Host Symbiont Research Articles

Evolution: Environmental conditions determine how Wolbachia interacts with its host

Wolbachia is a genus of globally distributed endosymbiotic bacteria found in thousands of species of arthropods and nematodes. One strain, wMel, has spread globally but varies dramatically in frequency. In a new paper, Hague etal. discovered that the interaction of host genotype, symbiont genotype, and environment explains natural wMel variation.

Roles of Bacterial Symbionts in Transmission of Plant Virus by Hemipteran Vectors

The majority of plant viruses are transmitted by hemipteran insects. Bacterial symbionts in hemipteran hosts have a significant impact on the host life, physiology and ecology. Recently, the involvement of bacterial symbionts in hemipteran vector-virus and vector-plant interactions has been documented. Thus, the exploitation and manipulation of bacterial symbionts have great potential for plant viral disease control. Herein, we review the studies performed on the impact of symbiotic bacteria on plant virus transmission, including insect-bacterial symbiont associations, the role of these bacterial symbionts in viral acquisition, stability and release during viral circulation in insect bodies, and in viral vertical transmission. Besides, we prospect further studies aimed to understand tripartite interactions of the virus-symbiotic microorganisms-insect vector.

Decreased Photosynthetic Efficiency in Response to Site Translocation and Elevated Temperature Is Mitigated with LPS Exposure in Porites astreoides Symbionts

Coral reefs have been detrimentally impacted causing health issues due to elevated ocean temperatures as a result of increased greenhouse gases. Extreme temperatures have also exacerbated coral diseases in tropical reef environments. Numerous studies have outlined the impacts of thermal stress and disease on coral organisms, as well as understanding the influence of site-based characteristics on coral physiology. However, few have discussed the interaction of all three. Laboratory out-planting restoration projects have been of importance throughout impacted areas such as the Caribbean and southern Florida in order to increase coral cover in these areas. This study analyzes photosynthetic efficiency of Porites astreoides from the lower Florida Keys after a two-year reciprocal transplant study at inshore (Birthday reef) and offshore (Acer24 reef) sites to understand acclimation capacity of this species. Laboratory experiments subjected these colonies to one of three treatments: control conditions, increases in temperature, and increases in temperature plus exposure to an immune stimulant (lipopolysaccharide (LPS)) to determine their influence on photosynthetic efficiency and how stress events impact these measurements. In addition, this study is a continuation of previous studies from this group. Here, we aim to understand if these results are static or if an acclimation capacity could be found. Overall, we observed site-specific influences from the Acer24 reef site, which had significant decreases in photosynthetic efficiencies in 32 °C treatments compared to Birthday reef colonies. We suggest that high irradiance and lack of an annual recovery period from the Acer24 site exposes these colonies to significant photoinhibition. In addition, we observed significant increases in photosynthetic efficiencies from LPS exposure. We suggest host-derived antioxidants can mitigate the negative impacts of increased thermal stress. Further research is required to understand the full complexity of host immunity and symbiont photosynthetic interactions.

Narrow Genetic Diversity of Wolbachia Symbionts in Acrididae Grasshopper Hosts (Insecta, Orthoptera).

Bacteria of the Wolbachia genus are maternally inherited symbionts of Nematoda and numerous Arthropoda hosts. There are approximately 20 lineages of Wolbachia, which are called supergroups, and they are designated alphabetically. Wolbachia strains of the supergroups A and B are predominant in arthropods, especially in insects, and supergroup F seems to rank third. Host taxa have been studied very unevenly for Wolbachia symbionts, and here, we turn to one of largely unexplored insect families: Acrididae. On the basis of five genes subject to multilocus sequence typing, we investigated the incidence and genetic diversity of Wolbachia in 41 species belonging three subfamilies (Gomphocerinae, Oedipodinae, and Podisminae) collected in Turkey, Kazakhstan, Tajikistan, Russia, and Japan, making 501 specimens in total. Our results revealed a high incidence and very narrow genetic diversity of Wolbachia. Although only the strains belonging to supergroups A and B are commonly present in present, the Acrididae hosts here proved to be infected with supergroups B and F without A-supergroup variants. The only trace of an A-supergroup lineage was noted in one case of an inter-supergroup recombinant haplotype, where the ftsZ gene came from supergroup A, and the others from supergroup B. Variation in the Wolbachia haplotypes in Acrididae hosts within supergroups B and F was extremely low. A comprehensive genetic analysis of Wolbachia diversity confirmed specific features of the Wolbachia allelic set in Acrididae hosts. This result can help to elucidate the crucial issue of Wolbachia biology: the route(s) and mechanism(s) of Wolbachia horizontal transmission.

Variability in Host Specificity and Functional Potential of Antarctic Sponge-Associated Bacterial Communities.

Sponge-associated microorganisms are essential for sponge survival. They play an important role in recycling nutrients and, therefore, in the maintenance of the ecosystem. These microorganisms are diverse, species-specific, and different from those in the surrounding seawater. Bacterial sponge symbionts have been extensively studied in the tropics; however, little is known about these microorganisms in sponges from high-latitude environments. Sponges can cover up to 80% of the benthos in Antarctica and are crucial architects for the marine food web. In this study, we present analyses of the bacterial symbionts of three sponges: Haliclona (Rhizoniera) sp., Hymeniacidon torquata, and Isodictya kerguelenensis from the Western Antarctic Peninsula (WAP) with the aim to determine variations on the specificity of the bacteria–sponge interactions and potential signatures on their predicted functional profiles. We use high-throughput 16S rRNA gene sequencing of 30 sponge individuals inhabiting South Bay (Palmer Archipelago, WAP) to describe their microbiome taxonomy and diversity and predict potential functional profiles based on this marker gene. Our work shows similar bacterial community composition profiles among the same sponge species, although the symbiotic relationship is not equally conserved among the three Antarctic sponges. The number of species-specific core operational taxonomic units (OTUs) of these Antarctic sponges was low, with important differences between the total abundance accounted for these OTUs. Only eight OTUs were shared between the three sponge species. Analyses of the functional potential revealed that despite the high host–symbiont specificity, the inferred functions are conserved among these microbiomes, although with differences in the abundance of specific functions. H. torquata showed the highest level of intra-specificity and a higher potential of pathways related to energy metabolism, metabolisms of terpenoids and polyketides, and biosynthesis of other secondary metabolites. Overall, this work shows variations in the specificity of the sponge-associated bacterial communities, differences in how hosts and symbionts establish their relations, and in their potential functional capabilities.

Симбиозы прокариот и эукариот: от взаимодействия генов к объединению геномов

Symbioses of prokaryotes and eukaryotes are considered as models to analyze the fundamental issues of genetics and evolutionary biology including trade-off between the reductionist and holistic approaches in the research of biosystems. Symbiotic models allow us to address: 1) emergence of cellular forms of life; 2) formation of multi-genomic eukaryotic cells; 3) evolution of multicellular eukaryotes as of holobionts in which microbial symbionts ensure the hosts’ nutrition, protection from antagonists and stresses as well as developmental regulation. We suggest that ecological and genetic factors responsible for natural history of super-species systems go far beyond the Darwinian paradigm of adaptive evolution based on accumulation of minor changes supported by natural (individual) selection. Specifically, partners’ cooperation involves: a) interspecies altruism, based on formation by symbionts of non-reproducible cellular forms implementing the host-beneficial functions; b) pangenesis assuming the inheritance by hosts of symbionts as of acquired genetic determinants, which are maintained in hologenomes and can donate their genes to nuclear genomes.

Acclimation to elevated temperatures in Acropora cervicornis: effects of host genotype and symbiont shuffling

Climate change is increasing the average surface temperatures of tropical waters, creating unfavorable conditions for corals. Some species of coral can physiologically acclimate to elevated temperatures, but the degree to which genetic variation underlies differences in this ability is currently unknown. Acclimation to elevated temperatures in coral has been hypothesized to be due to either alterations in the symbiont community or to changes in gene expression. The present study investigated the ability of Acropora cervicornis to acclimate to elevated temperatures, estimated the heritability of plasticity in upper thermal tolerance, and tested whether observed acclimation patterns could be explained by symbiont shuffling. Coral fragments from a nursery in the Florida Keys (USA) were acclimated at either ambient (27 ± 1°C) or elevated (30 ± 1°C) temperatures and then exposed to a second heat stress (32 ± 1°C) and monitored for mortality. Fragments acclimated to elevated temperatures showed significantly longer lifespans in the subsequent heat stress than did those acclimated at ambient temperature. The ability to acclimate to elevated temperatures differed significantly among coral genets, yielding low, but significant, estimates of broad-sense heritability. A subsequent experiment revealed no changes in either bacterial or dinoflagellate communities of symbionts as a result of acclimation, suggesting that symbiont shuffling did not account for the differences in lifespan between treatments. While estimates of heritability were low, the results suggest that plasticity in upper thermal tolerance significantly differs among coral genets, and that acclimation is likely a result of alterations in gene expression as opposed to symbiont shuffling.

Similar cost of Hamiltonella defensa in experimental and natural aphid-endosymbiont associations.

Endosymbiont‐conferred resistance to parasitoids is common in aphids, but comes at a cost to the host in the absence of parasitoids. In black bean aphids (Aphis fabae), costs in terms of reduced lifespan and lifetime reproduction were demonstrated by introducing 11 isolates of the protective symbiont Hamiltonella defensa into previously uninfected aphid clones. Transfection of H. defensa isolates into a common genetic background allows to compare the costs of different endosymbiont isolates unconfounded by host genetic variation, but has been suggested to overestimate the realized costs of the endosymbiont in natural populations, because transfection creates new and potentially maladapted host–symbiont combinations that would be eliminated by natural selection in the field. In this experiment, we show that removing H. defensa isolates from their natural host clones with antibiotics results in a fitness gain that is comparable to the fitness loss from their introduction into two new clones. This suggests that estimating cost by transfecting endosymbiont isolates into a shared host genotype does not lead to gross overestimates of their realized costs, at least not in the two recipient genotypes used here. By comparing our data with data reported in previous publications using the same lines, we show that symbiont‐induced costs may fluctuate over time. Thus, costs estimated after extended culture in the laboratory may not always be representative of the costs at the time of collection in the field. Finally, we report the accidental observation that two isolates from a distinct haplotype of H. defensa could not be removed by cefotaxime treatment, while all isolates from two other haplotypes were readily eliminated, which is suggestive of variation in susceptibility to this antibiotic in H. defensa.

Morphoanatomical and phylogenetic characterization of the ectomycorrhiza between Laccaria squarrosa with Pinus pseudostrobus and its relevance for reforestation programs

Background: Pinus (Coniferophyta) and Laccaria (Basidiomycota) establish ectomycorrhizal symbioses in natural forests . However, their detailed morphoanatomical and phylogenetic characterization have received little attention. Accurate identification of native host symbionts is of paramount relevance to the production of mycorrhized seedlings for successful reforestation programs.
 Questions/Objective: We aimed to determine if L. squarrosa is able to establish ectomycorrhizal symbiosis with gymnosperms, thereby widening its host range and highlighting its relevance as a potential inoculant for pine seedlings. Currently, L. squarrosa is only known from its type collection associated with the angiosperm Fagus grandifolia var. mexicana .
 Studied species: The fungus L. squarrosa and Pinus pseudostrobus , a tree endemic to Mexico. 
 Study site and dates: A Pinus - Quercus forest in Piedra Canteada, Nanacamilpa, Tlaxcala; 2018-2020.
 Methods: L. squarrosa basidiomata were identified and ectomycorrhizal roots were collected and morphoanatomically characterized. For molecular identification, DNA was extracted, PCR was performed targeting the nuclear ribosomal internal transcribed spacer region (nucrDNA ITS) for the mycobiont identification and the chloroplastic single-locus trnL region for the phytobiont.
 Results: In the phylogenetic analyses, our sequences from basidiomata and ectomycorrhizae clustered together with L. squarrosa with high values of supporting identity. Meanwhile, P. pseudostrobus was molecularly identified as the phytobiont.
 Conclusions: This is one of the few worldwide characterizations of Laccaria ectomycorrhiza under field conditions and contributes to the understanding of the ecology, distribution, and economic relevance of the symbiotic association. Our data suggest that L. squarrosa has potential for use as a native inoculant for P. pseudostrobus tree production.
 
 
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Carbon sources and trophic interactions of vent fauna in the Onnuri Vent Field, Indian Ocean, inferred from stable isotopes

Food web structures (source and flow of energy in biological communities) are linked to ecological niche and species interactions, which are essential for assessing ecosystem stability and resilience. This study investigated the carbon fixation pathways of the source microbes and trophic relations of the benthic faunal community in the Onnuri Vent Field (OVF), Central Indian Ridge using sulfur (δ34S), carbon (δ13C), and nitrogen (δ15N) stable isotopic compositions. OVF megafauna exhibited a common isotopic pattern in hydrothermal vents with increasing δ15N and δ13C values, but not with δ34S values. Chemosynthetic production was the predominant source of energy in the OVF, and we identified at least three potential pools of isotopically distinct chemosynthetic production pathways, namely, the Calvin–Benson–Bassham cycle, reverse tricarboxylic acid cycle, and methanotrophic production. Within phyla, the endosymbionts and episymbionts of host taxa had different bacterial groups or carbon fixation pathways, indicating that they utilize non-overlapping resources, mutually benefiting themselves from less competition. We identified seven trophic guilds and four trophic levels in the OVF: (i) basal species consisting of symbiont host and bacterivores, (ii) intermediate species consuming particulate organic matter and detritus in addition to bacteria, (iii) scavengers and carnivores that specialize in few diets, and (iv) top predators of crab Austinograea rodriguezensis and Munidopsis sp. that are omnivorous. However, there were some spatial variations in energy sources of megafauna within the OVF, suggesting that source availability affect their trophic relations. We also found similarities and differences in the energy source and feeding relationship of some OVF fauna from those in other Indian Ocean hydrothermal vents. Overall, we believe that the results of this study contribute to improving our understanding of the nature and variation of food web structure and feeding strategies used by vent fauna for survival in the Indian Ocean hydrothermal vents that are particularly less explored.

Social Behavior, Community Composition, Pathogen Strain, and Host Symbionts Influence Fungal Disease Dynamics in Salamanders.

The emerging fungal pathogen, Batrachochytrium dendrobatidis (Bd), which can cause a fatal disease called chytridiomycosis, is implicated in the collapse of hundreds of host amphibian species. We describe chytridiomycosis dynamics in two co-occurring terrestrial salamander species, the Santa Lucia Mountains slender salamander, Batrachoseps luciae, and the arboreal salamander, Aneides lugubris. We (1) conduct a retrospective Bd-infection survey of specimens collected over the last century, (2) estimate present-day Bd infections in wild populations, (3) use generalized linear models (GLM) to identify biotic and abiotic correlates of infection risk, (4) investigate susceptibility of hosts exposed to Bd in laboratory trials, and (5) examine the ability of host skin bacteria to inhibit Bd in culture. Our historical survey of 2,866 specimens revealed that for most of the early 20th century (~1920–1969), Bd was not detected in either species. By the 1990s the proportion of infected specimens was 29 and 17% (B. luciae and A. lugubris, respectively), and in the 2010s it was 10 and 17%. This was similar to the number of infected samples from contemporary populations (2014–2015) at 10 and 18%. We found that both hosts experience signs of chytridiomycosis and suffered high Bd-caused mortality (88 and 71% for B. luciae and A. lugubris, respectively). Our GLM revealed that Bd-infection probability was positively correlated with intraspecific group size and proximity to heterospecifics but not to abiotic factors such as precipitation, minimum temperature, maximum temperature, mean temperature, and elevation, or to the size of the hosts. Finally, we found that both host species contain symbiotic skin-bacteria that inhibit growth of Bd in laboratory trials. Our results provide new evidence consistent with other studies showing a relatively recent Bd invasion of amphibian host populations in western North America and suggest that the spread of the pathogen may be enabled both through conspecific and heterospecific host interactions. Our results suggest that wildlife disease studies should assess host-pathogen dynamics that consider the interactions and effects of multiple hosts, as well as the historical context of pathogen invasion, establishment, and epizootic to enzootic transitions to better understand and predict disease dynamics.

Insect-host control of obligate, intracellular symbiont density.

Many insects rely on intracellular bacterial symbionts to supplement their specialized diets with micronutrients. Using data from diverse and well-studied insect systems, we propose three lines of evidence suggesting that hosts have tight control over the density of their obligate, intracellular bacterial partners. First, empirical studies have demonstrated that the within-host symbiont density varies depending on the nutritional and developmental requirements of the host. Second, symbiont genomes are highly reduced and have limited capacity for self-replication or transcriptional regulation. Third, several mechanisms exist for hosts to tolerate, regulate and remove symbionts including physical compartmentalization and autophagy. We then consider whether such regulation is adaptive, by discussing the relationship between symbiont density and host fitness. We discuss current limitations of empirical studies for exploring fitness effects in host–symbiont relationships, and emphasize the potential for using mathematical models to formalize evolutionary hypotheses and to generate testable predictions for future work.

Fatty acid profiles of separated host–symbiont fractions from five symbiotic corals: applications of chemotaxonomic and trophic biomarkers

Fatty acids (FAs) are the main components of lipids in corals. We examined FAs profiles from five symbiotic coral species belonging to five different genera (Acropora, Pavona, Turbinaria, Favites, and Platygyra) and four different families (Acroporidae, Agariciidae, Dendrophyllidae, Faviidae). We separated symbionts from the coral host tissue to investigate the interaction of FA between symbionts and host tissue. After separation, we used FA profiles, in particular specific FAs (e.g. 16:0, 18:0, 18:3n-3, 20:5n-3, 22:6n-3) and their ratios (EPA:DHA, PUFA:SFA) as biomarkers (i.e. signature lipids) to examine chemotaxonomy and trophic level (autotrophy vs. heterotrophy) of each coral species. Gas chromatography–mass spectrometry (GC–MS) was performed to identify and quantify FA. For quantification, the dry weight of total lipids was used to normalize FA concentration (μg mg−1). We found that (1) the five different coral species showed define species-specific FA profiles; (2) certain FAs were valuable biomarkers to determine relative trophic strategies (i.e. autotrophy and/or heterotrophy; (3) the application of FA ratios to define trophic level requires caution in research application and data interpretation. Considering the limitations of FA ratios determined herein, we suggest it to be more appropriate to examine response to environmental change within species. Going forward, our study provides important FA baseline data that builds the foundation for future investigations on the impact of environmental changes related to nutrition and metabolism in symbiotic corals.

Capacity of soil bacteria to reach the phyllosphere and convergence of floral communities despite soil microbiota variation.

Leaves and flowers are colonized by diverse bacteria that impact plant fitness and evolution. Although the structure of these microbial communities is becoming well-characterized, various aspects of their environmental origin and selection by plants remain uncertain, such as the relative proportion of soilborne bacteria in phyllosphere communities. Here, to address this issue and to provide experimental support for bacteria being filtered by flowers, we conducted common-garden experiments outside and under gnotobiotic conditions. We grew Arabidopsis thaliana in a soil substitute and added two microbial communities from natural soils. We estimated that at least 25% of the phyllosphere bacteria collected from the plants grown in the open environment were also detected in the controlled conditions, in which bacteria could reach leaves and flowers only from the soil. These taxa represented more than 40% of the communities based on amplicon sequencing. Unsupervised hierarchical clustering approaches supported the convergence of all floral microbiota, and 24 of the 28 bacteria responsible for this pattern belonged to the Burkholderiaceae family, which includes known plant pathogens and plant growth-promoting members. We anticipate that our study will foster future investigations regarding the routes used by soil microbes to reach leaves and flowers, the ubiquity of the environmental filtering of Burkholderiaceae across plant species and environments, and the potential functional effects of the accumulation of these bacteria in the reproductive organs of flowering plants.

Positive selection on mitochondria may eliminate heritable microbes from arthropod populations.

Diverse eukaryotic taxa carry facultative heritable symbionts, microbes that are passed from mother to offspring. These symbionts are coinherited with mitochondria, and selection favouring either new symbionts, or new symbiont variants, is known to drive loss of mitochondrial diversity as a correlated response. More recently, evidence has accumulated of episodic directional selection on mitochondria, but with currently unknown consequences for symbiont evolution. We therefore employed a population genetic mean field framework to model the impact of selection on mitochondrial DNA (mtDNA) upon symbiont frequency for three generic scenarios of host–symbiont interaction. Our models predict that direct selection on mtDNA can drive symbionts out of the population where a positively selected mtDNA mutation occurs initially in an individual that is uninfected with the symbiont, and the symbiont is initially at low frequency. When, by contrast, the positively selected mtDNA mutation occurs in a symbiont-infected individual, the mutation becomes fixed and in doing so removes symbiont variation from the population. We conclude that the molecular evolution of symbionts and mitochondria, which has previously been viewed from a perspective of selection on symbionts driving the evolution of a neutral mtDNA marker, should be reappraised in the light of positive selection on mtDNA.

Host–symbiont transcriptomic changes during natural bleaching and recovery in the leaf coral Pavona decussata

Coral bleaching has become a major threat to coral reefs worldwide, but for most coral species little is known about their resilience to environmental changes. We aimed to understand the gene expressional regulation underlying natural bleaching and recovery in Pavona decussata, a dominant species of scleractinian coral in the northern South China Sea. Analyzing samples collected in 2017 from the field revealed distinct zooxanthellae density, chlorophyll a concentration and transcriptomic signatures corresponding to changes in health conditions of the coral holobiont. In the host, normal-looking tissues of partially bleached colonies were frontloaded with stress responsive genes, as indicated by upregulation of immune defense, response to endoplasmic reticulum, and oxidative stress genes. Bleaching was characterized by upregulation of apoptosis-related genes which could cause a reduction in algal symbionts, and downregulation of genes involved in stress responses and metabolic processes. The transcription factors stat5b and irf1 played key roles in bleaching by regulating immune and apoptosis pathways. Recovery from bleaching was characterized by enrichment of pathways involved in mitosis, DNA replication, and recombination for tissue repairing, as well as restoration of energy and metabolism. In the symbionts, bleaching corresponded to imbalance in photosystems I and II activities which enhanced oxidative stress and limited energy production and nutrient assimilation. Overall, our study revealed distinct gene expressional profiles and regulation in the different phases of the bleaching and recovery process, and provided new insight into the molecular mechanisms underlying the holobiont's resilience that may determine the species’ fate in response to global and regional environmental changes.

Identifying fungal-host associations in an amphibian host system.

Host-associated microbes can interact with macro-organisms in a number of ways that affect host health. Few studies of host-associated microbiomes, however, focus on fungi. In addition, it is difficult to discern whether a fungal organism found in or on an ectotherm host is associating with it in a durable, symbiotic interaction versus a transient one, and to what extent the habitat and host share microbes. We seek to identify these host-microbe interactions on an amphibian, the Colorado boreal toad (Anaxyrus boreas boreas). We sequenced the ITS1 region of the fungal community on the skin of wild toads (n = 124) from four sites in the Colorado Rocky Mountains, across its physiologically dynamic developmental life stages. We also sampled the common habitats used by boreal toads: water from their natal wetland and aquatic pond sediment. We then examined diversity patterns within different life stages, between host and habitat, and identified fungal taxa that could be putatively host-associated with toads by using an indicator species analysis on toad versus environmental samples. Host and habitat were strikingly similar, with the exception of toad eggs. Post-hatching toad life stages were distinct in their various fungal diversity measures. We identified eight fungal taxa that were significantly associated with eggs, but no other fungal taxa were associated with other toad life stages compared with their environmental habitat. This suggests that although pre- and post-metamorphic toad life stages differ from each other, the habitat and host fungal communities are so similar that identifying obligate host symbionts is difficult with the techniques used here. This approach does, however, leverage sequence data from host and habitat samples to predict which microbial taxa are host-associated versus transient microbes, thereby condensing a large set of sequence data into a smaller list of potential targets for further consideration.

Protistan grazing impacts microbial communities and carbon cycling at deep-sea hydrothermal vents

Microbial eukaryotes (or protists) in marine ecosystems are a link between primary producers and all higher trophic levels, and the rate at which heterotrophic protistan grazers consume microbial prey is a key mechanism for carbon transport and recycling in microbial food webs. At deep-sea hydrothermal vents, chemosynthetic bacteria and archaea form the base of a food web that functions in the absence of sunlight, but the role of protistan grazers in these highly productive ecosystems is largely unexplored. Here, we pair grazing experiments with a molecular survey to quantify protistan grazing and to characterize the composition of vent-associated protists in low-temperature diffuse venting fluids from Gorda Ridge in the northeast Pacific Ocean. Results reveal protists exert higher predation pressure at vents compared to the surrounding deep seawater environment and may account for consuming 28 to 62% of the daily stock of prokaryotic biomass within discharging hydrothermal vent fluids. The vent-associated protistan community was more species rich relative to the background deep sea, and patterns in the distribution and co-occurrence of vent microbes provide additional insights into potential predator-prey interactions. Ciliates, followed by dinoflagellates, Syndiniales, rhizaria, and stramenopiles, dominated the vent protistan community and included bacterivorous species, species known to host symbionts, and parasites. Our findings provide an estimate of protistan grazing pressure within hydrothermal vent food webs, highlighting the important role that diverse protistan communities play in deep-sea carbon cycling.

Development of a free radical scavenging bacterial consortium to mitigate oxidative stress in cnidarians.

SummaryCorals are colonized by symbiotic microorganisms that profoundly influence the animal’s health. One noted symbiont is a single‐celled alga (in the dinoflagellate family Symbiodiniaceae), which provides the coral with most of its fixed carbon. Thermal stress increases the production of reactive oxygen species (ROS) by Symbiodiniaceae during photosynthesis. ROS can both damage the algal symbiont’s photosynthetic machinery and inhibit its repair, causing a positive feedback loop for the toxic accumulation of ROS. If not scavenged by the antioxidant network, excess ROS may trigger a signaling cascade ending with the coral host and algal symbiont disassociating in a process known as bleaching. We use Exaiptasia diaphana as a model for corals and constructed a consortium comprised of E. diaphana–associated bacteria capable of neutralizing ROS. We identified six strains with high free radical scavenging (FRS) ability belonging to the families Alteromonadaceae, Rhodobacteraceae, Flavobacteriaceae and Micrococcaceae. In parallel, we established a consortium of low FRS isolates consisting of genetically related strains. Bacterial whole genome sequences were used to identify key pathways that are known to influence ROS.

Supply of acetyl-CoA to N<sub>2</sub>-fixing bacteroids: insights from the mutational and proteomic analyses of <em>Sinorhizobium meliloti</em>

Rhizobia represent a diverse group of gram-negative bacteria capable of fixing atmospheric nitrogen in symbiosis with leguminous plants. Mechanisms of symbiotic efficiency are important to study not only to reveal the “fine tuning” of the host–symbiont supra-organismal genetic system emergence, but also to develop agriculture with minimal environmental risks. In this paper we demonstrate that among seven genes whose inactivation by Tn5 insertions results in an increased efficiency of rhizobia (Sinorhizobium meliloti) symbiosis with alfalfa (Eff++ phenotype), six genes are involved in the metabolism of small molecules. One of them (SMc04399) encodes for acetate-CoA transferase catalyzing the formation of acetyl-CoA from acyl-CoA. Since acetyl-CoA is required for operation of the Krebs cycle, providing ATP for symbiotic N2 fixation, we suggest that a significant portion of this coenzyme utilized by bacteroids is provided by the plant cell supporting the energy-consuming nitrogenase reaction. Proteomic data analysis allow us to reveal the lability of enzymatic pathways which are involved in bacteroids in the production and catabolism of acetyl-CoA and which should be modified to obtain the Eff++ phenotype. This phenotype was developed also after inactivation of NoeB protein which is involved in the host-specific nodulation and is characterized by an elevated production in wild type S. meliloti bacteroids, suggesting a multifunctional role of noeB in the symbiosis operation.