luminosity density from z~3 to z~6. Second, we use these same V-dropout simulations to derive a selection function for our i-dropout sample and compute the UV-luminosity density (7.2+/-2.5 x 10^25 ergs/s/Hz/Mpc^3 down to z~27). We find a 39+/-21% drop over the same redshift range. This is our preferred value and suggests a star formation rate of 0.0090+/-0.0031 M_sol/yr/Mpc^3 to z~27, or ~0.036+/- 0.012 M_sol/yr/Mpc^3 extrapolating the LF to the faint limit. Third, we follow a very similar procedure, but assume no incompleteness, finding a luminosity density which is ~2-3X lower. This final estimate constitutes a lower limit. All three estimates are within the canonical range of luminosity densities necessary for reionization of the universe at this epoch. (abridged)Comment: 36 pages, 13 figures, 2 tables, accepted for publication in ApJ, postscript version with high-resolution figures can be downloaded at http://www.ucolick.org/~bouwens/idropout.p

Multi-agent Hierarchical Reinforcement Learning with Dynamic Termination

In a multi-agent system, an agent's optimal policy will typically depend on the policies chosen by others. Therefore, a key issue in multi-agent systems research is that of predicting the behaviours of others, and responding promptly to changes in such behaviours. One obvious possibility is for each agent to broadcast their current intention, for example, the currently executed option in a hierarchical reinforcement learning framework. However, this approach results in inflexibility of agents if options have an extended duration and are dynamic. While adjusting the executed option at each step improves flexibility from a single-agent perspective, frequent changes in options can induce inconsistency between an agent's actual behaviour and its broadcast intention. In order to balance flexibility and predictability, we propose a dynamic termination Bellman equation that allows the agents to flexibly terminate their options. We evaluate our model empirically on a set of multi-agent pursuit and taxi tasks, and show that our agents learn to adapt flexibly across scenarios that require different termination behaviours.Comment: PRICAI 201

Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe.Comment: 6 pages with embedded figures, ApJ in pres

Discovery of Globular Clusters in the Proto-Spiral NGC2915: Implications for Hierarchical Galaxy Evolution

We have discovered three globular clusters beyond the Holmberg radius in Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark matter dominated blue compact dwarf galaxy NGC2915. The clusters, all of which start to resolve into stars, have M_{V606} = -8.9 to -9.8 mag, significantly brighter than the peak of the luminosity function of Milky Way globular clusters. Their colors suggest a metallicity [Fe/H] ~ -1.9 dex, typical of metal-poor Galactic globular clusters. The specific frequency of clusters is at a minimum normal, compared to spiral galaxies. However, since only a small portion of the system has been surveyed it is more likely that the luminosity and mass normalized cluster content is higher, like that seen in elliptical galaxies and galaxy clusters. This suggests that NGC2915 resembles a key phase in the early hierarchical assembly of galaxies - the epoch when much of the old stellar population has formed, but little of the stellar disk. Depending on the subsequent interaction history, such systems could go on to build-up larger elliptical galaxies, evolve into normal spirals, or in rare circumstances remain suspended in their development to become systems like NGC2915.Comment: ApJ Letters accepted; 6 pages, 2 figures, 3 table