Defining the response of a microorganism to temperatures that span its complete growth temperature range (-2°C to 28°C) using multiplex quantitative proteomics

Uncovering the temporal dynamics and regulatory networks of thermal stress response in a hyperthermophile using transcriptomics and proteomics

Facing rapid fluctuations in their natural environment, extremophiles, like the hyperthermophilic archaeon Pyrococcus furiosus, exhibit remarkable adaptability to extreme conditions. However, our understanding of their dynamic cellular responses remains limited. This study integrates RNA-sequencing and mass spectrometry data, thereby elucidating transcriptomic and proteomic responses to heat and cold shock stress in P. furiosus. Our results reveal rapid and dynamic changes in gene and protein expression following these stress responses. Heat shock triggers extensive transcriptome reprogramming, orchestrated by the transcriptional regulator Phr, targeting a broader gene repertoire than previously demonstrated. For heat shock signature genes, RNA levels swiftly return to baseline upon recovery, while protein levels remain persistently upregulated, reflecting a rapid but sustained response. Intriguingly, cold shock at 4°C elicits distinct short- and long-term responses at both RNA and protein levels. Cluster analysis identified gene sets with either congruent or contrasting trends in RNA and protein changes, representing well-separated arCOG groups tailored to their individual cellular responses. Particularly, upregulation of ribosomal proteins and significant enrichment of 5'-leadered sequences in cold-shock responsive genes suggest that translation regulation is important during cold shock adaption. Further investigating transcriptomic features, we reveal that thermal stress genes are equipped with basal sequence elements, such as strong promoter and poly(U)-terminators, facilitating a regulated response of the respective transcription units. Our study provides a comprehensive overview of the cellular response to temperature stress, advancing our understanding of stress response mechanisms in hyperthermophilic archaea and providing valuable insights into the molecular adaptations that facilitate life in extreme environments.IMPORTANCEExtreme environments provide unique challenges for life, and the study of extremophiles can shed light on the mechanisms of adaptation to such conditions. Pyrococcus furiosus, a hyperthermophilic archaeon, is a model organism for studying thermal stress response mechanisms. In this study, we used an integrated analysis of RNA-sequencing and mass spectrometry data to investigate the transcriptomic and proteomic responses of P. furiosus to heat and cold shock stress and recovery. Our results reveal the rapid and dynamic changes in gene and protein expression patterns associated with these stress responses, as well as the coordinated regulation of different gene sets in response to different stressors. These findings provide valuable insights into the molecular adaptations that facilitate life in extreme environments and advance our understanding of stress response mechanisms in hyperthermophilic archaea.

Archaeal lipids

The major archaeal membrane glycerolipids are distinguished from those of bacteria and eukaryotes by the contrasting stereochemistry of their glycerol backbones, and by the use of ether-linked isoprenoid-based alkyl chains rather than ester-linked fatty acyl chains for their hydrophobic moieties. These fascinating compounds play important roles in the extremophile lifestyles of many species, but are also present in the growing numbers of recently discovered mesophilic archaea. The past decade has witnessed significant advances in our understanding of archaea in general and their lipids in particular. Much of the new information has come from the ability to screen large microbial populations via environmental metagenomics, which has revolutionised our understanding of the extent of archaeal biodiversity that is coupled with a strict conservation of their membrane lipid compositions. Significant additional progress has come from new culturing and analytical techniques that are gradually enabling archaeal physiology and biochemistry to be studied in real time. These studies are beginning to shed light on the much-discussed and still-controversial process of eukaryogenesis, which probably involved both bacterial and archaeal progenitors. Puzzlingly, although eukaryotes retain many attributes of their putative archaeal ancestors, their lipid compositions only reflect their bacterial progenitors. Finally, elucidation of archaeal lipids and their metabolic pathways have revealed potentially interesting applications that have opened up new frontiers for biotechnological exploitation of these organisms. This review is concerned with the analysis, structure, function, evolution and biotechnology of archaeal lipids and their associated metabolic pathways.

Evaluating Cellular Viability by iTRAQ Proteomic Profiling

Cellular health, functionality, response to environment, and other variables affecting cell, tissue, or organ viability are reflected in the cellular proteomes and metabolomes. These "omic" profiles are in constant flux even during normal cellular functioning, to maintain cellular homeostasis, in response to small environmental changes and maintenance of optimal cell viability. However proteomic "fingerprints" can also provide insight into cellular ageing, response to disease, adjustment to environmental changes, and other variables that impact cellular viability. A variety of proteomic methods can be used to determine qualitative and quantitative proteomic change. In this chapter, we will focus on a labeling method called isobaric tags for relative and absolute quantification (iTRAQ), which is frequently used to identify and quantify proteomic expression changes in cells and tissues.

A limit on the evolutionary rescue of an Antarctic bacterium from rising temperatures

Climate change is gradual, but it can also cause brief extreme heat waves that can exceed the upper thermal limit of any one organism. To study the evolutionary potential of upper thermal tolerance, we evolved the cold-adapted Antarctic bacterium Pseudoalteromonas haloplanktis to survive at 30°C, beyond its ancestral thermal limit. This high-temperature adaptation occurred rapidly and in multiple populations. It involved genomic changes that occurred in a highly parallel fashion and mitigated the effects of protein misfolding. However, it also confronted a physiological limit, because populations failed to grow beyond 30°C. Our experiments aimed to facilitate evolutionary rescue by using a small organism with large populations living at temperatures several degrees below their upper thermal limit. Larger organisms with smaller populations and living at temperatures closer to their upper thermal tolerances are even more likely to go extinct during extreme heat waves.

Heterotrophic Sulfur Oxidation of Halomonas titanicae SOB56 and Its Habitat Adaptation to the Hydrothermal Environment

Halomonas bacteria are ubiquitous in global marine environments, however, their sulfur-oxidizing abilities and survival adaptations in hydrothermal environments are not well understood. In this study, we characterized the sulfur oxidation ability and metabolic mechanisms of Halomonas titanicae SOB56, which was isolated from the sediment of the Tangyin hydrothermal field in the Southern Okinawa Trough. Physiological characterizations showed that it is a heterotrophic sulfur-oxidizing bacterium that can oxidize thiosulfate to tetrathionate, with the Na₂S₂O₃ degradation reaching 94.86%. Two potential thiosulfate dehydrogenase-related genes, tsdA and tsdB, were identified as encoding key catalytic enzymes, and their expression levels in strain SOB56 were significantly upregulated. Nine of fifteen examined Halomonas genomes possess TsdA- and TsdB-homologous proteins, whose amino acid sequences have two typical Cys-X2-Cys-His heme-binding regions. Moreover, the thiosulfate oxidation process in H. titanicae SOB56 might be regulated by quorum sensing, and autoinducer-2 synthesis protein LuxS was identified in its genome. Regarding the mechanisms underlying adaptation to hydrothermal environment, strain SOB56 was capable of forming biofilms and producing EPS. In addition, genes related to complete flagellum assembly system, various signal transduction histidine kinases, heavy metal transporters, anaerobic respiration, and variable osmotic stress regulation were also identified. Our results shed light on the potential functions of heterotrophic Halomonas bacteria in hydrothermal sulfur cycle and revealed possible adaptations for living at deep-sea hydrothermal fields by H. titanicae SOB56.

The life and death of RNA across temperatures

Temperature is an environmental condition that has a pervasive effect on cells along with all the molecules and reactions in them. The mechanisms by which prototypical RNA molecules sense and withstand heat have been identified mostly in bacteria and archaea. The relevance of these phenomena is, however, broader, and similar mechanisms have been recently found throughout the tree of life, from sex determination in reptiles to adaptation of viral RNA polymerases, to genetic disorders in humans. We illustrate the temperature dependence of RNA metabolism with examples from the synthesis to the degradation of mRNAs, and review recently emerged questions. Are cells exposed to greater temperature variations and gradients than previously surmised? How do cells reconcile the conflicting thermal stability requirements of primary and tertiary structures of RNAs? To what extent do enzymes contribute to the temperature compensation of the reaction rates in mRNA turnover by lowering the energy barrier of the catalyzed reactions? We conclude with the ecological, forensic applications of the temperature-dependence of RNA degradation and the biotechnological aspects of mRNA vaccine production.

Remarkably coherent population structure for a dominant Antarctic Chlorobium species

BACKGROUND

In Antarctica, summer sunlight enables phototrophic microorganisms to drive primary production, thereby "feeding" ecosystems to enable their persistence through the long, dark winter months. In Ace Lake, a stratified marine-derived system in the Vestfold Hills of East Antarctica, a Chlorobium species of green sulphur bacteria (GSB) is the dominant phototroph, although its seasonal abundance changes more than 100-fold. Here, we analysed 413 Gb of Antarctic metagenome data including 59 Chlorobium metagenome-assembled genomes (MAGs) from Ace Lake and nearby stratified marine basins to determine how genome variation and population structure across a 7-year period impacted ecosystem function.

RESULTS

A single species, Candidatus Chlorobium antarcticum (most similar to Chlorobium phaeovibrioides DSM265) prevails in all three aquatic systems and harbours very little genomic variation (≥ 99% average nucleotide identity). A notable feature of variation that did exist related to the genomic capacity to biosynthesize cobalamin. The abundance of phylotypes with this capacity changed seasonally ~ 2-fold, consistent with the population balancing the value of a bolstered photosynthetic capacity in summer against an energetic cost in winter. The very high GSB concentration (> 10⁸ cells ml^-1 in Ace Lake) and seasonal cycle of cell lysis likely make Ca. Chlorobium antarcticum a major provider of cobalamin to the food web. Analysis of Ca. Chlorobium antarcticum viruses revealed the species to be infected by generalist (rather than specialist) viruses with a broad host range (e.g., infecting Gammaproteobacteria) that were present in diverse Antarctic lakes. The marked seasonal decrease in Ca. Chlorobium antarcticum abundance may restrict specialist viruses from establishing effective lifecycles, whereas generalist viruses may augment their proliferation using other hosts.

CONCLUSION

The factors shaping Antarctic microbial communities are gradually being defined. In addition to the cold, the annual variation in sunlight hours dictates which phototrophic species can grow and the extent to which they contribute to ecosystem processes. The Chlorobium population studied was inferred to provide cobalamin, in addition to carbon, nitrogen, hydrogen, and sulphur cycling, as critical ecosystem services. The specific Antarctic environmental factors and major ecosystem benefits afforded by this GSB likely explain why such a coherent population structure has developed in this Chlorobium species. Video abstract.

Linking genomic and physiological characteristics of psychrophilic Arthrobacter to metagenomic data to explain global environmental distribution

BACKGROUND

Microorganisms drive critical global biogeochemical cycles and dominate the biomass in Earth's expansive cold biosphere. Determining the genomic traits that enable psychrophiles to grow in cold environments informs about their physiology and adaptive responses. However, defining important genomic traits of psychrophiles has proven difficult, with the ability to extrapolate genomic knowledge to environmental relevance proving even more difficult.

RESULTS

Here we examined the bacterial genus Arthrobacter and, assisted by genome sequences of new Tibetan Plateau isolates, defined a new clade, Group C, that represents isolates from polar and alpine environments. Group C had a superior ability to grow at -1°C and possessed genome G+C content, amino acid composition, predicted protein stability, and functional capacities (e.g., sulfur metabolism and mycothiol biosynthesis) that distinguished it from non-polar or alpine Group A Arthrobacter. Interrogation of nearly 1000 metagenomes identified an over-representation of Group C in Canadian permafrost communities from a simulated spring-thaw experiment, indicative of niche adaptation, and an under-representation of Group A in all polar and alpine samples, indicative of a general response to environmental temperature.

CONCLUSION

The findings illustrate a capacity to define genomic markers of specific taxa that potentially have value for environmental monitoring of cold environments, including environmental change arising from anthropogenic impact. More broadly, the study illustrates the challenges involved in extrapolating from genomic and physiological data to an environmental setting. Video Abstract.

Cadherins in early neural development

During early neural development, changes in signalling inform the expression of transcription factors that in turn instruct changes in cell identity. At the same time, switches in adhesion molecule expression result in cellular rearrangements that define the morphology of the emerging neural tube. It is becoming increasingly clear that these two processes influence each other; adhesion molecules do not simply operate downstream of or in parallel with changes in cell identity but rather actively feed into cell fate decisions. Why are differentiation and adhesion so tightly linked? It is now over 60 years since Conrad Waddington noted the remarkable "Constancy of the Wild Type" (Waddington in Nature 183: 1654-1655, 1959) yet we still do not fully understand the mechanisms that make development so reproducible. Conversely, we do not understand why directed differentiation of cells in a dish is sometimes unpredictable and difficult to control. It has long been suggested that cells make decisions as 'local cooperatives' rather than as individuals (Gurdon in Nature 336: 772-774, 1988; Lander in Cell 144: 955-969, 2011). Given that the cadherin family of adhesion molecules can simultaneously influence morphogenesis and signalling, it is tempting to speculate that they may help coordinate cell fate decisions between neighbouring cells in the embryo to ensure fidelity of patterning, and that the uncoupling of these processes in a culture dish might underlie some of the problems with controlling cell fate decisions ex-vivo. Here we review the expression and function of cadherins during early neural development and discuss how and why they might modulate signalling and differentiation as neural tissues are formed.

Genetic contribution to high temperature tolerance in Cryptococcus neoformans

The human fungal pathogen Cryptococcus neoformans relies on a complex signaling network for the adaptation and survival at the host temperature. Protein phosphatase calcineurin is central to proliferation at 37°C but its exact contributions remain ill-defined. To better define genetic contributions to the C. neoformans temperature tolerance, 4031 gene knockouts were screened for genes essential at 37°C and under conditions that keep calcineurin inactive. Identified 83 candidate strains, potentially sensitive to 37°C, were subsequently subject to technologically simple yet robust assay, in which cells are exposed to a temperature gradient. This has resulted in identification of 46 genes contributing to the maximum temperature at which C. neoformans can proliferate (Tmax). The 46 mutants, characterized by a range of Tmax on drug-free media, were further assessed for Tmax under conditions that inhibit calcineurin, which led to identification of several previously uncharacterized knockouts exhibiting synthetic interaction with the inhibition of calcineurin. A mutant that lacked septin Cdc11 was among those with the lowest Tmax and failed to proliferate in the absence of calcineurin activity. To further define connections with calcineurin and the role for septins in high temperature growth, the 46 mutants were tested for cell morphology at 37°C and growth in the presence of agents disrupting cell wall and cell membrane. Mutants sensitive to calcineurin inhibition were tested for synthetic lethal interaction with deletion of the septin-encoding CDC12 and the localization of the septin Cdc3-mCherry. The analysis described here pointed to previously uncharacterized genes that were missed in standard growth assays indicating that the temperature gradient assay is a valuable complementary tool for elucidating the genetic basis of temperature range at which microorganisms proliferate.

Fungal survival under temperature stress: a proteomic perspective

BACKGROUND

Increases in knowledge of climate change generally, and its impact on agricultural industries specifically, have led to a greater research effort aimed at improving understanding of the role of fungi in various fields. Fungi play a key role in soil ecosystems as the primary agent of decomposition, recycling of organic nutrients. Fungi also include important pathogens of plants, insects, bacteria, domestic animals and humans, thus highlighting their importance in many contexts. Temperature directly affects fungal growth and protein dynamics, which ultimately will cascade through to affect crop performance. To study changes in the global protein complement of fungi, proteomic approaches have been used to examine links between temperature stress and fungal proteomic profiles.

SURVEY METHODOLOGY AND OBJECTIVES

A traditional rather than a systematic review approach was taken to focus on fungal responses to temperature stress elucidated using proteomic approaches. The effects of temperature stress on fungal metabolic pathways and, in particular, heat shock proteins (HSPs) are discussed. The objective of this review is to provide an overview of the effects of temperature stress on fungal proteomes.

CONCLUDING REMARKS

Elucidating fungal proteomic response under temperature stress is useful in the context of increasing understanding of fungal sensitivity and resilience to the challenges posed by contemporary climate change processes. Although useful, a more thorough work is needed such as combining data from multiple -omics platforms in order to develop deeper understanding of the factor influencing and controlling cell physiology. This information can be beneficial to identify potential biomarkers for monitoring environmental changes in soil, including the agricultural ecosystems vital to human society and economy.

Enhanced glycosylation of an S-layer protein enables a psychrophilic methanogenic archaeon to adapt to elevated temperatures in abundant substrates

Adaptation to higher temperatures would increase the environmental competitiveness of psychrophiles, organisms that thrive in low-temperature environments. Methanolobus psychrophilus, a cold wetland methanogen, 'evolved' as a mesophile, growing optimally at 30 °C after subculturings, and cells grown with ample substrates exhibited higher integrity. Here, we investigated N-glycosylation of S-layer proteins, the major archaeal envelope component, with respect to mesophilic adaptation. Lectin affinity enriched a glycoprotein in cells grown at 30 °C under ample substrate availability, which was identified as the S-layer protein Mpsy_1486. Four N-glycosylation sites were identified on Mpsy_1486, which exhibited different glycosylation profiles, with N94 only found in cells cultured at 30 °C. An N-linked glycosylation inhibitor, tunicamycin, reduced glycosylation levels of Mpsy_1486 and growth at 30 °C, thus establishing a link between S-layer protein glycosylation and higher temperature adaptation of the psychrophilic archaeon M. psychrophilus.

The Response of Haloferax volcanii to Salt and Temperature Stress: A Proteome Study by Label-Free Mass Spectrometry

In-depth proteome analysis of the haloarchaeal model organism Haloferax volcanii has been performed under standard, low/high salt, and low/high temperature conditions using label-free mass spectrometry. Qualitative analysis of protein identification data from high-pH/reversed-phase fractionated samples indicates 61.1% proteome coverage (2509 proteins), which is close to the maximum recorded values in archaea. Identified proteins match to the predicted proteome in their physicochemical properties, with only a small bias against low-molecular-weight and membrane-associated proteins. Cells grown under low and high salt stress as well as low and high temperature stress are quantitatively compared to standard cultures by sequential window acquisition of all theoretical mass spectra (SWATH-MS). A total of 2244 proteins, or 54.7% of the predicted proteome, are quantified across all conditions at high reproducibility, which allowed for global analysis of protein expression changes under these stresses. Of these, 2034 are significantly regulated under at least one stress condition. KEGG pathway enrichment analysis shows that several major cellular pathways are part of H. volcanii's universal stress response. In addition, specific pathways (purine, cobalamin, and tryptophan) are affected by temperature stress. The most strongly downregulated proteins under all stress conditions, zinc finger protein HVO_2753 and ribosomal protein S14, are found oppositely regulated to their immediate genetic neighbors from the same operon.

Taxonomic and functional characterization of a microbial community from a volcanic englacial ecosystem in Deception Island, Antarctica

Glaciers are populated by a large number of microorganisms including bacteria, archaea and microeukaryotes. Several factors such as solar radiation, nutrient availability and water content greatly determine the diversity and abundance of these microbial populations, the type of metabolism and the biogeochemical cycles. Three ecosystems can be differentiated in glaciers: supraglacial, subglacial and englacial ecosystems. Firstly, the supraglacial ecosystem, sunlit and oxygenated, is predominantly populated by photoautotrophic microorganisms. Secondly, the subglacial ecosystem contains a majority of chemoautotrophs that are fed on the mineral salts of the rocks and basal soil. Lastly, the englacial ecosystem is the least studied and the one that contains the smallest number of microorganisms. However, these unknown englacial microorganisms establish a food web and appear to have an active metabolism. In order to study their metabolic potentials, samples of englacial ice were taken from an Antarctic glacier. Microorganisms were analyzed by a polyphasic approach that combines a set of -omic techniques: 16S rRNA sequencing, culturomics and metaproteomics. This combination provides key information about diversity and functions of microbial populations, especially in rare habitats. Several whole essential proteins and enzymes related to metabolism and energy production, recombination and translation were found that demonstrate the existence of cellular activity at subzero temperatures. In this way it is shown that the englacial microorganisms are not quiescent, but that they maintain an active metabolism and play an important role in the glacial microbial community.

Comparative proteomic analysis of Methanothermobacter thermautotrophicus reveals methane formation from H2 and CO2 under different temperature conditions

The growth of all methanogens is limited to a specific temperature range. However, Methanothermobacter thermautotrophicus can be found in a variety of natural and artificial environments, the temperatures of which sometimes even exceed the temperature growth ranges of thermophiles. As a result, the extent to which methane production and survival are affected by temperature remains unclear. To investigate the mechanisms of methanogenesis that Archaea have evolved to cope with drastic temperature shifts, the responses of Methanothermobacter thermautotrophicus to temperature were investigated under a high temperature growth (71°C) and cold shock (4°C) using Isobaric tags for relative and absolute quantitation (iTRAQ). The results showed that methane formation is decreased and that protein folding and degradation are increased in both high‐ and low‐temperature treatments. In addition, proteins predicted to be involved in processing environmental information processing and in cell membrane/wall/envelope biogenesis may play key roles in affecting methane formation and enhancing the response of M. thermautotrophicus to temperature stress. Analysis of the genomic locations of the genes corresponding to these temperature‐dependent proteins predicted that 77 of the genes likely to form 32 gene clusters. Here, we assess the response of M. thermautotrophicus to different temperatures and provide a new level of understanding of methane formation and cellular putative adaptive responses.

Genomes of ubiquitous marine and hypersaline Hydrogenovibrio, Thiomicrorhabdus and Thiomicrospira spp. encode a diversity of mechanisms to sustain chemolithoautotrophy in heterogeneous environments

Chemolithoautotrophic bacteria from the genera Hydrogenovibrio, Thiomicrorhabdus and Thiomicrospira are common, sometimes dominant, isolates from sulfidic habitats including hydrothermal vents, soda and salt lakes and marine sediments. Their genome sequences confirm their membership in a deeply branching clade of the Gammaproteobacteria. Several adaptations to heterogeneous habitats are apparent. Their genomes include large numbers of genes for sensing and responding to their environment (EAL- and GGDEF-domain proteins and methyl-accepting chemotaxis proteins) despite their small sizes (2.1-3.1 Mbp). An array of sulfur-oxidizing complexes are encoded, likely to facilitate these organisms' use of multiple forms of reduced sulfur as electron donors. Hydrogenase genes are present in some taxa, including group 1d and 2b hydrogenases in Hydrogenovibrio marinus and H. thermophilus MA2-6, acquired via horizontal gene transfer. In addition to high-affinity cbb₃ cytochrome c oxidase, some also encode cytochrome bd-type quinol oxidase or ba₃ -type cytochrome c oxidase, which could facilitate growth under different oxygen tensions, or maintain redox balance. Carboxysome operons are present in most, with genes downstream encoding transporters from four evolutionarily distinct families, which may act with the carboxysomes to form CO₂ concentrating mechanisms. These adaptations to habitat variability likely contribute to the cosmopolitan distribution of these organisms.

Going Green and Cold: Biosurfactants from Low-Temperature Environments to Biotechnology Applications

Approximately 80% of the Earth's biosphere is cold, at an average temperature of 5°C, and is populated by a diversity of microorganisms that are a precious source of molecules with high biotechnological potential. Biosurfactants from cold-adapted organisms can interact with multiple physical phases - water, ice, hydrophobic compounds, and gases - at low and freezing temperatures and be used in sustainable (green) and low-energy-impact (cold) products and processes. We review the biodiversity of microbial biosurfactants produced in cold habitats and provide a perspective on the most promising future applications in environmental and industrial technologies. Finally, we encourage exploring the cryosphere for novel types of biosurfactants via both culture screening and functional metagenomics.

Multiplex quantitative SILAC for analysis of archaeal proteomes: a case study of oxidative stress responses

Stable isotope labelling of amino acids in cell culture (SILAC) is a quantitative proteomic method that can illuminate new pathways used by cells to adapt to different lifestyles and niches. Archaea, while thriving in extreme environments and accounting for ∼20%-40% of the Earth's biomass, have not been analyzed with the full potential of SILAC. Here, we report SILAC for quantitative comparison of archaeal proteomes, using Haloferax volcanii as a model. A double auxotroph was generated that allowed for complete incorporation of ¹³ C/¹⁵ N-lysine and ¹³ C-arginine such that each peptide derived from trypsin digestion was labelled. This strain was found amenable to multiplex SILAC by case study of responses to oxidative stress by hypochlorite. A total of 2565 proteins was identified by LC-MS/MS analysis (q-value ≤ 0.01) that accounted for 64% of the theoretical proteome. Of these, 176 proteins were altered at least 1.5-fold (p-value < 0.05) in abundance during hypochlorite stress. Many of the differential proteins were of unknown function. Those of known function included transcription factor homologs related to oxidative stress by 3D-homology modelling and orthologous group comparisons. Thus, SILAC is found to be an ideal method for quantitative proteomics of archaea that holds promise to unravel gene function.

Glaciers and Ice Sheets As Analog Environments of Potentially Habitable Icy Worlds

Icy worlds in the solar system and beyond have attracted a remarkable attention as possible habitats for life. The current consideration about whether life exists beyond Earth is based on our knowledge of life in terrestrial cold environments. On Earth, glaciers and ice sheets have been considered uninhabited for a long time as they seemed too hostile to harbor life. However, these environments are unique biomes dominated by microbial communities which maintain active biochemical routes. Thanks to techniques such as microscopy and more recently DNA sequencing methods, a great biodiversity of prokaryote and eukaryote microorganisms have been discovered. These microorganisms are adapted to a harsh environment, in which the most extreme features are the lack of liquid water, extremely cold temperatures, high solar radiation and nutrient shortage. Here we compare the environmental characteristics of icy worlds, and the environmental characteristics of terrestrial glaciers and ice sheets in order to address some interesting questions: (i) which are the characteristics of habitability known for the frozen worlds, and which could be compatible with life, (ii) what are the environmental characteristics of terrestrial glaciers and ice sheets that can be life-limiting, (iii) What are the microbial communities of prokaryotic and eukaryotic microorganisms that can live in them, and (iv) taking into account these observations, could any of these planets or satellites meet the conditions of habitability? In this review, the icy worlds are considered from the point of view of astrobiological exploration. With the aim of determining whether icy worlds could be potentially habitable, they have been compared with the environmental features of glaciers and ice sheets on Earth. We also reviewed some field and laboratory investigations about microorganisms that live in analog environments of icy worlds, where they are not only viable but also metabolically active.

Unusual Lipid A from a Cold-Adapted Bacterium: Detailed Structural Characterization

Colwellia psychrerythraea 34H is a Gram-negative cold-adapted microorganism that adopts many strategies to cope with the limitations associated with the low temperatures of its habitat. In this study, we report the complete characterization of the lipid A moiety from the lipopolysaccharide of Colwellia. Lipid A and its partially deacylated derivative were completely characterized by high-resolution mass spectrometry, NMR spectroscopy, and chemical analysis. An unusual structure with a 3-hydroxy unsaturated tetradecenoic acid as a component of the primary acylation pattern was identified. In addition, the presence of a partially acylated phosphoglycerol moiety on the secondary acylation site at the 3-position of the reducing 2-amino-2-deoxyglucopyranose unit caused tremendous natural heterogeneity in the structure of lipid A. Biological-activity assays indicated that C. psychrerythraea 34H lipid A did not show an agonistic or antagonistic effect upon testing in human macrophages.

Characterisation of the Bordetella pertussis secretome under different media

Our understanding of the Bordetella pertussis secretome remains limited including the role of different growth conditions in the secretome. In this study the secretome of L1423, a clinical isolate from the 2008-2012 Australian epidemic, cultured on Stainer-Scholte (SS) and Thalen-IJssel (THIJS) media for 12h was characterised using liquid chromatography-mass spectrometry (LC-MS/MS). In the supernatant, LC-MS/MS identified 260 proteins with 143 bioinformatically predicted to be secreted. Eighty percent of proteins were identified in both media. Proteins secreted were functionally associated with cell surface (41%), pathogenicity (16%) and transport (17%). The most abundant proteins identified were pathogenic proteins including toxins (PtxA and CyaA), adhesins (TcfA) and type III secretion (T3SS) proteins. There were 46 proteins found uniquely in THIJS including 8 virulence associated proteins. These included T3SS proteins, adhesins (FhaL and FhaS) and a putative toxin (BP1251). Nine proteins were found uniquely in SS and these were metabolic and transport-related proteins. None of the unique proteins detected in SS were known to be virulence associated. This study found that THIJS promotes secretion of virulence factors based on the number of unique virulence proteins found and may be a growth media of choice for the study of B. pertussis virulence and vaccine development.

BIOLOGICAL SIGNIFICANCE

Over the past two decades, the number of B. pertussis notifications has risen despite vaccination. There is a greater need to understand the biology behind B. pertussis infections. The secretome of B. pertussis in two different media was characterised using LC-MS/MS. The results showed that THIJS promotes secretion of importance virulence factors which may be important for the development of vaccines.

Morphological and proteomic analysis of biofilms from the Antarctic archaeon, Halorubrum lacusprofundi

Biofilms enhance rates of gene exchange, access to specific nutrients, and cell survivability. Haloarchaea in Deep Lake, Antarctica, are characterized by high rates of intergenera gene exchange, metabolic specialization that promotes niche adaptation, and are exposed to high levels of UV-irradiation in summer. Halorubrum lacusprofundi from Deep Lake has previously been reported to form biofilms. Here we defined growth conditions that promoted the formation of biofilms and used microscopy and enzymatic digestion of extracellular material to characterize biofilm structures. Extracellular DNA was found to be critical to biofilms, with cell surface proteins and quorum sensing also implicated in biofilm formation. Quantitative proteomics was used to define pathways and cellular processes involved in forming biofilms; these included enhanced purine synthesis and specific cell surface proteins involved in DNA metabolism; post-translational modification of cell surface proteins; specific pathways of carbon metabolism involving acetyl-CoA; and specific responses to oxidative stress. The study provides a new level of understanding about the molecular mechanisms involved in biofilm formation of this important member of the Deep Lake community.

Characterization of a temperature-responsive two component regulatory system from the Antarctic archaeon, Methanococcoides burtonii

Cold environments dominate the Earth’s biosphere and the resident microorganisms play critical roles in fulfilling global biogeochemical cycles. However, only few studies have examined the molecular basis of thermosensing; an ability that microorganisms must possess in order to respond to environmental temperature and regulate cellular processes. Two component regulatory systems have been inferred to function in thermal regulation of gene expression, but biochemical studies assessing these systems in Bacteria are rare, and none have been performed in Archaea or psychrophiles. Here we examined the LtrK/LtrR two component regulatory system from the Antarctic archaeon, Methanococcoides burtonii, assessing kinase and phosphatase activities of wild-type and mutant proteins. LtrK was thermally unstable and had optimal phosphorylation activity at 10 °C (the lowest optimum activity for any psychrophilic enzyme), high activity at 0 °C and was rapidly thermally inactivated at 30 °C. These biochemical properties match well with normal environmental temperatures of M. burtonii (0–4 °C) and the temperature this psychrophile is capable of growing at in the laboratory (−2 to 28 °C). Our findings are consistent with a role for LtrK in performing phosphotransfer reactions with LtrR that could lead to temperature-dependent gene regulation.

Proteomics of Colwellia psychrerythraea at subzero temperatures - a life with limited movement, flexible membranes and vital DNA repair

The mechanisms that allow psychrophilic bacteria to remain metabolically active at subzero temperatures result from form and function of their proteins. We present first proteomic evidence of physiological changes of the marine psychrophile Colwellia psychrerythraea 34H (Cp34H) after exposure to subzero temperatures (-1, and -10°C in ice) through 8 weeks. Protein abundance was compared between different treatments to understand the effects of temperature and time, independently and jointly, within cells transitioning to, and being maintained in ice. Parallel [3H]-leucine and [3H]-thymidine incubations indicated active protein and DNA synthesis to -10°C. Mass spectrometry-based proteomics identified 1763 proteins across four experimental treatments. Proteins involved in osmolyte regulation and polymer secretion were found constitutively present across all treatments, suggesting that they are required for metabolic success below 0°C. Differentially abundant protein groups indicated a reallocation of resources from DNA binding to DNA repair and from motility to chemo-taxis and sensing. Changes to iron and nitrogen metabolism, cellular membrane structures, and protein synthesis and folding were also revealed. By elucidating vital strategies during life in ice, this study provides novel insight into the extensive molecular adaptations that occur in cold-adapted marine organisms to sustain cellular function in their habitat.

Quantitative proteomics of delirium cerebrospinal fluid

Delirium is a common cause and complication of hospitalization in older people, being associated with higher risk of future dementia and progression of existing dementia. However relatively little data are available on which biochemical pathways are dysregulated in the brain during delirium episodes, whether there are protein expression changes common among delirium subjects and whether there are any changes which correlate with the severity of delirium. We now present the first proteomic analysis of delirium cerebrospinal fluid (CSF), and one of few studies exploring protein expression changes in delirium. More than 270 proteins were identified in two delirium cohorts, 16 of which were dysregulated in at least 8 of 17 delirium subjects compared with a mild Alzheimer's disease neurological control group, and 31 proteins were significantly correlated with cognitive scores (mini-mental state exam and acute physiology and chronic health evaluation III). Bioinformatics analyses revealed expression changes in several protein family groups, including apolipoproteins, secretogranins/chromogranins, clotting/fibrinolysis factors, serine protease inhibitors and acute-phase response elements. These data not only provide confirmatory evidence that the inflammatory response is a component of delirium, but also reveal dysregulation of protein expression in a number of novel and unexpected clusters of proteins, in particular the granins. Another surprising outcome of this work is the level of similarity of CSF protein profiles in delirium patients, given the diversity of causes of this syndrome. These data provide additional elements for consideration in the pathophysiology of delirium as well as potential biomarker candidates for delirium diagnosis.

Proteomic analysis of the adaptation to warming in the Antarctic bacteria Shewanella frigidimarina

Antarctica is subjected to extremely variable conditions, but the importance of the temperature increase in cold adapted bacteria is still unknown. To study the molecular adaptation to warming of Antarctic bacteria, cultures of Shewanella frigidimarina were incubated at temperatures ranging from 0°C to 30°C, emulating the most extreme conditions that this strain could tolerate. A proteomic approach was developed to identify the soluble proteins obtained from cells growing at 4°C, 20°C and 28°C. The most drastic effect when bacteria were grown at 28°C was the accumulation of heat shock proteins as well as other proteins related to stress, redox homeostasis or protein synthesis and degradation, and the decrease of enzymes and components of the cell envelope. Furthermore, two main responses in the adaptation to warm temperature were detected: the presence of diverse isoforms in some differentially expressed proteins, and the composition of chaperone interaction networks at the limits of growth temperature. The abundance changes of proteins suggest that warming induces a stress situation in S. frigidimarina forcing cells to reorganize their molecular networks as an adaptive response to these environmental conditions.

Some like it cold: understanding the survival strategies of psychrophiles

Much of the Earth's surface, both marine and terrestrial, is either periodically or permanently cold. Although habitats that are largely or continuously frozen are generally considered to be inhospitable to life, psychrophilic organisms have managed to survive in these environments. This is attributed to their innate adaptive capacity to cope with cold and its associated stresses. Here, we review the various environmental, physiological and molecular adaptations that psychrophilic microorganisms use to thrive under adverse conditions. We also discuss the impact of modern "omic" technologies in developing an improved understanding of these adaptations, highlighting recent work in this growing field.

Microbial ecology of an Antarctic hypersaline lake: genomic assessment of ecophysiology among dominant haloarchaea

Deep Lake in Antarctica is a cold, hypersaline system where four types of haloarchaea representing distinct genera comprise >70% of the lake community: strain tADL ∼44%, strain DL31 ∼18%, Halorubrum lacusprofundi ∼10% and strain DL1 ∼0.3%. By performing comparative genomics, growth substrate assays, and analyses of distribution by lake depth, size partitioning and lake nutrient composition, we were able to infer important metabolic traits and ecophysiological characteristics of the four Antarctic haloarchaea that contribute to their hierarchical persistence and coexistence in Deep Lake. tADL is characterized by a capacity for motility via flagella (archaella) and gas vesicles, a highly saccharolytic metabolism, a preference for glycerol, and photoheterotrophic growth. In contrast, DL31 has a metabolism specialized in processing proteins and peptides, and appears to prefer an association with particulate organic matter, while lacking the genomic potential for motility. H. lacusprofundi is the least specialized, displaying a genomic potential for the utilization of diverse organic substrates. The least abundant species, DL1, is characterized by a preference for catabolism of amino acids, and is the only one species that lacks genes needed for glycerol degradation. Despite the four haloarchaea being distributed throughout the water column, our analyses describe a range of distinctive features, including preferences for substrates that are indicative of ecological niche partitioning. The individual characteristics could be responsible for shaping the composition of the haloarchaeal community throughout the lake by enabling selection of ecotypes and maintaining sympatric speciation.

Contribution of transcriptomics to systems-level understanding of methanogenic Archaea

Methane-producing Archaea are of interest due to their contribution to atmospheric change and for their roles in technological applications including waste treatment and biofuel production. Although restricted to anaerobic environments, methanogens are found in a wide variety of habitats, where they commonly live in syntrophic relationships with bacterial partners. Owing to tight thermodynamic constraints of methanogenesis alone or in syntrophic metabolism, methanogens must carefully regulate their catabolic pathways including the regulation of RNA transcripts. The transcriptome is a dynamic and important control point in microbial systems. This paper assesses the impact of mRNA (transcriptome) studies on the understanding of methanogenesis with special consideration given to how methanogenesis is regulated to cope with nutrient limitation, environmental variability, and interactions with syntrophic partners. In comparison with traditional microarray-based transcriptome analyses, next-generation high-throughput RNA sequencing is greatly advantageous in assessing transcription start sites, the extent of 5′ untranslated regions, operonic structure, and the presence of small RNAs. We are still in the early stages of understanding RNA regulation but it is already clear that determinants beyond transcript abundance are highly relevant to the lifestyles of methanogens, requiring further study.

Psychrophilic enzymes: from folding to function and biotechnology

Psychrophiles thriving permanently at near-zero temperatures synthesize cold-active enzymes to sustain their cell cycle. Genome sequences, proteomic, and transcriptomic studies suggest various adaptive features to maintain adequate translation and proper protein folding under cold conditions. Most psychrophilic enzymes optimize a high activity at low temperature at the expense of substrate affinity, therefore reducing the free energy barrier of the transition state. Furthermore, a weak temperature dependence of activity ensures moderate reduction of the catalytic activity in the cold. In these naturally evolved enzymes, the optimization to low temperature activity is reached via destabilization of the structures bearing the active site or by destabilization of the whole molecule. This involves a reduction in the number and strength of all types of weak interactions or the disappearance of stability factors, resulting in improved dynamics of active site residues in the cold. These enzymes are already used in many biotechnological applications requiring high activity at mild temperatures or fast heat-inactivation rate. Several open questions in the field are also highlighted.

The genome and transcriptome of a newly described psychrophilic archaeon, Methanolobus psychrophilus R15, reveal its cold adaptive characteristics

We analysed the cold-responsive gene repertoire for a psychrophilic methanogen, Methanolobus psychrophilus R15 through genomic and RNA-seq assayed transcriptomic comparisons for cultures at 18°C (optimal temperature) versus 4°C. The differences found by RNA-seq analysis were verified using quantitative real time-PCR assay. The results showed that as in the Antarctic methanogen, Methanococcoides burtonii, genes for methanogenesis, biosynthesis and protein synthesis were all downregulated by the cold in R15. However, the RNA polymerase complex was upregulated at cold, as well as a gene cluster for a putative exosome complex, suggesting that exosome-mediated RNA decay may be cold-accelerated. Unexpectedly, the chaperonin genes for both thermosome and GroES/EL were all upregulated at 4°C. Strain R15 possessed eight protein families for oxygen detoxification, including both anaerobe-specific superoxide reductase (SOR) and the aerobe-typical superoxide dismutase (SOD)-catalase oxidant-removing system, implying the higher oxidative tolerance. Compared with a mesophilic methanogen, R15 survived in higher paraquat, a redox-cycling drug. Moreover, 71 one-component systems and 50 two-component systems for signal transduction ranked strain R15, together with M. burtonii, as being highly adaptive among archaea. Most of them exhibited cold-enhanced expression, indicating their involvement in cold adaptation. This study has added new perspectives on the cold adaptation of methanogenic archaea.

Proteome characterization of the unsequenced psychrophile Pedobacter cryoconitis using 15N metabolic labeling, tandem mass spectrometry, and a new bioinformatic workflow

Organisms without a sequenced genome and lacking a complete protein database encounter an added level of complexity to protein identification and quantitation. De novo sequencing, new bioinformatics tools, and mass spectrometry (MS) techniques allow for advances in this area. Here, the proteomic characterization of an unsequenced psychrophilic bacterium, Pedobacter cryoconitis, is presented employing a novel workflow based on (15) N metabolic labelling, 2DE, MS/MS, and bioinformatics tools. Two bioinformatics pipelines, based on nitrogen constraint (N-constraint), ortholog searching, and de novo peptide sequencing with N-constraint similarity database search, are compared based on proteome coverage and throughput. Results demonstrate the effect of different growth temperatures (1°C, 20°C) and different carbon sources (glucose, maltose) on the proteome. Seventy-six and 69 proteins were identified and validated from the glucose- and maltose-grown bacterium, respectively, from which 21 and 22 were differentially expressed at different growth temperatures. Differentially expressed proteins are involved in stress response and carbohydrate metabolism, with higher expression at 20°C than at 1°C, while antioxidants were upregulated at 1°C. This study provides an alternative workflow to identify, validate, and quantify proteins from unsequenced organisms distantly related to other species in the protein database. Furthermore, it provides further understanding on bacterial adaptation mechanisms to cold environments, and a comparative proteomic analyses with other psychrophilic microorganisms.

Draft genome sequence of a psychrotolerant sulfur-oxidizing bacterium, Sulfuricella denitrificans skB26, and proteomic insights into cold adaptation

Except for several conspicuous cases, very little is known about sulfur oxidizers living in natural freshwater environments. Sulfuricella denitrificans skB26 is a psychrotolerant sulfur oxidizer recently isolated from a freshwater lake as a representative of a new genus in the class Betaproteobacteria. In this study, an approximately 3.2-Mb draft genome sequence of strain skB26 was obtained. In the draft genome, consisting of 23 contigs, a single rRNA operon, 43 tRNA genes, and 3,133 coding sequences were identified. The identified genes include those required for sulfur oxidation, denitrification, and carbon fixation. Comparative proteomic analysis was conducted to assess cold adaptation mechanisms of this organism. From cells grown at 22°C and 5°C, proteins were extracted for analysis by nano-liquid chromatography-electrospray ionization-tandem mass spectrometry. In the cells cultured at 5°C, relative abundances of ribosomal proteins, cold shock proteins, and DEAD/DEAH box RNA helicases were increased in comparison to those at 22°C. These results suggest that maintenance of proper translation is critical for growth under low-temperature conditions, similar to the case for other cold-adapted prokaryotes.

Microbiology and proteomics, getting the best of both worlds!

High-throughput identification of proteins with the latest generation of hybrid high-resolution mass spectrometers is opening new perspectives in microbiology. I present, here, an overview of tandem mass spectrometry technology and bioinformatics for shotgun proteomics that make 2D-PAGE approaches obsolete. Non-labelling quantitative approaches have become more popular than labelling techniques on most proteomic platforms because they are easier to carry out while their quantitative outcome is rather robust. Parameters for recording mass spectrometry data, however, need to be chosen carefully and statistics to assess the confidence of the results should not be neglected. Interestingly, next-generation sequencing methodologies make any microbial model quickly amenable to proteomics, leading to the documentation of a wide range of organisms from diverse environments. Some recent discoveries made using microbial proteomics have challenged some biological dogma, such as: (i) initiation of the translation does not occur predominantly from ATG codons in some microorganisms, (ii) non-canonical initiation codons are used to regulate the production of specific but important proteins and (iii) a gene may code for multiple polypeptide species, heterogeneous in terms of sequences. Microbial diversity and microbial physiology can now be revisited by means of exhaustive comparative proteomic surveys where thousands of proteins are detected and quantified. Proteogenomics, consisting of better annotating of genomes with the help of proteomic evidence, is paving the way for integrated multi-omic approaches in microbiology. Finally, meta-proteomic tools and approaches are emerging for tackling the high complexity of the microbial world as a whole, opening new perspectives for assessing how microbial communities function.

A metagenomic assessment of winter and summer bacterioplankton from Antarctica Peninsula coastal surface waters

Antarctic surface oceans are well-studied during summer when irradiance levels are high, sea ice is melting and primary productivity is at a maximum. Coincident with this timing, the bacterioplankton respond with significant increases in secondary productivity. Little is known about bacterioplankton in winter when darkness and sea-ice cover inhibit photoautotrophic primary production. We report here an environmental genomic and small subunit ribosomal RNA (SSU rRNA) analysis of winter and summer Antarctic Peninsula coastal seawater bacterioplankton. Intense inter-seasonal differences were reflected through shifts in community composition and functional capacities encoded in winter and summer environmental genomes with significantly higher phylogenetic and functional diversity in winter. In general, inferred metabolisms of summer bacterioplankton were characterized by chemoheterotrophy, photoheterotrophy and aerobic anoxygenic photosynthesis while the winter community included the capacity for bacterial and archaeal chemolithoautotrophy. Chemolithoautotrophic pathways were dominant in winter and were similar to those recently reported in global ‘dark ocean' mesopelagic waters. If chemolithoautotrophy is widespread in the Southern Ocean in winter, this process may be a previously unaccounted carbon sink and may help account for the unexplained anomalies in surface inorganic nitrogen content.

Chaperonins from an Antarctic archaeon are predominantly monomeric: crystal structure of an open state monomer

Archaea are abundant in permanently cold environments. The Antarctic methanogen, Methanococcoides burtonii, has proven an excellent model for studying molecular mechanisms of cold adaptation. Methanococcoides burtonii contains three group II chaperonins that diverged prior to its closest orthologues from mesophilic Methanosarcina spp. The relative abundance of the three chaperonins shows little dependence on organism growth temperature, except at the highest temperatures, where the most thermally stable chaperonin increases in abundance. In vitro and in vivo, the M. burtonii chaperonins are predominantly monomeric, with only 23-33% oligomeric, thereby differing from other archaea where an oligomeric ring form is dominant. The crystal structure of an N-terminally truncated chaperonin reveals a monomeric protein with a fully open nucleotide binding site. When compared with closed state group II chaperonin structures, a large-scale ≈ 30° rotation between the equatorial and intermediate domains is observed resulting in an open nucleotide binding site. This is analogous to the transition observed between open and closed states of group I chaperonins but contrasts with recent archaeal group II chaperonin open state ring structures. The predominance of monomeric form and the ability to adopt a fully open nucleotide site appear to be unique features of the M. burtonii group II chaperonins.