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1. Spider webs, stable isotopes and molecular gut content analysis: Multiple lines of evidence support trophic niche differentiation in a community of Hawaiian spiders

   https://doi.org/10.1111/1365-2435.13361  

   Kennedy, Susan ; Lim, Jun Ying ; Clavel, Joanne ; Krehenwinkel, Henrik ; Gillespie, Rosemary G. ; Godoy, ed., Oscar ( June 2019 , Functional Ecology)

   Adaptive radiations are typically characterized by niche partitioning among their constituent species. Trophic niche partitioning is particularly important in predatory animals, which rely on limited food resources for survival.

   We test for trophic niche partitioning in an adaptive radiation of HawaiianTetragnathaspiders, which have diversified in situ on the Hawaiian Islands. We focus on a community of nine species belonging to two different clades, one web‐building and the other actively hunting, which co‐occur in wet forest on East Maui. We hypothesize that trophic niches differ significantly both: (a) among species within a clade, indicating food resource partitioning, and (b) between the two clades, corresponding to their differences in foraging strategy.

   To assess niches of the spider species, we measure: (a) web architecture, the structure of the hunting tool, and (b) site choice, the physical placement of the web in the habitat. We then test whether differences in these parameters translate into meaningful differences in trophic niche by measuring (c) stable isotope signatures of carbon and nitrogen in the spiders’ tissues, and (d) gut content of spiders based on metabarcoding data.

   We find significant interspecific differences in web architecture and site choice. Importantly, these differences are reflected in stable isotope signatures among the five web‐building species, as well as significant isotopic differences between web‐builders and active hunters. Gut content data also show interspecific and inter‐clade differences. Pairwise overlaps of web architecture between species are positively correlated with overlaps of isotopic signature.

   Our results reveal trophic niche partitioning among species within each clade, as well as between the web‐building and actively hunting clades. Based on the correlation between web architecture and stable isotopes, it appears that the isotopic signatures of spiders’ tissues are influenced by architectural differences among their webs. Our findings indicate an important link between web structure, microhabitat preference and diet in the HawaiianTetragnatha.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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2. Nitrogen stable isotope turnover and discrimination in lizards

   https://doi.org/10.1002/rcm.9030  

   Warne, Robin W. ; Wolf, Blair O. ( January 2021 , Rapid Communications in Mass Spectrometry)

   Nitrogen stable isotope ratio (δ¹⁵N) processes are not well described in reptiles, which limits reliable inference of trophic and nutrient dynamics. In this study we detailed δ¹⁵N turnover and discrimination (Δ¹⁵N) in diverse tissues of two lizard species, and compared these results with previously published carbon data (δ¹³C) to inform estimates of reptilian foraging ecology and nutrient physiology.

   We quantified¹⁵N incorporation and discrimination dynamics over 360 days in blood fractions, skin, muscle, and liver ofSceloporus undulatusandCrotaphytus collaristhat differed in body mass. Tissue samples were analyzed on a continuous flow isotope ratio mass spectrometer.

   Δ¹⁵N for plasma and red blood cells (RBCs) ranged between +2.7 and +3.5‰; however, skin, muscle, and liver did not equilibrate, hindering estimates for these somatic tissues.¹⁵N turnover in plasma and RBCs ranged from 20.7 ± 4 to 303 ± 166 days among both species. Comparison with previously published δ¹³C results for these same samples showed that¹⁵N and¹³C incorporation patterns were uncoupled, especially during winter when hibernation physiology could have played a role.

   Our results provide estimates of¹⁵N turnover rates and discrimination values that are essential to using and interpreting isotopes in studies of diet reconstruction, nutrient allocation, and trophic characterization in reptiles. These results also suggest that somatic tissues can be unreliable, while life history shifts in nutrient routing and metabolism potentially cause¹⁵N and¹³C dynamics to be decoupled.

    

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3. Head shape predicts isotopic diet in anoles and day geckos

   https://doi.org/10.1111/1365-2435.14331  

   Figueira, Tyler J. ; Kennedy‐Gold, Stevie ; Piantoni, Carla ; Screen, Robyn M. ; Wright, Amber N. ( April 2023 , Functional Ecology)

   Trophic morphology affects resource acquisition; therefore, species differences in such traits may be informative for inferring resource use overlap and potential species interactions.

   In lizards, head size and shape determine the size and hardness of prey that can be consumed. Lizards with large differences in head morphology are expected to overlap less in prey use than lizards with more similar traits.

   Stable isotopes are increasingly being used to describe diet, yet how traditional functional traits affect isotopic diet is often not clear a priori.

   We measured head size, head shape, 𝛿¹⁵N, and 𝛿¹³C under controlled resource availability in an enclosure experiment using introduced lizards in Hawaiʻi to test whether functional traits predict isotopic diet.

   Brown anolesAnolis sagreihad the tallest and narrowest heads, the highest values of 𝛿¹³C, and the lowest values of 𝛿¹⁵N. Gold dust day geckosPhelsuma laticaudahad the shortest and widest heads, the lowest values of 𝛿¹³C, and the highest values of 𝛿¹⁵N. Green anolesAnolis carolinensiswere intermediate in both diet and morphology.

   As a result of isotopic diet overlap, green anoles have reduced competitor‐free resource space in the presence of both of the other lizard species.

   Head shape was the best predictor of diet and the only trait that explained variation within as well as among species. Head size was sexually dimorphic, and therefore the weaker diet correlations with this trait may be explained by sexual selection.

   Breadth in morphospace did not correlate with isotopic diet breadth, nor did the amount of overlap in morphospace predict the amount of overlap in isotopic diet space.

   While lizards were able to locally depress prey in experimental enclosures, no shifts in diet were detected in response to the presence of heterospecifics.

   The generality of head shape in predicting isotopic diet, and whether it does so independent of habitat use, warrants additional study. Head shape provides a potentially fruitful avenue for trait‐based approaches to studying ecology and evolution in lizards.

   Read the freePlain Language Summaryfor this article on the Journal blog.

    

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4. Defining the Sphagnum Core Microbiome across the North American Continent Reveals a Central Role for Diazotrophic Methanotrophs in the Nitrogen and Carbon Cycles of Boreal Peatland Ecosystems

   https://doi.org/10.1128/mbio.03714-21  

   Kolton, Max ; Weston, David J. ; Mayali, Xavier ; Weber, Peter K. ; McFarlane, Karis J. ; Pett-Ridge, Jennifer ; Somoza, Mark M. ; Lietard, Jory ; Glass, Jennifer B. ; Lilleskov, Erik A. ; et al ( February 2022 , mBio)

   Martiny, Jennifer B. (Ed.)

   ABSTRACT Peat mosses of the genus Sphagnum are ecosystem engineers that frequently predominate over photosynthetic production in boreal peatlands. Sphagnum spp. host diverse microbial communities capable of nitrogen fixation (diazotrophy) and methane oxidation (methanotrophy), thereby potentially supporting plant growth under severely nutrient-limited conditions. Moreover, diazotrophic methanotrophs represent a possible “missing link” between the carbon and nitrogen cycles, but the functional contributions of the Sphagnum -associated microbiome remain in question. A combination of metagenomics, metatranscriptomics, and dual-isotope incorporation assays was applied to investigate Sphagnum microbiome community composition across the North American continent and provide empirical evidence for diazotrophic methanotrophy in Sphagnum -dominated ecosystems. Remarkably consistent prokaryotic communities were detected in over 250 Sphagnum SSU rRNA libraries from peatlands across the United States (5 states, 17 bog/fen sites, 18 Sphagnum species), with 12 genera of the core microbiome comprising 60% of the relative microbial abundance. Additionally, nitrogenase ( nifH ) and SSU rRNA gene amplicon analysis revealed that nitrogen-fixing populations made up nearly 15% of the prokaryotic communities, predominated by Nostocales cyanobacteria and Rhizobiales methanotrophs. While cyanobacteria comprised the vast majority (>95%) of diazotrophs detected in amplicon and metagenome analyses, obligate methanotrophs of the genus Methyloferula (order Rhizobiales ) accounted for one-quarter of transcribed nifH genes. Furthermore, in dual-isotope tracer experiments, members of the Rhizobiales showed substantial incorporation of 13 CH 4 and 15 N 2 isotopes into their rRNA. Our study characterizes the core Sphagnum microbiome across large spatial scales and indicates that diazotrophic methanotrophs, here defined as obligate methanotrophs of the rare biosphere ( Methyloferula spp. of the Rhizobiales ) that also carry out diazotrophy, play a keystone role in coupling of the carbon and nitrogen cycles in nutrient-poor peatlands. IMPORTANCE Nitrogen availability frequently limits photosynthetic production in Sphagnum moss-dominated high-latitude peatlands, which are crucial carbon-sequestering ecosystems at risk to climate change effects. It has been previously suggested that microbial methane-fueled fixation of atmospheric nitrogen (N 2 ) may occur in these ecosystems, but this process and the organisms involved are largely uncharacterized. A combination of omics (DNA and RNA characterization) and dual-isotope incorporation approaches illuminated the functional diversity of Sphagnum -associated microbiomes and defined 12 bacterial genera in its core microbiome at the continental scale. Moreover, obligate diazotrophic methanotrophs showed high nitrogen fixation gene expression levels and incorporated a substantial amount of atmospheric nitrogen and methane-driven carbon into their biomass. Thus, these results point to a central role for members of the rare biosphere in Sphagnum microbiomes as keystone species that couple nitrogen fixation to methane oxidation in nutrient-poor peatlands. 

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5. Proteomic profile of bone “collagen” extracted for stable isotopes: Implications for bulk and single amino acid analyses

   https://doi.org/10.1002/rcm.9025  

   Cleland, Timothy P. ; Sarancha, Julianne J. ; France, Christine A. M. ( January 2021 , Rapid Communications in Mass Spectrometry)

   Protein studies in archaeology and paleontology have been dominated by stable isotope studies to understand diet and trophic levels, but recent applications of proteomic techniques have resulted in a more complete understanding of protein diagenesis than stable isotopes alone. In stable isotope analyses, samples are retained or discarded based on their properties. Proteomics can directly determine what proteins are present within the sample and may be able to allow previously discarded samples to be analyzed.

   Protein samples that had been previously analyzed for stable isotopes, including those with marginal and poor sample quality, were characterized by liquid chromatography/mass spectrometry using an LTQ Orbitrap Velos mass spectrometer after separation on a Dionex Ultimate 3000 LC system. Data were analyzed using MetaMorpheus and custom R scripts.

   We found a variety of proteins in addition to collagen, although collagen I was found in the majority of the samples (most samples >80%). We also found a positive correlation between total deamidation and wt% N, suggesting that deamidation may impact the overall nitrogen signal in bulk analyses. The amino acid profiles of samples, including those of marginal or poor stable isotope quality, reflect the expected collagen I percentages, allowing their use in single amino acid stable isotope analyses.

   All the samples regardless of quality were found to have high concentrations of collagen I, making interpretations of dietary routing based on collagen I reasonably valid. The amino acid profiles on the marginal and poor samples reflect an expected collagen I profile and allow these samples to be recovered for single amino acid analyses.

    

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