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1.
Archaeal microorganisms that grow optimally at Na+ concentrations of 1.7 M, or the equivalent of 10% (w/v) NaCl, and greater are considered to be extreme halophiles. This review
encompasses extremely halophilic archaea and their growth characteristics with respect to the correlation between the extent
of alkaline pH and elevated temperature optima and the extent of salt tolerance. The focus is on poly-extremophiles, i.e.,
taxa growing optimally at a Na+ concentration at or above 1.7 M (approximately 10% w/v NaCl); alkaline pH, at or above 8.5; and elevated temperature optima,
at or above 50°C. So far, only a very few extreme halophiles that are able to grow optimally under alkaline conditions as
well as at elevated temperatures have been isolated. The distribution of extremely halophilic archaea growing optimally at
3.4 M Na+ (approximately 20% w/v NaCl) is bifurcated with respect to pH optima, either they are neutrophilic, with a pHopt of approximately 7, or strongly alkaliphilic, with pHopt at or above 8.5. Amongst these extreme halophiles which have elevated pH optima, only four taxa have an optimum temperature
above 50°C: Haloarcula quadrata (52°C), Haloferax elongans (53°C), Haloferax mediterranei (51°C) and Natronolimnobius
‘aegyptiacus’ (55°C). 相似文献
2.
Bacterial microorganisms that grow optimally at Na+ concentrations of 1.7 M, or the equivalent of 10% (w/v) NaCl, and greater are considered to be extreme halophiles. This review focuses on the correlation between the extent of alkaline pH and elevated temperature optima and the extent of salt tolerance of extremely halophilic eubacteria; the focus is on those with alkaline pH optima, above 8.5, and elevated temperature optima, above 50°C. If all three conditions are required for optimal growth, these microorganisms are termed "poly-extremophiles". However, only a very few extreme halophiles able to grow optimally under alkaline conditions as well as at elevated temperatures have been isolated so far. Therefore the question is: do the combined extreme growth conditions of the recently isolated poly-extremophiles, i.e., anaerobic halophilic alkalithermophiles, approach a physico-chemical boundary for life? These poly-extremophiles are of interest, as their adaptive mechanisms give insight into organisms' abilities to survive in environments which were previously considered prohibitive to life, as well as to possible properties of early evolutionary and extraterrestrial life forms. 相似文献
3.
The food pathogen Bacillus cereus is likely to encounter acidic environments (i) in food when organic acids are added for preservation purposes, and (ii) during the stomachal transit of aliments. In order to characterise the acid stress response of B. cereus ATCC14579, cells were grown in chemostat at different pH values (pHo from 9.0 to 5.5) and different growth rates (μ from 0.1 to 0.8 h−1), and were submitted to acid shock at pH 4.0. Cells grown at low pHo were adapted to acid media and induced a significant acid tolerance response (ATR). The ATR induced was modulated by both pHo and μ, and the μ effect was more marked at pHo 5.5. Intracellular pH (pHi) was affected by both pHo and μ. At a pHo above 6, the pHi decreased with the decrease of pHo and the increase of μ. At pHo 5.5, pHi was higher compared to pHo 6.0, suggesting that mechanisms of pHi homeostasis were induced. The acid survival of B. cereus required protein neo-synthesis and the capacity of cells to maintain their pHi and ΔpH (pHi - pHo). Haemolysin BL and non-haemolytic enterotoxin production were both influenced by pHo and μ. 相似文献
4.
Summary The intracellular pH (pH
i
) of Ehrlich ascites tumor cells, both in the steady state and under conditions of acid loading or recovery from acid loading, was investigated by measuring the transmembrane flux of H+ equivalents and correlating this with changes in the distribution ratio of dimethyloxazolidine-2,4-dione (DMO). The pH
i
of cells placed in an acidic medium (pH
o
below 7.15) decreases and reaches a steady-state value that is more alkaline than the outside. For example when pH
o
is acutely reduced to 5.5, pH
i
falls exponentially from 7.20 ± 0.06 to 6.29 ± 0.04 with a halftime of 5.92 ± 1.37 min, suggesting a rapid influx of H+. The unidirectional influx of H+ exhibits saturation kinetics with respect to extracellular [H+]; the maximal flux is 15.8 ± 0.05 mmol/(kg dry wt · min) andK
m
is 0.74 ± 0.09 × 10–6
m.Steady-state cells with pH
i
above 6.8 continuously extrude H+ by a process that is not dependent on ATP but is inhibited by anaerobiosis. Acid-loaded cells (pH
i
6.3) when returned to pH
o
7.3 medium respond by transporting H+, resulting in a rapid rise in pH
i
. The halftime for this process is 1.09 ± 0.22 min. The H+ efflux measured under similar conditions increases as the intracellular acid load increases. An ATP-independent as well as an ATP-dependent efflux contributes to the restoration of pH
i
to its steady-state value. 相似文献
5.
Banciu HL Sorokin DY Tourova TP Galinski EA Muntyan MS Kuenen JG Muyzer G 《Extremophiles : life under extreme conditions》2008,12(3):391-404
A chemolithoautotrophic sulfur-oxidizing bacterium (SOB) strain ALCO 1 capable of growing at both near-neutral and extremely
alkaline pH was isolated from hypersaline soda lakes in S-W Siberia (Altai, Russia). Strain ALCO 1 represents a novel separate
branch within the halothiobacilli in the Gammaproteobacteria, which, so far, contained only neutro-halophilic SOB. On the basis of its unique phenotypic properties and distant phylogeny,
strain ALCO 1 is proposed as a new genus and species Thioalkalibacter halophilus gen. nov. sp. nov. ALCO 1 was able to grow within a broad range of salinity (0.5–3.5 M of total sodium) with an optimum at
around 1 M Na+, and pH (7.2–10.2, pHopt at around 8.5). Na+ was required for sulfur-dependent respiration in ALCO 1. The neutral (NaCl)-grown chemostat culture had a much lower maximum
growth rate (μmax), respiratory activity and total cytochrome c content than its alkaline-grown counterpart. The specific concentration of osmolytes (ectoine and glycine-betaine) produced
at neutral pH and 3 M NaCl was roughly two times higher than at pH 10 in soda. Altogether, strain ALCO 1 represents an interesting
chemolithoautotrophic model organism for comparative investigations of bacterial adaptations to high salinity and pH.
Nucleotide sequence accession number: The GenBank/EMBL accession number of the 16S rRNA gene sequence of strain ALCO1T is EU124668. 相似文献
6.
J.I. Kourie 《The Journal of membrane biology》1999,167(1):73-83
The understanding of the role of cytoplasmic pH in modulating sarcoplasmic reticulum (SR) ion channels involved in Ca2+ regulation is important for the understanding of the function of normal and adversely affected muscles. The dependency of
the SR small chloride (SCl) channel from rabbit skeletal muscle on cytoplasmic pH (pH
cis
) and luminal pH (pH
trans
) was investigated using the lipid bilayer-vesicle fusion technique. Low pH
cis
6.75–4.28 modifies the operational mode of this multiconductance channel (conductance levels between 5 and 75 pS). At pH
cis
7.26–7.37 the channel mode is dominated by the conductance and kinetics of the main conductance state (65–75 pS) whereas
at low pH
cis
6.75–4.28 the channel mode is dominated by the conductance and kinetics of subconductance states (5–40 pS). Similarly, low
pH
trans
4.07, but not pH
trans
6.28, modified the activity of SCl channels. The effects of low pH
cis
are pronounced at 10−3 and 10−4
m [Ca2+]
cis
but are not apparent at 10−5
m [Ca2+]
cis
, where the subconductances of the channel are already prominent. Low pH
cis
-induced mode shift in the SCl channel activity is due to modification of the channel proteins that cause the uncoupling of
the subconductance states. The results in this study suggest that low pH
cis
can modify the functional properties of the skeletal SR ion channels and hence contribute, at least partly, to the malfunction
in the contraction-relaxation mechanism in skeletal muscle under low cytoplasmic pH levels.
Received: 20 May 1998/Revised: 24 September 1998 相似文献
7.
Wenhui Wang Yang Wang Stefan Silbernagl Hans Oberleithner 《The Journal of membrane biology》1988,101(1):259-265
Summary Experiments were performed in intact proximal tubules of the doubly perfused kidney and in fused proximal tubule cells ofRaha esculenta to evaluate the dependence of intracellular pH (pHi) on cell membrane potential applying pH-sensitive and conventional microelectrodes. In proximal tubules an increase of the K– concentration in the peritubular perfusate from 3 to 15 mmol/liter decreased the peritubular cell membrane potential from –55±2 to –38±1 mV paralleled by an increase of pH
i
, from 7.54±0.02 to 7.66±0.02. The stilbene derivative DIDS hyperpolarized the cell membrane potential from –57 ± 2 to –71 ±4 mV and led to a significant increase of the K–-induced cell membrane depolarization, but prevented the K–-induced intracellular alkalinization. Fused proximal tubule cells were impaled by three microelectrodes simultaneously and cell voltage was clamped stepwise while pH
i
changes were monitored. Cell membrane hyperpolarization acidified the cell cytoplasm in a linear relationship. This voltage-induced intracellular acidification was reduced to about one-third when HCO3 ions were omitted from the extracellular medium. We conclude that in proximal tubule cells pH
i
depends on cell voltage due to the rheogenicity of the HCO
3
–
transport system. 相似文献
8.
9.
《Journal of thermal biology》1999,24(3):199-208
A technique based on homogenisation of rapidly frozen tissue was used to investigate the regulation of intracellular pH (pHi) in freshwater and marine fish from diverse environmental temperatures. The following species were held at ambient temperatures of ca. 1°C (Notothenia coriiceps; Antarctica), 5°C (Pleuronectes platessa, Myoxocephalus scorpius; North Sea), and 26°C (Oreochromis niloticus; African lakes). The effects of seasonal acclimatisation to 4, 11 and 18°C were also examined in rainbow trout in the winter, autumn and summer, respectively. Extracellular (whole blood) pH (pHe) did not follow the constant relative alkalinity relationship, where pH+=pOH− for any particular temperature, over a range of 1–26°C (overall δpHe/δT=0.009±0.002 U °C−1; P<0.001), apparently being regulated by ionic fluxes and ventilation. Intracellular pH (pHi) was also regulated independently of pN(=0.5 pK water) in all species of fish examined. The inverse relationship between pHi and environmental temperature gave an overall δpHi/δT of −0.010±0.001 U °C−1 (for both white and red muscle) and −0.004±0.003 U °C−1 (cardiac muscle). However, between 1 and 11°C δpHi/δT was much higher (P<0.001), −0.022±0.003 U °C−1 (white muscle) and −0.022±0.004 U °C−1 (red muscle). The possible adaptive roles for these different acid–base responses to environmental temperature variation among tissues and species, and the potential difficulties of estimating pHi, are discussed. 相似文献
10.
Summary Recently we proposed that cytoplasmic acidification of low K+ (LK) sheep erythrocytes may stimulate ouabain-resistant Cl–-dependent K+ flux (K+Cl– cotransport), also known to be activated by cell swelling, treatment with N-ethylmaleimide (NEM), or removal of cellular bivalent cations. Here we studied the dependence of K+ transport on intracellular and extracellular pH (pH
i
, pH
o
) varied either simultaneously or independently using the Cl–/HCO
3
–
exchange inhibitor 4,4, diisothiocyanatostilbene-3,2-disulfonic acid (DIDS). In both control and NEM-treated LK cells volumes were kept near normal by varying extracellular sucrose. Using DIDS as an effective pH clamp, both K+ efflux and influx of Rb+ used as K+ congener were strongly activated at acid pH
i
and alkaline pH
o
. A small stimulation of K+ (Rb+) flux was also seen at acid pH
i
in the absence of DIDS, i.e., when pH
i
pH
o
. Anti-L
l
serum, known to inhibit K+Cl– cotransport, prevented the pH
i
-stimulated K+ (Rb+) fluxes. Subsequent to NEM treatment at pH 6, K+ (Rb+) fluxes were activated only by raising pH, and thus were similar to the pH activation profile of K+ (Rb+) fluxes in DIDS-treated cells with pH
o
varied at constant physiologic pH
i
. Anti-L
l
, which inhibited NEM-stimulated K+ (Rb+) fluxes, failed to do so in NEM-plus DIDS-treated cells. Thus, NEM treatment interferes with the internal but not with the external pH-sensitive site. 相似文献
11.
D. B. Kintner M. E. Anderson‡ K. A. Sailor G. Dienel James H. FitzpatrickJr. § & D. D. Gilboe† 《Journal of neurochemistry》1999,72(1):405-412
Abstract : A unique method for simultaneously measuring interstitial (pHe) as well as intracellular (pHi) pH in the brains of lightly anesthetized rats is described. A 4-mm microdialysis probe was inserted acutely into the right frontal lobe in the center of the area sampled by a surface coil tuned for the collection of 31P-NMR spectra. 2-Deoxyglucose 6-phosphate (2-DG-6-P) was microdialyzed into the rat until a single NMR peak was detected in the phosphomonoester region of the 31P spectrum. pHe and pHi values were calculated from the chemical shift of 2-DG-6-P and inorganic phosphate, respectively, relative to the phosphocreatine peak. The average in vivo pHe was 7.24 ± 0.01, whereas the average pHi was 7.05 ± 0.01 (n = 7). The average pHe value and the average CSF bicarbonate value (23.5 ± 0.1 mEq/L) were used to calculate an interstitial Pco2 of 55 mm Hg. Rats were then subjected to a 15-min period of either hypercapnia, by addition of CO2 (2.5, 5, or 10%) to the ventilator gases, or hypocapnia (Pco2 < 30 mm Hg), by increasing the ventilation rate and volume. pHe responded inversely to arterial Pco2 and was well described (r2 = 0.91) by the Henderson-Hassel-balch equation, assuming a pKa for the bicarbonate buffer system of 6.1 and a solubility coefficient for CO2 of 0.031. This confirms the view that the bicarbonate buffer system is dominant in the interstitial space. pHi responded inversely and linearly to arterial Pco2. The intracellular effect was muted as compared with pHe (slope = -0.0025, r2 = 0.60). pHe and pHi values were also monitored during the first 12 min of ischemia produced by cardiac arrest. pHe decreases more rapidly than pHi during the first 5 min of ischemia. After 12 min of ischemia, pHe and pHi values were not significantly different (6.44 ± 0.02 and 6.44 ± 0.03, respectively). The limitations, advantages, and future uses of the combined microdialysis/31P-NMR method for measurement of pHe and pHi are discussed. 相似文献
12.
Ionic responses to alteration in external and internal pH were examined in an organism from a marine-like environment. Vacuolar pH (pHv) is about 4.9–5.1, constant at external pH (pHo) 5–8, while cytoplasmic pH (pHc) increases from 7.3 to 7.7. pHc regulation fails above pHo 9, and this is accompanied by failure of turgor regulation. Na+ increases above pHo 9, while K+ and Cl– decrease. These changes alone cannot however explain the alterations in turgor. Agents known to affect internal pH are also tested for their effect on ion relations.Abbreviations Ci
ion concentration
- CCCP
carbonyl cyanide m-chlorophenyl hydrazone
- DCCD
dicyclohexylcarbodiimide
- DES
diethylstilbestrol
- DMO
5,5-dimethyloxazolidine-2,4-dione
- DNP
2,4-dinitrophenol
- pHo
external pH
- pHc
cytoplasmic pH
- pHv
vacuolar pH
- i
osmotic pressure
-
turgor pressure 相似文献
13.
Combined gasification and fermentation technologies can potentially produce biofuels from renewable biomass. Gasification
generates synthesis gas consisting primarily of CO, CO2, H2, N2, with smaller amounts of CH4, NOx, O2, C2 compounds, ash and tars. Several anaerobic bacteria species can ferment bottled mixtures of pure synthesis gas constituents.
However, there are challenges to maintaining culture viability of synthesis gas exposed cells. This study was designed to
enhance culture stability and improve ethanol-to-acetate ratios using resting (non-growing) cells in synthesis gas fermentation.
Resting cell states were induced in autotrophic Clostridium ljungdahlii cultures with minimal ethanol and acetate production due to low metabolic activity compared to growing cell production levels
of 5.2 and 40.1 mM of ethanol and acetate. Clostridium autoethanogenum cultures were not induced into true resting states but did show improvement in total ethanol production (from 5.1 mM in growing
cultures to 9.4 in one nitrogen-limited medium) as well as increased shifts in ethanol-to-acetate production ratios. 相似文献
14.
Ira Kurtz 《The Journal of membrane biology》1988,106(3):253-260
Summary The present study was designed to investigate the apical and basolateral transport processes responsible for intracellular pH regulation in the thin descending limb of Henle. Rabbit thin descending limbs of long-loop nephrons were perfused in vitro and intracellular pH (pH
i
) was measured using BCECF. Steady-state pH
i
in HEPES buffered solutions (pH 7.4) was 7.18±0.03. Following the removal of luminal Na+, pH
i
decreased at a rate of 1.96±0.37 pH/min. In the presence of luminal amiloride (1mm), the rate of decrease of pH
i
was significantly less, 0.73±0.18 pH/min. Steady-state pH
i
decreased 0.18 pH units following the addition of amiloride (1mm) to the lumen (Na+ 140mm lumen and bath). When Na+ was removed from the basolateral side of the tubule, pH
i
decreased at a rate of 0.49±0.05 pH/min. The rate of decrease of pH
i
was significantly less in the presence of 1mm basolateral amiloride, 0.29±0.04 pH/min. Addition of 1mm amiloride to the basolateral side (Na+ 140mm lumen and bath) caused steady-state pH
i
to decrease significantly by 0.06 pH units. When pH
i
was acutely decreased to 5.87±0.02 following NH4Cl removal (lumen, bath), pH
i
failed to recover in the absence of Na+ (lumen, bath). Addition of 140mm Na+ to the lumen caused pH
i
to recover at a rate of 2.17±0.59 pH/min. The rate of pH
i
recovery was inhibited 93% by 1mm luminal amiloride. When 140mm Na+ was added to the basolateral side, pH
i
recovered only partially at 0.38±0.07 pH/min. Addition of 1mm basolateral amiloride inhibited the recovery of pH
i
, by 97%. The results demonstrate that the rabbit thin descending limb of long-loop nephrons possesses apical and basolateral Na+/N+ antiporters. In the steady state, the rate of Na+-dependent H+ flux across the apical antiporter exceeds the rate of Na+-dependent H+ flux via the basolateral antiporter. Recovery of pH
i
following acute intracellular acidification is Na+ dependent and mediated primarily by the luminal antiporter. 相似文献
15.
The patch-clamp technique was used to investigate regulation of anion channel activity in the tonoplast of Chara corallina in response to changing proton and calcium concentrations on both sides of the membrane. These channels are known to be Ca2+-dependent, with conductances in the range of 37 to 48 pS at pH 7.4. By using low pH at the vacuolar side (either pHvac 5.3 or 6.0) and a cytosolic pH (pHcyt) varying in a range of 4.3 to 9.0, anion channel activity and single-channel conductance could be reversibly modulated. In
addition, Ca2+-sensitivity of the channels was markedly influenced by pH changes. At pHcyt values of 7.2 and 7.4 the half-maximal concentration (EC
50) for calcium activation was 100–200 μm, whereas an EC
50 of about 5 μm was found at a pHcyt of 6.0. This suggests an improved binding of Ca2+ ions to the channel protein at more acidic cytoplasm. At low pHcyt, anion channel activity and mean open times were voltage-dependent. At pipette potentials (V
p) of +100 mV, channel activity was approximately 15-fold higher than activity at negative pipette potentials and the mean
open time of the channel increased. In contrast, at pHcyt 7.2, anion channel activity and the opening behavior seemed to be independent of the applied V
p. The kinetics of the channel could be further controlled by the Ca2+ concentration at the cytosolic membrane side: the mean open time significantly increased in the presence of a high cytosolic
Ca2+ concentration. These results show that tonoplast anion channels are maintained in a highly active state in a narrow pH range,
below the resting pHcyt. A putative physiological role of the pH-dependent modulation of these anion channels is discussed.
Received: 14 March 2001/Revised: 16 July 2001 相似文献
16.
Acclimation of photosynthetic temperature optima of temperate and boreal tree species in response to experimental forest warming
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Kerrie M. Sendall Peter B. Reich Changming Zhao Hou Jihua Xiaorong Wei Artur Stefanski Karen Rice Roy L. Rich Rebecca A. Montgomery 《Global Change Biology》2015,21(3):1342-1357
Rising temperatures caused by climate change could negatively alter plant ecosystems if temperatures exceed optimal temperatures for carbon gain. Such changes may threaten temperature‐sensitive species, causing local extinctions and range migrations. This study examined the optimal temperature of net photosynthesis (Topt) of two boreal and four temperate deciduous tree species grown in the field in northern Minnesota, United States under two contrasting temperature regimes. We hypothesized that Topt would be higher in temperate than co‐occurring boreal species, with temperate species exhibiting greater plasticity in Topt, resulting in better acclimation to elevated temperatures. The chamberless experiment, located at two sites in both open and understory conditions, continuously warmed plants and soils during three growing seasons. Results show a modest, but significant shift in Topt of 1.1 ± 0.21 °C on average for plants subjected to a mean 2.9 ± 0.01 °C warming during midday hours in summer, and shifts with warming were unrelated to species native ranges. The 1.1 °C shift in Topt with 2.9 °C warming might be interpreted as suggesting limited capacity to shift temperature response functions to better match changes in temperature. However, Topt of warmed plants was as well‐matched with prior midday temperatures as Topt of plants in the ambient treatment, and Topt in both treatments was at a level where realized photosynthesis was within 90–95% of maximum. These results suggest that seedlings of all species were close to optimizing photosynthetic temperature responses, and equally so in both temperature treatments. Our study suggests that temperate and boreal species have considerable capacity to match their photosynthetic temperature response functions to prevailing growing season temperatures that occur today and to those that will likely occur in the coming decades under climate change. 相似文献
17.
F. Yonsel 《Engineering in Life Science》2001,21(2):195-203
Reaction enthalpy for propionate degradationΔG0 is only negative when the partial pressure ofhydrogen pH2 is less than 10—4 bar. This means that for pH2 more than 10—4 bar, a total anaerobic degradation of propionate is impossible for thermodynamic reasons. Therefore, with increasing pH2, the anaerobic degradation rate of propionate via acetate is inhibited. There are two ways to investigate the inhibitory effect of pH2: to keep the concentration of hydrogen consuming bacteria low or to increase the mass transfer by feeding the hydrogen at higher flow rates. The author used an extended fixed bed reactor filled with polyurethane particles as a carrier for the bacteria, aerated with pure H2 gas. The results, compared with the literature by using model equations in order to standardize the data, correspond well: The addition of pure H2 gas has no observable effects on propionate degradation.In the fixed bed reactor with immobilized bacteria, it was not possible to reach an inhibitory concentration of H2 and high process stability could be maintained. 相似文献
18.
Since the major mechanisms responsible for regulation of intracellular pH of enterocytes are located in the basolateral membrane,
respective effects may be expected on pH in the compartment near the basolateral membrane. A method was established to estimate
the pH at the basolateral membrane (pH
b
) of isolated caecal epithelia of guinea pig using pH-sensitive fluorescein attached to lectin (lens culinaris). In the presence of bicarbonate and a perfusion solution-pH of 7.4, pH
b
was 7.70 ± 0.15. In the absence of bicarbonate or chloride as well as by inhibition of the basolateral Cl−-HCO−
3 exchange with H2-DIDS, pH
b
was reduced near to solution-pH. Inhibition of the basolateral Na+-H+ exchanger by adding a sodium- and bicarbonate-free, low-buffered solution increased pH
b
. Decrease of pH of serosal perfusion solution to 6.4 provoked a similar decrease of pH
b
to solution pH. Short-chain fatty acids (SCFA) added to the mucosal solution caused a slight decrease of pH
b
. SCFA added to the serosal side alkalized pH
b
. However, in the presence of bicarbonate pH
b
returned quickly to the initial pH
b
, and after removal of SCFA a transient acidification of pH
b
was seen. These responses could not be inhibited by MIA or H2-DIDS. We conclude that no constant pH-microclimate exists at the basolateral side. The regulation of the intracellular pH
of enterocytes reflects pH
b
. The slightly alkaline pH
b
is due to the bicarbonate efflux. Data support the presence of an SCFA−-HCO−
3 exchange.
Received: 17 December 1998/Revised: 24 February 1999 相似文献
19.
Elena Bañares-España Victoria López-Rodas Concepción Salgado Eduardo Costas Antonio Flores-Moya 《Aquatic Botany》2006
Inter-strain variability in pH compensation point (pHc) in the cyanobacterium Microcystis aeruginosa has been investigated. The pHc allows one to discriminate whether the organism is able to take up HCO3− as inorganic carbon (Ci) source in photosynthesis. Eight subgroups were found according to the pHc value, ranging from 10.44 ± 0.22 to 11.67 ± 0.05. The high variability in pHc (and consequently, in the capacity to use HCO3− as Ci source) suggested that different HCO3− use mechanisms could occur in M. aeruginosa and, from an evolutionary point of view, this trait is not under high natural selective pressure. 相似文献
20.
Summary Homeostasis of intracellular calcium ([Ca++]i) and pH (pHi) is important in the cell's ability to respond to growth factors, to initiate differentiation and proliferation, and to maintain
normal metabolic pathways. Because of the importance of these ions to cellular functions, we investigated the effects of changes
of [Ca++]i and pHi on each other in primary cultures of rabbit corneal epithelial cells. Digitized fluorescence imaging was used to measure
[Ca++]i with fura-2 and pHi with 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Resting pHi in these cells was 7.37±0.05 (n=20 cells) and resting [Ca++]i was 129±10 nM (n=35 cells) using a nominally bicarbonate-free Krebs Ringer HEPES buffer (KRHB), pH 7.4. On exposure to 20 mM NH4Cl, which rapidly alkalinized cells by 0.45 pH units, an increase in [Ca++]i to 215±14 nM occurred. Pretreatment of the cells with 100 μM verapamil or exposure to 1 mM ethylene bis-(oxyethylenenitrilo)-tetraacetic acid (EGTA) without extracellular calcium before addition of 20 mM NH4Cl did not abolish the calcium increase, suggesting that the source of the calcium transient was from intracellular calcium
stores. On removal of NH4Cl or addition of 20 mM sodium lactate, there were minimal changes in calcium even though pHi decreased. Treatment of CE cells with the calcium ionophores, ionomycin and 4-bromo A23187, increased [Ca++]i, but produced a biphasic change in pHi. Initially, there was an acidification of the cytosol, and then an alkalinization of 0.10 to 0.11 pH units above initial
values. When [Ca++]i was decreased by treating the cells with 5 mM EGTA and 20 μM ionomycin, pHi decreased by 0.35±0.02 units. We conclude that an increase in pHi leads to an increase in [Ca++]i in rabbit corneal epithelial cells; however, a decrease in pHi leads to minor changes in [Ca++]i. The ability of CE cells to maintain proper calcium homeostasis when pHi is decreased may represent an adaptive mechanism to maintain physiological calcium levels during periods of acidification,
which occur during prolonged eye closure. 相似文献