Evolutionary Adaptations of Bacteria in Arid Environments
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EVOLUTIONARY ADAPTATIONS OF BACTERIA IN ARID ENVIRONMENTS By: Reihaneh Shekari 1 BRIEF HISTORY OF LIFE ON EARTH • The Earth is about 4.6 billion years old the most ancient rocks were dated to be 3.5 to 3.86 billion years old. • Studies have shown that the initial conditions of the earth were extreme. 2 WHAT ARE EXTREMOPHILES? Extremophiles are organisms that are adapted to grow optimally at or near to the extreme ranges of environmental variables. Most extremophiles are microorganisms that thrive under conditions that, from a human perspective, are clearly hostile. 3 Octopus Spring is an alkaline hot spring, located in Yellowstone National Park, that supports the growth of thermophilic bacteria. 4 TYPES OF EXTREME ENVIRONMENTS Alkaline Acitic Extremely cold Extremely hot Hypersaline Under pressure Radiation Without water Without oxygen 5 Organisms that can survive in dry conditions are called xerophiles. xero in Greek means dry. 7.5% of Earth is the desert region. North and South Poles may be dry or very dry. Atacama Desert in northern Chile and the Negev Desert in Israel are example dry environment. 6 ENVIRONMENTS WITHOUT WATER • Scanning electron micrograph of cyanobacterial sheath material, holding sand grains together 7 There are no obligated Xerophiles. it is better to call them Xerotolerant. Areas of the earth where Xerotolerant organisms are able to survive make up 10% of the planet. Xerotolerant microorganisms are not limited to deserts and can live in hypersaline environments. 8 • Xerotolerant microorganisms can be found in distinct niches along the surface and in the sub-layers of rocky landscapes of semi-arid and hyper-arid environments. 9 BACTERIAL MECHANISMS FOR DESICCATION TOLERANCE • production of bacterial spores: • endospores of Firmicutes • exospores of Actinobacteria • akinetes Cyanobacteria: for example Anabaena cylindrical • Azotobacter (Gammaproteobacteria) • myxospores of Myxococcus (Deltaproteobacteria). • VBNC in geram negative bacteria 10 • A completely different mechanism was more recently described: no cell differentiation involved in Deinococcus but a highly efficient and rapid ability to repair DNA damage.mechanism for tolerance of desiccation, which is correlated to tolerance of gamma radiation, is discussed in Sect • Rhizobia is deserts, especially strains able to nodulate the legume tree Acacia. Rhizobia synthesis of osmoprotectants (glycine-betaine, sucrose, ectoine). mannosucrose and trehalose are also involved in the desiccation tolerance of Rhizobium 11 • Garcia-Pichel and Pringault showed that the filamentous cyanobacterium Oscillatoria could migrate to the soil surface in response to wetting events or retreating below in response to drying event. This nomadic behaviour seems to be the main explanation of Oscillatoria adaptation to desert life. 12 • Nostoc group of cyanobacteria forms colony morphotypes with filamentous, lamellate, spherical, or spheroid shapes • These include terrestrial (Nostoc flagelliforme and Nostoc commune) and aquatic (Nostoc sphaeroides, Nostoc verrucosum, Nostoc zetterstedtii, and Nostoc pruniforme) species. • Much of the success of terrestrial Nostoc species is related to their ability to withstand long-term desiccation and to recover metabolic activity after rewetting . Exopolysaccharide (EPS) is believed toplay a key role in providing protection for cyanobacteria against environmental stresses 13 • In arid regions with intense solar radiation and periodic desiccation rehydration cycles, the density of the (Exopolysaccharide )EPS matrix appears to be crucial for colonial Nostoc species. • A dense EPS matrix accommodates additional extracellular components, such as Uv absorbing pigments and antioxidant proteins, prevents membrane fusions during desiccation and freeze-drying, and, most importantly, facilitates the absorption and retention of moisture. • dense EPS has an important acclimation strategy for extreme dryness. 14 • The cyanobacterial EPS matrix accommodates many exoproteins, including antioxidant proteins. • a particularly attractive protein is the acidic water stress protein, WspA, first reported in N. commune • WspA is induced after desiccation or UV irradiation of cells, accumulates in cells, and is secreted into the EPS matrix upon rehydration. • WspA is found N. flagelliforme, N. sphaeroides, and N. verrucosum 15 WspA is a novel -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. EPS biosynthesis mutants of Escherichia coli, Pantoea stewartii and Acinetobacter calcoaceticus showed a sixfold decrease in their survival rates under desiccating conditions . 16 • The role of EPS in resistance to desiccation can be linked to their hygroscopic nature. • When Pseudomonas putida is subjected to water stress, it over-produces the anionic EPS alginate, which can hold several times its weight of water. • The hydrophilic properties of EPS also contribute to the rapid water absorption rates and restoration of photosynthetic activity that are observed in Nostoc commune following rehydration after prolonged desiccation. 17 EPS AND BIOFILM FORMATION • Biofilms are found in various xeric environments as dominant community structures in which algae, fungi, archaea and bacteria coexist and have thus been strongly correlated with the survival of microorganisms in soils that have low moisture content. • Most of the biomass of biofilms is composed of EPS. EPS are crucial for biofilm-mediated desiccation tolerance. • Transcriptome studies have shown that the formation of biofilms is directly involved in desiccation tolerance in L. monocytogenes, S. enterica and Bradyrhizobium japonicum, in which genes that are associated with the formation of biofilms are upregulated in response to desiccation 18 PHYSIOLOGICAL ADAPTATIONS TO WATER STRESS • Cell membrane adaptations : • increases in the ratio of saturated to unsaturated fatty acids • trans-mono-unsaturated to cis-mono-unsaturated fatty acids • increased conversion of monoenoic fatty acids to cyclopropane fatty acids. • increasing phosphatidylglycerol lipids and cardiolipin • expense of zwitterionic lipids, such as phosphatidylethanolamine • production of pigments • Carotenoid mutants of C. sakazakii showed decreased survival in response to long-term desiccation 19 ACCUMULATION OF COMPATIBLE SOLUTES. • Under desiccating conditions, the intracellular accumulation of small molecules and salts restores the osmotic equilibrium and permits continued protein function at low water activity, which enables cells to adapt to xeric stress. • Extreme halophiles use a ‘salting-in’ strategy Conversely, xerotolerant bacteria use a bi-phasic ‘salting-out’ process. Initially, they rapidly accumulate charged solutes (such as potassium and glutamate) in response to osmotic stress. • These low-molecular-weight compounds are either acquired from the environment or biosynthesized. • Compatible solutes, such as trehalose and glycine betaine, are part of the general adaptive strategy in most xerotolerant microorganisms. 20 METABOLIC ADAPTATIONS • Photosynthesis and the fixation of carbon dioxide are downregulated in response to desiccation in Anabaena sp. PCC7120 • The expression of fermentative enzymes and those of the oxidative pentose phosphate pathway are induced in C. sakazakii and B. japonicum • glucose is diverted to trehalose synthesis and fatty acid oxidation becomes a highly efficient alternative energy source for the cell • flagellar motility and chemotaxis, are downregulated in cells 21 MOLECULAR ADAPTATIONS • accumulation of late embryogenesis abundant (LEA) under conditions of water stress in plants. • This proteins are found in other organisms, such as E. coli and Saccharomyces cerevisiae . • so far ,six families of LEA proteins that have been described. • family 3 proteins and their homologues are present in a wide range of organisms that are well adapted to desiccation. • Deinococcus radiodurans and Haemophilus influenzae, have LEA and resistant under xeric conditions. • All six families of LEA proteins share a common glycine-rich and highly hydrophilic backbone that remains in an unstructured coiled state under normal hydrated conditions but transitions into an ordered structure under desiccating conditions. 22 EFFECTS OF DESICCATION ON THE PHYSIOLOGY AND BIOCHEMISTRY OF BACTERIALCELLS 23 Adaptive mechanisms of xerotolerant bacteria 24 DEINOCOCCUS, PROTEIN PROTECTION, AND DNA REPAIR • Deinococcus bacteria is tolerance to gamma and UV radiation, desiccation, and other oxidative stress- generating conditions. • Natural sources of ionizing radiation on Earth exist at levels much lower than those tolerated by Deinococcus. • tolerance is a consequence of adaptation to natural non-radioactive DNA-damaging conditions such as desiccation. • several radiation-tolerant Deinococcus strains have been isolated from arid desert soil. 25 • For D. radiodurans, efficient reconstitution of an intact genome is dependent on extended synthesis-dependent strand annealing (ESDSA) and homologous recombination. • DNA repair mechanism requires homologs of proteins that are