What distinguishes cyanobacteria able to revive after desiccation from those that cannot: the genome aspect

The global speciation continuum of the cyanobacterium Microcoleus

Speciation is a continuous process driven by genetic, geographic, and ecological barriers to gene flow. It is widely investigated in multicellular eukaryotes, yet we are only beginning to comprehend the relative importance of mechanisms driving the emergence of barriers to gene flow in microbial populations. Here, we explored the diversification of the nearly ubiquitous soil cyanobacterium Microcoleus. Our dataset consisted of 291 genomes, of which 202 strains and eight herbarium specimens were sequenced for this study. We found that Microcoleus represents a global speciation continuum of at least 12 lineages, which radiated during Eocene/Oligocene aridification and exhibit varying degrees of divergence and gene flow. The lineage divergence has been driven by selection, geographical distance, and the environment. Evidence of genetic divergence and selection was widespread across the genome, but we identified regions of exceptional differentiation containing candidate genes associated with stress response and biosynthesis of secondary metabolites.

Microbiome of seventh-century old Parsurameswara stone monument of India and role of desiccation-tolerant cyanobacterium Lyngbya corticicola on its biodeterioration

The Parsurameswara stone monument, built in the seventh century, is one of the oldest stone monuments in Odisha, India. Metagenomic analysis of the biological crust samples collected from the stone monument revealed 17 phyla in the microbiome, with Proteobacteria being the most dominant phylum, followed by cyanobacteria. Eight cyanobacteria were isolated. Lyngbya corticicola was the dominant cyanobacterium in all crust samples and could tolerate six months of desiccation in vitro. With six months of desiccation, chlorophyll-a decreased; however, carotenoid and cellular carbohydrate contents of this organism increased in the desiccated state. Resistance to desiccation, high carotenoid content, and effective trehalose biosynthesis in this cyanobacterium provide a distinct advantage over other microbiomes. Comparative metabolic profiles of the biological crust and L. corticicola show strongly corrosive organic acids such as dichloroacetic acid, which might be responsible for the biocorrosion of stone monuments.

Unveiling metabolic pathways involved in the extreme desiccation tolerance of an Atacama cyanobacterium

Gloeocapsopsis dulcis strain AAB1 is an extremely xerotolerant cyanobacterium isolated from the Atacama Desert (i.e., the driest and oldest desert on Earth) that holds astrobiological significance due to its ability to biosynthesize compatible solutes at ultra-low water activities. We sequenced and assembled the G. dulcis genome de novo using a combination of long- and short-read sequencing, which resulted in high-quality consensus sequences of the chromosome and two plasmids. We leveraged the G. dulcis genome to generate a genome-scale metabolic model (iGd895) to simulate growth in silico. iGd895 represents, to our knowledge, the first genome-scale metabolic reconstruction developed for an extremely xerotolerant cyanobacterium. The model's predictive capability was assessed by comparing the in silico growth rate with in vitro growth rates of G. dulcis, in addition to the synthesis of trehalose. iGd895 allowed us to explore simulations of key metabolic processes such as essential pathways for water-stress tolerance, and significant alterations to reaction flux distribution and metabolic network reorganization resulting from water limitation. Our study provides insights into the potential metabolic strategies employed by G. dulcis, emphasizing the crucial roles of compatible solutes, metabolic water, energy conservation, and the precise regulation of reaction rates in their adaptation to water stress.

Phormidium ambiguum and Leptolyngbya ohadii Exopolysaccharides under Low Water Availability

Cyanobacteria can cope with various environmental stressors, due to the excretion of exopolysaccharides (EPS). However, little is known about how the composition of these polymers may change according to water availability. This work aimed at characterizing the EPS of Phormidium ambiguum (Oscillatoriales; Oscillatoriaceae) and Leptolyngbya ohadii (Pseudanabaenales; Leptolyngbyaceae), when grown as biocrusts and biofilms, subject to water deprivation. The following EPS fractions were quantified and characterized: soluble (loosely bound, LB) and condensed (tightly bound, TB) for biocrusts, released (RPS), and sheathed in P. ambiguum and glycocalyx (G-EPS) in L. ohadii for biofilms. For both cyanobacteria upon water deprivation, glucose was the main monosaccharide present and the amount of TB-EPS resulted was significantly higher, confirming its importance in these soil-based formations. Different profiles of monosaccharides composing the EPSs were observed, as for example the higher concentration of deoxysugars observed in biocrusts compared to biofilms, demonstrating the plasticity of the cells to modify EPS composition as a response to different stresses. For both cyanobacteria, both in biofilms and biocrusts, water deprivation induced the production of simpler carbohydrates, with an increased dominance index of the composing monosaccharides. The results obtained are useful in understanding how these very relevant cyanobacterial species are sensitively modifying the EPS secreted when subject to water deprivation and could lead to consider them as suitable inoculants in degraded soils.

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii, a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.

Light and carbon: Synthetic biology toward new cyanobacteria-based living biomaterials

Cyanobacteria are ideal candidates to use in developing carbon neutral and carbon negative technologies; they are efficient photosynthesizers and amenable to genetic manipulation. Over the past two decades, researchers have demonstrated that cyanobacteria can make sustainable, useful biomaterials, many of which are engineered living materials. However, we are only beginning to see such technologies applied at an industrial scale. In this review, we explore the ways in which synthetic biology tools enable the development of cyanobacteria-based biomaterials. First we give an overview of the ecological and biogeochemical importance of cyanobacteria and the work that has been done using cyanobacteria to create biomaterials so far. This is followed by a discussion of commonly used cyanobacteria strains and synthetic biology tools that exist to engineer cyanobacteria. Then, three case studies-bioconcrete, biocomposites, and biophotovoltaics-are explored as potential applications of synthetic biology in cyanobacteria-based materials. Finally, challenges and future directions of cyanobacterial biomaterials are discussed.

Metabolites Facilitating Adaptation of Desert Cyanobacteria to Extremely Arid Environments

Desert is one of the harshest environments on the planet, characterized by exposure to daily fluctuations of extreme conditions (such as high temperature, low nitrogen, low water, high salt, etc.). However, some cyanobacteria are able to live and flourish in such conditions, form communities, and facilitate survival of other organisms. Therefore, to ensure survival, desert cyanobacteria must develop sophisticated and comprehensive adaptation strategies to enhance their tolerance to multiple simultaneous stresses. In this review, we discuss the metabolic pathways used by desert cyanobacteria to adapt to extreme arid conditions. In particular, we focus on the extracellular polysaccharides and compatible solutes biosynthesis pathways and their evolution and special features. We also discuss the role of desert cyanobacteria in the improvement of soil properties and their ecological and environmental impact on soil communities. Finally, we summarize recent achievements in the application of desert cyanobacteria to prevent soil erosion and desertification.

Coevolution of tandemly repeated hlips and RpaB-like transcriptional factor confers desiccation tolerance to subaerial Nostoc species

Desert-inhabiting cyanobacteria can tolerate extreme desiccation and quickly revive after rehydration. The regulatory mechanisms that enable their vegetative cells to resurrect upon rehydration are poorly understood. In this study, we identified a single gene family of high light-inducible proteins (Hlips) with dramatic expansion in the Nostoc flagelliforme genome and found an intriguingly special convergence formed through four tandem gene duplication. The emerged four independent hlip genes form a gene cluster (hlips-cluster) and respond to dehydration positively. The gene mutants in N. flagelliforme were successfully generated by using gene-editing technology. Phenotypic analysis showed that the desiccation tolerance of hlips-cluster-deleted mutant decreased significantly due to impaired photosystem II repair, whereas heterologous expression of hlips-cluster from N. flagelliforme enhanced desiccation tolerance in Nostoc sp. PCC 7120. Furthermore, a transcription factor Hrf1 (hlips-cluster repressor factor 1) was identified and shown to coordinately regulate the expression of hlips-cluster and desiccation-induced psbAs. Hrf1 acts as a negative regulator for the adaptation of N. flagelliforme to the harsh desert environment. Phylogenetic analysis revealed that most species in the Nostoc genus possess both tandemly repeated Hlips and Hrf1. Our results suggest convergent evolution of desiccation tolerance through the coevolution of tandem Hlips duplication and Hrf1 in subaerial Nostoc species, providing insights into the mechanism of desiccation tolerance in photosynthetic organisms.

Divergence of Biocrust Active Bacterial Communities in the Negev Desert During a Hydration-Desiccation Cycle

Rain events in arid environments are highly unpredictable and intersperse extended periods of drought. Therefore, tracking changes in desert soil bacterial communities during rain events, in the field, was seldom attempted. Here, we assessed rain-mediated dynamics of active bacterial communities in the Negev Desert biological soil crust (biocrust). Biocrust samples were collected during, and after a medium rainfall and dry soil was used as a control; we evaluated the changes in active bacterial composition, potential function, potential photosynthetic activity, and extracellular polysaccharide (EPS) production. We hypothesized that rain would activate the biocrust phototrophs (mainly Cyanobacteria), while desiccation would inhibit their activity. In contrast, the biocrust Actinobacteria would decline during rewetting and revive with desiccation. Our results showed that hydration increased chlorophyll content and EPS production. As expected, biocrust rewetting activated Cyanobacteria, which replaced the former dominant Actinobacteria, boosting potential autotrophic functions. However, desiccation of the biocrust did not immediately change the bacterial composition or potential function and was followed by a delayed decrease in chlorophyll and EPS levels. This dramatic shift in the community upon rewetting led to modifications in ecosystem services. We propose that following a rain event, the response of the active bacterial community lagged behind the biocrust water content due to the production of EPS which delayed desiccation and temporarily sustained the biocrust community activity.

Expansion of bilin-based red light sensors in the subaerial desert cyanobacterium Nostoc flagelliforme

Light is the crucial environmental signal for desiccation-tolerant cyanobacteria to activate photosynthesis and prepare for desiccation at dawn. However, the photobiological characteristics of desert cyanobacteria adaptation to one of the harshest habitats on Earth remain unresolved. In this study, we surveyed the genome of a subaerial desert cyanobacterium Nostoc flagelliforme and identified two phytochromes and seven cyanobacteriochromes (CBCRs) with one or more bilin-binding GAF (cGMP phosphodiesterase/adenylyl cyclase/FhlA) domains. Biochemical and spectroscopic analyses of 69 purified GAF-containing proteins from recombinant phycocyanobilin (PCB), biliverdin or phycoerythrobilin-producing Escherichia coli indicated that nine of these proteins bind chromophores. Further investigation revealed that 11 GAFs form covalent adducts responsive to near-UV and visible light: eight GAFs contained PCB chromophores, three GAFs contained biliverdin chromophores and one contained the PCB isomer, phycoviolobilin. Interestingly, COO91_03972 is the first-ever reported GAF-only CBCR capable of sensing five wavelengths of light. Bioinformatics and biochemical analyses revealed that residue P132 of COO91_03972 is essential for chromophore binding to dual-cysteine CBCRs. Furthermore, the complement of N. flagelliforme CBCRs is enriched in red light sensors. We hypothesize that these sensors are critical for the acclimatization of N. flagelliforme to weak light environments at dawn.

Red/far-red light signals regulate the activity of the carbon-concentrating mechanism in cyanobacteria

Desiccation-tolerant cyanobacteria can survive frequent hydration/dehydration cycles likely affecting inorganic carbon (Ci) levels. It was recently shown that red/far-red light serves as signal-preparing cells toward dehydration. Here, the effects of desiccation on Ci assimilation by Leptolyngbya ohadii isolated from Israel's Negev desert were investigated. Metabolomic investigations indicated a decline in ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation activity, and this was accelerated by far-red light. Far-red light negatively affected the Ci affinity of L. ohadii during desiccation and in liquid cultures. Similar effects were evident in the non-desiccation-tolerant cyanobacterium Synechocystis The Synechocystis Δcph1 mutant lacking the major phytochrome exhibited reduced photosynthetic Ci affinity when exposed to far-red light, whereas the mutant ΔsbtB lacking a Ci uptake inhibitory protein lost the far-red light inhibition. Collectively, these results suggest that red/far-red light perception likely via phytochromes regulates Ci uptake by cyanobacteria and that this mechanism contributes to desiccation tolerance in strains such as L. ohadii.

Reading and surviving the harsh conditions in desert biological soil crust: The cyanobacterial viewpoint

Biological soil crusts (BSCs) are found in drylands, cover ∼12% of the Earth's surface in arid and semi-arid lands and their destruction is considered an important promoter of desertification. These crusts are formed by the adhesion of soil particles to polysaccharides excreted mostly by filamentous cyanobacteria, which are the pioneers and main primary producers in BSCs. Desert BSCs survive in one of the harshest environments on Earth, and are exposed to daily fluctuations of extreme conditions. The cyanobacteria inhabiting these habitats must precisely read the changing conditions and predict, for example, the forthcoming desiccation. Moreover, they evolved a comprehensive regulation of multiple adaptation strategies to enhance their stress tolerance. Here we focus on what distinguishes cyanobacteria able to revive after dehydration from those that cannot. While important progress has been made in our understanding of physiological, biochemical and omics aspects, clarification of the sensing, signal transduction and responses enabling desiccation tolerance are just emerging. We plot the trajectory of current research and open questions ranging from general strategies and regulatory adaptations in the hydration/desiccation cycle, to recent advances in our understanding of photosynthetic adaptation. The acquired knowledge provides new insights to mitigate desertification and improve plant productivity under drought conditions.

Physiological and Proteomic Studies of the Cyanobacterium Anabaena sp. Acclimated to Desiccation Stress

Agricultural productivity is threatened by increasing incidence of drought and the drought tolerant cyanobacteria offer a better solution in the restoration of soil fertility and productivity. The present study describes the comparative physiological response of the cyanobacterium Anabaena sp. acclimated and un-acclimated to desiccation stress induced by polyethylene glycol (10% PEG). While, the acclimated cyanobacterial cells grew luxuriantly with optimal chlorophyll content, photosynthetic activities and nitrogen fixation, the un-acclimated cells exhibited reduced growth rate, chlorophyll content, photosynthetic activities and nitrogen fixation. Distinct differences in the accumulation of lipid peroxidation products, proline and activity of superoxide dismutase were observed under identical growth conditions in the acclimated and un-acclimated cells. Desiccation-acclimated and un-acclimated cyanobacteria showed significant alterations in the abundance of important proteins in the proteome. Two-dimensional gel electrophoresis followed by MALDI-TOF-MS/MS analysis identified twelve proteins. The acclimated cells showed the up regulation of proteins such as Rubisco, fructose-bis-phosphate aldolase, fructose 1-6 bisphosphatase, phosphoglycerate dehydrogenase and elongation factors Tu and Ts as compared to un-acclimated cells. Therefore, the ability to maintain photosynthesis, antioxidants and increased accumulation of proteins related to energy metabolism helped the acclimated cyanobacterium Anabaena sp. to grow optimally under desiccation stress conditions.

Comparative genomics reveals insights into cyanobacterial evolution and habitat adaptation

Cyanobacteria are photosynthetic prokaryotes that inhabit diverse aquatic and terrestrial environments. However, the evolutionary mechanisms involved in the cyanobacterial habitat adaptation remain poorly understood. Here, based on phylogenetic and comparative genomic analyses of 650 cyanobacterial genomes, we investigated the genetic basis of cyanobacterial habitat adaptation (marine, freshwater, and terrestrial). We show: (1) the expansion of gene families is a common strategy whereby terrestrial cyanobacteria cope with fluctuating environments, whereas the genomes of many marine strains have undergone contraction to adapt to nutrient-poor conditions. (2) Hundreds of genes are strongly associated with specific habitats. Genes that are differentially abundant in genomes of marine, freshwater, and terrestrial cyanobacteria were found to be involved in light sensing and absorption, chemotaxis, nutrient transporters, responses to osmotic stress, etc., indicating the importance of these genes in the survival and adaptation of organisms in specific habitats. (3) A substantial fraction of genes that facilitate the adaptation of Cyanobacteria to specific habitats are contributed by horizontal gene transfer, and such genetic exchanges are more frequent in terrestrial cyanobacteria. Collectively, our results further our understandings of the adaptations of Cyanobacteria to different environments, highlighting the importance of ecological constraints imposed by the environment in shaping the evolution of Cyanobacteria.

Multi-omics reveals mechanisms of total resistance to extreme illumination of a desert alga

The unparalleled performance of Chlorella ohadii under irradiances of twice full sunlight underlines the gaps in our understanding of how the photosynthetic machinery operates, and what sets its upper functional limit. Rather than succumbing to photodamage under extreme irradiance, unique features of photosystem II function allow C. ohadii to maintain high rates of photosynthesis and growth, accompanied by major changes in composition and cellular structure. This remarkable resilience allowed us to investigate the systems response of photosynthesis and growth to extreme illumination in a metabolically active cell. Using redox proteomics, transcriptomics, metabolomics and lipidomics, we explored the cellular mechanisms that promote dissipation of excess redox energy, protein S-glutathionylation, inorganic carbon concentration, lipid and starch accumulation, and thylakoid stacking. C. ohadii possesses a readily available capacity to utilize a sudden excess of reducing power and carbon for growth and reserve formation, and post-translational redox regulation plays a pivotal role in this rapid response. Frequently the response in C. ohadii deviated from that of model species, reflecting its life history in desert sand crusts. Comparative global and case-specific analyses provided insights into the potential evolutionary role of effective reductant utilization in this extreme resistance of C. ohadii to extreme irradiation.

Evo-physio: on stress responses and the earliest land plants

Embryophytes (land plants) can be found in almost any habitat on the Earth's surface. All of this ecologically diverse embryophytic flora arose from algae through a singular evolutionary event. Traits that were, by their nature, indispensable for the singular conquest of land by plants were those that are key for overcoming terrestrial stressors. Not surprisingly, the biology of land plant cells is shaped by a core signaling network that connects environmental cues, such as stressors, to the appropriate responses-which, thus, modulate growth and physiology. When did this network emerge? Was it already present when plant terrestrialization was in its infancy? A comparative approach between land plants and their algal relatives, the streptophyte algae, allows us to tackle such questions and resolve parts of the biology of the earliest land plants. Exploring the biology of the earliest land plants might shed light on exactly how they overcame the challenges of terrestrialization. Here, we outline the approaches and rationale underlying comparative analyses towards inferring the genetic toolkit for the stress response that aided the earliest land plants in their conquest of land.

The Ecology of Subaerial Biofilms in Dry and Inhospitable Terrestrial Environments

The ecological relationship between minerals and microorganisms arguably represents one of the most important associations in dry terrestrial environments, since it strongly influences major biochemical cycles and regulates the productivity and stability of the Earth’s food webs. Despite being inhospitable ecosystems, mineral substrata exposed to air harbor form complex and self-sustaining communities called subaerial biofilms (SABs). Using life on air-exposed minerals as a model and taking inspiration from the mechanisms of some microorganisms that have adapted to inhospitable conditions, we illustrate the ecology of SABs inhabiting natural and built environments. Finally, we advocate the need for the convergence between the experimental and theoretical approaches that might be used to characterize and simulate the development of SABs on mineral substrates and SABs’ broader impacts on the dry terrestrial environment.

Actinobacteria and Cyanobacteria Diversity in Terrestrial Antarctic Microenvironments Evaluated by Culture-Dependent and Independent Methods

Bacterial diversity from McMurdo Dry Valleys in Antarctica, the coldest desert on earth, has become more easily assessed with the development of High Throughput Sequencing (HTS) techniques. However, some of the diversity remains inaccessible by the power of sequencing. In this study, we combine cultivation and HTS techniques to survey actinobacteria and cyanobacteria diversity along different soil and endolithic micro-environments of Victoria Valley in McMurdo Dry Valleys. Our results demonstrate that the Dry Valleys actinobacteria and cyanobacteria distribution is driven by environmental forces, in particular the effect of water availability and endolithic environments clearly conditioned the distribution of those communities. Data derived from HTS show that the percentage of cyanobacteria decreases from about 20% in the sample closest to the water source to negligible values on the last three samples of the transect with less water availability. Inversely, actinobacteria relative abundance increases from about 20% in wet soils to over 50% in the driest samples. Over 30% of the total HTS data set was composed of actinobacterial strains, mainly distributed by 5 families: Sporichthyaceae, Euzebyaceae, Patulibacteraceae, Nocardioidaceae, and Rubrobacteraceae. However, the 11 actinobacterial strains isolated in this study, belonged to Micrococcaceae and Dermacoccaceae families that were underrepresented in the HTS data set. A total of 10 cyanobacterial strains from the order Synechococcales were also isolated, distributed by 4 different genera (Nodosilinea, Leptolyngbya, Pectolyngbya, and Acaryochloris-like). In agreement with the cultivation results, Leptolyngbya was identified as dominant genus in the HTS data set. Acaryochloris-like cyanobacteria were found exclusively in the endolithic sample and represented 44% of the total 16S rRNA sequences, although despite our efforts we were not able to properly isolate any strain from this Acaryochloris-related group. The importance of combining cultivation and sequencing techniques is highlighted, as we have shown that culture-dependent methods employed in this study were able to retrieve actinobacteria and cyanobacteria taxa that were not detected in HTS data set, suggesting that the combination of both strategies can be usefull to recover both abundant and rare members of the communities.

Genomic Features for Desiccation Tolerance and Sugar Biosynthesis in the Extremophile Gloeocapsopsis sp. UTEX B3054

For tolerating extreme desiccation, cyanobacteria are known to produce both compatible solutes at intracellular level and a copious amount of exopolysaccharides as a protective coat. However, these molecules make cyanobacterial cells refractory to a broad spectrum of cell disruption methods, hindering genome sequencing, and molecular studies. In fact, few genomes are already available from cyanobacteria from extremely desiccated environments such as deserts. In this work, we report the 5.4 Mbp draft genome (with 100% of completeness in 105 contigs) of Gloeocapsopsis sp. UTEX B3054 (subsection I; Order Chroococcales), a cultivable sugar-rich and hardly breakable hypolithic cyanobacterium from the Atacama Desert. Our in silico analyses focused on genomic features related to sugar-biosynthesis and adaptation to dryness. Among other findings, screening of Gloeocapsopsis genome revealed a unique genetic potential related to the biosynthesis and regulation of compatible solutes and polysaccharides. For instance, our findings showed for the first time a novel genomic arrangement exclusive of Chroococcaceae cyanobacteria associated with the recycling of trehalose, a compatible solute involved in desiccation tolerance. Additionally, we performed a comparative genome survey and analyses to entirely predict the highly diverse pool of glycosyltransferases enzymes, key players in polysaccharide biosynthesis and the formation of a protective coat to dryness. We expect that this work will set the fundamental genomic framework for further research on microbial tolerance to desiccation and to a wide range of other extreme environmental conditions. The study of microorganisms like Gloeocapsopsis sp. UTEX B3054 will contribute to expand our limited understanding regarding water optimization and molecular mechanisms allowing extremophiles to thrive in xeric environments such as the Atacama Desert.

Coping with low water activities and osmotic stress in Acinetobacter baumannii: significance, current status and perspectives

Multidrug resistant (MDR) pathogens are one of the most pressing challenges of contemporary health care. Acinetobacter baumannii takes a predominant position, emphasized in 2017 by the World Health Organization. The increasing emergence of MDR strains strengthens the demand for new antimicrobials. Possible targets for such compounds might be proteins involved in resistance against low water activity environments, since A. baumannii is known for its pronounced resistance against desiccation stress. Despite the importance of desiccation resistance for persistence of this pathogen in hospitals, comparable studies and precise data on this topic are rare and the mechanisms involved are largely unknown. This review aims to give an overview of the studies performed so far and the current knowledge on genes and proteins important for desiccation survival. 'Osmotic stress' is not identical to 'desiccation stress', but the two share the response of bacteria to low water activities. Osmotic stress resistance is in general studied much better, and in recent years it turned out that accumulation of compatible solutes in A. baumannii comprises some special features such as the bifunctional enzyme MtlD synthesizing the unusual solute mannitol. Furthermore, the regulatory pathways, as understood today, will be discussed.

Dawn illumination prepares desert cyanobacteria for dehydration

Desert biological soil crusts (BSC), among the harshest environments on Earth, are formed by the adhesion of soil particles to polysaccharides excreted mainly by filamentous cyanobacteria (see [1] and references therein). These species are the main primary producers in this habitat where they cope with various stressors including frequent hydration-dehydration cycles. Water is mainly provided as early-morning dew, followed by dehydration with rising temperatures and declining relative humidity. Earlier studies focused on community structure and cyanobacterial activities in various BSCs [1,2]. They identified genes present in dehydration-tolerant, but not -sensitive cyanobacteria [3], and suggested that abiotic conditions during natural dehydration (Figure 1A) are critical for the recovery upon rewetting. Inability of Leptolyngbya ohadii, which is abundant in the BSC examined here, to recover after rapid desiccation (Figure 1B) [4] suggested that the cells must prepare themselves toward forthcoming dehydration, but the nature of the signal involved was unknown. We show here that the rising dawn illumination, perceived by photo-sensors, serves as the signal inciting BSC-inhabiting cyanobacteria to prepare for forthcoming dehydration. L. ohadii filaments were exposed to simulated natural conditions from the morning of October 14^th 2009, using our environmental chamber that enables accurate reproduction of BSC environment [4] (Supplemental Figure S1A). Samples were withdrawn at specific time points (Figure 1A), followed by RNA extraction and global transcript profiling (accession PRJNA391854). Four hours of dehydration led to up-regulation of 567 genes and down-regulation of 1597 (by more than 2-fold). Since BSC-inhabiting organisms have not been used as genetic models, the functions of 3258 (43.5% of the 7487 L. ohadii genes [3]) are unknown. Nevertheless, a pronounced rise in transcript levels of genes involved in carbon metabolism, transport, osmolyte production, energy dissipation and other cellular activities was observed. On the other hand, a declining transcript abundance for genes involved in light harvesting, photosynthetic metabolism, protein biosynthesis, cell division and other pathways was detected. The analysis unraveled clear distinctions between early- and late-responding genes. Supplemental Table S1 lists the 40 strongest differentially expressed genes verified by RT-qPCR and used in further analyses.

Flexibility-Rigidity Coordination of the Dense Exopolysaccharide Matrix in Terrestrial Cyanobacteria Acclimated to Periodic Desiccation

A dense exopolysaccharide (EPS) matrix is crucial for cyanobacterial survival in terrestrial xeric environments, in which cyanobacteria undergo frequent expansion and shrinkage processes during environmental desiccation-rehydration cycles. However, it is unclear how terrestrial cyanobacteria coordinate the structural dynamics of the EPS matrix upon expansion and shrinkage to avoid potential mechanical stress while benefiting from the matrix. In the present study, we sought to answer this question by investigating the gene expression, protein dynamics, enzymatic characteristics, and biological roles of WspA, an abundantly secreted protein, in the representative terrestrial cyanobacterium Nostoc flagelliforme The results demonstrated that WspA is a novel β-galactosidase that facilitates softening of the EPS matrix by breaking the polysaccharide backbone under substantial moisture or facilitates the thickening and relinkage of the broken matrix during the drying process, and thus these regulations are well correlated with moisture availability or desiccation-rehydration cycles. This coordination of flexibility and rigidity of the cyanobacterial extracellular matrix may contribute to a favorable balance of cell growth and stress resistance in xeric environments.IMPORTANCE How the exopolysaccharide matrix is dynamically coordinated by exoproteins to cope with frequent expansion and shrinkage processes in terrestrial colonial cyanobacteria remains unclear. Here we elucidated the biochemical identity and biological roles of a dominant exoprotein in these regulation processes. Our study thus gained insight into this regulative mechanism in cyanobacteria to combat periodic desiccation. In addition, the filamentous drought-adapted cyanobacterium Nostoc flagelliforme serves as an ideal model for us to explore this issue in this study.

On the cradle of CCM research: discovery, development, and challenges ahead

Herein, 40 years after its discovery, I briefly and critically survey the development of ideas that propelled research on CO2-concentrating mechanisms (CCMs; a term proposed by Dean Price) of phytoplankton, mainly focusing on cyanobacteria. This is not a comprehensive review on CCM research, but a personal view on the past developments and challenges that lie ahead.