Boreal feather mosses secrete chemical signals to gain nitrogen

Exploring the Metatranscriptome of Bacterial Communities of Two Moss Species Thriving in Different Environments-Terrestrial and Aquatic

Mosses host diverse bacterial communities essential for their fitness, nutrient acquisition, stress tolerance, and pathogen defense. Understanding the microbiome's taxonomic composition is the first step, but unraveling their functional capabilities is crucial for grasping their ecological significance. Metagenomics characterizes microbial communities by composition, while metatranscriptomics explores gene expression, providing insights into microbiome functionality beyond the structure. Here, we present for the first time a metatranscriptomic study of two moss species, Hypnum cupressiforme (Hedw.) and Platyhypnidium riparioides (Hedw.) Dixon., renowned as key biomonitors of atmospheric and water pollution. Our investigation extends beyond taxonomic profiling and offers a profound exploration of moss bacterial communities. Pseudomonadota and Actinobacteria are the dominant bacterial phyla in both moss species, but their proportions differ. In H. cupressiforme, Actinobacteria make up 62.45% and Pseudomonadota 32.48%, while in P. riparioides, Actinobacteria account for only 25.67% and Pseudomonadota 69.08%. This phylum-level contrast is reflected in genus-level differences. Our study also shows the expression of most genes related to nitrogen cycling across both microbiomes. Additionally, functional annotation highlights disparities in pathway prevalence, including carbon dioxide fixation, photosynthesis, and fatty acid biosynthesis, among others. These findings hint at potential metabolic distinctions between microbial communities associated with different moss species, influenced by their specific genotypes and habitats. The integration of metatranscriptomic data holds promise for enhancing our understanding of bryophyte-microbe partnerships, opening avenues for novel applications in conservation, bioremediation, and sustainable agriculture.

Harnessing biological nitrogen fixation in plant leaves

The importance of biological nitrogen fixation (BNF) in securing food production for the growing world population with minimal environmental cost has been increasingly acknowledged. Leaf surfaces are one of the biggest microbial habitats on Earth, harboring diverse free-living N2-fixers. These microbes inhabit the epiphytic and endophytic phyllosphere and contribute significantly to plant N supply and growth. Here, we summarize the contribution of phyllosphere-BNF to global N cycling, evaluate the diversity of leaf-associated N2-fixers across plant hosts and ecosystems, illustrate the ecological adaptation of N2-fixers to the phyllosphere, and identify the environmental factors driving BNF. Finally, we discuss potential BNF engineering strategies to improve the nitrogen uptake in plant leaves and thus sustainable food production.

Interplay of moss cover and seasonal variation regulate soil physicochemical properties and net nitrogen mineralization rates in Central Himalayas, India

Mosses (Class- Bryopsida) are vital to ecosystem dynamics in numerous biomes, although their effects on soil processes are poorly understood. The interplay of moss cover and seasonal variations in soil processes is still unclear in the Indian Central Himalayas. Therefore, we examined the seasonal variations in net nitrogen (N) mineralization rates and several soil properties under two ground covers (with and without moss cover). We used the ex-situ incubation technique to determine N mineralization rates (Rmin) and standard methodology for soil physical and chemical analysis. During the rainy season, the physical properties of the soil and its nutrients, apart from phosphorus, were higher under moss cover. The winter season, however, showed a different pattern, with soil properties exhibiting higher values in soils without moss cover. Ammonium concentrations were higher under moss cover, while nitrate concentrations were higher in soil without moss cover during rainy and winter seasons. The Rmin rates were higher in soil under moss cover, indicating that moss cover promotes N transformation. In contrast, Rmin rates were negative in soil without moss cover, indicating that N immobilization was dominant in N transformation under this ground cover during the rainy season. Our research shows that mosses positively impact the nutrient status and N mineralization rates in various temperate forest types. The seasonal patterns of soil properties are strongly influenced by soil temperature, moisture, and organic carbon. Therefore, we advocate the conservation of mosses and their integration into forest management plans for better ecosystem processes and services in the ecologically fragile Himalayas.

Species of associated bryophytes and their effects on the yield and quality of Dendrobium nobile

BACKGROUND

Dendrobium nobile has unique growth environment requirements, and unstable yields and high management costs are the key factors restricting the development of its imitation wild cultivation industry. The present study explored the effects of different associated bryophyte species on the yield and quality of D. nobile to clarify the dominant bryophyte species associated with D. nobile and to provide a scientific basis for the rational cultivation and quality evaluation of D. nobile.

RESULTS

The growth of D. nobile was closely related to the microenvironment of the Danxia stone, and the different associated bryophytes had different effects on D. nobile growth. There was a rich variety of bryophytes associated with D. nobile, with a total of 15 families, 24 genera and 31 species of bryophytes identified in the study area, including 13 families, 22 genera and 29 species of mosses and 2 families, 2 genera and 2 species of liverworts, and mosses predominated in the association with D. nobile. Usually, 3-9 species of bryophytes were growing in association with D. nobile, among which associations of 5-6 bryophytes species were more common, and the bryophytes associated with D. nobile were only related to the species to which they belonged. The dry matter accumulation, quality and mineral content of D. nobile differed significantly among different bryophyte species. The coefficients of variation of dry matter accumulation, dendrobine content and content of 11 mineral elements of D. nobile in the 35 sample quadrats were 25.00%, 21.08%, and 11.33-57.96%, respectively. The biomass, dendrobine content and mineral content of D. nobile were analysed by principal component analysis (PCA) and membership function. The results showed that each evaluation method initially screened Trachycystis microphylla and Leucobryum juniperoideum as the dominant associated bryophytes in the preliminary identification analysis, and the frequency of occurrence and coverage of the two bryophytes were significantly higher than those of the remaining bryophytes. It was determined that T. microphylla and L. juniperoideum were the dominant associated bryophytes.

CONCLUSIONS

There is a rich variety of bryophytes associated with D. nobile. The yield and quality of D. nobile differed significantly among different bryophyte species. T. microphylla and L. juniperoideum were the dominant associated bryophytes, and were the two bryophytes associated with D. nobile through mixed growth.

Diversity and regulation of symbiotic nitrogen fixation in plants

Plants associate with nitrogen-fixing bacteria to secure nitrogen, which is generally the most limiting nutrient for plant growth. Endosymbiotic nitrogen-fixing associations are widespread among diverse plant lineages, ranging from microalgae to angiosperms, and are primarily one of three types: cyanobacterial, actinorhizal or rhizobial. The large overlap in the signaling pathways and infection components of arbuscular mycorrhizal, actinorhizal and rhizobial symbioses reflects their evolutionary relatedness. These beneficial associations are influenced by environmental factors and other microorganisms in the rhizosphere. In this review, we summarize the diversity of nitrogen-fixing symbioses, key signal transduction pathways and colonization mechanisms relevant to such interactions, and compare and contrast these interactions with arbuscular mycorrhizal associations from an evolutionary standpoint. Additionally, we highlight recent studies on environmental factors regulating nitrogen-fixing symbioses to provide insights into the adaptation of symbiotic plants to complex environments.

Nitrogen fixation and nifH gene diversity in cyanobacteria living on feather mosses in a subalpine forest of Mt. Fuji

In the boreal forests, feather mosses such as Hylocomium splendens and Pleurozium schreberi are colonized by cyanobacteria, which provide large amounts of nitrogen to forest ecosystems through nitrogen fixation. Although these feather mosses are also ubiquitous in subalpine forests of East Asia, little is known regarding their associated cyanobacteria and their ability to fix nitrogen. In this study, we investigated (1) whether cyanobacteria co-exist and fix nitrogen in the two species of feather mosses that cover the ground surface in a subalpine forest of Mt. Fuji, (2) whether cyanobacteria belonging to a common cluster with boreal forests are found in feather mosses in Mt. Fuji, and (3) whether moss-associated nitrogen fixation rates differed among moss growing substrates, canopy openness, and moss nitrogen concentrations in the same forest area. Our results showed that cyanobacteria colonized feather mosses in the subalpine forests of Mt. Fuji and acetylene reduction rates as an index of nitrogen fixation tended to be higher in H. splendens than in P. schreberi. Based on analysis of the nifH gene, 43 bacterial operational taxonomic units (OTUs) were identified, 28 of which represented cyanobacteria. Among the five clusters of cyanobacteria classified based on their nifH gene and identified in northern Europe, four (Nostoc cluster I, Nostoc cluster II, Stigonema cluster, and nifH2 cluster) were also found at Mt. Fuji. The acetylene reduction rate differed depending on the moss growing substrate and the total nitrogen concentration of moss shoots, and a strong negative correlation was observed with the total nitrogen concentration.

The multiscale feedback theory of biodiversity

Plants and their environments engage in feedback loops that not only affect individuals, but also scale up to the ecosystem level. Community-level negative feedback facilitates local diversity, while the ability of plants to engineer ecosystem-wide conditions for their own benefit enhances local dominance. Here, we suggest that local and regional processes influencing diversity are inherently correlated: community-level negative feedback predominates among large species pools formed under historically common conditions; ecosystem-level positive feedback is most apparent in historically restricted habitats. Given enough time and space, evolutionary processes should lead to transitions between systems dominated by positive and negative feedbacks: species-poor systems should become richer due to diversification of dominants and adaptation of subordinates; however, new monodominants may emerge due to migration or new adaptations.

Revealing the transfer pathways of cyanobacterial-fixed N into the boreal forest through the feather-moss microbiome

INTRODUCTION

Biological N₂ fixation in feather-mosses is one of the largest inputs of new nitrogen (N) to boreal forest ecosystems; however, revealing the fate of newly fixed N within the bryosphere (i.e. bryophytes and their associated organisms) remains uncertain.

METHODS

Herein, we combined ¹⁵N tracers, high resolution secondary ion mass-spectrometry (NanoSIMS) and a molecular survey of bacterial, fungal and diazotrophic communities, to determine the origin and transfer pathways of newly fixed N₂ within feather-moss (Pleurozium schreberi) and its associated microbiome.

RESULTS

NanoSIMS images reveal that newly fixed N₂, derived from cyanobacteria, is incorporated into moss tissues and associated bacteria, fungi and micro-algae.

DISCUSSION

These images demonstrate that previous assumptions that newly fixed N₂ is sequestered into moss tissue and only released by decomposition are not correct. We provide the first empirical evidence of new pathways for N₂ fixed in feather-mosses to enter the boreal forest ecosystem (i.e. through its microbiome) and discuss the implications for wider ecosystem function.

Unraveling global and diazotrophic bacteriomes of boreal forest floor feather mosses and their environmental drivers at the ecosystem and at the plant scale in North America

Feather mosses are abundant cryptogams of the boreal forest floor and shelter a broad diversity of bacteria who have important ecological functions (e.g., decomposition, nutrient cycling). In particular, nitrogen (N₂-) fixation performed by feather moss-associated diazotrophs constitutes an important entry of nitrogen in the boreal forest ecosystem. However, the composition of the feather moss bacteriome and its environmental drivers are still unclear. Using cDNA amplicon sequencing of the 16S rRNA and nifH genes and cyanobacterial biomass quantification, we explored the active global and diazotrophic bacterial communities of two dominant feather moss species (i) at the ecosystem scale, along a 500-km climatic and nutrient deposition gradient in the North American boreal forest, and (ii) at the plant scale, along the moss shoot senescence gradient. We found that cyanobacteria were major actors of the feather moss bacteriome, accounting for 33% of global bacterial communities and 65% of diazotrophic communities, and that several cyanobacterial and methanotrophic genera were contributing to N₂-fixation. Moreover, we showed that bacteria were occupying ecological niches along the moss shoot, with phototrophs being dominant in the apical part and methanotrophs being dominant in the basal part. Finally, climate (temperature, precipitation), environmental variables (moss species, month, tree density) and nutrients (nitrogen, phosphorus, molybdenum, vanadium, iron) strongly shaped global and diazotrophic bacteriomes. In summary, this work presents evidence that the feather moss bacteriome plays crucial roles in supporting moss growth, health, and decomposition, as well as in the boreal forest carbon and nitrogen cycles. This study also highlights the substantial effects of climate and nutrients on the feather moss bacteriome, suggesting the importance of understanding the impacts of global change on moss-associated bacterial growth and activity.

Are legumes different? Origins and consequences of evolving nitrogen fixing symbioses

Nitrogen fixing symbioses between plants and bacteria are ancient and, while not numerous, are formed in diverse lineages of plants ranging from microalgae to angiosperms. One symbiosis stands out as the most widespread one is that between legumes and rhizobia, leading to the formation of nitrogen-fixing nodules. The legume family is one of the largest and most diverse group of plants and legumes have been used by humans since the beginning of agriculture, both as high nitrogen food, as well as pastures and rotation crops. One open question is whether their ability to form a nitrogen-fixing symbiosis has contributed to legumes' success, and whether legumes have any unique characteristics that have made them more diverse and widespread than other groups of plants. This review examines the evolutionary journey that has led to the diversification of legumes, in particular its nitrogen-fixing symbiosis, and asks four questions to investigate which legume traits might have contributed to their success: 1. In what ways do legumes differ from other plant groups that have evolved nitrogen-fixing symbioses? In order to answer this question, the characteristics of the symbioses, and efficiencies of nitrogen fixation are compared between different groups of nitrogen fixing plants. 2. Could certain unique features of legumes be a reason for their success? This section examines the manifestations and possible benefits of a nitrogen-rich 'lifestyle' in legumes. 3. If nitrogen fixation was a reason for such a success, why have some species lost the symbiosis? Formation of symbioses has trade-offs, and while these are less well known for non-legumes, there are known energetic and ecological reasons for loss of symbiotic potential in legumes. 4. What can we learn from the unique traits of legumes for future crop improvements? While exploiting some of the physiological properties of legumes could be used to improve legume breeding, our increasing molecular understanding of the essential regulators of root nodule symbioses raise hope of creating new nitrogen fixing symbioses in other crop species.

Evolutionary genomic insights into cyanobacterial symbioses in plants

Photosynthesis, the ability to fix atmospheric carbon dioxide, was acquired by eukaryotes through symbiosis: the plastids of plants and algae resulted from a cyanobacterial symbiosis that commenced more than 1.5 billion years ago and has chartered a unique evolutionary path. This resulted in the evolutionary origin of plants and algae. Some extant land plants have recruited additional biochemical aid from symbiotic cyanobacteria; these plants associate with filamentous cyanobacteria that fix atmospheric nitrogen. Examples of such interactions can be found in select species from across all major lineages of land plants. The recent rise in genomic and transcriptomic data has provided new insights into the molecular foundation of these interactions. Furthermore, the hornwort Anthoceros has emerged as a model system for the molecular biology of cyanobacteria-plant interactions. Here, we review these developments driven by high-throughput data and pinpoint their power to yield general patterns across these diverse symbioses.

Biotic and abiotic controls of nitrogen fixation in cyanobacteria-moss associations

Most mosses are colonized by nitrogen (N)-fixing cyanobacteria. This discovery is relatively recent, which can explain the large knowledge gaps the field is now tackling. For instance, while we have a good understanding of the abiotic controls (e.g. nutrient availability, increased temperature), we still do not know much about the biotic controls of N₂ fixation in mosses. I propose here that we should endeavour to position moss-cyanobacteria associations along the mutualism-parasitism continuum under varying abiotic conditions (e.g. nutrient availability). This would finally unravel the nature of the relationship between the partners and will be a big leap in our understanding of the evolution of plant-bacteria interactions using moss-cyanobacteria associations as a model system.

Photosynthetic microorganisms effectively contribute to bryophyte CO2 fixation in boreal and tropical regions

Photosynthetic microbes are omnipresent in land and water. While they critically influence primary productivity in aquatic systems, their importance in terrestrial ecosystems remains largely overlooked. In terrestrial systems, photoautotrophs occur in a variety of habitats, such as sub-surface soils, exposed rocks, and bryophytes. Here, we study photosynthetic microbial communities associated with bryophytes from a boreal peatland and a tropical rainforest. We interrogate their contribution to bryophyte C uptake and identify the main drivers of that contribution. We found that photosynthetic microbes take up twice more C in the boreal peatland (~4.4 mg CO2.h-1.m-2) than in the tropical rainforest (~2.4 mg CO2.h-1.m-2), which corresponded to an average contribution of 4% and 2% of the bryophyte C uptake, respectively. Our findings revealed that such patterns were driven by the proportion of photosynthetic protists in the moss microbiomes. Low moss water content and light conditions were not favourable to the development of photosynthetic protists in the tropical rainforest, which indirectly reduced the overall photosynthetic microbial C uptake. Our investigations clearly show that photosynthetic microbes associated with bryophyte effectively contribute to moss C uptake despite species turnover. Terrestrial photosynthetic microbes clearly have the capacity to take up atmospheric C in bryophytes living under various environmental conditions, and therefore potentially support rates of ecosystem-level net C exchanges with the atmosphere.

Chemical Stimulation of Heterocyte Differentiation by the Feather Moss Hylocomium splendens: a Potential New Step in Plant-Cyanobacteria Symbioses

Cyanobacteria associated with mosses play a key role in the nitrogen (N) cycle in unpolluted ecosystems. Mosses have been found to release molecules that induce morphophysiological changes in epiphytic cyanobionts. Nevertheless, the extent of moss influence on these microorganisms remains unknown. To evaluate how mosses or their metabolites influence N₂ fixation rates by cyanobacteria, we assessed the nitrogenase activity, heterocyte frequency and biomass of a cyanobacterial strain isolated from the feather moss Hylocomium splendens and a non-symbiotic strain when they were either growing by themselves, together with H. splendens or exposed to H. splendens water, acetone, ethanol, or isopropanol extracts. The same cyanobacterial strains were added to another moss (Taxiphyllum barbieri) and a liverwort (Monosolenium tenerum) to assess if these bryophytes affect N₂ fixation differently. Although no significant increases in nitrogenase activity by the cyanobacteria were observed when in contact with H. splendens shoots, both the symbiotic and non-symbiotic cyanobacteria increased nitrogenase activity as well as heterocyte frequency significantly upon exposure to H. splendens ethanol extracts. Contact with T. barbieri shoots, on the other hand, did lead to increases in nitrogenase activity, indicating low host-specificity to cyanobacterial activity. These findings suggest that H. splendens produces heterocyte-differentiating factors (HDFs) that are capable of stimulating cyanobacterial N₂ fixation regardless of symbiotic competency. Based on previous knowledge about the chemical ecology and dynamics of moss-cyanobacteria interactions, we speculate that HDF expression by the host takes place in a hypothetical new step occurring after plant colonization and the repression of hormogonia.

Unraveling host-microbe interactions and ecosystem functions in moss-bacteria symbioses

Mosses are non-vascular plants usually found in moist and shaded areas, with great ecological importance in several ecosystems. This is especially true in northern latitudes, where mosses are responsible for up to 100% of primary production in some ecosystems. Mosses establish symbiotic associations with unique bacteria that play key roles in the carbon and nitrogen cycles. For instance, in boreal environments, more than 35% of the nitrogen fixed by diazotrophic symbionts in peatlands is transferred to mosses, directly affecting carbon fixation by the hosts, while moss-associated methanotrophic bacteria contribute 10-30% of moss carbon. Further, half of ecosystem N input may derive from moss-cyanobacteria associations in pristine ecosystems. Moss-bacteria interactions have consequences on a global scale since northern environments sequester 20% of all the carbon generated by forests in the world and stock at least 32% of global terrestrial carbon. Different moss hosts influence bacteria in distinct ways, which suggests that threats to mosses also threaten unique microbial communities with important ecological and biogeochemical consequences. Since their origin ~500 Ma, mosses have interacted with bacteria, making these associations ideal models for understanding the evolution of plant-microbe associations and their contribution to biogeochemical cycles.

Dominance of coniferous and broadleaved trees drives bacterial associations with boreal feather mosses

The composition of ecologically important moss-associated bacterial communities seems to be mainly driven by host species but may also be shaped by environmental conditions related with tree dominance. The moss phyllosphere has been studied in coniferous forests while broadleaf forests remain understudied. To determine if host species or environmental conditions defined by tree dominance drives the bacterial diversity in the moss phyllosphere, we used 16S rRNA gene amplicon sequencing to quantify changes in bacterial communities as a function of host species (Pleurozium schreberi and Ptilium crista-castrensis) and forest type (coniferous black spruce versus deciduous broadleaf trembling aspen) in eastern Canada. The overall composition of moss phyllosphere was defined by the interaction of both factors, though most of bacterial phyla were determined by a strong effect of forest type. Bacterial α-diversity was highest in spruce forests, while there was greater turnover (β-diversity) and higher γ-diversity in aspen forests. Unexpectedly, Cyanobacteria were much more relatively abundant in aspen than in spruce forests, with the cyanobacteria family Nostocaceae differing the most between forest types. Our results advance the understanding of moss-associated microbial communities among coniferous and broadleaf deciduous forests, which are important with the increasing changes in tree dominance in the boreal system. This article is protected by copyright. All rights reserved.

Novel metabolic interactions and environmental conditions mediate the boreal peatmoss-cyanobacteria mutualism

Interactions between Sphagnum (peat moss) and cyanobacteria play critical roles in terrestrial carbon and nitrogen cycling processes. Knowledge of the metabolites exchanged, the physiological processes involved, and the environmental conditions allowing the formation of symbiosis is important for a better understanding of the mechanisms underlying these interactions. In this study, we used a cross-feeding approach with spatially resolved metabolite profiling and metatranscriptomics to characterize the symbiosis between Sphagnum and Nostoc cyanobacteria. A pH gradient study revealed that the Sphagnum–Nostoc symbiosis was driven by pH, with mutualism occurring only at low pH. Metabolic cross-feeding studies along with spatially resolved matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) identified trehalose as the main carbohydrate source released by Sphagnum, which were depleted by Nostoc along with sulfur-containing choline-O-sulfate, taurine and sulfoacetate. In exchange, Nostoc increased exudation of purines and amino acids. Metatranscriptome analysis indicated that Sphagnum host defense was downregulated when in direct contact with the Nostoc symbiont, but not as a result of chemical contact alone. The observations in this study elucidated environmental, metabolic, and physiological underpinnings of the widespread plant–cyanobacterial symbioses with important implications for predicting carbon and nitrogen cycling in peatland ecosystems as well as the basis of general host-microbe interactions.

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

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 ¹³CH₄ and ¹⁵N₂ 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.

The moss traits that rule cyanobacterial colonization

BACKGROUND AND AIMS

Cyanobacteria associated with mosses represent a main nitrogen (N) source in pristine, high-latitude and -altitude ecosystems due to their ability to fix N2. However, despite progress made regarding moss-cyanobacteria associations, the factors driving the large interspecific variation in N2 fixation activity between moss species remain elusive. The aim of the study was to identify the traits of mosses that determine cyanobacterial colonization and thus N2 fixation activity.

METHODS

Four moss species varying in N2 fixation activity were used to assess cyanobacterial abundance and activity to correlate it with moss traits (morphological, chemical, water-balance traits) for each species.

KEY RESULTS

Moss hydration rate was one of the pivotal traits, explaining 56 and 38 % of the variation in N2 fixation and cyanobacterial colonization, respectively, and was linked to morphological traits of the moss species. Higher abundance of cyanobacteria was found on shoots with smaller leaves, and with a high frequency of leaves. High phenol concentration inhibited N2 fixation but not colonization. These traits driving interspecific variation in cyanobacterial colonization, however, are also affected by the environment, and lead to intraspecific variation. Approximately 24 % of paraphyllia, filamentous appendages on Hylocomium splendens stems, were colonized by cyanobacteria.

CONCLUSIONS

Our findings show that interspecific variations in moss traits drive differences in cyanobacterial colonization and thus, N2 fixation activity among moss species. The key traits identified here that control moss-associated N2 fixation and cyanobacterial colonization could lead to improved predictions of N2 fixation in different moss species as a function of their morphology.

Empirical and Earth system model estimates of boreal nitrogen fixation often differ: A pathway toward reconciliation

The impacts of global environmental change on productivity in northern latitudes will be contingent on nitrogen (N) availability. In circumpolar boreal ecosystems, nonvascular plants (i.e., bryophytes) and associated N₂ -fixing diazotrophs provide one of the largest known N inputs but are rarely accounted for in Earth system models. Instead, most models link N₂ -fixation with the functioning of vascular plants. Neglecting nonvascular N₂ -fixation may be contributing toward high uncertainty that currently hinders model predictions in northern latitudes, where nonvascular N₂ -fixing plants are more common. Adequately accounting for nonvascular N₂ -fixation and its drivers could subsequently improve predictions of future N availability and ultimately, productivity, in northern latitudes. Here, we review empirical evidence of boreal nonvascular N₂ -fixation responses to global change factors (elevated CO₂ , N deposition, warming, precipitation, and shading by vascular plants), and compare empirical findings with model predictions of N₂ -fixation using nine Earth system models. The majority of empirical studies found positive effects of CO₂ , warming, precipitation, or light on nonvascular N₂ -fixation, but N deposition strongly downregulated N₂ -fixation in most empirical studies. Furthermore, we found that the responses of N₂ -fixation to elevated CO₂ were generally consistent between models and very limited empirical data. In contrast, empirical-model comparisons suggest that all models we assessed, and particularly those that scale N₂ -fixation with net primary productivity or evapotranspiration, may be overestimating N₂ -fixation under increasing N deposition. Overestimations could generate erroneous predictions of future N stocks in boreal ecosystems unless models adequately account for the drivers of nonvascular N₂ -fixation. Based on our comparisons, we recommend that models explicitly treat nonvascular N₂ -fixation and that field studies include more targeted measurements to improve model structures and parameterization.

A review of root exudates and rhizosphere microbiome for crop production

Increasing crop yields and ensuring food security is a major global challenge. In order to increase crop production, chemical fertilizers and pesticides are excessively used. However, the significance of root exudates is understudied. Beneficial interactions between plant and rhizosphere microbiome are critical for plant fitness and health. In this review, we discuss the application and progress of current research methods and technologies in terms of root exudates and rhizosphere microbiome. We summarize how root exudates promote plant access to nitrogen, phosphorus, and iron, and how root exudates strengthen plant immunity to cope with biotic stress by regulating the rhizosphere microbiome, and thereby reducing dependence on fertilizers and pesticides. Optimizing these interactions to increase plant nutrient uptake and resistance to biotic stresses offers one of the few untapped opportunities to confront sustainability issues in food security. To overcome the limitations of current research, combination of multi-omics, imaging technology together with synthetic communities has the potential to uncover the interaction mechanisms and to fill the knowledge gap for their applications in agriculture to achieve sustainable development.

The relationship of C and N stable isotopes to high-latitude moss-associated N2 fixation

Moss-associated N₂ fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N₂ fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N₂ fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N₂ fixation with ¹⁵N₂ gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that δ¹⁵N values would increase toward 0‰ with higher N₂ fixation to reflect the increasing contribution of fixed N₂ in moss biomass. Second, we hypothesized that δ¹³C and N₂ fixation would be positively related, as enriched δ¹³C signatures reflect abiotic conditions favorable to N₂ fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N₂ fixation rates and δ¹⁵N in a structural equation model. We found a significant positive relationship between δ¹³C and N₂ fixation only in Hypnales, where the probability of N₂ fixation activity reached 95% when δ¹³C values exceeded - 30.4‰. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N₂ fixation rates.

Ecological stoichiometric characteristics of soil-moss C, N, and P in restoration stages of karst rocky desertification

Rocky desertification is the most serious ecological disaster in karst areas. Comprehensive control of rocky desertification plays an important role in promoting the economic development of karst areas. Studying the stoichiometric characteristics of mosses and soil can provide a powerful reference for the ecological restoration and evaluation of ecosystems experiencing rocky desertification. Soil and mosses were collected from sites representing different stages of ecological restoration (bare rock, grassland, shrubland, and secondary forest), and the contents of carbon (C), nitrogen (N), and phosphorus (P) were detected for ecological stoichiometric analysis. The results indicate that in different restoration stages following karst rocky desertification, the contents of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) and the stoichiometric ratios in the shrub habitat are higher than those in the bare rock, grassland, and secondary forest habitats. However, the TP and available P contents were low at all stages (0.06 g/kg and 0.62 mg/kg, respectively). The N and P contents and stoichiometric ratios in the mosses showed no significant differences among the succession stages. The C contents in the mosses had a significant positive correlation with SOC and TN and TP content, and the P content had a significant positive correlation with the soil available P. However, there was a significant negative correlation between the C: N and C:P ratios of the bryophytes and soil C: N. In summary, during the process of natural restoration of karst rocky desertification areas, SOC and soil TN contents accumulate with each succession stage. Soil nutrients are higher in shrub habitats than in other succession stages. Mosses have a strong effect on improving soil nutrients in rocky desertification areas.

The bacterial communities of Alaskan mosses and their contributions to N2-fixation

BACKGROUND

Mosses in high-latitude ecosystems harbor diverse bacterial taxa, including N2-fixers which are key contributors to nitrogen dynamics in these systems. Yet the relative importance of moss host species, and environmental factors, in structuring these microbial communities and their N2-fixing potential remains unclear. We studied 26 boreal and tundra moss species across 24 sites in Alaska, USA, from 61 to 69° N. We used cultivation-independent approaches to characterize the variation in moss-associated bacterial communities as a function of host species identity and site characteristics. We also measured N2-fixation rates via 15N2 isotopic enrichment and identified potential N2-fixing bacteria using available literature and genomic information.

RESULTS

Host species identity and host evolutionary history were both highly predictive of moss microbiome composition, highlighting strong phylogenetic coherence in these microbial communities. Although less important, light availability and temperature also influenced composition of the moss microbiome. Finally, we identified putative N2-fixing bacteria specific to some moss hosts, including potential N2-fixing bacteria outside well-studied cyanobacterial clades.

CONCLUSIONS

The strong effect of host identity on moss-associated bacterial communities demonstrates mosses' utility for understanding plant-microbe interactions in non-leguminous systems. Our work also highlights the likely importance of novel bacterial taxa to N2-fixation in high-latitude ecosystems. Video Abstract.

Environmental patterns of brown moss- and Sphagnum-associated microbial communities

Northern peatlands typically develop through succession from fens dominated by the moss family Amblystegiaceae to bogs dominated by the moss genus Sphagnum. How the different plants and abiotic environmental conditions provided in Amblystegiaceae and Sphagnum peat shape the respective moss associated microbial communities is unknown. Through a large-scale molecular and biogeochemical study spanning Arctic, sub-Arctic and temperate regions we assessed how the endo- and epiphytic microbial communities of natural northern peatland mosses relate to peatland type (Sphagnum and Amblystegiaceae), location, moss taxa and abiotic environmental variables. Microbial diversity and community structure were distinctly different between Amblystegiaceae and Sphagnum peatlands, and within each of these two peatland types moss taxon explained the largest part of microbial community variation. Sphagnum and Amblystegiaceae shared few (< 1% of all operational taxonomic units (OTUs)) but strikingly abundant (up to 65% of relative abundance) OTUs. This core community overlapped by one third with the Sphagnum-specific core-community. Thus, the most abundant microorganisms in Sphagnum that are also found in all the Sphagnum plants studied, are the same OTUs as those few shared with Amblystegiaceae. Finally, we could confirm that these highly abundant OTUs were endophytes in Sphagnum, but epiphytes on Amblystegiaceae. We conclude that moss taxa and abiotic environmental variables associate with particular microbial communities. While moss taxon was the most influential parameter, hydrology, pH and temperature also had significant effects on the microbial communities. A small though highly abundant core community is shared between Sphagnum and Amblystegiaceae.

Bidirectional C and N transfer and a potential role for sulfur in an epiphytic diazotrophic mutualism

In nitrogen-limited boreal forests, associations between feathermoss and diazotrophic cyanobacteria control nitrogen inputs and thus carbon cycling, but little is known about the molecular regulators required for initiation and maintenance of these associations. Specifically, a benefit to the cyanobacteria is not known, challenging whether the association is a nutritional mutualism. Targeted mutagenesis of the cyanobacterial alkane sulfonate monooxygenase results in an inability to colonize feathermosses by the cyanobacterium Nostoc punctiforme, suggesting a role for organic sulfur in communication or nutrition. Isotope probing paired with high-resolution imaging mass spectrometry (NanoSIMS) demonstrated bidirectional elemental transfer between partners, with carbon and sulfur both being transferred to the cyanobacteria, and nitrogen transferred to the moss. These results support the hypothesis that moss and cyanobacteria enter a mutualistic exosymbiosis with substantial bidirectional material exchange of carbon and nitrogen and potential signaling through sulfur compounds.

Experimental assessment of tree canopy and leaf litter controls on the microbiome and nitrogen fixation rates of two boreal mosses

Nitrogen (N₂ )-fixing moss microbial communities play key roles in nitrogen cycling of boreal forests. Forest type and leaf litter inputs regulate moss abundance, but how they control moss microbiomes and N₂ -fixation remains understudied. We examined the impacts of forest type and broadleaf litter on microbial community composition and N₂ -fixation rates of Hylocomium splendens and Pleurozium schreberi. We conducted a moss transplant and leaf litter manipulation experiment at three sites with paired paper birch (Betula neoalaskana) and black spruce (Picea mariana) stands in Alaska. We characterized bacterial communities using marker gene sequencing, determined N₂ -fixation rates using stable isotopes (¹⁵ N₂ ) and measured environmental covariates. Mosses native to and transplanted into spruce stands supported generally higher N₂ -fixation and distinct microbial communities compared to similar treatments in birch stands. High leaf litter inputs shifted microbial community composition for both moss species and reduced N₂ -fixation rates for H. splendens, which had the highest rates. N₂ -fixation was positively associated with several bacterial taxa, including cyanobacteria. The moss microbiome and environmental conditions controlled N₂ -fixation at the stand and transplant scales. Predicted shifts from spruce- to deciduous-dominated stands will interact with the relative abundances of mosses supporting different microbiomes and N₂ -fixation rates, which could affect stand-level N inputs.

Genome Sequencing of Pleurozium schreberi: The Assembled and Annotated Draft Genome of a Pleurocarpous Feather Moss

The pleurocarpous feather moss Pleurozium schreberi is a ubiquitous moss species which plays a fundamental role in many terrestrial ecosystems, for instance within the boreal forest, the Earth's largest terrestrial biome, this species plays a significant role in driving ecosystem nitrogen and carbon inputs and fluxes. By hosting dinitrogen (N₂)-fixing cyanobacteria, the moss-cyanobacteria symbiosis constitutes the main nitrogen input into the ecosystem and by the high productivity and the low decomposability of the moss litter, P schreberi contributes significantly to build-up soil organic matter, and therefore long-term C sequestration. Knowledge on P. schreberi genome will facilitate the development of 'omics' and system's biology approaches to gain a more complete understanding of the physiology and ecological adaptation of the moss and the mechanisms underpinning the establishment of the symbiosis. Here we present the de novo assembly and annotation of P. schreberi genome that will help investigating these questions. The sequencing was performed using the HiSeq X platform with Illumina paired-end and mate-pair libraries prepared with CTAB extracted DNA. In total, the assembled genome was approximately 318 Mb, while repetitive elements account for 28.42% of the genome and 15,992 protein-coding genes were predicted from the genome, of which 84.23% have been functionally annotated. We anticipate that the genomic data generated will constitute a significant resource to study ecological and evolutionary genomics of P. schreberi, and will be valuable for evo-devo investigations as well as our understanding of the evolution of land plants by providing the genome of a pleurocarpous moss.

N2 fixation associated with the bryophyte layer is suppressed by low levels of nitrogen deposition in boreal forests

Biological fixation of atmospheric nitrogen (N₂) by bryophyte-associated cyanobacteria is an important source of plant-available N in the boreal biome. Information on the factors that drive biological N₂ fixation (BNF) rates is needed in order to understand the N dynamics of forests under a changing climate. We assessed the potential of several cryptogam species (the feather mosses Hylocomium splendens and Pleurozium schreberi, a group of Dicranum bryophytes, two liverworts, and Cladina lichens) to serve as associates of cyanobacteria or other N₂-fixing bacteria (diazotrophs) using acetylene reduction assay (ARA). We tested the hypotheses that the legacy of chronic atmospheric N deposition reduces BNF in the three bryophyte species, sampled from 12 coniferous forests located at latitudes 60-68° N in Finland. In addition, we tested the effect of moisture and temperature on BNF. All species studied showed a BNF signal in the north, with the highest rates in feather mosses. In moss samples taken along the north-south gradient with an increasing N bulk deposition from 0.8 to 4.4 kg ha^-1 year^-1, we found a clear decrease in BNF in both feather mosses and Dicranum group. BNF turned off at N deposition of 3-4 kg ha^-1 year^-1. Inorganic N (NH₄-N + NO₃-N) best predicted the BNF rate among regression models with different forms of N deposition as explanatory variables. However, in southern spruce stands, tree canopies modified the N in throughfall so that dissolved organic N (DON) leached from canopies compensated for inorganic N retained therein. Here, both DON and inorganic N negatively affected BNF in H. splendens. In laboratory experiments, BNF increased with increasing temperature and moisture. Our results suggest that even relatively low N deposition suppresses BNF in bryophyte-associated diazotrophs. Further, BNF could increase in northern low-deposition areas, especially if climate warming leads to moister conditions, as predicted.

Novel bacterial lineages associated with boreal moss species

Mosses are critical components of boreal ecosystems where they typically account for a large proportion of net primary productivity and harbour diverse bacterial communities that can be the major source of biologically-fixed nitrogen in these ecosystems. Despite their ecological importance, we have limited understanding of how microbial communities vary across boreal moss species and the extent to which local site conditions may influence the composition of these bacterial communities. We used marker gene sequencing to analyze bacterial communities associated with seven boreal moss species collected near Fairbanks, AK, USA. We found that host identity was more important than site in determining bacterial community composition and that mosses harbour diverse lineages of potential N₂ -fixers as well as an abundance of novel taxa assigned to understudied bacterial phyla (including candidate phylum WPS-2). We performed shotgun metagenomic sequencing to assemble genomes from the WPS-2 candidate phylum and found that these moss-associated bacteria are likely anoxygenic phototrophs capable of carbon fixation via RuBisCo with an ability to utilize byproducts of photorespiration from hosts via a glyoxylate shunt. These results give new insights into the metabolic capabilities of understudied bacterial lineages that associate with mosses and the importance of plant hosts in shaping their microbiomes.

Ecology and evolution of metabolic cross-feeding interactions in bacteria

Literature covered: early 2000s to late 2017Bacteria frequently exchange metabolites with other micro- and macro-organisms. In these often obligate cross-feeding interactions, primary metabolites such as vitamins, amino acids, nucleotides, or growth factors are exchanged. The widespread distribution of this type of metabolic interactions, however, is at odds with evolutionary theory: why should an organism invest costly resources to benefit other individuals rather than using these metabolites to maximize its own fitness? Recent empirical work has shown that bacterial genotypes can significantly benefit from trading metabolites with other bacteria relative to cells not engaging in such interactions. Here, we will provide a comprehensive overview over the ecological factors and evolutionary mechanisms that have been identified to explain the evolution and maintenance of metabolic mutualisms among microorganisms. Furthermore, we will highlight general principles that underlie the adaptive evolution of interconnected microbial metabolic networks as well as the evolutionary consequences that result for cells living in such communities.

Genomic Changes Associated with the Evolutionary Transitions of Nostoc to a Plant Symbiont

Cyanobacteria belonging to the genus Nostoc comprise free-living strains and also facultative plant symbionts. Symbiotic strains can enter into symbiosis with taxonomically diverse range of host plants. Little is known about genomic changes associated with evolutionary transition of Nostoc from free-living to plant symbiont. Here, we compared the genomes derived from 11 symbiotic Nostoc strains isolated from different host plants and infer phylogenetic relationships between strains. Phylogenetic reconstructions of 89 Nostocales showed that symbiotic Nostoc strains with a broad host range, entering epiphytic and intracellular or extracellular endophytic interactions, form a monophyletic clade indicating a common evolutionary history. A polyphyletic origin was found for Nostoc strains which enter only extracellular symbioses, and inference of transfer events implied that this trait was likely acquired several times in the evolution of the Nostocales. Symbiotic Nostoc strains showed enriched functions in transport and metabolism of organic sulfur, chemotaxis and motility, as well as the uptake of phosphate, branched-chain amino acids, and ammonium. The genomes of the intracellular clade differ from that of other Nostoc strains, with a gain/enrichment of genes encoding proteins to generate l-methionine from sulfite and pathways for the degradation of the plant metabolites vanillin and vanillate, and of the macromolecule xylan present in plant cell walls. These compounds could function as C-sources for members of the intracellular clade. Molecular clock analysis indicated that the intracellular clade emerged ca. 600 Ma, suggesting that intracellular Nostoc symbioses predate the origin of land plants and the emergence of their extant hosts.

Feathermoss and epiphytic Nostoc cooperate differently: expanding the spectrum of plant-cyanobacteria symbiosis

Dinitrogen (N₂)-fixation by cyanobacteria in symbiosis with feathermosses is the primary pathway of biological nitrogen (N) input into boreal forests. Despite its significance, little is known about the cyanobacterial gene repertoire and regulatory rewiring needed for the establishment and maintenance of the symbiosis. To determine gene acquisitions and regulatory changes allowing cyanobacteria to form and maintain this symbiosis, we compared genomically closely related symbiotic-competent and -incompetent Nostoc strains using a proteogenomics approach and an experimental set up allowing for controlled chemical and physical contact between partners. Thirty-two gene families were found only in the genomes of symbiotic strains, including some never before associated with cyanobacterial symbiosis. We identified conserved orthologs that were differentially expressed in symbiotic strains, including protein families involved in chemotaxis and motility, NO regulation, sulfate/phosphate transport, and glycosyl-modifying and oxidative stress-mediating exoenzymes. The physical moss–cyanobacteria epiphytic symbiosis is distinct from other cyanobacteria–plant symbioses, with Nostoc retaining motility, and lacking modulation of N₂-fixation, photosynthesis, GS-GOGAT cycle and heterocyst formation. The results expand our knowledge base of plant–cyanobacterial symbioses, provide a model of information and material exchange in this ecologically significant symbiosis, and suggest new currencies, namely nitric oxide and aliphatic sulfonates, may be involved in establishing and maintaining the cyanobacteria–feathermoss symbiosis.

Adaptation Mechanisms in the Evolution of Moss Defenses to Microbes

Bryophytes, including mosses, liverworts and hornworts are early land plants that have evolved key adaptation mechanisms to cope with abiotic stresses and microorganisms. Microbial symbioses facilitated plant colonization of land by enhancing nutrient uptake leading to improved plant growth and fitness. In addition, early land plants acquired novel defense mechanisms to protect plant tissues from pre-existing microbial pathogens. Due to its evolutionary stage linking unicellular green algae to vascular plants, the non-vascular moss Physcomitrella patens is an interesting organism to explore the adaptation mechanisms developed in the evolution of plant defenses to microbes. Cellular and biochemical approaches, gene expression profiles, and functional analysis of genes by targeted gene disruption have revealed that several defense mechanisms against microbial pathogens are conserved between mosses and flowering plants. P. patens perceives pathogen associated molecular patterns by plasma membrane receptor(s) and transduces the signal through a MAP kinase (MAPK) cascade leading to the activation of cell wall associated defenses and expression of genes that encode proteins with different roles in plant resistance. After pathogen assault, P. patens also activates the production of ROS, induces a HR-like reaction and increases levels of some hormones. Furthermore, alternative metabolic pathways are present in P. patens leading to the production of a distinct metabolic scenario than flowering plants that could contribute to defense. P. patens has acquired genes by horizontal transfer from prokaryotes and fungi, and some of them could represent adaptive benefits for resistance to biotic stress. In this review, the current knowledge related to the evolution of plant defense responses against pathogens will be discussed, focusing on the latest advances made in the model plant P. patens.

A Genetic and Chemical Perspective on Symbiotic Recruitment of Cyanobacteria of the Genus Nostoc into the Host Plant Blasia pusilla L

Liverwort Blasia pusilla L. recruits soil nitrogen-fixing cyanobacteria of genus Nostoc as symbiotic partners. In this work we compared Nostoc community composition inside the plants and in the soil around them from two distant locations in Northern Norway. STRR fingerprinting and 16S rDNA phylogeny reconstruction showed a remarkable local diversity among isolates assigned to several Nostoc clades. An extensive web of negative allelopathic interactions was recorded at an agricultural site, but not at the undisturbed natural site. The cell extracts of the cyanobacteria did not show antimicrobial activities, but four isolates were shown to be cytotoxic to human cells. The secondary metabolite profiles of the isolates were mapped by MALDI-TOF MS, and the most prominent ions were further analyzed by Q-TOF for MS/MS aided identification. Symbiotic isolates produced a great variety of small peptide-like substances, most of which lack any record in the databases. Among identified compounds we found microcystin and nodularin variants toxic to eukaryotic cells. Microcystin producing chemotypes were dominating as symbiotic recruits but not in the free-living community. In addition, we were able to identify several novel aeruginosins and banyaside-like compounds, as well as nostocyclopeptides and nosperin.

Seasonal variation in nifH abundance and expression of cyanobacterial communities associated with boreal feather mosses

Dinitrogen (N2)-fixation by cyanobacteria living in symbiosis with pleurocarpous feather mosses (for example, Pleurozium schreberi and Hylocomium splendens) represents the main pathway of biological N input into N-depleted boreal forests. Little is known about the role of the cyanobacterial community in contributing to the observed temporal variability of N2-fixation. Using specific nifH primers targeting four major cyanobacterial clusters and quantitative PCR, we investigated how community composition, abundance and nifH expression varied by moss species and over the growing seasons. We evaluated N2-fixation rates across nine forest sites in June and September and explored the abundance and nifH expression of individual cyanobacterial clusters when N2-fixation is highest. Our results showed temporal and host-dependent variations of cyanobacterial community composition, nifH gene abundance and expression. N2-fixation was higher in September than June for both moss species, explained by higher nifH gene expression of individual clusters rather than higher nifH gene abundance or differences in cyanobacterial community composition. In most cases, 'Stigonema cluster' made up less than 29% of the total cyanobacterial community, but accounted for the majority of nifH gene expression (82-94% of total nifH expression), irrespective of sampling date or moss species. Stepwise multiple regressions showed temporal variations in N2-fixation being greatly explained by variations in nifH expression of the 'Stigonema cluster'. These results suggest that Stigonema is potentially the most influential N2-fixer in symbiosis with boreal forest feather mosses.

The Sensitivity of Moss-Associated Nitrogen Fixation towards Repeated Nitrogen Input

Nitrogen (N₂) fixation is a major source of available N in ecosystems that receive low amounts of atmospheric N deposition. In boreal forest and subarctic tundra, the feather moss Hylocomium splendens is colonized by N₂ fixing cyanobacteria that could contribute fundamentally to increase the N pool in these ecosystems. However, N₂ fixation in mosses is inhibited by N input. Although this has been shown previously, the ability of N₂ fixation to grow less sensitive towards repeated, increased N inputs remains unknown. Here, we tested if N₂ fixation in H. splendens can recover from increased N input depending on the N load (0, 5, 20, 80, 320 kg N ha^-1 yr^-1) after a period of N deprivation, and if sensitivity towards increased N input can decrease after repeated N additions. Nitrogen fixation in the moss was inhibited by the highest N addition, but was promoted by adding 5 kg N ha^-1 yr^-1, and increased in all treatments during a short period of N deprivation. The sensitivity of N₂ fixation towards repeated N additions seem to decrease in the 20 and 80 kg N additions, but increased in the highest N addition (320 kg N ha^-1 yr^-1). Recovery of N in leachate samples increased with increasing N loads, suggesting low retention capabilities of mosses if N input is above 5 kg N ha^-1 yr^-1. Our results demonstrate that the sensitivity towards repeated N additions is likely to decrease if N input does not exceed a certain threshold.

Across-habitat comparison of diazotroph activity in the subarctic

Nitrogen (N) fixation by N2-fixing bacteria (diazotrophs) is the primary N input to pristine ecosystems like boreal forests and subarctic and arctic tundra. However, the contribution by the various diazotrophs to habitat N2 fixation remains unclear. We present results from in situ assessments of N2 fixation of five diazotroph associations (with a legume, lichen, feather moss, Sphagnum moss and free-living) incorporating the ground cover of the associations in five typical habitats in the subarctic (wet and dry heath, polygon-heath, birch forest, mire). Further, we assessed the importance of soil and air temperature, as well as moisture conditions for N2 fixation. Across the growing season, the legume had the highest total as well as the highest fraction of N2 fixation rates at habitat level in the heaths (>85 % of habitat N2 fixation), whereas the free-living diazotrophs had the highest N2 fixation rates in the polygon heath (56 %), the lichen in the birch forest (87 %) and Sphagnum in the mire (100 %). The feather moss did not contribute more than 15 % to habitat N2 fixation in any of the habitats despite its high ground cover. Moisture content seemed to be a major driver of N2 fixation in the lichen, feather moss and free-living diazotrophs. Our results show that the range of N2 fixers found in pristine habitats contribute differently to habitat N2 fixation and that ground cover of the associates does not necessarily mirror contribution.

Slow recovery of High Arctic heath communities from nitrogen enrichment

Arctic ecosystems are strongly nutrient limited and exhibit dramatic responses to nitrogen (N) enrichment, the reversibility of which is unknown. This study uniquely assesses the potential for tundra heath to recover from N deposition and the influence of phosphorus (P) availability on recovery. We revisited an experiment in Svalbard, established in 1991, in which N was applied at rates representing atmospheric N deposition in Europe (10 and 50 kg N ha(-1)  yr(-1) ; 'low' and 'high', respectively) for 3-8 yr. We investigated whether significant effects on vegetation composition and ecosystem nutrient status persisted up to 18 yr post-treatment. Although the tundra heath is no longer N saturated, N treatment effects persist and are strongly P-dependent. Vegetation was more resilient to N where no P was added, although shrub cover is still reduced in low-N plots. Where P was also added (5 kg P ha(-1)  yr(-1) ), there are still effects of low N on community composition and nutrient dynamics. High N, with and without P, has many lasting impacts. Importantly, N + P has caused dramatically increased moss abundance, which influences nutrient dynamics. Our key finding is that Arctic ecosystems are slow to recover from even small N inputs, particularly where P is not limiting.

Evolution of microbial markets

Biological market theory has been used successfully to explain cooperative behavior in many animal species. Microbes also engage in cooperative behaviors, both with hosts and other microbes, that can be described in economic terms. However, a market approach is not traditionally used to analyze these interactions. Here, we extend the biological market framework to ask whether this theory is of use to evolutionary biologists studying microbes. We consider six economic strategies used by microbes to optimize their success in markets. We argue that an economic market framework is a useful tool to generate specific and interesting predictions about microbial interactions, including the evolution of partner discrimination, hoarding strategies, specialized versus diversified mutualistic services, and the role of spatial structures, such as flocks and consortia. There is untapped potential for studying the evolutionary dynamics of microbial systems. Market theory can help structure this potential by characterizing strategic investment of microbes across a diversity of conditions.