Mosquito Microbiomes of Rwanda: Characterizing Mosquito Host and Microbial Communities in the Land of a Thousand Hills.

Mosquitoes are a complex nuisance around the world and tropical countries bear the brunt of the burden of mosquito-borne diseases. Rwanda has had success in reducing malaria and some arboviral diseases over the last few years, but still faces challenges to elimination. By building our understanding of in situ mosquito communities in Rwanda at a disturbed, human-occupied site and at a natural, preserved site, we can build our understanding of natural mosquito microbiomes toward the goal of implementing novel microbial control methods. Here, we examined the composition of collected mosquitoes and their microbiomes at two diverse sites using Cytochrome c Oxidase I sequencing and 16S V4 high-throughput sequencing. The majority (36 of 40 species) of mosquitoes captured and characterized in this study are the first-known record of their species for Rwanda but have been characterized in other nations in East Africa. We found significant differences among mosquito genera and among species, but not between mosquito sexes or catch method. Bacteria of interest for arbovirus control, Asaia, Serratia, and Wolbachia, were found in abundance at both sites and varied greatly by species.

The Mycobiome of Bats in the American Southwest Is Structured by Geography, Bat Species, and Behavior.

Bats are widespread mammals that play key roles in ecosystems as pollinators and insectivores. However, there is a paucity of information about bat-associated microbes, in particular their fungal communities, despite the important role microbes play in host health and overall host function. The emerging fungal disease, white-nose syndrome, presents a potential challenge to the bat microbiome and understanding healthy bat-associated taxa will provide valuable information about potential microbiome-pathogen interactions. To address this knowledge gap, we collected 174 bat fur/skin swabs from 14 species of bats captured in five locations in New Mexico and Arizona and used high-throughput sequencing of the fungal internal transcribed (ITS) region to characterize bat-associated fungal communities. Our results revealed a highly heterogeneous bat mycobiome that was structured by geography and bat species. Furthermore, our data suggest that bat-associated fungal communities are affected by bat foraging, indicating the bat skin microbiota is dynamic on short time scales. Finally, despite the strong effects of site and species, we found widespread and abundant taxa from several taxonomic groups including the genera Alternaria and Metschnikowia that have the potential to be inhibitory towards fungal and bacterial pathogens.

Abiotic Treatment to Common Bean Plants Results in an Altered Endophytic Seed Microbiome.

There has been a growing interest in the seed microbiome due to its important role as an end and starting point of plant microbiome assembly that can have consequences for plant health. However, the effect of abiotic conditions on the seed microbial community remains unknown. We performed a pilot study in a controlled growth chamber to investigate how the endophytic seed microbiome of the common bean (Phaseolus vulgaris L. [var. Red Hawk]) was altered under abiotic treatments relevant for crop management with changing climate. Bean plants were subjected to one of three treatments: 66% water withholding to simulate mild drought, 50% Hoagland nutrient solution to simulate fertilization, or control with sufficient water and baseline nutrition. We performed 16S rRNA gene amplicon sequencing and Internal Transcribed Spacer 1 (ITS1) amplicon sequencing of the endophytic DNA to assess seed bacterial/archaeal and fungal community structure, respectively. We found that variability in the seed microbiome structure was high, while α-diversity was low, with tens of taxa present. Water withholding and nutrient addition significantly altered the seed microbiome structure for bacterial/archaeal communities compared to the control, and each treatment resulted in a distinct microbiome structure. Conversely, there were no statistically supported differences in the fungal microbiome across treatments. These promising results suggest that further investigation is needed to better understand abiotic or stress-induced changes in the seed microbiome, the mechanisms that drive those changes, and their implications for the health and stress responses of the next plant generation. IMPORTANCE Seed microbiome members initiate the assembly of plant-associated microbial communities, but the environmental drivers of endophytic seed microbiome composition are unclear. Here, we exposed plants to short-term drought and fertilizer treatments during early vegetative growth and quantified the microbiome composition of the seeds that were ultimately produced. We found that seeds produced by plants stressed by water limitation or receiving nutrient addition had statistically different endophytic bacterial/archaeal microbiome compositions from each other and from seeds produced by control plants. This work suggests that the abiotic experience of a parental plant can influence the composition of its seed microbiome, with unknown consequences for the next plant generation.

Chronic stress and captivity alter the cloacal microbiome of a wild songbird.

There are complex interactions between an organism's microbiome and its response to stressors, often referred to as the 'gut-brain axis'; however, the ecological relevance of this axis in wild animals remains poorly understood. Here, we used a chronic mild stress protocol to induce stress in wild-caught house sparrows (Passer domesticus), and compared microbial communities among stressed animals, those recovering from stress, captive controls (unstressed) and a group not brought into captivity. We assessed changes in microbial communities and abundance of shed microbes by culturing cloacal samples on multiple media to select for aerobic and anaerobic bacteria and fungi. We complemented this with cultivation-independent 16S and ITS rRNA gene amplification and sequencing, pairing these results with host physiological and immune metrics, including body mass change, relative spleen mass and plasma corticosterone concentrations. We found significant effects of stress and captivity on the house sparrow microbiomes, with stress leading to an increased relative abundance of endotoxin-producing bacteria - a possible mechanism for the hyperinflammatory response observed in captive avians. While we found evidence that the microbiome community partially recovers after stress cessation, animals may lose key taxa, and the abundance of endotoxin-producing bacteria persists. Our results suggest an overall link between chronic stress, host immune system and the microbiome, with the loss of potentially beneficial taxa (e.g. lactic acid bacteria), and an increase in endotoxin-producing bacteria due to stress and captivity. Ultimately, consideration of the host's microbiome may be useful when evaluating the impact of stressors on individual and population health.

Long-Term Fertilization Alters Nitrous Oxide Cycling Dynamics in Salt Marsh Sediments.

Salt marsh sediments are known hotspots for nitrogen cycling, including the production and consumption of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent. Coastal eutrophication, particularly elevated nitrogen loading from the application of fertilizers, is accelerating nitrogen cycling processes in salt marsh sediments. Here, we examine the impact of long-term fertilization on nitrogen cycling processes with a focus on N2O dynamics in a New England salt marsh. By combining 15N-tracer experiments with numerical modeling, we found that both nitrification and denitrification contribute to net N2O production in fertilized sediments. Long-term fertilization increased the relative importance of nitrification to N2O production, likely a result of increased oxygen penetration from nutrient-induced increases in marsh elevation. Substrate utilization rates of key nitrogen cycling processes revealed links between functions and the corresponding microbial communities. Higher specific substrate utilization rates leading to N2O production from nitrification in fertilized sediments indicate a shift in the community composition of ammonia oxidizers, whereas the lack of change in specific substrate utilization of N2O production from denitrification under long-term fertilization suggests resilience of the denitrifying communities. Both are consistent with previous studies on the functional gene community composition in these experimental plots.

Host-associated microbiomes are predicted by immune system complexity and climate.

BACKGROUND: Host-associated microbiomes, the microorganisms occurring inside and on host surfaces, influence evolutionary, immunological, and ecological processes. Interactions between host and microbiome affect metabolism and contribute to host adaptation to changing environments. Meta-analyses of host-associated bacterial communities have the potential to elucidate global-scale patterns of microbial community structure and function. It is possible that host surface-associated (external) microbiomes respond more strongly to variations in environmental factors, whereas internal microbiomes are more tightly linked to host factors. RESULTS: Here, we use the dataset from the Earth Microbiome Project and accumulate data from 50 additional studies totaling 654 host species and over 15,000 samples to examine global-scale patterns of bacterial diversity and function. We analyze microbiomes from non-captive hosts sampled from natural habitats and find patterns with bioclimate and geophysical factors, as well as land use, host phylogeny, and trophic level/diet. Specifically, external microbiomes are best explained by variations in mean daily temperature range and precipitation seasonality. In contrast, internal microbiomes are best explained by host factors such as phylogeny/immune complexity and trophic level/diet, plus climate. CONCLUSIONS: Internal microbiomes are predominantly associated with top-down effects, while climatic factors are stronger determinants of microbiomes on host external surfaces. Host immunity may act on microbiome diversity through top-down regulation analogous to predators in non-microbial ecosystems. Noting gaps in geographic and host sampling, this combined dataset represents a global baseline available for interrogation by future microbial ecology studies.

Publisher Correction: Host-associated microbiomes are predicted by immune system complexity and climate.

Following publication of the original paper [1], it was reported that an error in the processing of Fig. 8 occurred. In the online HTML version of the article, Fig. 8 was presented as a duplication of Fig. 7. The original article [1] has been corrected.

Establishment Limitation Constrains the Abundance of Lactic Acid Bacteria in the Napa Cabbage Phyllosphere.

Patterns of phyllosphere diversity have become increasingly clear with high-throughput sequencing surveys, but the processes that control phyllosphere diversity are still emerging. Through a combination of lab and field experiments using Napa cabbage and lactic acid bacteria (LAB), we examined how dispersal and establishment processes shape the ecological distributions of phyllosphere bacteria. We first determined the abundance and diversity of LAB on Napa cabbage grown at three sites using both culture-based approaches and 16S rRNA gene amplicon sequencing. Across all sites, LAB made up less than 0.9% of the total bacterial community abundance. To assess whether LAB were low in abundance in the Napa cabbage phyllosphere due to a limited abundance in local species pools (source limitation), we quantified LAB in leaf and soil samples across 51 vegetable farms and gardens throughout the northeastern United States. Across all sites, LAB comprised less than 3.2% of the soil bacterial communities and less than 1.6% of phyllosphere bacterial communities. To assess whether LAB are unable to grow in the phyllosphere even if they dispersed at high rates (establishment limitation), we used a gnotobiotic Napa cabbage system in the lab with experimental communities mimicking various dispersal rates of LAB. Even at high dispersal rates, LAB became rare or completely undetectable in experimental communities, suggesting that they are also establishment limited. Collectively, our data demonstrate that the low abundance of LAB in phyllosphere communities may be explained by establishment limitation.IMPORTANCE The quality and safety of vegetable fermentations are dependent on the activities of LAB naturally present in the phyllosphere. Despite their critical role in determining the success of fermentation, the processes that determine the abundance and diversity of LAB in vegetables used for fermentation are poorly characterized. Our work demonstrates that the limited ability of LAB to grow in the cabbage phyllosphere environment may constrain their abundance on cabbage leaves. These results suggest that commercial fermentation of Napa cabbage proceeds despite low and variable abundances of LAB across different growing regions. Propagule limitation may also explain ecological distributions of other rare members of phyllosphere microbes.

16S rRNA/rRNA Gene Ratios and Cell Activity Staining Reveal Consistent Patterns of Microbial Activity in Plant-Associated Soil.

At any given time, only a subset of microbial community members are active in their environment. The others are in a state of dormancy, with strongly reduced metabolic rates. It is of interest to distinguish active and inactive microbial cells and taxa to understand their functional contributions to ecosystem processes and to understand shifts in microbial activity in response to change. Of the methods used to assess microbial activity-dormancy dynamics, 16S rRNA/rRNA gene amplicons (16S ratios) and active cell staining with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) are two of the most common, yet each method has limitations. Given that in situ activity-dormancy dynamics are proxied only by laboratory methods, further study is needed to assess the level of agreement and potential complementarity of these methods. We conducted two experiments investigating microbial activity in plant-associated soils. First, we treated corn field soil with phytohormones to simulate plant soil stress signaling, and second, we used rhizosphere soil from common bean plants exposed to drought or nutrient enrichment. Overall, the 16S ratio and CTC methods exhibited similar patterns of relative activity across treatments when treatment effects were large, and the instances in which they differed could be attributed to changes in community size (e.g., cell death or growth). Therefore, regardless of the method used to assess activity, we recommend quantifying community size to inform ecological interpretation. Our results suggest that the 16S ratio and CTC methods report comparable patterns of activity that can be applied to observe ecological dynamics over time, space, or experimental treatment. IMPORTANCE Although the majority of microorganisms in natural ecosystems are dormant, relatively little is known about the dynamics of the active and dormant microbial pools through both space and time. The limited knowledge of microbial activity-dormancy dynamics is in part due to uncertainty in the methods currently used to quantify active taxa. Here, we directly compared two of the most common methods (16S ratios and active cell staining) for estimating microbial activity in plant-associated soil and found that they were largely in agreement in the overarching patterns. Our results suggest that 16S ratios and active cell staining provide complementary information for measuring and interpreting microbial activity-dormancy dynamics in soils. They also support the idea that 16S rRNA/rRNA gene ratios have comparative value and offer a high-throughput, sequencing-based option for understanding relative changes in microbiome activity, as long as this method is coupled with quantification of community size.

Community richness of amphibian skin bacteria correlates with bioclimate at the global scale.

Animal-associated microbiomes are integral to host health, yet key biotic and abiotic factors that shape host-associated microbial communities at the global scale remain poorly understood. We investigated global patterns in amphibian skin bacterial communities, incorporating samples from 2,349 individuals representing 205 amphibian species across a broad biogeographic range. We analysed how biotic and abiotic factors correlate with skin microbial communities using multiple statistical approaches. Global amphibian skin bacterial richness was consistently correlated with temperature-associated factors. We found more diverse skin microbiomes in environments with colder winters and less stable thermal conditions compared with environments with warm winters and less annual temperature variation. We used bioinformatically predicted bacterial growth rates, dormancy genes and antibiotic synthesis genes, as well as inferred bacterial thermal growth optima to propose mechanistic hypotheses that may explain the observed patterns. We conclude that temporal and spatial characteristics of the host's macro-environment mediate microbial diversity.

Nutrient Enrichment Alters Salt Marsh Fungal Communities and Promotes Putative Fungal Denitrifiers.

Enrichment of ecosystems with excess nutrients is occurring at an alarming rate and has fundamentally altered ecosystems worldwide. Salt marshes, which lie at the land-sea interface, are highly effective at removing anthropogenic nutrients through the action of macrophytes and through microbial processes in coastal sediments. The response of salt marsh bacteria to excess nitrogen has been documented; however, the role of fungi and their response to excess nitrogen in salt marsh sediments is not fully understood. Here, we document the response of salt marsh fungal communities to long-term excess nitrate in four distinct marsh habitats within a northern temperate marsh complex. We show that salt marsh fungal communities varied as a function of salt marsh habitat, with both fungal abundance and diversity increasing with carbon quantity. Nutrient enrichment altered fungal communities in all habitats through an increase in fungal abundance and the proliferation of putative fungal denitrifiers. Nutrient enrichment also altered marsh carbon quality in low marsh surface sediments where fungal response to nutrient enrichment was most dramatic, suggesting nutrient enrichment can alter organic matter quality in coastal sediments. Our results indicate that fungi, in addition to bacteria, likely play an important role in anaerobic decomposition of salt marsh sediment organic matter.

Nitrifying and Denitrifying Bacterial Communities in Advanced Nitrogen-Removal Onsite Wastewater Treatment Systems.

Advanced N-removal onsite wastewater treatment systems (OWTS) rely on nitrification and denitrification to remove N from wastewater. Despite their use to reduce N contamination, we know little about microbial communities controlling N removal in these systems. We used quantitative polymerase chain reaction and high-throughput sequencing targeting nitrous oxide reductase () and bacterial ammonia monooxygenase () to determine the size, structure, and composition of communities containing these genes. We analyzed water samples from three advanced N-removal technologies in 38 systems in five towns in Rhode Island in August 2016, and in nine systems from one town in June, August, and October 2016. Abundance of ranged from 9.1 × 10 to 9 × 10 copies L and differed among technologies and over time, whereas bacterial abundance ranged from 0 to 1.9 × 10 copies L and was not different among technologies or over time. Richness and diversity of -but not -differed over time, with median Shannon diversity indices ranging from 2.61 in October to 4.53 in August. We observed weak community similarity patterns driven by geography and technology in The most abundant and containing bacteria were associated with water distribution and municipal wastewater treatment plants, such as and species. Our results show that communities in N-removal OWTS technologies differ slightly in terms of size and diversity as a function of time, but not geography, whereas communities are similar across time, technology, and geography. Furthermore, community composition appears to be stable across technologies, geography, and time for .

Trait-based patterns of microbial dynamics in dormancy potential and heterotrophic strategy: case studies of resource-based and post-press succession.

Understanding the relationship between microbial community structure and function is a major challenge in microbial ecology. Recent work has shown that community weighted mean 16S rRNA gene copies, as a proxy for heterotrophic growth strategy, is a microbial community trait that decreases predictably over successional trajectories that are underpinned by changes in resource availability. However, it has been challenging to identify other microbial traits that are predictive of community functions and have consistent patterns with succession. Trait-based patterns of secondary succession (e.g., after a disturbance) are less often considered, and these responses may be underpinned by abiotic drivers other than changes in resources. In this perspectives piece, we present hypotheses about microbial traits important for microbial succession in resource-based and post-press disturbance scenarios, as synthesized from previous works and extended within this work. Using four case studies, we compare two traits, heterotrophic strategy and dormancy potential, and two different types of succession, resource-based (endogenous heterotrophic) and post-press. There were decreases in weighted ribosomal operon counts and in dormancy genes over resource-based succession. Both traits also were lower in post-press succession as compared to reference conditions, but increased with time from disturbance. Thus, dormancy potential may be an additional trait that changes predictably with succession. Finally, considering changes in microbial community traits over post-press succession is as important as over resource-based succession. These patterns need to be interpreted carefully and reference and recovering samples can be collected to improve interpretation of changes in community traits over post-press succession.

Genetic Analysis of Flooding Tolerance in an Andean Diversity Panel of Dry Bean (Phaseolus vulgaris L.).

Climate change models predict temporal and spatial shifts in precipitation resulting in more frequent incidents of flooding, particularly in regions with poor soil drainage. In these flooding conditions, crop losses are inevitable due to exposure of plants to hypoxia and the spread of root rot diseases. Improving the tolerance of bean cultivars to flooding is crucial to minimize crop losses. In this experiment, we evaluated the phenotypic responses of 277 genotypes from the Andean Diversity Panel to flooding at germination and seedling stages. A randomized complete block design, with a split plot arrangement, was employed for phenotyping germination rate, total weight, shoot weight, root weight, hypocotyl length, SPAD index, adventitious root rate, and survival score. A subset of genotypes (n = 20) were further evaluated under field conditions to assess correlations between field and greenhouse data and to identify the most tolerant genotypes. A genome-wide association study (GWAS) was performed using ~203 K SNP markers to understand the genetic architecture of flooding tolerance in this panel. Survival scores between field and greenhouse data were significantly correlated (r = 0.55, P = 0.01). Subsequently, a subset of the most tolerant and susceptible genotypes were evaluated under pathogenic Pythium spp. pressure. This experiment revealed a potential link between flooding tolerance and Pythium spp. resistance. Several tolerant genotypes were identified that could be used as donor parents in breeding pipelines, especially ADP-429 and ADP-604. Based on the population structure analysis, a subpopulation consisting of 20 genotypes from the Middle American gene pool was detected that also possessed the highest root weight, hypocotyl length, and adventitious root development under flooding conditions. Genomic regions associated with flooding tolerance were identified including a region on Pv08/3.2 Mb, which is associated with germination rate and resides in vicinity of SnRK1.1, a central gene involved in response of plants to hypoxia. Furthermore, a QTL at Pv07/4.7 Mb was detected that controls survival score of seedlings under flooding conditions. The association of these QTL with the survivability traits including germination rate and survival score, indicates that these loci can be used in marker-assisted selection breeding to improve flooding tolerance in the Andean germplasm.

Community Composition of Nitrous Oxide-Related Genes in Salt Marsh Sediments Exposed to Nitrogen Enrichment.

Salt marshes provide many key ecosystem services that have tremendous ecological and economic value. One critical service is the removal of fixed nitrogen from coastal waters, which limits the negative effects of eutrophication resulting from increased nutrient supply. Nutrient enrichment of salt marsh sediments results in higher rates of nitrogen cycling and, commonly, a concurrent increase in the flux of nitrous oxide, an important greenhouse gas. Little is known, however, regarding controls on the microbial communities that contribute to nitrous oxide fluxes in marsh sediments. To address this disconnect, we generated profiles of microbial communities and communities of micro-organisms containing specific nitrogen cycling genes that encode several enzymes (amoA, norB, nosZ) related to nitrous oxide flux from salt marsh sediments. We hypothesized that communities of microbes responsible for nitrogen transformations will be structured by nitrogen availability. Taxa that respond positively to high nitrogen inputs may be responsible for the elevated rates of nitrogen cycling processes measured in fertilized sediments. Our data show that, with the exception of ammonia-oxidizing archaea, the community composition of organisms involved in the production and consumption of nitrous oxide was altered under nutrient enrichment. These results suggest that previously measured rates of nitrous oxide production and consumption are likely the result of changes in community structure, not simply changes in microbial activity.

Lineage overwhelms environmental conditions in determining rhizosphere bacterial community structure in a cosmopolitan invasive plant.

Plant-microbe interactions play crucial roles in species invasions but are rarely investigated at the intraspecific level. Here, we study these interactions in three lineages of a globally distributed plant, Phragmites australis. We use field surveys and a common garden experiment to analyze bacterial communities in the rhizosphere of P. australis stands from native, introduced, and Gulf lineages to determine lineage-specific controls on rhizosphere bacteria. We show that within-lineage bacterial communities are similar, but are distinct among lineages, which is consistent with our results in a complementary common garden experiment. Introduced P. australis rhizosphere bacterial communities have lower abundances of pathways involved in antimicrobial biosynthesis and degradation, suggesting a lower exposure to enemy attack than native and Gulf lineages. However, lineage and not rhizosphere bacterial communities dictate individual plant growth in the common garden experiment. We conclude that lineage is crucial for determination of both rhizosphere bacterial communities and plant fitness.Environmental factors often outweigh host heritable factors in structuring host-associated microbiomes. Here, Bowen et al. show that host lineage is crucial for determination of rhizosphere bacterial communities in Phragmites australis, a globally distributed invasive plant.

The skin microbiome of cow-nose rays (Rhinoptera bonasus) in an aquarium touch-tank exhibit.

Public aquaria offer numerous educational opportunities for visitors while touch-tank exhibits offer guests the ability to directly interact with marine life via physical contact. Despite the popularity of touch-tanks, there is a paucity of research about animal health in these exhibits and, in particular, there is little research on the microbial communities in these highly interactive exhibits. Microbial community structure can have implications for both host health and habitat function. To better understand the microbiome of a touch-tank we used high-throughput sequencing of the 16S rRNA gene to analyze the microbial community on the dorsal and ventral surfaces of cow-nose rays (Rhinoptera bonasus) as well as their environment in a frequently visited touch-tank exhibit at the New England Aquarium. Our analyses revealed a distinct microbial community associated with the skin of the ray that had lower diversity than the surrounding habitat. The ray skin was dominated by three orders: Burkholderiales (∼55%), Flavobacteriales (∼19%), and Pseudomonadales (∼12%), taxonomic groups commonly associated with other fish species. Our results provide a survey of ray-associated bacterial communities in a touch-tank environment, thereby laying the foundation for future studies examining the role of potential challenges to ray microbiota and their associated health.

Fight Fungi with Fungi: Antifungal Properties of the Amphibian Mycobiome.

Emerging infectious diseases caused by fungal taxa are increasing and are placing a substantial burden on economies and ecosystems worldwide. Of the emerging fungal diseases, chytridomycosis caused by the fungus Batrachochytrium dendrobatidis (hereafter Bd) is linked to global amphibian declines. Amphibians have innate immunity, as well as additional resistance through cutaneous microbial communities. Despite the targeting of bacteria as potential probiotics, the role of fungi in the protection against Bd infection in unknown. We used a four-part approach, including high-throughput sequencing of bacterial and fungal communities, cultivation of fungi, Bd challenge assays, and experimental additions of probiotic to Midwife Toads (Altyes obstetricans), to examine the overlapping roles of bacterial and fungal microbiota in pathogen defense in captive bred poison arrow frogs (Dendrobates sp.). Our results revealed that cutaneous fungal taxa differed from environmental microbiota across three species and a subspecies of Dendrobates spp. frogs. Cultivation of host-associated and environmental fungi realved numerous taxa with the ability to inhibit or facilitate the growth of Bd. The abundance of cutaneous fungi contributed more to Bd defense (~45% of the fungal community), than did bacteria (~10%) and frog species harbored distinct inhibitory communities that were distinct from the environment. Further, we demonstrated that a fungal probiotic therapy did not induce an endocrine-immune reaction, in contrast to bacterial probiotics that stressed amphibian hosts and suppressed antimicrobial peptide responses, limiting their long-term colonization potential. Our results suggest that probiotic strategies against amphibian fungal pathogens should, in addition to bacterial probiotics, focus on host-associated and environmental fungi such as Penicillium and members of the families Chaetomiaceae and Lasiosphaeriaceae.

Nutrient enrichment induces dormancy and decreases diversity of active bacteria in salt marsh sediments.

Microorganisms control key biogeochemical pathways, thus changes in microbial diversity, community structure and activity can affect ecosystem response to environmental drivers. Understanding factors that control the proportion of active microbes in the environment and how they vary when perturbed is critical to anticipating ecosystem response to global change. Increasing supplies of anthropogenic nitrogen to ecosystems globally makes it imperative that we understand how nutrient supply alters active microbial communities. Here we show that nitrogen additions to salt marshes cause a shift in the active microbial community despite no change in the total community. The active community shift causes the proportion of dormant microbial taxa to double, from 45 to 90%, and induces diversity loss in the active portion of the community. Our results suggest that perturbations to salt marshes can drastically alter active microbial communities, however these communities may remain resilient by protecting total diversity through increased dormancy.

Recent Trends in Local-Scale Marine Biodiversity Reflect Community Structure and Human Impacts.

The modern biodiversity crisis reflects global extinctions and local introductions. Human activities have dramatically altered rates and scales of processes that regulate biodiversity at local scales. Reconciling the threat of global biodiversity loss with recent evidence of stability at fine spatial scales is a major challenge and requires a nuanced approach to biodiversity change that integrates ecological understanding. With a new dataset of 471 diversity time series spanning from 1962 to 2015 from marine coastal ecosystems, we tested (1) whether biodiversity changed at local scales in recent decades, and (2) whether we can ignore ecological context (e.g., proximate human impacts, trophic level, spatial scale) and still make informative inferences regarding local change. We detected a predominant signal of increasing species richness in coastal systems since 1962 in our dataset, though net species loss was associated with localized effects of anthropogenic impacts. Our geographically extensive dataset is unlikely to be a random sample of marine coastal habitats; impacted sites (3% of our time series) were underrepresented relative to their global presence. These local-scale patterns do not contradict the prospect of accelerating global extinctions but are consistent with local species loss in areas with direct human impacts and increases in diversity due to invasions and range expansions in lower impact areas. Attempts to detect and understand local biodiversity trends are incomplete without information on local human activities and ecological context.