Possible nitrogen fertilization of the early Earth Ocean by microbial continental ecosystems

Enhancing Soil Health Through Biocrusts: A Microbial Ecosystem Approach for Degradation Control and Restoration

Escalating global concerns about soil degradation, driven by erosion, salinization, compaction, pollution, and organic matter loss, highlights the critical need for sustainable remediation. Biocrusts-complex communities of cyanobacteria, algae, lichens, bryophytes, and fungi-play a pivotal role in soil stabilization, erosion prevention, and nutrient cycling. This study presents recent advancements in biocrust application for soil management and restoration, focusing on artificial biocrusts as a nature-based solution biotechnology. It emphasizes their effectiveness in enhancing soil quality, biodiversity, and ecosystem functionality. Researchers are leveraging these microbial communities to develop strategies that improve soil health and rehabilitate degraded landscapes. The review concludes that biocrusts are a viable strategy for boosting soil resilience and enhancing soil health against environmental stressors. It recommends future research on their long-term ecological impacts and methods to enhance their functionality.

Carbon- and Nitrogen-Based Complexes as Photocatalysts for Prebiotic and Oxygen Chemistry during Earth Evolution

Sunlight has long served as primary energy source on our planet, shaping the behavior of living organisms. Extensive research has been dedicated to unraveling the evolutionary pathways involved. When the formation of Earth atmosphere, it primarily consisted of small gas molecules, which are considered crucial for the emergence of life. Recent demonstrations have shown that these molecules can also be transformed into semiconductors, with the potential to harness solar energy and catalyze chemical reactions as photocatalysts. Building upon this research, this minireview focuses on the potential revolutionary impact of photocatalysis on Earth. Initially, it examines key reactions, such as the formation of prebiotic molecules and the oxygen evolution reaction via water oxidation. Additionally, various C-N complexes in photocatalysts are explored, showcasing their roles in catalyzing chemical reactions. The conclusion and outlook provide a potential pathway for the evolution of Earth, shedding light on the significance of metal-free photocatalysts in development of Earth.

Volatile Organic Compound Metabolism on Early Earth

Biogenic volatile organic compounds (VOCs) constitute a significant portion of gas-phase metabolites in modern ecosystems and have unique roles in moderating atmospheric oxidative capacity, solar radiation balance, and aerosol formation. It has been theorized that VOCs may account for observed geological and evolutionary phenomena during the Archaean, but the direct contribution of biology to early non-methane VOC cycling remains unexplored. Here, we provide an assessment of all potential VOCs metabolized by the last universal common ancestor (LUCA). We identify enzyme functions linked to LUCA orthologous protein groups across eight literature sources and estimate the volatility of all associated substrates to identify ancient volatile metabolites. We hone in on volatile metabolites with confirmed modern emissions that exist in conserved metabolic pathways and produce a curated list of the most likely LUCA VOCs. We introduce volatile organic metabolites associated with early life and discuss their potential influence on early carbon cycling and atmospheric chemistry.

Nitrate transformation and source tracking of Yarlung Tsangpo River using a multi-tracer approach combined with Bayesian stable isotope mixing model

Excessive levels of nitrate nitrogen (NO3--N) could lead to ecological issues, particularly in the Yarlung Tsangpo River (YTR) region located on the Qinghai Tibet Plateau. Therefore, it is crucial to understand the fate and sources of nitrogen to facilitate pollution mitigation efforts. Herein, multiple isotopes and source resolution models were applied to analyze key transformation processes and quantify the sources of NO3-. The δ15N-NO3- and δ18O-NO3- isotopic compositions in the YTR varied between 1.23‰ and 13.64‰ and -7.88‰-11.19‰, respectively. The NO3--N concentrations varied from 0.08 to 0.86 mg/L in the dry season and 0.20-1.19 mg/L during the wet season. Nitrification remained the primary process for nitrogen transformation in both seasons. However, the wet season had a widespread effect on increasing nitrate levels, while denitrification had a limited ability to reduce nitrate. The elevated nitrate concentrations during the flood season were caused by increased release of NO3- from manure & sewage (M&S) and chemical fertilizers (CF). Future endeavors should prioritize enhancing management strategies to improve the utilization efficiency of CF and hinder the direct entry of untreated sewage into the water system.

Dating Ammonia-Oxidizing Bacteria with Abundant Eukaryotic Fossils

Evolution of a complete nitrogen (N) cycle relies on the onset of ammonia oxidation, which aerobically converts ammonia to nitrogen oxides. However, accurate estimation of the antiquity of ammonia-oxidizing bacteria (AOB) remains challenging because AOB-specific fossils are absent and bacterial fossils amenable to calibrate molecular clocks are rare. Leveraging the ancient endosymbiosis of mitochondria and plastid, as well as using state-of-the-art Bayesian sequential dating approach, we obtained a timeline of AOB evolution calibrated largely by eukaryotic fossils. We show that the first AOB evolved in marine Gammaproteobacteria (Gamma-AOB) and emerged between 2.1 and 1.9 billion years ago (Ga), thus postdating the Great Oxidation Event (GOE; 2.4 to 2.32 Ga). To reconcile the sedimentary N isotopic signatures of ammonia oxidation occurring near the GOE, we propose that ammonia oxidation likely occurred at the common ancestor of Gamma-AOB and Gammaproteobacterial methanotrophs, or the actinobacterial/verrucomicrobial methanotrophs which are known to have ammonia oxidation activities. It is also likely that nitrite was transported from the terrestrial habitats where ammonia oxidation by archaea took place. Further, we show that the Gamma-AOB predated the anaerobic ammonia-oxidizing (anammox) bacteria, implying that the emergence of anammox was constrained by the availability of dedicated ammonia oxidizers which produce nitrite to fuel anammox. Our work supports a new hypothesis that N redox cycle involving nitrogen oxides evolved rather late in the ocean.

Mapping Geological Events and Nitrogen Fixation Evolution Onto the Timetree of the Evolution of Nitrogen-Fixation Genes

Nitrogen is essential for all organisms, but biological nitrogen fixation (BNF) occurs only in a small fraction of prokaryotes. Previous studies divided nitrogenase-gene-carrying prokaryotes into Groups I to IV and provided evidence that BNF first evolved in bacteria. This study constructed a timetree of the evolution of nitrogen-fixation genes and estimated that archaea evolved BNF much later than bacteria and that nitrogen-fixing cyanobacteria evolved later than 1,900 MYA, considerably younger than the previous estimate of 2,200 MYA. Moreover, Groups III and II/I diverged ∼2,280 MYA, after the Kenorland supercontinent breakup (∼2,500-2,100 MYA) and the Great Oxidation Event (∼2,400-2,100 MYA); Groups III and Vnf/Anf diverged ∼2,086 MYA, after the Yarrabubba impact (∼2,229 MYA); and Groups II and I diverged ∼1,920 MYA, after the Vredefort impact (∼2,023 MYA). In summary, this study provided a timescale of BNF events and discussed the possible effects of geological events on BNF evolution.

Modern analogs for ammonia flux from terrestrial hydrothermal features to the Archean atmosphere

The isotopic composition of nitrogen in the rock record provides valuable evidence of reactive nitrogen sources and processing on early Earth, but the wide range of δ15N values (- 10.2 to + 50.4‰) leads to ambiguity in defining the early Precambrian nitrogen cycle. The high δ15N values have been explained by large fractionation associated with the onset of nitrification and/or fractionation produced by ammonia-ammonium equilibrium and water-air flux in alkaline paleolakes. Previous flux sensitivity studies in modern water bodies report alkaline pH is not a prerequisite and temperature can be the dominate parameter driving water-air flux. Here, I use the chemical and physical components of 1022 modern hydrothermal features to provide evidence that water-air NH3 flux produced a significant source of fixed nitrogen to early Earth's atmosphere and biosphere. With regard to the modeled average NH3 flux (2.1 kg N m-2 year-1) and outlier removed average flux (1.2 kg N m-2 year-1), the Archean Earth's surface would need to be 0.0092, and 0.017% terrestrial hydrothermal features, respectively, for the flux to match the annual amount of N produced by biogenic fixation on modern Earth. Water-air NH3 flux from terrestrial hydrothermal features may have played a significant role in supplying bioavailable nitrogen to early life.

The Microbiology of Biological Soil Crusts

Biological soil crusts are thin, inconspicuous communities along the soil atmosphere ecotone that, until recently, were unrecognized by ecologists and even more so by microbiologists. In its broadest meaning, the term biological soil crust (or biocrust) encompasses a variety of communities that develop on soil surfaces and are powered by photosynthetic primary producers other than higher plants: cyanobacteria, microalgae, and cryptogams like lichens and mosses. Arid land biocrusts are the most studied, but biocrusts also exist in other settings where plant development is constrained. The minimal requirement is that light impinge directly on the soil; this is impeded by the accumulation of plant litter where plants abound. Since scientists started paying attention, much has been learned about their microbial communities, their composition, ecological extent, and biogeochemical roles, about how they alter the physical behavior of soils, and even how they inform an understanding of early life on land. This has opened new avenues for ecological restoration and agriculture.

Spatial distribution and core community of diazotrophs in Biological soil crusts and subsoils in temperate semi-arid and arid deserts of China

Introduction

Biological soil crusts (BSCs) are distributed in arid and semiarid regions, and they function as important microhabitats for nitrogen fixation. The diazotroph community is critical for nitrogen fixation in BSCs and their subsoils. However, little is known about the key groups in different types of BSCs and subsoils in temperate semi-arid or arid deserts.

Methods

Here, we sampled three types of BSCs and their subsoils from the Inner Mongolian plateau, investigated the distribution characteristics of the diazotroph community by high-throughput sequencing, predicted keystone species using the molecular ecological network analyses pipeline (MENAP), and verified their close relationship with the available nitrogen (AN) content.

Results

The results showed that available nitrogen content in BSCs was higher than that in subsoils in three different types of BSCs, and there were differences among seasons and according to the mean annual precipitation. The abundance of diazotrophs was higher in Cyano-BSCs, while diversity had no significant difference among BSCs and subsoils. Cyanobacteria and Proteobacteria, Nostocaceae and Scytonemataceae, Skermanella, Scytonema, Azohydromonas, Nostoc and Trichormus were the dominant phyla, families, and genera, respectively. The dominant groups belong to Skermanella, Scytonema, and Nostoc formed the core diazotroph community in the three types of BSCs and subsoils, and each had a close relationship with AN.

Discussion

These results indicate that diazotrophs in BSCs and subsoils had high diversity, and the core diazotroph communities have a close relationship with nitrogen fixation and that they may be the main contributor to nitrogen fixing in BSCs and subsoils in temperate deserts.

Origin and evolution of nitrogen fixation in prokaryotes

The origin of nitrogen fixation is an important issue in evolutionary biology. While nitrogen is required by all living organisms, only a small fraction of bacteria and archaea can fix nitrogen. The prevailing view is that nitrogen fixation first evolved in archaea and was later transferred to bacteria. However, nitrogen-fixing (Nif) bacteria are far larger in number and far more diverse in ecological niches than Nif archaea. We, therefore, propose the bacteria-first hypothesis, which postulates that nitrogen fixation first evolved in bacteria and was later transferred to archaea. As >30,000 prokaryotic genomes have been sequenced, we conduct an in-depth comparison of the two hypotheses. We first identify the six genes involved in nitrogen fixation in all sequenced prokaryotic genomes and then reconstruct phylogenetic trees using the six Nif proteins individually or in combination. In each of these trees, the earliest lineages are bacterial Nif protein sequences and in the oldest clade (group) the archaeal sequences are all nested inside bacterial sequences, suggesting that the Nif proteins first evolved in bacteria. The bacteria-first hypothesis is further supported by the observation that the majority of Nif archaea carry the major bacterial Mo (molybdenum) transporter (ModABC) rather than the archaeal Mo transporter (WtpABC). Moreover, in our phylogeny of all available ModA and WtpA protein sequences, the earliest lineages are bacterial sequences while archaeal sequences are nested inside bacterial sequences. Furthermore, the bacteria-first hypothesis is supported by available isotopic data. In conclusion, our study strongly supports the bacteria-first hypothesis.

Contrasting nutrient availability between marine and brackish waters in the late Mesoproterozoic: Evidence from the Paranoá Group, Brazil

Understanding the delayed rise of eukaryotic life on Earth is one of the most fundamental questions about biological evolution. Numerous studies have presented evidence for oxygen and nutrient limitations in seawater during the Mesoproterozoic era, indicating that open marine settings may not have been able to sustain a eukaryotic biosphere with complex, multicellular organisms. However, many of these data sets represent restricted marine basins, which may bias our view of habitability. Furthermore, it remains untested whether rivers could have supplied significant nutrient fluxes to coastal habitats. To better characterize the sources of the major nutrients nitrogen and phosphorus, we turned to the late Mesoproterozoic Paranoá Group in Brazil (~1.1 Ga), which was deposited on a passive margin of the São Francisco craton. We present carbon, nitrogen and sulphur isotope data from an open shelf setting (Fazenda Funil) and from a brackish-water environment with significant riverine input (São Gabriel). Our results show that waters were well-oxygenated and nitrate was bioavailable in the open ocean setting at Fazenda Funil; the redoxcline appears to have been deeper and further offshore compared to restricted marine basins elsewhere in the Mesoproterozoic. In contrast, the brackish site at São Gabriel received only limited input of marine nitrate and sulphate. Nevertheless, previous reports of acritarchs reveal that this brackish-water setting was habitable to eukaryotic life. Paired with previously published cadmium isotope data, which can be used as a proxy for phosphorus cycling, our results suggest that complex organisms were perhaps not strictly dependent on marine nutrient supplies. Riverine influxes of P and possibly other nutrients likely rendered coastal waters perhaps equally habitable to the Mesoproterozoic open ocean. This conclusion supports the notion that eukaryotic organisms may have thrived in brackish or perhaps even freshwater environments.

An overview of experimental simulations of microbial activity in early Earth

Microbial activity has shaped the evolution of the ocean and atmosphere throughout the Earth history. Thus, experimental simulations of microbial metabolism under the environment conditions of the early Earth can provide vital information regarding biogeochemical cycles and the interaction and coevolution between life and environment, with important implications for extraterrestrial exploration. In this review, we discuss the current scope and knowledge of experimental simulations of microbial activity in environments representative of those of early Earth, with perspectives on future studies. Inclusive experimental simulations involving multiple species, and cultivation experiments with more constraints on environmental conditions similar to early Earth would significantly advance our understanding of the biogeochemical cycles of the geological past.

Using modern low-oxygen marine ecosystems to understand the nitrogen cycle of the Paleo- and Mesoproterozoic oceans

During the productive Paleoproterozoic (2.4-1.8 Ga) and less productive Mesoproterozoic (1.8-1.0 Ga), the ocean was suboxic to anoxic and multicellular organisms had not yet evolved. Here, we link geologic information about the Proterozoic ocean to microbial processes in modern low-oxygen systems. High iron concentrations and rates of Fe cycling in the Proterozoic are the largest differences from modern oxygen-deficient zones. In anoxic waters, which composed most of the Paleoproterozoic and ~40% of the Mesoproterozoic ocean, nitrogen cycling dominated. Rates of N₂ production by denitrification and anammox were likely linked to sinking organic matter fluxes and in situ primary productivity under anoxic conditions. Additionally autotrophic denitrifiers could have used reduced iron or methane. 50% of the Mesoproterozoic ocean may have been suboxic, promoting nitrification and metal oxidation in the suboxic water and N₂ O and N₂ production by partial and complete denitrification in anoxic zones in organic aggregates. Sulfidic conditions may have composed ~10% of the Mesoproterozoic ocean focused along continental margins. Due to low nitrate concentrations in offshore regions, anammox bacteria likely dominated N₂ production immediately above sulfidic zones, but in coastal regions, higher nitrate concentrations probably promoted complete S-oxidizing autotrophic denitrification at the sulfide interface.

In situ S-isotope compositions of sulfate and sulfide from the 3.2 Ga Moodies Group, South Africa: A record of oxidative sulfur cycling

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO₄ ). The origin of this barite remains controversially debated. The mass-independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen-poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S-isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S-isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long-term increase in Δ³³ S_sulfate values through the Paleoarchean.

Biological Soil Crusts as Modern Analogs for the Archean Continental Biosphere: Insights from Carbon and Nitrogen Isotopes

Stable isotope signatures of elements related to life such as carbon and nitrogen can be powerful biomarkers that provide key information on the biological origin of organic remains and their paleoenvironments. Marked advances have been achieved in the last decade in our understanding of the coupled evolution of biological carbon and nitrogen cycling and the chemical evolution of the early Earth thanks, in part, to isotopic signatures preserved in fossilized microbial mats and organic matter of marine origin. However, the geologic record of the early continental biosphere, as well as its evolution and biosignatures, is still poorly constrained. Following a recent report of direct fossil evidence of life on land at 3.22 Ga, we compare here the carbon and nitrogen isotopic signals of this continental Archean biosphere with biosignatures of cyanobacteria biological soil crusts (cyanoBSCs) colonizing modern arid environments. We report the first extended δ¹³C and δ¹⁵N data set from modern cyanoBSCs and show that these modern communities harbor specific isotopic biosignatures that compare well with continental Archean organic remains. We therefore suggest that cyanoBSCs are likely relevant analogs for the earliest continental ecosystems. As such, they can provide key information on the timing, extent, and possibly mechanism of colonization of the early Earth's emergent landmasses.

Niche Partitioning with Temperature among Heterocystous Cyanobacteria (Scytonema spp., Nostoc spp., and Tolypothrix spp.) from Biological Soil Crusts

Heterocystous cyanobacteria of biocrusts are key players for biological fixation in drylands, where nitrogen is only second to water as a limiting resource. We studied the niche partitioning among the three most common biocrust heterocystous cyanobacteria sts using enrichment cultivation and the determination of growth responses to temperature in 30 representative isolates. Isolates of Scytonema spp. were most thermotolerant, typically growing up to 40 °C, whereas only those of Tolypothrix spp. grew at 4 °C. Nostoc spp. strains responded well at intermediate temperatures. We could trace the heat sensitivity in Nostoc spp. and Tolypothrix spp. to N₂-fixation itself, because the upper temperature for growth increased under nitrogen replete conditions. This may involve an inability to develop heterocysts (specialized N₂-fixing cells) at high temperatures. We then used a meta-analysis of biocrust molecular surveys spanning four continents to test the relevance of this apparent niche partitioning in nature. Indeed, the geographic distribution of the three types was clearly constrained by the mean local temperature, particularly during the growth season. This allows us to predict a potential shift in dominance in many locales as a result of global warming, to the benefit of Scytonema spp. populations.

Mesophilic microorganisms build terrestrial mats analogous to Precambrian microbial jungles

Development of Archean paleosols and patterns of Precambrian rock weathering suggest colonization of continents by subaerial microbial mats long before evolution of land plants in the Phanerozoic Eon. Modern analogues for such mats, however, have not been reported, and possible biogeochemical roles of these mats in the past remain largely conceptual. We show that photosynthetic, subaerial microbial mats from Indonesia grow on mafic bedrocks at ambient temperatures and form distinct layers with features similar to Precambrian mats and paleosols. Such subaerial mats could have supported a substantial aerobic biosphere, including nitrification and methanotrophy, and promoted methane emissions and oxidative weathering under ostensibly anoxic Precambrian atmospheres. High C-turnover rates and cell abundances would have made these mats prime locations for early microbial diversification. Growth of landmass in the late Archean to early Proterozoic Eons could have reorganized biogeochemical cycles between land and sea impacting atmospheric chemistry and climate.

How did the evolution of oxygenic photosynthesis influence the temporal and spatial development of the microbial iron cycle on ancient Earth?

Iron is the most abundant redox active metal on Earth and thus provides one of the most important records of the redox state of Earth's ancient atmosphere, oceans and landmasses over geological time. The most dramatic shifts in the Earth's iron cycle occurred during the oxidation of Earth's atmosphere. However, tracking the spatial and temporal development of the iron cycle is complicated by uncertainties about both the timing and location of the evolution of oxygenic photosynthesis, and by the myriad of microbial processes that act to cycle iron between redox states. In this review, we piece together the geological evidence to assess where and when oxygenic photosynthesis likely evolved, and attempt to evaluate the influence of this innovation on the microbial iron cycle.

Timing the Evolutionary Advent of Cyanobacteria and the Later Great Oxidation Event Using Gene Phylogenies of a Sunscreen

The biosynthesis of the unique cyanobacterial (oxyphotobacterial) indole-phenolic UVA sunscreen, scytonemin, is coded for in a conserved operon that contains both core metabolic genes and accessory, aromatic amino acid biosynthesis genes dedicated to supplying scytonemin's precursors. Comparative genomics shows conservation of this operon in many, but not all, cyanobacterial lineages. Phylogenetic analyses of the operon's aromatic amino acid genes indicate that five of them were recruited into the operon after duplication events of their respective housekeeping cyanobacterial cognates. We combined the fossil record of cyanobacteria and relaxed molecular clock models to obtain multiple estimates of these duplication events, setting a minimum age for the evolutionary advent of scytonemin at 2.1 ± 0.3 billion years. The same analyses were used to estimate the advent of cyanobacteria as a group (and thus the appearance of oxygenic photosynthesis), at 3.6 ± 0.2 billion years before present. Post hoc interpretation of 16S rRNA-based Bayesian analyses was consistent with these estimates. Because of physiological constraints on the use of UVA sunscreens in general, and the biochemical constraints of scytonemin in particular, scytonemin's age must postdate the time when Earth's atmosphere turned oxic, known as the Great Oxidation Event (GOE). Indeed, our biological estimate is in agreement with independent geochemical estimates for the GOE. The difference between the estimated ages of oxygenic photosynthesis and the GOE indicates the long span (on the order of a billion years) of the era of "oxygen oases," when oxygen was available locally but not globally.IMPORTANCE The advent of cyanobacteria, with their invention of oxygenic photosynthesis, and the Great Oxidation Event are arguably among the most important events in the evolutionary history of life on Earth. Oxygen is a significant toxicant to all life, but its accumulation in the atmosphere also enabled the successful development and proliferation of many aerobic organisms, especially metazoans. The currently favored dating of the Great Oxidation Event is based on the geochemical rock record. Similarly, the advent of cyanobacteria is also often drawn from the same estimates because in older rocks paleontological evidence is scarce or has been discredited. Efforts to obtain molecular evolutionary alternatives have offered widely divergent estimates. Our analyses provide a novel means to circumvent these limitations and allow us to estimate the large time gap between the two events.

Spatial segregation of the biological soil crust microbiome around its foundational cyanobacterium, Microcoleus vaginatus, and the formation of a nitrogen-fixing cyanosphere

BACKGROUND

Biological soil crusts (biocrusts) are a key component of arid land ecosystems, where they render critical services such as soil surface stabilization and nutrient fertilization. The bundle-forming, filamentous, non-nitrogen-fixing cyanobacterium Microcoleus vaginatus is a pioneer primary producer, often the dominant member of the biocrust microbiome, and the main source of leaked organic carbon. We hypothesized that, by analogy to the rhizosphere of plant roots, M. vaginatus may shape the microbial populations of heterotrophs around it, forming a specialized cyanosphere.

RESULTS

By physically isolating bundles of M. vaginatus from biocrusts, we were able to study the composition of the microbial populations attached to it, in comparison to the bulk soil crust microbiome by means of high-throughput 16S rRNA sequencing. We did this in two M. vaginatus-dominated biocrust from distinct desert biomes. We found that a small, selected subset of OTUs was significantly enriched in close proximity to M. vaginatus. Furthermore, we also found that a majority of bacteria (corresponding to some two thirds of the reads) were significantly more abundant away from this cyanobacterium. Phylogenetic placements suggest that all typical members of the cyanosphere were copiotrophs and that many were diazotrophs (Additional file 1: Tables S2 and S3). Nitrogen fixation genes were in fact orders of magnitude more abundant in this cyanosphere than in the bulk biocrust soil as assessed by qPCR. By contrary, competition for light, CO2, and low organic carbon concentrations defined at least a part of the OTUs segregating from the cyanobacterium.

CONCLUSIONS

We showed that M. vaginatus acts as a significant spatial organizer of the biocrust microbiome. On the one hand, it possesses a compositionally differentiated cyanosphere that concentrates the nitrogen-fixing function. We propose that a mutualism based on C for N exchange between M. vaginatus and copiotrophic diazotrophs helps sustains this cyanosphere and that this consortium constitutes the true pioneer community enabling the colonization of nitrogen-poor soils. On the other hand, a large number of biocrust community members segregate away from the vicinity of M. vaginatus, potentially through competition for light or CO2, or because of a preference for oligotrophy.