f(R) theories

Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in Relativity, Published version, Comments are welcom

Modularity and predicted functions of the global sponge-microbiome network

Defining the organisation of species interaction networks and unveiling the processes behind their assembly is fundamental to understanding patterns of biodiversity, community stability and ecosystem functioning. Marine sponges host complex communities of microorganisms that contribute to their health and survival, yet the mechanisms behind microbiome assembly are largely unknown. We present the global marine sponge-microbiome network and reveal a modular organisation in both community structure and function. Modules are linked by a few sponge species that share microbes with other species around the world. Further, we provide evidence that abiotic factors influence the structuring of the sponge microbiome when considering all microbes present, but biotic interactions drive the assembly of more intimately associated 'core' microorganisms. These findings suggest that both ecological and evolutionary processes are at play in host-microbe network assembly. We expect mechanisms behind microbiome assembly to be consistent across multicellular hosts throughout the tree of life

Metagenomic binning of a marine sponge microbiome reveals unity in defense but metabolic specialization

Marine sponges are ancient metazoans that are populated by distinct and highly diverse microbial communities. In order to obtain deeper insights into the functional gene repertoire of the Mediterranean sponge Aplysina aerophoba, we combined Illumina short-read and PacBio long-read sequencing followed by un-targeted metagenomic binning. We identified a total of 37 high-quality bins representing 11 bacterial phyla and two candidate phyla. Statistical comparison of symbiont genomes with selected reference genomes revealed a significant enrichment of genes related to bacterial defense (restriction-modification systems, toxin-antitoxin systems) as well as genes involved in host colonization and extracellular matrix utilization in sponge symbionts. A within-symbionts genome comparison revealed a nutritional specialization of at least two symbiont guilds, where one appears to metabolize carnitine and the other sulfated polysaccharides, both of which are abundant molecules in the sponge extracellular matrix. A third guild of symbionts may be viewed as nutritional generalists that perform largely the same metabolic pathways but lack such extraordinary numbers of the relevant genes. This study characterizes the genomic repertoire of sponge symbionts at an unprecedented resolution and it provides greater insights into the molecular mechanisms underlying microbial-sponge symbiosis

Complex ecological associations: Evidence for competition and facilitation in a sponge-algal interaction

Over the past few decades, Caribbean coral reefs have undergone a phase shift from coral-dominated to algal dominated communities, due to several factors including increased anthropogenic nutrients. With the decline in coral cover, sponges have also become more dominant members of Caribbean coral reef communities. Increased algal and sponge dominance on Caribbean reefs has led to an increase in the frequency of interaction between these two groups. This study used a factorial design to assess the independent and interactive effects of contact and elevated nutrient levels on two common members of these communities, the sponge Aplysina cauliformis, and the macroalga Microdictyon marinum. Algal contact had a significant negative physiological effect on A. cauliformis, affecting both the host sponge and its cyanobacterial symbionts. While elevated nutrient levels had some positive effects on the sponge photosymbionts, this only occurred in the absence of algal contact or a shading/abrasion control, and elevated nutrient levels had a negative effect on the sponge holobiont. In contrast, M. marinum responded positively to experimentally enhanced nutrient levels and to sponge contact under ambient nutrient regimes, but was not affected by sponge contact under elevated nutrient concentrations. Stable isotope enrichment experiments showed that the alga’s positive response to sponge contact was associated with nitrogen transfer from the sponge over the course of the experiment. Thus, while A. cauliformis facilitates increased productivity in M. marinum, algal contact competitively inhibits sponge condition

Diversity in sponge holobiont functioning: the role of microbial abundance, host sponge identity, and geography

Oral presentation - Session #:12: Abstract ID: 29719Although marine sponges are frequently placed into functional categories based on their overall microbial abundance (High and Low Microbial Abundance [HMA and LMA]), recent research has shown that these communities and overall interactions are highly variable across individual species. To elucidate the role that overall microbial abundance, photosymbiont abundance, host sponge identity, and geography play in holobiont C and N cycling, we used a stable isotope approach in conjunction with multivariate and ellipse-based analyses. This allowed us to evaluate the core isotopic niche of a species or group and investigate factors driving the placement of individuals within isotopic space. Up to 14 sponge species were collected from three Caribbean sites (Panama, Honduras, and Belize), the Florida Keys, and a temperate reef off Georgia, USA. Some variation was attributed to geographic location, but this effect was greatly minimized when data from the temperate reef were excluded. Within individual sites, HMA and LMA groups filled unique niche space. However, host identity accounted for at least 70 percent of the variation in isotope values within a site, while photosymbiont and overall microbial abundance accounted for a much lower proportion of this variation. Together these data suggest that microbial symbionts may allow sponges to expand into novel niche space on tropical reefs, but species placement within isotopic space is relatively stable and is likely structured more by host-specific microbial community composition than by overall microbial or photosymbiont abundance