Bacterial life and dinitrogen fixation at a gypsum rock

Microbial colonization of gypsum: from the fossil record to the present day

Microorganisms inhabiting gypsum have been observed in environments that differ greatly in water availability. Gypsum colonized by microorganisms, including cyanobacteria, eukaryotic algae, and diverse heterotrophic communities, occurs in hot, arid or even hyperarid environments, in cold environments of the Antarctic and Arctic zones, and in saline and hypersaline lakes and ponds where gypsum precipitates. Fossilized microbial remnants preserved in gypsum were also reported. Gypsum protects the endolithic microbial communities against excessive insolation and ultraviolet radiation, while allowing photosynthetically active radiation to penetrate through the mineral substrate. We here review the worldwide occurrences of microbially colonized gypsum and the specific properties of gypsum related to its function as a substrate and habitat for microbial life on Earth and possibly beyond. Methods for detecting and characterizing endolithic communities and their biomarkers in gypsum are discussed, including microscopic, spectroscopic, chemical, and molecular biological techniques. The modes of adaptation of different microorganisms to life within gypsum crystals under different environmental conditions are described. Finally, we discuss gypsum deposits as possible targets for the search for microbial life or its remnants beyond Earth, especially on Mars, where sulfate-rich deposits occur, and propose strategies to detect them during space exploration missions.

Effect of experimentally increased nutrient availability on the structure, metabolic activities, and potential microbial functions of a maritime Antarctic microbial mat

The role of competitive interactions based on resource utilisation was explored in a phototrophic microbial mat from Byers Peninsula (Maritime Antarctica). Shotgun metagenomic profiling of the mat showed a taxonomic and functionally diverse microbial community. The heterotrophic bacterial community was dominated by Proteobacteria, where genera typically found in polar habitats, such as Janthinobacterium, Pseudomonas, and Polaromonas, were highly prevalent. Cyanobacteria played the main role as primary producers, accompanied by diatoms and chlorophytes. To test the potential effects of the inorganic nutrient (N and P) availability on this community, a fully factorial nitrate and phosphorus addition experiment was conducted in situ. The mat exhibited a functional and structural response to the nutrient amendments. Compared to the undisturbed mat, phosphorus fertilisation favoured the growth of (non-heterocystous) cyanobacteria relative to that of diatoms, as indicated by changes in the carotenoid pigment biomarkers. Although no mat accretion was visible, fertilisation improved the phototrophic activity, and, mainly, when P was amended, the production of exopolymeric substances was favoured, whereas further changes in the vertical distribution of primary production activity were observed as well. Illumina amplicon sequencing of the 16S rRNA gene also demonstrated changes in the relative abundance of heterotrophic prokaryotes, which were detectable from the phylum to the genus level and mainly related to the amendment of nitrogen. Predictions made on the functional skills of these shifted prokaryotic communities indicated changes in abundance selecting taxa with a metabolic adaptation to the new nutrient scenarios. They mainly consisted of the enhancement of ecological strategies and metabolic regulatory mechanisms related to the uptake and metabolising of either nitrogen or phosphorus, regulated by its availability whether in a balanced way or not. This study is a pioneer in demonstrating how shifts in the regional dynamic of nutrients might alter the metabolic equilibrium of these initially considered homeostatic benthic communities. They can be accordingly considered as taxonomically diverse microbiomes with a functional repertoire still inclined to respond to the biogeochemical alteration of nutrient cycles, although occurring in a cold extreme environment where biological activity is partially restricted by environmental harshness.

Opening the Gap: Rare Lichens With Rare Cyanobionts - Unexpected Cyanobiont Diversity in Cyanobacterial Lichens of the Order Lichinales

The last decades of research led to a change in understanding of lichens that are now seen as self-sustaining micro-ecosystems, harboring diverse microbial organisms in tight but yet not fully understood relationships. Among the diverse interdependencies, the relationship between the myco- and photobiont is the most crucial, determining the shape, and ecophysiological properties of the symbiotic consortium. Roughly 10% of lichens associate with cyanobacteria as their primary photobiont, termed cyanolichens. Up to now, the diversity of cyanobionts of bipartite lichens resolved by modern phylogenetic approaches is restricted to the filamentous and heterocytous genera of the order Nostocales. Unicellular photobionts were placed in the orders Chroococcales, Pleurocapsales, and Chroococcidiopsidales. However, especially the phylogeny and taxonomy of the Chroococcidiopsidales genera remained rather unclear. Here we present new data on the identity and phylogeny of photobionts from cyanolichens of the genera Gonohymenia, Lichinella, Peccania, and Peltula from a broad geographical range. A polyphasic approach was used, combining morphological and cultivation-depending characteristics (microscopy, staining techniques, life cycle observation, baeocyte motility, and nitrogen fixation test) with phylogenetic analyses of the 16S rRNA and 16S–23S ITS gene region. We found an unexpectedly high cyanobiont diversity in the cyanobacterial lichens of the order Lichinales, including two new genera and seven new species, all of which were not previously perceived as lichen symbionts. As a result, we describe the novel unicellular Chroococcidiopsidales genera Pseudocyanosarcina gen. nov. with the species Pseudocyanosarcina phycocyania sp. nov. (from Peltula clavata, Australia) and Compactococcus gen. nov. with the species Compactococcus sarcinoides sp. nov. (from Gonohymenia sp., Australia) and the new Chroococcidiopsidales species Aliterella compacta sp. nov. (from Peltula clavata, Australia), Aliterella gigantea sp. nov. (from Peltula capensis; South Africa), Sinocapsa ellipsoidea sp. nov. (from Peccania cerebriformis, Austria), as well as the two new Nostocales species Komarekiella gloeocapsoidea sp. nov. (from Gonohymenia sp., Czechia) and Komarekiella globosa sp. nov. (from Lichinella cribellifera, Canary Islands, Spain). Our study highlights the role of cyanolichens acting as a key in untangling cyanobacterial taxonomy and diversity. With this study, we hope to stimulate further research on photobionts, especially of rare cyanolichens.

Final Destination? Pinpointing Hyella disjuncta sp. nov. PCC 6712 (Cyanobacteria) Based on Taxonomic Aspects, Multicellularity, Nitrogen Fixation and Biosynthetic Gene Clusters

Unicellular cyanobacteria inhabit a wide range of ecosytems and can be found throughout the phylum offering space for taxonomic confusion. One example is strain PCC 6712 that was described as Chlorogloea sp. (Nostocales) and later assigned to the genus Chroococcidiopsis (Chroococcidiopsidales). We now show that this strain belongs to the order Pleurocapsales and term it Hyella disjuncta based on morphology, genome analyses and 16S-23S ITS rRNA phylogeny. Genomic analysis indicated that H. disjuncta PCC 6712 shared about 44.7% orthologue genes with its closest relative H. patelloides. Furthermore, 12 cryptic biosynthetic gene clusters (BGCs) with potential bioactivity, such as a mycosporine-like amino acid BGC, were detected. Interestingly, the full set of nitrogen fixation genes was found in H. disjuncta PCC 6712 despite its inability to grow on nitrogen-free medium. A comparison of genes responsible for multicellularity was performed, indicating that most of these genes were present and related to those found in other cyanobacterial orders. This is in contrast to the formation of pseudofilaments-a main feature of the genus Hyella-which is weakly expressed in H. disjuncta PCC 6712 but prominent in Hyella patelloides LEGE 07179. Thus, our study pinpoints crucial but hidden aspects of polyphasic cyanobacterial taxonomy.

Microscopic and biomolecular complementary approaches to characterize bioweathering processes at petroglyph sites from the Negev Desert, Israel

Throughout the Negev Desert highlands, thousands of ancient petroglyphs sites are susceptible to deterioration processes that may result in the loss of this unique rock art. Therefore, the overarching goal of the current study was to characterize the composition, diversity and effects of microbial colonization of the rocks to find ways of protecting these unique treasures. The spatial organization of the microbial colonizers and their relationships with the lithic substrate were analysed using scanning electron microscopy. This approach revealed extensive epilithic and endolithic colonization and close microbial-mineral interactions. Shotgun sequencing analysis revealed various taxa from the archaea, bacteria and some eukaryotes. Metagenomic coding sequences (CDS) of these microbial lithobionts exhibited specific metabolic pathways involved in the rock elements' cycles and uptake processes. Thus, our results provide evidence for the potential participation of the microorganisms colonizing these rocks during different solubilization and mineralization processes. These damaging actions may contribute to the deterioration of this extraordinary rock art and thus threaten this valuable heritage. Shotgun metagenomic sequencing, in conjunction with the in situ scanning electron microscopy study, can thus be considered an effective strategy to understand the complexity of the weathering processes occurring at petroglyph sites and other cultural heritage assets.

Connectivity of Edaphic and Endolithic Microbial Niches in Cold Mountain Desert of Eastern Pamir (Tajikistan)

Microbial communities found in arid environments are commonly represented by biological soil crusts (BSCs) and endolithic assemblages. There is still limited knowledge concerning endoliths and BSCs occurring in the cold mountain desert of Pamir. The aim of the study was to investigate the composition and structure of endolithic bacterial communities in comparison to surrounding BSCs in three subregions of the Eastern Pamir (Tajikistan). The endolithic and BSC communities were studied using culture-independent and culture-dependent techniques. The structure of the endolithic bacterial communities can be characterized as Actinobacteria-Proteobacteria-Bacteroidetes-Chloroflexi-Cyanobacteria, while the BSCs' can be described as Proteobacteria-Actinobacteria-Bacteroidetes-Cyanobacteria assemblages with low representation of other bacteria. The endolithic cyanobacterial communities were characterized by the high percentage of Chroococcidiopsaceae, Nodosilineaceae, Nostocaceae and Thermosynechococcaceae, while in the BSCs were dominated by Nodosilineaceae, Phormidiaceae and Nostocaceae. The analysis of 16S rRNA genes of the cyanobacterial cultures revealed the presence of possibly novel species of Chroococcidiopsis, Gloeocapsopsis and Wilmottia. Despite the niches' specificity, which is related to the influence of microenvironment factors on the composition and structure of endolithic communities, our results illustrate the interrelation between the endoliths and the surrounding BSCs in some regions. The structure of cyanobacterial communities from BSC was the only one to demonstrate some subregional differences.

Genetic diversity of diazotrophs and total bacteria in the phyllosphere of Pyrus serotina, Prunus armeniaca, Prunus avium, and Vitis vinifera

The phyllosphere, which supports a large number of microorganisms, represents the interface between the aboveground parts of plants and air. In this study, four nifH clone libraries were constructed from the phyllosphere of Pyrus serotina (L), Vitis vinifera (P), Prunus armeniaca (X), and Prunus avium (Y). Clones related to Skermanella (L, 12.1%; X, 15.6%; Y, 62.5%; P 70.8%), Bradyrhizobium (X, 2.1%; P, 15.1%; L, 63.7%), Erwinia (X, 68.8%), Pseudomonas (L, 3.3%; P, 7.6%), and Chroococcidiopsis (P, 0.9%; L, 4.4%, X; 5.2%, Y; 19.6%) were present at high percentages, highlighting their critical role in contributing nitrogen to the phyllosphere ecosystem. The 16S rDNA sequence analysis suggested that phyllosphere-associated bacteria were affiliated with a wide range of taxa, encompassing members from Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Cyanobacteria, Tenericutes, and Deinococcus-Thermus. Additionally, the abundance of the nifH gene and 16S rDNA was assessed with quantitative PCR. The number of copies of nifH and 16S rDNA ranged from 1.14 × 10³ to 1.49 × 10⁴ and from 3.72 × 10⁶ to 7.02 × 10⁷ copies/g fresh leaf sample, respectively. In conclusion, our work sheds light on the microbial communities of the phyllosphere that are important for plant growth. Moreover, we observed a unique composition of nitrogen-fixing bacteria in each phyllosphere sample, suggesting the existence of specific interactions between these functional microorganism and plants, which may provide information or be a reference for the development of bacterial fertilizers.

Stress effects of cyanotoxin β-methylamino-L-alanine (BMAA) on cyanobacterial heterocyst formation and functionality

Various species of cyanobacteria, diatoms and dinoflagellates are capable of synthesizing the non-proteinogenic neurotoxic amino acid β-N-methylamino-L-alanine (BMAA), which is known to be a causative agent of human neurodegeneration. Similar to most cyanotoxins, the biological and ecological functions of BMAA in cyanobacteria are unknown. In this study, we show for the first time that BMAA, in micromolar amounts, inhibits the formation of heterocysts (specialized nitrogen-fixing cells) in heterocystous, diazotrophic cyanobacteria [Anabaena sp. PCC 7120, Nostoc punctiforme PCC 73102 (ATCC 29133), Nostoc sp. strain 8963] under conditions of nitrogen starvation. The inhibitory effect of BMAA is abolished by the addition of glutamate. To understand the genetic reason for the observed phenomenon, we used qPCR to study the expression of key genes involved in cell differentiation and nitrogen metabolism in the model cyanobacterium Anabaena sp. PCC 7120. We observed that in the presence of BMAA, Anabaena sp. PCC 7120 does not express two essential genes associated with heterocyst differentiation, namely, hetR and hepA. We also found that addition of BMAA to cyanobacterial cultures with mature heterocysts inhibits nifH gene expression and nitrogenase activity.

Diel-scale temporal dynamics recorded for bacterial groups in Namib Desert soil

Microbes in hot desert soil partake in core ecosystem processes e.g., biogeochemical cycling of carbon. Nevertheless, there is still a fundamental lack of insights regarding short-term (i.e., over a 24-hour [diel] cycle) microbial responses to highly fluctuating microenvironmental parameters like temperature and humidity. To address this, we employed T-RFLP fingerprinting and 454 pyrosequencing of 16S rRNA-derived cDNA to characterize potentially active bacteria in Namib Desert soil over multiple diel cycles. Strikingly, we found that significant shifts in active bacterial groups could occur over a single 24-hour period. For instance, members of the predominant Actinobacteria phyla exhibited a significant reduction in relative activity from morning to night, whereas many Proteobacterial groups displayed an opposite trend. Contrary to our leading hypothesis, environmental parameters could only account for 10.5% of the recorded total variation. Potential biotic associations shown through co-occurrence networks indicated that non-random inter- and intra-phyla associations were ‘time-of-day-dependent’ which may constitute a key feature of this system. Notably, many cyanobacterial groups were positioned outside and/or between highly interconnected bacterial associations (modules); possibly acting as inter-module ‘hubs’ orchestrating interactions between important functional consortia. Overall, these results provide empirical evidence that bacterial communities in hot desert soils exhibit complex and diel-dependent inter-community associations.

Adaptation strategies of endolithic chlorophototrophs to survive the hyperarid and extreme solar radiation environment of the Atacama Desert

The Atacama Desert, northern Chile, is one of the driest deserts on Earth and, as such, a natural laboratory to explore the limits of life and the strategies evolved by microorganisms to adapt to extreme environments. Here we report the exceptional adaptation strategies of chlorophototrophic and eukaryotic algae, and chlorophototrophic and prokaryotic cyanobacteria to the hyperarid and extremely high solar radiation conditions occurring in this desert. Our approach combined several microscopy techniques, spectroscopic analytical methods, and molecular analyses. We found that the major adaptation strategy was to avoid the extreme environmental conditions by colonizing cryptoendolithic, as well as, hypoendolithic habitats within gypsum deposits. The cryptoendolithic colonization occurred a few millimeters beneath the gypsum surface and showed a succession of organized horizons of algae and cyanobacteria, which has never been reported for endolithic microbial communities. The presence of cyanobacteria beneath the algal layer, in close contact with sepiolite inclusions, and their hypoendolithic colonization suggest that occasional liquid water might persist within these sub-microhabitats. We also identified the presence of abundant carotenoids in the upper cryptoendolithic algal habitat and scytonemin in the cyanobacteria hypoendolithic habitat. This study illustrates that successful lithobiontic microbial colonization at the limit for microbial life is the result of a combination of adaptive strategies to avoid excess solar irradiance and extreme evapotranspiration rates, taking advantage of the complex structural and mineralogical characteristics of gypsum deposits-conceptually called "rock's habitable architecture." Additionally, self-protection by synthesis and accumulation of secondary metabolites likely produces a shielding effect that prevents photoinhibition and lethal photooxidative damage to the chlorophototrophs, representing another level of adaptation.

Biota and biomolecules in extreme environments on Earth: implications for life detection on Mars

The three main requirements for life as we know it are the presence of organic compounds, liquid water, and free energy. Several groups of organic compounds (e.g., amino acids, nucleobases, lipids) occur in all life forms on Earth and are used as diagnostic molecules, i.e., biomarkers, for the characterization of extant or extinct life. Due to their indispensability for life on Earth, these biomarkers are also prime targets in the search for life on Mars. Biomarkers degrade over time; in situ environmental conditions influence the preservation of those molecules. Nonetheless, upon shielding (e.g., by mineral surfaces), particular biomarkers can persist for billions of years, making them of vital importance in answering questions about the origins and limits of life on early Earth and Mars. The search for organic material and biosignatures on Mars is particularly challenging due to the hostile environment and its effect on organic compounds near the surface. In support of life detection on Mars, it is crucial to investigate analogue environments on Earth that resemble best past and present Mars conditions. Terrestrial extreme environments offer a rich source of information allowing us to determine how extreme conditions affect life and molecules associated with it. Extremophilic organisms have adapted to the most stunning conditions on Earth in environments with often unique geological and chemical features. One challenge in detecting biomarkers is to optimize extraction, since organic molecules can be low in abundance and can strongly adsorb to mineral surfaces. Methods and analytical tools in the field of life science are continuously improving. Amplification methods are very useful for the detection of low concentrations of genomic material but most other organic molecules are not prone to amplification methods. Therefore, a great deal depends on the extraction efficiency. The questions “what to look for”, “where to look”, and “how to look for it” require more of our attention to ensure the success of future life detection missions on Mars.

Hypolithic microbial communities: between a rock and a hard place

Drylands are the largest terrestrial biome on Earth and a ubiquitous feature is desert pavement terrain, comprising rocks embedded in the mineral soil surface. Quartz and other translucent rocks are common and microbial communities termed hypoliths develop as biofilms on their ventral surfaces. In extreme deserts these represent major concentrations of biomass, and are emerging as key to geobiological processes and soil stabilization. These highly specialized communities are dominated by cyanobacteria that support diverse heterotrophic assemblages. Here we identify global-scale trends in the ecology of hypoliths that are strongly related to climate, particularly with regard to shifts in cyanobacterial assemblages. A synthesis of available data revealed a linear trend for colonization with regard to climate, and we suggest potential application for hypoliths as 'biomarkers' of aridity on a landscape scale. The potential to exploit the soil-stabilizing properties of hypolithic colonization in environmental engineering on dryland soils is also discussed.

Hypolithic communities: important nitrogen sources in Antarctic desert soils

Hypolithic microbial communities (i.e. cryptic microbial assemblages found on the undersides of translucent rocks) are major contributors of carbon input into the oligotrophic hyper-arid desert mineral soils of the Eastern Antarctic Dry Valleys. Here we demonstrate, for the first time, that hypolithic microbial communities possess both the genetic capacity for nitrogen fixation (i.e. the presence of nifH genes) and the ability to catalyse acetylene reduction, an accepted proxy for dinitrogen fixation. An estimate of the total contribution of these communities suggests that hypolithic communities are important contributors to fixed nitrogen budgets in Antarctic desert soils.

Cyanobacteria and chloroflexi-dominated hypolithic colonization of quartz at the hyper-arid core of the Atacama Desert, Chile

Quartz stones are ubiquitous in deserts and are a substrate for hypoliths, microbial colonists of the underside of such stones. These hypoliths thrive where extreme temperature and moisture stress limit the occurrence of higher plant and animal life. Several studies have reported the occurrence of green hypolithic colonization dominated by cyanobacteria. Here, we describe a novel red hypolithic colonization from Yungay, at the hyper-arid core of the Atacama Desert in Chile. Comparative analysis of green and red hypoliths from this site revealed markedly different microbial community structure as revealed by 16S rRNA gene clone libraries. Green hypoliths were dominated by cyanobacteria (Chroococcidiopsis and Nostocales phylotypes), whilst the red hypolith was dominated by a taxonomically diverse group of chloroflexi. Heterotrophic phylotypes common to all hypoliths were affiliated largely to desiccation-tolerant taxa within the Actinobacteria and Deinococci. Alphaproteobacterial phylotypes that affiliated with nitrogen-fixing taxa were unique to green hypoliths, whilst Gemmatimonadetes phylotypes occurred only on red hypolithon. Other heterotrophic phyla recovered with very low frequency were assumed to represent functionally relatively unimportant taxa.

Nitrogen fixation and hydrogen metabolism in cyanobacteria

This review summarizes recent aspects of (di)nitrogen fixation and (di)hydrogen metabolism, with emphasis on cyanobacteria. These organisms possess several types of the enzyme complexes catalyzing N(2) fixation and/or H(2) formation or oxidation, namely, two Mo nitrogenases, a V nitrogenase, and two hydrogenases. The two cyanobacterial Ni hydrogenases are differentiated as either uptake or bidirectional hydrogenases. The different forms of both the nitrogenases and hydrogenases are encoded by different sets of genes, and their organization on the chromosome can vary from one cyanobacterium to another. Factors regulating the expression of these genes are emerging from recent studies. New ideas on the potential physiological and ecological roles of nitrogenases and hydrogenases are presented. There is a renewed interest in exploiting cyanobacteria in solar energy conversion programs to generate H(2) as a source of combustible energy. To enhance the rates of H(2) production, the emphasis perhaps needs not to be on more efficient hydrogenases and nitrogenases or on the transfer of foreign enzymes into cyanobacteria. A likely better strategy is to exploit the use of radiant solar energy by the photosynthetic electron transport system to enhance the rates of H(2) formation and so improve the chances of utilizing cyanobacteria as a source for the generation of clean energy.

The microbe-mineral environment and gypsum neogenesis in a weathered polar evaporite

Evaporitic deposits are a globally widespread habitat for micro-organisms. The microbe-mineral environment in weathered and remobilized gypsum from exposed mid-Ordovician marine evaporite beds in the polar desert of Devon Island, Nunavut, Canadian High Arctic was examined. The gypsum is characterized by internal green zones of cyanobacterial colonization (dominated by Gloeocapsa/Aphanothece and Chroococcidiopsis spp. morphotypes) and abundant black zones, visible from the surface, that contain pigmented cyanobacteria and fungi. Bioessential elements in the gypsum are primarily provided by allochthonous material from the present-day polar desert. The disruption, uplift and rotation of the evaporite beds by the Haughton meteorite impact 39 Ma have facilitated gypsum weathering and its accessibility as a habitat. No cultured cyanobacteria, bacteria and fungi were halophilic consistent with the expectation that halophily is not required to persist in gypsum habitats. Heterotrophic bacteria from the evaporite were slightly or moderately halotolerant, as were heterotrophs isolated from soil near the gypsum outcrop showing that halotolerance is common in arctic bacteria in this location. Psychrotolerant Arthrobacter species were isolated. No psychrophilic organisms were isolated. Two Arthrobacter isolates from the evaporite were used to mediate gypsum neogenesis in the laboratory, demonstrating a potential role for microbial biomineralization processes in polar environments.

Highly specialized microbial diversity in hyper-arid polar desert

The McMurdo Dry Valleys in Antarctica are a cold hyperarid polar desert that present extreme challenges to life. Here, we report a culture-independent survey of multidomain microbial biodiversity in McKelvey Valley, a pristine example of the coldest desert on Earth. We demonstrate that life has adapted to form highly-specialized communities in distinct lithic niches occurring concomitantly within this terrain. Endoliths and chasmoliths in sandstone displayed greatest diversity, whereas soil was relatively depauperate and lacked a significant photoautotrophic component, apart from isolated islands of hypolithic cyanobacterial colonization on quartz rocks in soil contact. Communities supported previously unreported polar bacteria and fungi, but archaea were absent from all niches. Lithic community structure did not vary significantly on a landscape scale and stochastic moisture input due to snowmelt resulted in increases in colonization frequency without significantly affecting diversity. The findings show that biodiversity near the cold-arid limit for life is more complex than previously appreciated, but communities lack variability probably due to the high selective pressures of this extreme environment.

Microclimate and limits to photosynthesis in a diverse community of hypolithic cyanobacteria in northern Australia

Hypolithic microbes, primarily cyanobacteria, inhabit the highly specialized microhabitats under translucent rocks in extreme environments. Here we report findings from hypolithic cyanobacteria found under three types of translucent rocks (quartz, prehnite, agate) in a semiarid region of tropical Australia. We investigated the photosynthetic responses of the cyanobacterial communities to light, temperature and moisture in the laboratory, and we measured the microclimatic variables of temperature and soil moisture under rocks in the field over an annual cycle. We also used molecular techniques to explore the diversity of hypolithic cyanobacteria in this community and their phylogenetic relationships within the context of hypolithic cyanobacteria from other continents. Based on the laboratory experiments, photosynthetic activity required a minimum soil moisture of 15% (by mass). Peak photosynthetic activity occurred between approximately 8 degrees C and 42 degrees C, though some photosynthesis occurred between -1 degrees C and 51 degrees C. Maximum photosynthesis rates also occurred at light levels of approximately 150-550 micromol m(-2) s(-1). We used the field microclimatic data in conjunction with these measurements of photosynthetic efficiency to estimate the amount of time the hypolithic cyanobacteria could be photosynthetically active in the field. Based on these data, we estimated that conditions were appropriate for photosynthetic activity for approximately 942 h (approximately 75 days) during the year. The hypolithic cyanobacteria community under quartz, prehnite and agate rocks was quite diverse both within and between rock types. We identified 115 operational taxonomic units (OTUs), with each rock hosting 8-24 OTUs. A third of the cyanobacteria OTUs from northern Australia grouped with Chroococcidiopsis, a genus that has been identified from hypolithic and endolithic communities from the Gobi, Mojave, Atacama and Antarctic deserts. Several OTUs identified from northern Australia have not been reported to be associated with hypolithic communities previously.

Cryptic photosynthesis--extrasolar planetary oxygen without a surface biological signature

On Earth, photosynthetic organisms are responsible for the production of virtually all the oxygen in the atmosphere. On land, vegetation reflects in the visible and leads to a "red edge," which developed about 450 million years ago on Earth and has been proposed as a biosignature for life on extrasolar planets. However, in many regions on Earth, particularly where surface conditions are extreme--in hot and cold deserts, for example--photosynthetic organisms can be driven into and under substrates where light is still sufficient for photosynthesis. These communities exhibit no detectable surface spectral signature to indicate life. The same is true of the assemblages of photosynthetic organisms at more than a few meters' depth in water bodies. These communities are widespread and dominate local photosynthetic productivity. We review known cryptic photosynthetic communities and their productivity. We have linked geomicrobiology with observational astronomy by calculating the disk-averaged spectra of cryptic habitats and identifying detectable features on an exoplanet dominated by such a biota. The hypothetical cryptic photosynthesis worlds discussed here are Earth analogues that show detectable atmospheric biosignatures like our own planet but do not exhibit a discernable biological surface feature in the disc-averaged spectrum.

DEWFALL AS A WATER SOURCE FREQUENTLY ACTIVATES THE ENDOLITHIC CYANOBACTERIAL COMMUNITIES IN THE GRANITES OF TAYLOR VALLEY, ANTARCTICA(1)

Endolithic photosynthetic microorganisms like cyanobacteria and algae are well known from savannas and deserts of the world, the high Arctic, and also Antarctic habitats like the Dry Valleys in the Ross Dependency. These endolithic microbial communities are thought to be at the limits of life with reported ages in the order of thousands of years. Here we report on an extensive chasmoendolithic cyanobacterial community inside granite rocks of Mt. Falconer in the lower Taylor Valley, Dry Valleys. On average, the cyanobacterial community was 4.49 ± 0.95 mm below the rock surface, where it formed a blue-green layer. The community was composed mainly of the cyanobacterium Chroococcidiopsis sp., with occasional Cyanothece cf. aeruginosa (Nägeli) Komárek and Nostoc sp. Mean biomass was 168 ± 44 g carbon · m(-2) , and the mean chl a content was 24.3 ± 34.2 mg · m(-2) . In situ chl fluorescence measurements-a relative measure of photosynthetic activity-showed that they were active over long periods each day and also showed activity the next day in the absence of any moisture. Radiocarbon dating gave a relatively young age (175-280 years) for the community. Calculations from microclimate data demonstrated that formation of dew or rime was possible and could frequently activate the cyanobacteria and may explain the younger age of microbial communities at Mt. Falconer compared to older and less active endolithic microorganisms reported earlier from Linnaeus Terrace, a higher altitude region that experiences colder, drier conditions.

Diazotrophic bacterioplankton in a coral reef lagoon: phylogeny, diel nitrogenase expression and response to phosphate enrichment

We investigated diazotrophic bacterioplankton assemblage composition in the Heron Reef lagoon (Great Barrier Reef, Australia) using culture-independent techniques targeting the nifH fragment of the nitrogenase gene. Seawater was collected at 3 h intervals over a period of 72 h (i.e. over diel as well as tidal cycles). An incubation experiment was also conducted to assess the impact of phosphate (PO(4)3*) availability on nifH expression patterns. DNA-based nifH libraries contained primarily sequences that were most similar to nifH from sediment, microbial mat and surface-associated microorganisms, with a few sequences that clustered with typical open ocean phylotypes. In contrast to genomic DNA sequences, libraries prepared from gene transcripts (mRNA amplified by reverse transcription-polymerase chain reaction) were entirely cyanobacterial and contained phylotypes similar to those observed in open ocean plankton. The abundance of Trichodesmium and two uncultured cyanobacterial phylotypes from previous studies (group A and group B) were studied by quantitative-polymerase chain reaction in the lagoon samples. These were detected as transcripts, but were not detected in genomic DNA. The gene transcript abundance of these phylotypes demonstrated variability over several diel cycles. The PO(4)3* enrichment experiment had a clearer pattern of gene expression over diel cycles than the lagoon sampling, however PO(4)3* additions did not result in enhanced transcript abundance relative to control incubations. The results suggest that a number of diazotrophs in bacterioplankton of the reef lagoon may originate from sediment, coral or beachrock surfaces, sloughing into plankton with the flooding tide. The presence of typical open ocean phylotype transcripts in lagoon bacterioplankton may indicate that they are an important component of the N cycle of the coral reef.

Cyanobacterial ecology across environmental gradients and spatial scales in China's hot and cold deserts

Lithic photoautotrophic communities function as principal primary producers in the world's driest deserts, yet many aspects of their ecology remain unknown. This is particularly true for Asia, where some of the Earth's oldest and driest deserts occur. Using methods derived from plant landscape ecology, we measured the abundance and spatial distribution of cyanobacterial colonization on quartz stony pavement across environmental gradients of rainfall and temperature in the isolated Taklimakan and Qaidam Basin deserts of western China. Colonization within available habitat ranged from 0.37+/-0.16% to 12.6+/-1.8%, with cold dry desert sites exhibiting the lowest abundance. Variation between sites was most strongly correlated with moisture-related variables and was independent of substrate availability. Cyanobacterial communities were spatially aggregated at multiple scales in patterns distinct from the underlying rock pattern. Site-level differences in cyanobacterial spatial pattern (e.g. mean inter-patch distance) were linked with rainfall, whereas patchiness within sites was correlated with local geology (greater colonization frequency of large rocks) and biology (dispersal during rainfall). We suggest that cyanobacterial patchiness may also in part be self-organized - that is, an outcome of soil water-biological feedbacks. We propose that landscape ecology concepts and models linking desert vegetation, biological feedbacks and ecohydrological processes are applicable to microbial communities.

Hypolithic community shifts occur as a result of liquid water availability along environmental gradients in China's hot and cold hyperarid deserts

Hypolithic cyanobacterial communities occur in hot and cold hyperarid environments but the physical factors determining their diversity are not well understood. Here we report hypolithic diversity and colonization of a common quartz substrate at several hyperarid locations in the ancient deserts of north-western China, that experience varying mean annual temperature, rainfall and concomitant availability of liquid water in soil. Microscopy and enrichment culture resulted only in Chroococcidiopsis morphotypes which were ubiquitous, but community phylogenetic analysis revealed considerable cyanobacterial and heterotrophic bacterial diversity. Species Richness and Shannon's Diversity Index displayed a significant positive linear correlation with availability of liquid water but not temperature or rainfall alone. Several taxonomic groups occurred only in specific climatically defined locations, while for Chroococcidiopsis, Deinococcus and Phormidium location specific lineages within these genera were also evident. Multivariate analysis was used to illustrate pronounced community shifts due to liquid water availability, although these did not significantly affect the predicted functional relationships within any given assemblage in either hot or cold, wet or dry hyperarid deserts. This study clearly demonstrates that availability of liquid water, rather than temperature or rainfall per se is the key determinant of hypolithic diversity in hyperarid locations, and furthermore that functionally similar yet taxonomically distinct communities occur, characterized by the presence of taxa that are specific to defined levels of aridity.

Watering, fertilization, and slurry inoculation promote recovery of biological crust function in degraded soils

Biological soil crusts are very sensitive to human-induced disturbances and are in a degraded state in many areas throughout their range. Given their importance in the functioning of arid and semiarid ecosystems, restoring these crusts may contribute to the recovery of ecosystem functionality in degraded areas. We conducted a factorial microcosm experiment to evaluate the effects of inoculation type (discrete fragments vs slurry), fertilization (control vs addition of composted sewage sludge), and watering frequency (two vs five times per week) on the cyanobacterial composition, nitrogen fixation, chlorophyll content, and net CO2 exchange rate of biological soil crusts inoculated on a semiarid degraded soil from SE Spain. Six months after the inoculation, the highest rates of nitrogen fixation and chlorophyll a content were found when the biological crusts were inoculated as slurry, composted sewage sludge was added, and the microcosms were watered five times per week. Net CO2 exchange rate increased when biological crusts were inoculated as slurry and the microcosms were watered five times per week. Denaturing gradient gel electrophoresis fingerprints and phylogenetic analyses indicated that most of the cyanobacterial species already present in the inoculated crust had the capability to spread and colonize the surface of the surrounding soil. These analyses showed that cyanobacterial communities were less diverse when the microcosms were watered five times per week, and that watering frequency (followed in importance by the addition of composted sewage sludge and inoculation type) was the treatment that most strongly influenced their composition. Our results suggest that the inoculation of biological soil crusts in the form of slurry combined with the addition of composted sewage sludge could be a suitable technique to accelerate the recovery of the composition and functioning of biological soil crusts in drylands.

The rice field cyanobacteria Anabaena azotica and Anabaena sp. CH1 express vanadium-dependent nitrogenase

Anabaena azotica FACHB-118 and Anabaena sp. CH1, heterocystous cyanobacteria isolated from Chinese and Taiwanese rice fields, expressed vanadium-containing nitrogenase when under molybdenum deficiency. This is the second direct observation of an alternative nitrogenase in cyanobacteria. The vanadium nitrogenase-specific genes vnfDG are fused and clustered in a phylogenetic tree next to the corresponding genes of Methanosarcina. The expression of vnfH in cells cultured in Mo-free medium and of nifH in Mo-grown cells was shown for the first time by sequencing cDNA derived from cultures of A. azotica and Anabaena sp. CH1. The vnfH sequences clustered with that of Anabaena variabilis. The vnf genes were strongly transcribed only in cultures grown either in Mo-free medium, or in W-containing medium, but also weakly in Mo-containing medium. NifH was transcribed in all media. On-line measurements of acetylene reduction by Mo-free A. azotica cultures demonstrated that the V-nitrogenase was active. Ethane was formed continuously at a rate of 2.1% of that of ethylene. Acetylene reduction of cultures grown either with or without Mo had a high temperature optimum of 42.5 degrees C. The uptake hydrogenase gene hupL was expressed in Mo-free medium concomitantly with vnfDG in A. azotica, Anabaena sp. CH1, and A. variabilis.