Polysome turnover in immobilized cells of Nostoc commune (cyanobacteria) exposed to water stress

Cyanobacteria and Alphaproteobacteria May Facilitate Cooperative Interactions in Niche Communities

Hypoliths, microbial assemblages found below translucent rocks, provide important ecosystem services in deserts. While several studies have assessed microbial diversity of hot desert hypoliths and whether these communities are metabolically active, the interactions among taxa remain unclear. Here, we assessed the structure, diversity, and co-occurrence patterns of hypolithic communities from the hyperarid Namib Desert by comparing total (DNA) and potentially active (RNA) communities. The potentially active and total hypolithic communities differed in their composition and diversity, with significantly higher levels of Cyanobacteria and Alphaproteobacteria in potentially active hypoliths. Several phyla known to be abundant in total hypolithic communities were metabolically inactive, indicating that some hypolithic taxa may be dormant or dead. The potentially active hypolith network was highly modular in structure with almost exclusively positive co-occurrences (>95% of the total) between taxa. Members of the Cyanobacteria and Alphaproteobacteria were identified as potential keystone taxa, and exhibited numerous positive co-occurrences with other microbes, suggesting that these groups might have important roles in maintaining network topological structure despite their low abundance.

Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments

BACKGROUND

The cyanobacterial genus Nostoc includes several species forming centimetre-large gelatinous colonies in nutrient-poor freshwaters and harsh semi-terrestrial environments with extended drought or freezing. These Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates.

SCOPE

The objective of this review is to evaluate the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources.

CONCLUSIONS

Simple models predict the decline in uptake of dissolved inorganic carbon (DIC) and a decline in the growth rate of spherical freshwater colonies of N. pruniforme and N. zetterstedtii and sheet-like colonies of N. commune in response to a thicker diffusion boundary layer, lower external DIC concentration and higher organic carbon mass per surface area (CMA) of the colony. Measured growth rates of N. commune and N. pruniforme at high DIC availability comply with general empirical predictions of maximum growth rate (i.e. doubling time 10-14 d) as functions of CMA for marine macroalgae and as functions of tissue thickness for aquatic and terrestrial plants, while extremely low growth rates of N. zetterstedtii (i.e. doubling time 2-3 years) are 10-fold lower than model predictions, either because of very low ambient DIC and/or an extremely costly colony matrix. DIC uptake is limited by diffusion at low concentrations for all species, although they exhibit efficient HCO3(-) uptake, accumulation of respiratory DIC within the colonies and very low CO2 compensation points. Long light paths and light attenuation by structural substances in large Nostoc colonies cause lower quantum efficiency and assimilation number and higher light compensation points than in unicells and other aquatic macrophytes. Extremely low growth and mortality rates of N. zetterstedtii reflect stress-selected adaptation to nutrient- and DIC-poor temperate lakes, while N. pruniforme exhibits a mixed ruderal- and stress-selected strategy with slow growth and year-long survival prevailing in sub-Arctic lakes and faster growth and shorter longevity in temperate lakes. Nostoc commune and its close relative N. flagelliforme have a mixed stress-disturbance strategy not found among higher plants, with stress selection to limiting water and nutrients and disturbance selection in quiescent dry or frozen stages. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range (0-35 °C; maximum at 25 °C). Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.

Tolerance of the widespread cyanobacterium Nostoc commune to extreme temperature variations (-269 to 105°C), pH and salt stress

Nostoc commune is a widespread colonial cyanobacterium living on bare soils that alternate between frost and thaw, drought and inundation and very low and high temperatures. We collected N. commune from alternating wet and dry limestone pavements in Sweden and tested its photosynthesis and respiration at 20°C after exposure to variations in temperature (-269 to 105°C), pH (2-10) and NaCl (0.02-50 g NaCl kg(-1)). We found that dry field samples and rewetted specimens tolerated exposure beyond that experienced in natural environmental conditions: -269 to 70°C, pH 3-10 and 0-20 g NaCl kg(-1), with only a modest reduction of respiration, photosynthesis and active carbon uptake at 20°C. (14)CO(2) uptake from air declined markedly below zero and above 55°C, but remained positive. Specimens maintained a high metabolism with daily exposure to 6 h of rehydration and 18 h of desiccation at -18 and 20°C, but died at 40°C. The field temperature never exceeded the critical 40°C threshold during the wet periods, but it frequently exceeded this temperature during dry periods when N. commune is already dry and unaffected. We conclude that N. commune has an excellent tolerance to low temperatures, long-term desiccation and recurring cycles of desiccation and rewetting. These traits explain why it is the pioneer species in extremely harsh, nutrient-poor and alternating wet and dry environments.

LIGHT DEPENDENCY OF PHOTOSYNTHETIC RECOVERY DURING WETTING AND THE ACCLIMATION OF PHOTOSYNTHETIC APPARATUS TO LIGHT FLUCTUATION IN A TERRESTRIAL CYANOBACTERIUM NOSTOC COMMUNE(1)

The PSII photochemical activity in a terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault during rewetting was undetectable in the dark but was immediately recognized in the light. The maximum quantum yield of PSII (Fv /Fm ) during rewetting in the light rose to 85% of the maximum within ∼30 min and slowly reached the maximum within 6 h, while with rewetting in the darkness for 6 h and then exposure to light the recovery of Fv /Fm required only ∼3 min. These results suggested that recovery of photochemical activity might depend on two processes, light dependence and light independence, and the activation of photosynthetic recovery in the initial phase was severely light dependent. The inhibitor experiments showed that the recovery of Fv /Fm was not affected by chloramphenicol (CMP), but severely inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) in the light, suggesting that the light-dependent recovery of photochemical activity did not require de novo protein synthesis but required activation of PSII associated with electron flow to plastoquinone. Furthermore, the test indicated that the lower light intensity and the red light were of benefit to its activation of photochemical activity. In an outdoor experiment of diurnal changes of photochemical activity, our results showed that PSII photochemical activity was sensitive to light fluctuation, and the nonphotochemical quenching (NPQ) was rapidly enhanced at noon. Furthermore, the test suggested that the repair of PSII by de novo protein synthesis played an important role in the acclimation of photosynthetic apparatus to high light, and the heavily cloudy day was more beneficial for maintaining high photochemical activity.

Accumulation of trehalose and sucrose in cyanobacteria exposed to matric water stress

The drought-resistant cyanobacteria Phormidium autumnale, strain LPP(4), and a Chroococcidiopsis sp. accumulated trehalose, sucrose, and both trehalose and sucrose, respectively, in response to matric water stress. Accumulated sugar concentrations reached values of up to 6.2 mug of trehalose per mug of chlorophyll in P. autumnale, 6.9 mug of sucrose per mug of chlorophyll in LPP(4), and 4.1 mug of sucrose and 3.2 mug of trehalose per mug of chlorophyll in the Chroococcidiopsis sp. The same sugars were accumulated by these cyanobacteria in similar concentrations under osmotic water stress. Cyanobacteria that did not show drought resistance (Plectonema boryanum and Synechococcus strain PCC 7942) did not accumulate significant amounts of sugars when matric water stress was applied.

Effects of periodic desiccation on the synthesis of the UV-screening compound, scytonemin, in cyanobacteria

Scytonemin is an ultraviolet radiation (UVR)-screening compound synthesized by some sheathed cyanobacteria exposed to high solar and sky radiation. It is primarily produced in response to UVA radiation, but certain environmental stresses can enhance synthesis. This study focuses on the effects of periodic desiccation on scytonemin synthesis in three desiccation-tolerant cyanobacterial strains, Nostoc punctiforme PCC 73102, Chroococcidiopsis CCMEE 5056 and Chroococcidiopsis CCMEE 246. Nostoc punctiforme and Chroococcidiopsis CCMEE 5056 exposed to UVA radiation produced more concentrated scytonemin screens when experiencing periodic desiccation (i.e. 1 day desiccated for every 2 days hydrated) than when continuously hydrated. A more concentrated scytonemin screen would reduce the amount of UVR damage accrued when cells are desiccated and metabolically inactive. This might allow the cyanobacteria to allocate more energy to systems other than UVR damage repair during rehydration, which would facilitate recovery. The scytonemin screen is extremely stable, remaining largely intact in the sheaths of desiccated N. punctiforme even when continuously exposed to UVA radiation for about 2 months. In contrast to the above findings, scytonemin synthesis in Chroococcidiopsis CCMEE 246, a strain that produces scytonemin constitutively under low visible light (no UVA), was partially inhibited by periodic desiccation.

Characterization of the cold stress-induced cyanobacterial DEAD-box protein CrhC as an RNA helicase

We have shown previously that CrhC is a unique member of the DEAD-box family of RNA helicases whose expression occurs specifically under conditions of cold stress. Here we show that recombinant His-tagged CrhC, purified from Escherichia coli, is an ATP-independent RNA binding protein possessing RNA-dependent ATPase activity which is stimulated most efficiently by rRNA and polysome preparations. RNA strand displacement assays indicate that CrhC possesses RNA unwinding activity that is adenosine nucleotide specific. Unwinding of partially duplexed RNA proceeds in the 5'-->3' but not the 3'-->5' direction using standard assay conditions. Immunoprecipitation and far-western analysis indicate that CrhC is a component of a multisubunit complex, interacting specifically with a 37 kDa polypeptide. We propose that CrhC unwinds cold-stabilized secondary structure in the 5'-UTR of RNA during cold stress.

Ionizing-radiation resistance in the desiccation-tolerant cyanobacterium Chroococcidiopsis

The effect of X-ray irradiation on cell survival, induction, and repair of DNA damage was studied by using 10 Chroococcidiopsis strains isolated from desert and hypersaline environments. After exposure to 2.5 kGy, the percentages of survival for the strains ranged from 80 to 35%. In the four most resistant strains, the levels of survival were reduced by 1 or 2 orders of magnitude after irradiation with 5 kGy; viable cells were recovered after exposure to 15 kGy but not after exposure to 20 kGy. The severe DNA damage evident after exposure to 2.5 kGy was repaired within 3 h, and the severe DNA damage evident after exposure to 5 kGy was repaired within 24 h. The increase in trichloroacetic acid-precipitable radioactivity in the culture supernatant after irradiation with 2.5 kGy might have been due to cell lysis and/or an excision process involved in DNA repair. The radiation resistance of Chroococcidiopsis strains may reflect the ability of these cyanobacteria to survive prolonged desiccation through efficient repair of the DNA damage that accumulates during dehydration.

Active Fe-containing superoxide dismutase and abundant sodF mRNA in Nostoc commune (Cyanobacteria) after years of desiccation

Active Fe-superoxide dismutase (SodF) was the third most abundant soluble protein in cells of Nostoc commune CHEN/1986 after prolonged (13 years) storage in the desiccated state. Upon rehydration, Fe-containing superoxide disumutase (Fe-SOD) was released and the activity was distributed between rehydrating cells and the extracellular fluid. The 21-kDa Fe-SOD polypeptide was purified, the N terminus was sequenced, and the data were used to isolate sodF from the clonal isolate N. commune DRH1. sodF encodes an open reading frame of 200 codons and is expressed as a monocistronic transcript (of approximately 750 bases) from a region of the genome which includes genes involved in nucleic acid synthesis and repair, including dipyrimidine photolyase (phr) and cytidylate monophosphate kinase (panC). sodF mRNA was abundant and stable in cells after long-term desiccation. Upon rehydration of desiccated cells, there was a turnover of sodF mRNA within 15 min and then a rise in the mRNA pool to control levels (quantity of sodF mRNA in cells in late logarithmic phase of growth) over approximately 24 h. The extensive extracellular polysaccharide (glycan) of N. commune DRH1 generated superoxide radicals upon exposure to UV-A or -B irradiation, and these were scavenged by SOD. Despite demonstrated roles for the glycan in the desiccation tolerance of N. commune, it may in fact be a significant source of damaging free radicals in vivo. It is proposed that the high levels of SodF in N. commune, and release of the enzyme from dried cells upon rehydration, counter the effects of oxidative stress imposed by multiple cycles of desiccation and rehydration during UV-A or -B irradiation in situ.

Desiccation tolerance of prokaryotes

The removal of cell-bound water through air drying and the addition of water to air-dried cells are forces that have played a pivotal role in the evolution of the prokaryotes. In bacterial cells that have been subjected to air drying, the evaporation of free cytoplasmic water (Vf) can be instantaneous, and an equilibrium between cell-bound water (Vb) and the environmental water (vapor) potential (psi wv) may be achieved rapidly. In the air-dried state some bacteria survive only for seconds whereas others can tolerate desiccation for thousands, perhaps millions, of years. The desiccated (anhydrobiotic) cell is characterized by its singular lack of water--with contents as low as 0.02 g of H2O g (dry weight)-1. At these levels the monolayer coverage by water of macromolecules, including DNA and proteins, is disturbed. As a consequence the mechanisms that confer desiccation tolerance upon air-dried bacteria are markedly different from those, such as the mechanism of preferential exclusion of compatible solutes, that preserve the integrity of salt-, osmotically, and freeze-thaw-stressed cells. Desiccation tolerance reflects a complex array of interactions at the structural, physiological, and molecular levels. Many of the mechanisms remain cryptic, but it is clear that they involve interactions, such as those between proteins and co-solvents, that derive from the unique properties of the water molecule. A water replacement hypothesis accounts for how the nonreducing disaccharides trehalose and sucrose preserve the integrity of membranes and proteins. Nevertheless, we have virtually no insight into the state of the cytoplasm of an air-dried cell. There is no evidence for any obvious adaptations of proteins that can counter the effects of air drying or for the occurrence of any proteins that provide a direct and a tangible contribution to cell stability. Among the prokaryotes that can exist as anhydrobiotic cells, the cyanobacteria have a marked capacity to do so. One form, Nostoc commune, encompasses a number of the features that appear to be critical to the withstanding of a long-term water deficit, including the elaboration of a conspicuous extracellular glycan, synthesis of abundant UV-absorbing pigments, and maintenance of protein stability and structural integrity. There are indications of a growing technology for air-dried cells and enzymes. Paradoxically, desiccation tolerance of bacteria has virtually been ignored for the past quarter century. The present review considers what is known, and what is not known, about desiccation, a phenomenon that impinges upon every facet of the distributions and activities of prokaryotic cells.

Desiccation tolerance of prokaryotes

The removal of cell-bound water through air drying and the addition of water to air-dried cells are forces that have played a pivotal role in the evolution of the prokaryotes. In bacterial cells that have been subjected to air drying, the evaporation of free cytoplasmic water (Vf) can be instantaneous, and an equilibrium between cell-bound water (Vb) and the environmental water (vapor) potential (psi wv) may be achieved rapidly. In the air-dried state some bacteria survive only for seconds whereas others can tolerate desiccation for thousands, perhaps millions, of years. The desiccated (anhydrobiotic) cell is characterized by its singular lack of water--with contents as low as 0.02 g of H2O g (dry weight)-1. At these levels the monolayer coverage by water of macromolecules, including DNA and proteins, is disturbed. As a consequence the mechanisms that confer desiccation tolerance upon air-dried bacteria are markedly different from those, such as the mechanism of preferential exclusion of compatible solutes, that preserve the integrity of salt-, osmotically, and freeze-thaw-stressed cells. Desiccation tolerance reflects a complex array of interactions at the structural, physiological, and molecular levels. Many of the mechanisms remain cryptic, but it is clear that they involve interactions, such as those between proteins and co-solvents, that derive from the unique properties of the water molecule. A water replacement hypothesis accounts for how the nonreducing disaccharides trehalose and sucrose preserve the integrity of membranes and proteins. Nevertheless, we have virtually no insight into the state of the cytoplasm of an air-dried cell. There is no evidence for any obvious adaptations of proteins that can counter the effects of air drying or for the occurrence of any proteins that provide a direct and a tangible contribution to cell stability. Among the prokaryotes that can exist as anhydrobiotic cells, the cyanobacteria have a marked capacity to do so. One form, Nostoc commune, encompasses a number of the features that appear to be critical to the withstanding of a long-term water deficit, including the elaboration of a conspicuous extracellular glycan, synthesis of abundant UV-absorbing pigments, and maintenance of protein stability and structural integrity. There are indications of a growing technology for air-dried cells and enzymes. Paradoxically, desiccation tolerance of bacteria has virtually been ignored for the past quarter century. The present review considers what is known, and what is not known, about desiccation, a phenomenon that impinges upon every facet of the distributions and activities of prokaryotic cells.

Cloning of nifHD from Nostoc commune UTEX 584 and of a flanking region homologous to part of the Azotobacter vinelandii nifU gene

The heterocystous cyanobacterium Nostoc commune UTEX 584 contains two nifH-like sequences (nifH1 and nifH2) in addition to nifHD. A region of DNA 1 kilobase upstream from the 5' end of nifH showed considerable sequence similarity to part of the published nifU sequences of Azotobacter vinelandii and Klebsiella pneumoniae.

Variation in Phospholipid Ester-Linked Fatty Acids and Carotenoids of Desiccated Nostoc commune (Cyanobacteria) from Different Geographic Locations

Profiles of phospholipid fatty acids and carotenoids in desiccated Nostoc commune (cyanobacteria) collected from China, Federal Republic of Germany, and Antarctica and in axenic cultures of the desiccation-tolerant strains N. commune UTEX 584 and Hydrocoleum strain GOEI were analyzed. The phospholipid fatty acid contents of the three samples of desiccated Nostoc species were all similar, and the dominant compounds were 16:1ω7c, 16:0, 18:2ω6, 18:3ω3, and 18:1ω7c. In comparison with the field materials, N. commune UTEX 584 had a much higher ratio of 18:2ω6 to 18:3ω3 (5.36) and a significantly lower ratio of 18:1ω7c to 18:1ω9c (1.86). Compound 18:3 was present in large amounts in the samples of desiccated Nostoc species which had been subject, in situ, to repeated cycles of drying and rewetting, but represented only a small fraction of the total fatty acids of the strains grown in liquid culture. This finding is in contrast to the data obtained from studies on the effects of drought and water stress on higher plants. Field materials of Nostoc species contained, in contrast to the axenic strains, significant amounts of apocarotenoids and a P384 pigment which, upon reduction with NaBH 4 , yielded a mixture of a chlorophyll derivative and a compound with an absorption maximum of 451 nm. A clear distinction can be made between the carotenoid contents of the axenic cultures and the desiccated field materials. In the former, β-carotene and echinenone predominate; in the latter, canthaxanthin and the β-γ series of carotenoids are found.