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PubMed1969

Preservation of the ultrastructure of Bacillus subtilis by chemical fixation as verified by freeze-etching.

The present study on the ultrastructure of Bacillus subtilis was undertaken in order to examine by means of the freeze-etching technique possible structural changes occurring during the chemical fixation procedure (Ryter-Kellenberger (R-K) fixation). Three stages were followed by freeze-etching, viz.: (a) fixation in osmium tetroxide, (b) fixation in osmium tetroxide and posttreatment with uranyl acetate, and (c) fixation in osmium tetroxide, posttreatment in uranyl acetate, and dehydration in a graded series of acetone. Preparations were made after each stage in the presence of 20% glycerol. Good preservation of ultrastructure was observed, after any of the three treatments, of the outer surface of the plasma membrane, and the inner surface of the plasma membrane. No alteration in fracturing properties could be observed. However, if we are to judge by the results of freeze-etching, any of the successive steps of the chemical fixation procedure achieve strong contrast between the nucleoplasmic region and the cytoplasm. Dependent on the quality of fixation, very delicately preserved DNA fibrils or strongly aggregated ones were seen. It appears that R-K fixation is capable of producing more or less distinctly visible changes in the native state of the nucleoplasm in young cells of B. subtilis.

Bacillus subtilis↗

PubMed2003

Probability of fixation of an advantageous mutant in a viral quasispecies.

The probability that an advantageous mutant rises to fixation in a viral quasispecies is investigated in the framework of multitype branching processes. Whether fixation is possible depends on the overall growth rate of the quasispecies that will form if invasion is successful rather than on the individual fitness of the invading mutant. The exact fixation probability can be calculated only if the fitnesses of all potential members of the invading quasispecies are known. Quasispecies fixation has two important characteristics: First, a sequence with negative selection coefficient has a positive fixation probability as long as it has the potential to grow into a quasispecies with an overall growth rate that exceeds that of the established quasispecies. Second, the fixation probabilities of sequences with identical fitnesses can nevertheless vary over many orders of magnitudes. Two approximations for the probability of fixation are introduced. Both approximations require only partial knowledge about the potential members of the invading quasispecies. The performance of these two approximations is compared to the exact fixation probability on a network of RNA sequences with identical secondary structure.

Animals↗

PubMed1998

Effectiveness of homologous cadaveric fascia lata and role of suture fixation to tarsus in frontalis suspension.

The results of frontalis suspension (double rhomboid technique with preserved homologous cadaver fascia) were compared in two surgical groups: 1) patients in whom the fascia was sutured to the tarsus and 2) patients in whom the fascia was not sutured to the tarsus. The study was performed to determine the optimum surgical technique and to determine whether preserved cadaver fascia is a suitable suspensory material. All data retrospectively reviewed included 1) predisposing cause of severe blepharoptosis, 2) results in the two groups of patients, suture fixation and nonsuture fixation, and 3) surgical complications. Of the 27 patients (36 eyelids), the fascia was sutured to the tarsus in 15 patients (20 eyelids) and not sutured to the tarsus in 12 patients (16 eyelids). In the suture fixation group, no undercorrections occurred, but four of 20 eyelids had lower than expected eyelid creases and six had residual dermatochalasis. Six patients had lagophthalmos with corneal exposure that required intense corneal lubrication, and three such patients required temporary tarsorrhaphy for 3 weeks. In the group without suture fixation, two patients had undercorrection (one with ocular cicatricial pemphigoid and the other with myotonic dystrophy). The mean followup period was 44 months. We conclude that preserved fascia provides excellent results with or without fixation of the fascia to the tarsus. In patients with suture fixation, the eyelid crease may form just above the point at which the fascia is sutured to tarsus and result in a low eyelid crease. Excision of excess skin should be considered at the time of frontalis suspension in selected patients in whom the fascia is fixated to the tarsus or who have preexisting dermatochalasis. Patients with suture fixation may have significant temporary postoperative lagophthalmos.

Adult↗

PubMed1998

Fixation of the craniofacial skeleton with butyl-2-cyanoacrylate and its effects on histotoxicity and healing.

Butyl-2-cyanoacrylate is an easily applied, biocompatible, bioresorbable polymer glue that provides an alternative to conventional rigid fixation techniques. Our aim was to determine if cyanoacrylate fixation of the bone flap in a rabbit craniotomy model provides the healing and strength afforded by plate and screw fixation. We also investigated the inflammatory responses of adjacent tissues including the scalp, cranium, and brain. A unilateral parietal bone flap was elevated in 33 adult New Zealand rabbits. The bone was fixed in position with cyanoacrylate (n = 13), fixed with a microplate and screws (n = 14), or was replaced without fixation (sham-control, n = 6). Normal scar formation and no residual polymer were found in scalp specimens. Neuropathologic analysis identified the presence of residual polymer on the surface of 2 of the 13 rabbit brains. Histopathologic analysis of the bone flap-to-skull interface revealed no difference in the degree but rather in the quality of inflammation and healing between the plate and screw and polymer fixation groups. Microdensitometric analysis of the bone gap revealed nearly equivalent bone density in the cyanoacrylate and plated groups, tending to less density in the sham group (p = 0.11 and 0.09, respectively). An additional study focusing on neurotoxicity was performed in 20 adult rabbits with 3-week and 11-week recovery periods and similarly found the absence of a marked inflammatory response to the polymer. In conclusion, bone healing and soft-tissue inflammation were comparable between cyanoacrylate and plate and screw fixation groups. Although butyl-2-cyanoacrylate glue fixation may provide a reasonable alternative to hardware fixation, further investigations are necessary to identify its ideal utilization.

Animals↗

PubMed1994

The role of plate and screw fixation in occipitocervical fusion in rheumatoid arthritis.

STUDY DESIGN: In a clinical retrospective study, the results of occipitocervical fusion in patients with rheumatoid arthritis were studied and analyzed. OBJECTIVES: The results of two different operative techniques were compared. The advantages of screw fixation compared with wiring techniques in this population of patients were investigated. SUMMARY OF BACKGROUND DATA: Numerous different implants have been presented in the literature for occipitocervical fusion in patients with rheumatoid arthritis. The use of wires being the standard fixation technique. METHODS: Occipitocervical fusion was performed in patients with rheumatoid arthritis: 26 patients with the wiring technique and 33 patients with a new Y-plate fixation. The results were compared at a follow-up period of 24 months and 50 months, respectively. Clinical and radiologic results were investigated. RESULTS: The atlantodental distance could be significantly better reduced in the group with the Y-plate fixation and the neurologic improvement in the wiring group was 40%, whereas in the Y-plate fixation 86% of neurologic improvement was observed. Pseudarthrosis was seen in 27% of the wiring technique and in 6% in the plate and screw fixation technique. CONCLUSIONS: In occipitocervical fusion for patients with rheumatoid arthritis, the screw and plate fixation technique provides superior results than other techniques using wire fixations.

Arthritis, Rheumatoid↗

PubMed1995

Predicting the integrity of vertebral bone screw fixation in anterior spinal instrumentation.

STUDY DESIGN: Using 2-week-old calf lumbar vertebrae as a model for the human adolescent spine, the strength and rigidity of different methods of anterior spinal screw fixation and the use of noninvasive techniques for predicting bone screw stability before surgery and screw insertional torque intraoperatively were investigated. OBJECTIVES: The objectives were to determine what factors most affect the strength and rigidity of screw fixation, to determine the strongest and most rigid type of screw fixation for anterior spinal instrumentation, and to determine if noninvasive measurements of bone density before surgery by dual energy x-ray absorptiometry or quantitative computed tomography and if intraoperative measurement of screw insertional torque can be used to predict the in vivo strength and rigidity of vertebral screw fixation. SUMMARY OF BACKGROUND DATA: Anterior spinal instrumentation is an efficient method to correct spinal deformities in the thoracolumbar and lumbar spine. Fewer vertebrae are instrumented and arthrodesed, allowing for greater spine mobility. The forces transmitted to each vertebra are higher, perhaps accounting for the clinical failure rate of 13-30% at the metal bone interface from screw cut out. METHODS: Quantitative computed tomography and dual energy x-ray absorptiometry were used to assess the bone density of 24, 2-week-old calf lumbar vertebrae. Four different methods of vertebral screw fixation were evaluated: unicortical screw, bicortical screw, bicortical screw and washer, and bicortical screws and staple. The maximal screw insertional torque was measured for each specimen. Each vertebral body-screw construct failed in a mode simulating in vivo screw cut out. The applied moment and rotatory displacement were recorded. Ash density was measured for each vertebral body after removing all hardware. RESULTS: Noninvasive measures of bone density varied linearly with ash density (P < 0.01). Screw insertional torque varied linearly with bone mineral content and bone mineral density (r2 = 0.50) and was correlated with the yield moment for all types of fixation except the staple. Density measured by quantitative computed tomography did not affect rigidity or yield moment. Post hoc analysis showed that the screw-staple construct was the strongest and most rigid form of fixation. CONCLUSIONS: Measurement of bone density before surgery using dual energy x-ray absorptiometry and intraoperative measurement of screw insertional torque can be used to assess the stability of anterior spinal instrumentation. A bicortical screw inserted through a Dwyer-type staple provided the strongest and most rigid form of fixation.

Absorptiometry, Photon↗

PubMed1996

The role of pediculolaminar fixation in compromised pedicle bone.

STUDY DESIGN: This in vitro study analyzed the effects of a supralaminar hook on pedicle screw fixation in compromised pedicle bone. OBJECTIVES: To determine the ability of pediculolaminar fixation to restore pedicle screw pull-out strength after stripping of senile pedicle bone. SUMMARY OF BACKGROUND DATA: Despite improvements in pedicle screw design, the bone-screw interface remains the "weakest link" in pedicle screw fixation. This interface is especially vulnerable in osteoporotic bone previously instrumented pedicles, and at the ends of long instrumentation constructs. METHODS: Side-to-side testing between a pedicle screw and a pedicle screw supplemented with a supralaminar hook (pediculolaminar fixation) was performed in human cadaveric lumbar vertebrae. Comparisons were made for intact and compromised pedicle bone. RESULTS: Pediculolaminar fixation restored 89% of intact pedicle screw pull-out strength whereas the pedicle screw alone restored only 19% of intact pull-out strength. The role of pediculolaminar fixation was greatest in weaker bone. Significant differences were noted in energy to failure and post-failure energy. In intact bone, the pediculolaminar construct did not increase pull-out strength or energy to failure, although it did have a greater post-failure energy. CONCLUSIONS: Pediculolaminar fixation can augment pedicle screw fixation in pedicle bone compromised by previous stripping or significant osteoporosis or both.

Aged↗

PubMed2000

The effects of cyclic loading on pull-out strength of sacral screw fixation: an in vitro biomechanical study.

STUDY DESIGN: The pull-out strength of sacral screw fixation after cyclic loading was tested using young human cadaveric specimens. OBJECTIVES: To evaluate the effects of fatigue loading on the pull-out strength of medial and lateral unicortical and bicortical sacral screws and to correlate the pull-out strength with sacral bone density and the screw insertion torque. SUMMARY OF BACKGROUND DATA: The immediate biomechanical effects of depth of penetration, screw orientation, and bone density on sacral screw fixation have been studied in aged cadaveric specimens. The effect of cyclic loading on the pull-out strength of sacral screw fixation is unknown, however, and data from young specimens is rare. METHODS: Eleven fresh specimens of human sacrum were used in this study. Bone mineral density at the vertebral body and the ala were determined by peripheral quantitative computed tomography. Seven-millimeter compact Cotrel-Dubousset sacral screws were inserted into the sacrum anteromedially and anterolaterally, both unicortically and bicortically, and the insertion torque for each screw was measured. Cyclic loading from 40 to 400 N was applied to each screw at a frequency of 2 Hz up to 20,000 cycles. Pull-out tests were conducted after completion of the fatigue tests. RESULTS: The average bone density was 0.38 +/- 0.08 g/mL at the S1 body and 0.24 +/- 0.05 g/mL at the S1 ala. The insertion torque and average pull-out force after cyclic loading were significantly higher for bicortical fixation than for unicortical fixation for a particular screw alignment. The pull-out strength and insertion torque of medially oriented fixation was always higher than that for lateral fixation, however, regardless of whether the insertion was unicortical or bicortical. The pull-out force of unicortical and bicortical medial screw fixations after cyclic loading showed significant linear correlations with both the insertion torque and the bone mineral density of the S1 body. CONCLUSIONS: In a young population, screw orientation (anterolateral or anteromedial) was more important in determining pull-out strength than screw depth (unicortical or bicortical) after fatigue loading, anteromedially directed screws being significantly stronger than laterallyplaced screws. Bone mineral density of the S1 body andinsertion torque were good preoperative and intraoperative indicators of screw pull-out strength.

Adult↗

PubMed2001

Transarticular screw fixation for atlanto-occipital dislocation.

STUDY DESIGN: A case report of traumatic atlanto-occipital dislocation managed by transarticular screw fixation. OBJECTIVES: To present a case with initial failed wire fixation but successful reduction and fixation with transarticular screw fixation and occipitocervical plate stabilization. SUMMARY OF BACKGROUND DATA: Atlanto-occipital dislocation is generally a fatal injury, except when it occurs in children. Management of this injury is difficult because of multidirectional instability and the problems associated with technical stabilization. METHODS: Intraoperative atlanto-occipital reduction was achieved and maintained by direct C0-C1 transarticular screw fixation. To protect this fixation, a posterior occipitocervical fusion at C0-C2 using a Y-plate was performed. RESULTS: The internal fixation and reduction were maintained, indicating a good surgical outcome, at examination 2 years after surgery. The initially severe neurologic deficit was reduced to some motor weakness of the right hand and weakness of oculomotor function. CONCLUSIONS: Anatomic reduction and reliable fixation with transarticular screws may provide satisfactory clinical results with important neurologic recovery in cases of atlanto-occipital dislocation.

Atlanto-Occipital Joint↗

PubMed2001

Posterior C1-C2 fusion with polyaxial screw and rod fixation.

STUDY DESIGN: A novel technique of atlantoaxial stabilization using individual fixation of the C1 lateral mass and the C2 pedicle with minipolyaxial screws and rods is described. In addition, the initial results of this technique on 37 patients are described. OBJECTIVES: To describe the technique and the initial clinical and radiographic results for posterior C1-C2 fixation with a new implant system. SUMMARY OF BACKGROUND DATA: Stabilization of the atlantoaxial complex is a challenging procedure because of the unique anatomy of this region. Fixation by transarticular screws combined with posterior wiring and structural bone grafting leads to excellent fusion rates. The technique is technically demanding and has a potential risk of injury to the vertebral artery. In addition, this procedure cannot be used in the presence of fixed subluxation of C1 on C2 and in the case of an aberrant path of the vertebral artery. To address these limitations, a new technique of C1-C2 fixation has been developed: bilateral insertion of polyaxial-head screws in the lateral mass of C1 and through the pars interarticularis into the pedicle of C2, followed by a fluoroscopically controlled reduction maneuver and rod fixation. METHODS: After posterior exposure of the C1-C2 complex, the 3.5-mm polyaxial screws are inserted in the lateral masses of C1. Two polyaxial screws are then inserted into the pars interarticularis of C2. Drilling is guided by anatomic landmarks and fluoroscopy. If necessary, reduction of C1 onto C2 can be accomplished by manipulation of the implants, followed by fixation to the 3-mm rod. For definitive fusion, cancellous bone can be added. No structural bone graft or wiring is required. In selected cases, e.g., C1-C2 subluxation or fractures in young patients in whom only temporary fixation is necessary, the instrumentation can be removed after an appropriate time. Because the joint surfaces stay intact, the patient can regain motion in the C1-C2 joints. RESULTS: Thirty-seven patients underwent this procedure. No neural or vascular damage related to this technique has been observed. The early clinical and radiologic follow-up data indicate solid fusion in all patients. CONCLUSION: Fixation of the atlantoaxial complex using polyaxial-head screws and rods seems to be a reliable technique and should be considered an efficient alternative to the previously reported techniques.

Adolescent↗

PubMed2004

Mechanical evaluation of posterior wiring as a supplement to anterior cervical plate fixation.

STUDY DESIGN: An in vitro experimental study was performed to examine 3-dimensional biomechanical stability of cervical fixations. OBJECTIVES: To determine whether posterior interspinous wiring contributes to the rigidity of a single-level motion segment that has been plated anteriorly, and to determine the effects of this combined fixation on intradiscal pressure and spinal motion at the adjacent segments. SUMMARY OF BACKGROUND DATA: Combined anterior and posterior column fixation is being increasingly used in a variety of clinical situations that do not involve complete disruption of the motion segment. The biomechanical validity of combined anterior posterior fixation in the absence of overt posterior ligamentous disruption has not been studied. METHODS: Six human fresh-frozen cadaveric cervical spines (C3-T1) were used. Three-dimensional intersegmental motion and intradiscal pressure were measured while the spine was loaded in flexion, extension, lateral bending, and torsion (up to 2.5 Nm). Fixation stability at the operative level (C5-C6) and influence of the fixation on adjacent segments were evaluated after an anterior plating procedure and combined anterior plating and posterior wiring. RESULTS: Comparing the combined approach with anterior plating alone, significant reductions in C5-C6 motion was noted: 49% in flexion (P <0.05), 48% in extension (P <0.003), and 33% and 39%, respectively, in left and right torsion (P <0.05). Reduction in the left and right lateral bending was not significant (18% and 12%, respectively). The improved fixation had minimal influence on the adjacent segments. CONCLUSIONS: Combined anterior posterior fixation further reduces the segmental motion by almost 50% in flexion and extension, 33% and 39% in torsion, and does not significantly alter intradiscal pressure and spinal motion at adjacent segments.

Aged↗

PubMed1968

The acetylene-ethylene assay for n(2) fixation: laboratory and field evaluation.

The methodology, characteristics and application of the sensitive C(2)H(2)-C(2)H(4) assay for N(2) fixation by nitrogenase preparations and bacterial cultures in the laboratory and by legumes and free-living bacteria in situ is presented in this comprehensive report. This assay is based on the N(2)ase-catalyzed reduction of C(2)H(2) to C(2)H(4), gas chromatographic isolation of C(2)H(2) and C(2)H(4), and quantitative measurement with a H(2)-flame analyzer. As little as 1 mumumole C(2)H(4) can be detected, providing a sensitivity 10(3)-fold greater than is possible with (15)N analysis.A simple, rapid and effective procedure utilizing syringe-type assay chambers is described for the analysis of C(2)H(2)-reducing activity in the field. Applications to field samples included an evaluation of N(2) fixation by commercially grown soybeans based on over 2000 analyses made during the course of the growing season. Assay values reflected the degree of nodulation of soybean plants and indicated a calculated seasonal N(2) fixation rate of 30 to 33 kg N(2) fixed per acre, in good agreement with literature estimates based on Kjeldahl analyses. The assay was successfully applied to measurements of N(2) fixation by other symbionts and by free living soil microorganisms, and was also used to assess the effects of light and temperature on the N(2) fixing activity of soybeans. The validity of measuring N(2) fixation in terms of C(2)H(2) reduction was established through extensive comparisons of these activities using defined systems, including purified N(2)ase preparations and pure cultures of N(2)-fixing bacteria.With this assay it now becomes possible and practicable to conduct comprehensive surveys of N(2) fixation, to make detailed comparisons among different N(2)-fixing symbionts, and to rapidly evaluate the effects of cultural practices and environmental factors on N(2) fixation. The knowledge obtained through extensive application of this assay should provide the basis for efforts leading to the maximum agricultural exploitation of the N(2) fixation reaction.

Journal Article↗

PubMed1981

CO(2) Exchange and Dinitrogen Fixation of Subterranean Clover in Response to Light Level.

Small swards of nodulated subterranean clover plants were grown in pots to a common dry weight under controlled conditions. The rooting medium was a porous calcined clay. All mineral nutrients except nitrogen were supplied daily in solution. Pots then were placed in an assimilation chamber for 3 days for the measurement of net CO(2) exchange at light levels ranging from 0.1 to 2.0 millieinsteins per square meter per second. N(2)-fixation (acetylene reduction) of each pot was measured subsequently. H(2)-evolution and N(2)-fixation were measured for similar treatments in separate experiments using smaller pots.CO(2)-assimilation rate and N(2)-fixation rate responded similarly to increase in light level so that N(2)-fixation was found to be a linear function of both the daily (12-hour) net CO(2)-uptake and of the growth rate (24 hours) of the sward over a wide range. N-concentration of the plants was independent of the light level under which they were grown. The rate of N(2)-fixation per unit weight of nodule varied over a wide range depending on the number of nodules present and on the demand for nitrogen as determined by the growth rate of the sward. It is concluded that the amount of assimilate available to the whole plant is a major factor determining the rate of N(2)-fixation by the whole plant, although it may not be important in determining specific rates of nodule activity. N(2)-fixation per unit weight of nodule therefore is considered to have limited utility for comparative purposes.The relative efficiency of N(2)-fixation was found to be about 0.7 and independent of the level of the incident light after growth at two contrasting light levels, 0.4 and 1.0 millieinsteins per square meter per second.

Journal Article↗

PubMed1987

Nonphotosynthetic CO(2) Fixation by Alfalfa (Medicago sativa L.) Roots and Nodules.

The dependence of alfalfa (Medicago sativa L.) root and nodule nonphotosynthetic CO(2) fixation on the supply of currently produced photosynthate and nodule nitrogenase activity was examined at various times after phloem-girdling and exposure of nodules to Ar:O(2). Phloemgirdling was effected 20 hours and exposure to Ar:O(2) was effected 2 to 3 hours before initiation of experiments. Nodule and root CO(2) fixation rates of phloem-girdled plants were reduced to 38 and 50%, respectively, of those of control plants. Exposure to Ar:O(2) decreased nodule CO(2) fixation rates to 45%, respiration rates to 55%, and nitrogenase activities to 51% of those of the controls. The products of nodule CO(2) fixation were exported through the xylem to the shoot mainly as amino acids within 30 to 60 minutes after exposure to (14)CO(2). In contrast to nodules, roots exported very little radioactivity, and most of the (14)C was exported as organic acids. The nonphotosynthetic CO(2) fixation rate of roots and nodules averaged 26% of the gross respiration rate, i.e. the sum of net respiration and nonphotosynthetic CO(2) assimilation. Nodules fixed CO(2) at a rate 5.6 times that of roots, but since nodules comprised a small portion of root system mass, roots accounted for 76% of the nodulated root system CO(2) fixation. The results of this study showed that exposure of nodules to Ar:O(2) reduced nodule-specific respiration and nitrogenase activity by similar amounts, and that phloem-girdling significantly reduced nodule CO(2) fixation, nitrogenase activity, nodule-specific respiration, and transport of (14)C photoassimilate to nodules. These results indicate that nodule CO(2) fixation in alfalfa is associated with N assimilation.

Journal Article↗

PubMed2000

Serial lead impedance measurements confirm fixation of helical screw electrodes during pacemaker implantation.

The purpose of this study was to determine whether serial measurements of helical screw pacemaker lead impedance could reliably confirm electrode fixation in the right atrium and right ventricle. Fixation is generally assessed fluoroscopically, which can be misleading because the myocardium is radiolucent. Alternatively, because the electrical conductivity of blood is greater than that of myocardium, serial measurements of the lead impedance might be expected to show an impedance increase with appropriate fixation of the pacemaker electrode when the electrode becomes embedded in myocardial tissue. Impedance measurements were made during the placement of 23 atrial and 28 ventricular active fixation electrodes in 31 consecutive patients. Impedance measurements were recorded in unipolar and bipolar electrode configurations with the electrode free floating in the chamber, unfixed (with exposed screws) but touching the endocardial surface, and after fixation. No significant impedance differences were found between free-floating and unfixed electrode positions. With fixation, the lead impedance increased significantly in the ventricle (P = 0.0001, unipolar and bipolar) and the atrium (P = 0.0069 unipolar and 0.0052 bipolar). Typical increases, reflected by median values, were 197 ohms unipolar and 203 ohms bipolar in the ventricle and 47 ohms unipolar and 53 ohms bipolar in the atrium for electrodes with permanently exposed or retractable screw designs. Comparing serial measurements of lead impedance before and after electrode fixation is a valid electrical method of confirming appropriate fixation of helical screw electrodes.

Aged↗

PubMed1990

Diel nitrogen fixation by cyanobacterial surface blooms in sanctuary lake, pennsylvania.

Diel nitrogen fixation studies were conducted with assemblages of cyanobacteria sampled from surface blooms on Sanctuary Lake, Pa. The studies were conducted between July and September of 1982 to 1985 by using the acetylene reduction technique. Assemblages with the lowest cell concentrations (0.9 x 10 to 1.0 x 10 cells per liter) exhibited nitrogen fixation activity throughout the day, with maximum fixation rates occurring in mid to late afternoon; fixation proceeded throughout the night at rates equivalent to 23 to 28% of the afternoon maximum. In studies conducted with the highest cell concentrations (3.7 x 10 to 6.7 x 10 cells per liter), fixation rates reached maximum values in mid to late morning. The rates declined rapidly throughout the midday period and subsequently ceased from late afternoon until sunrise on the following day. The afternoon decline and cessation of fixation exhibited by high cell concentrations correlated with photosynthetically induced low total CO(2) and supersaturating O(2) concentrations. The midday decline could be prevented and partially reversed by experimentally lowering O(2) and increasing total CO(2) concentrations. Under experimental conditions which simultaneously prevented supersaturating O(2) concentrations and maintained high total CO(2) availability, nitrogen fixation continued throughout the solar day, with maximum rates occurring at midday. These observations indicate that temporal changes in photosynthetic activity may affect diel fluctuations in nitrogen fixation.

Journal Article↗

PubMed1990

Nitrogen fixation dynamics of two diazotrophic communities in mono lake, california.

Two types of diazotrophic microbial communities were found in the littoral zone of alkaline hypersaline Mono Lake, California. One consisted of anaerobic bacteria inhabiting the flocculent surface layers of sediments. Nitrogen fixation (acetylene reduction) by flocculent surface layers occurred under anaerobic conditions, was not stimulated by light or by additions of organic substrates, and was inhibited by O(2), nitrate, and ammonia. The second community consisted of a ball-shaped association of a filamentous chlorophyte (Ctenocladus circinnatus) with diazotrophic, nonheterocystous cyanobacteria, as well as anaerobic bacteria (Ctenocladus balls). Nitrogen fixation by Ctenocladus balls was usually, but not always, stimulated by light. Rates of anaerobic dark fixation equaled those in the light under air. Fixation in the light was stimulated by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea and by propanil [N-(3,4-dichlorophenyl)propanamide]. 3-(3,4-Dichlorophenyl)-1,1-dimethyl urea-elicited nitrogenase activity was inhibited by ammonia (96%) and nitrate (65%). Fixation was greatest when Ctenocladus balls were incubated anaerobically in the light with sulfide. Dark anaerobic fixation was not stimulated by organic substrates in short-term (4-h) incubations, but was in long-term (67-h) ones. Areal estimates of benthic N(2) fixation were measured seasonally, using chambers. Highest rates ( approximately 29.3 mumol of C(2)H(4) m h) occurred under normal diel regimens of light and dark. These estimates indicate that benthic N(2) fixation has the potential to be a significant nitrogen source in Mono Lake.

Journal Article↗

PubMed1963

QUANTITATIVE STUDIES OF THE EFFECT OF ORGANIC SUBSTRATES AND 2,4-DINITROPHENOL ON HETEROTROPHIC CARBON DIOXIDE FIXATION IN HYDROGENOMONAS FACILIS.

McFadden, Bruce A. (Washington State University, Pullman), and H. Robert Homann. Quantitative studies of the effect of organic substrates and 2,4-dinitrophenol on heterotrophic carbon dioxide fixation in Hydrogenomonas facilis. J. Bacteriol. 86:971-977. 1963.-Whole cells of Hydrogenomonas facilis under heterotrophic conditions fixed levels of C(14)O(2) which depended upon the nature of the carbon source being oxidized. It was established that oxidative rates varied as a function of p(CO2). Therefore, all studies were conducted in the presence of 1.5 mole% CO(2) in the gas phase. With glucose-grown cells supplied with glucose as substrate, the heterotrophic fixation was curtailed 98% by the addition of 8.3 x 10(-4)m 2,4-dinitrophenol (DNP). A coupling between reductive fixation of CO(2) and heterotrophic oxidation of substrate is consistent with the observed effect of DNP. The efficiency of coupling of fixation with oxidation was studied for acetate, d-glucose, l-glutamate, d,l-lactate, d-ribose, and succinate as substrates. Kinetic studies showed that the efficiency of coupling (expressed as disintegrations per minute of C(14) per microliter of O(2)) was initially time-variable for all substrates; however, it approached a constant value after 30 to 45 min for acetate, glutamate, lactate, and succinate. The initial variation of the ratio with time was due primarily to C(14)O(2) uptake, which was nonlinear with time. Control studies in the absence of exogenous substrate indicated that CO(2) fixation may also be linked to oxidation of endogenous stores accumulated during heterotrophic growth. d-Ribose appears to be the most promising substrate for short-term fixation studies owing to the rapid incorporation of C(14) and the unusually low endogenous fixation rate by cells grown on ribose. Calculations reveal that, after isotopic equilibrium has occurred, the amount of CO(2) utilized during glucose oxidation is almost 50% of O(2) uptake during the same interval. Even during succinate oxidation, which was shown to be coupled much less effectively with CO(2) fixation, the CO(2) utilized during the same interval is 8% of O(2) uptake.

2,4-Dinitrophenol↗