Fish gill water boundary layer: a site of linkage between carbon dioxide and ammonia excretion

Summary Carbon dioxide excreted across fish gills is hydrated catalytically to form HCO ₃ ^– and H⁺ ions in water near the gill surface. We tested the possibility that CO₂ excretion is functionally linked to ammonia excretion through chemical reactions in the gill-water boundary layer. A bloodperfused trout head preparation was utilized in which the convective and diffusive components of branchial gas transfer were controlled. Pre-incubation of blood perfusate with the carbonic anhydrase inhibitor, acetazolamide, reduced both carbon dioxide and ammonia excretion in the blood-perfused preparation. Increasing the buffering capacity of inspired ventilatory water significantly reduced ammonia excretion, but carbon dioxide excretion was unaffected. Each of these experimental treatments significantly reduced the acidification of ventilatory water flowing over the gills. It is proposed that the catalysed conversion of excreted CO₂ to form HCO ₃ ^– and H⁺ ions provides a continual supply of H⁺ ions need for the removal of NH₃ as NH ₄ ⁺ . We suggest, therefore, that acidification of boundary layer water by CO₂ enhances blood-to-water NH₃ diffusion gradients and facilitates ammonia excretion.     

Effects of carbon dioxide and oxygen on the regulation of photosynthetic carbon metabolism by ammonia in spinach mesophyll cells

Photosynthetic carbon metabolism of isolated spinach mesophyll cells was characterized under conditions favoring photorespiratory (PR; 0.04% CO₂ and 20% O₂) and nonphotorespiratory (NPR; 0.2% CO₂ and 2% O₂) metabolism, as well as intermediate conditions. Comparisons were made between the metabolic effects of extracellularly supplied NH₄⁺ and intracellular NH₄⁺, produced primarily via PR metabolism. The metabolic effects of ¹⁴CO₂ fixation under PR conditions were similar to perturbations of photosynthetic metabolism brought about by externally supplied NH₄⁺; both increased labeling and intracellular concentrations of glutamine at the expense of glutamate and increased anaplerotic synthesis through α-ketoglutarate. The metabolic effects of added NH₄⁺ during NPR fixation were greater than those during PR fixation, presumably due to lower initial NH₄⁺ levels during NPR fixation. During PR fixation, addition of ammonia caused decreased pools and labeling of glutamate and serine and increased glycolate, glyoxylate, and glycine labeling. The glycolate pathway was thus affected by increased rates of carbon flow and decreased glutamate availability for glyoxylate transamination, resulting in increased usage of serine for transamination. Sucrose labeling decreased with NH₄⁺ addition only during PR fixation, suggesting that higher photosynthetic rates under NPR conditions can accommodate the increased drain of carbon toward amino acid synthesis while maintaining sucrose synthesis.     

Conversion of sodium cyanide to carbon dioxide and ammonia by immobilized cells ofPseudomonas putida

Summary Pseudomonas putida, isolated from contaminated industrial wastewaters and soil sites, was found to utilize sodium cyanide (NaCN) as a sole source of carbon and nitrogen. Cells, immobilized in calcium alginate beads (1–2 mm diameter) were aerated in air-uplift-type fluidized batch bioreactor containing 100–400 ppm of NaCN. Degradation of NaCN was monitored for 168 h by analyzing gaseous and dissolved ammonia (NH₃), CO₂, pH and optical density. The results indicated that the alginate-immobilized cells ofP. putida were able to degrade NaCN into NH₃ and CO₂ in a time-dependent manner.     

Regulation of non-autotrophic carbon dioxide assimilation by ammonia in cultured cells of Acer pseudoplatanus L

The assimilation of ¹⁴CO₂ by Acer pseudoplatanus cells in the dark was stimulated by the addition of either NH₄Cl or methylamine. Results were obtained demonstrating that methylamine was not metabolised to any appreciable extent by Acer cells. This suggests that the mechanism of stimulation of dark fixation by methylamine does not involve metabolism via the glutamine synthetase reaction. NH₄⁺ stimulation of CO₂ fixation also occurred in cells pretreated with the glutamine-synthetase inhibitor methionine sulfoximine. This further supports the conclusion that neither NH₄⁺ nor methylamine exerts its effect on CO₂-assimilation via a mechanism that depends upon the assimilation of NH₃ by glutamine synthetase.     

Biodegradation of cyanides, cyanates and thiocyanates to ammonia and carbon dioxide by immobilized cells of Pseudomonas putida

Pseudomonas putida utilizes cyanide as the sole source of carbon and nitrogen. Agar, alginate, and carrageenan were screened as the encapsulating matrices for P. putida. Alginate-immobilized cells of P. putida degraded sodium cyanide (NaCN) more efficiently than non-immobilized cells or cells immobilized in agar or carrageenan. The end products of biodegradation of cyanide were identified as ammonia (NH₃) and carbon dioxide (CO₂). These products changed the medium pH. In bioreactors, the rate of cyanide degradation increased with an increase in the rate of aeration. Maximum utilization of cyanide was observed at 200 ml min⁻¹ of aeration. Immobilized cells of P. putida degraded cyanides, cyanates and thiocyanates to NH₃ and CO₂. Use of Na[¹⁴C]-CN showed that 70% of carbon of Na[¹⁴C]-CN was converted into ¹⁴CO₂ and only 10% was associated with the cell biomass. The substrate-dependent kinetics indicated that the K _m and V _max values of P. putida for the substrate, NaCN were 14 mM and 29 nmol of oxygen consumed mg protein⁻¹ min⁻¹ respectively. Received 29 January 1996/ Accepted in revised form 19 September 1997     

In vitro carbon dioxide excretion from erythrocytes of two species of Antarctic fishes and its inhibition by catecholamines

This study was designed to investigate whether the blood of Pagothenia borchgrevinki , exhibits a Haldane effect, and whether activation of a Na⁺/H⁺ antiporter increases transport of intracellular protons and Bohr protons out of the erythrocytes resulting in inhibition of CO₂ excretion in both P. borchgrevinki , and Dissostichus mawsoni. When carbon dioxide dissociation curves were determined from blood samples pooled from three fish under oxygenated and deoxygenated conditions a Haldane effect was observed. Using an in vitro , CO₂ excretion assay, the rate of HCO₃⁻ dehydration was determined on blood and plasma equilibrated under an N₂atmosphere then rapidly oxygenated with air in the presence of 10⁻⁵ M noradrenaline or acetazolamide (10⁰⁴M). Whole blood and plasma from P. borchgrevinki , and D. mawsoni , were equilibrated with 0·5% CO₂ in air and assayed in the presence of 10⁻⁵ M noradrenaline. Erythrocyte CO₂ excretion rates were depressed significantly by noradrenaline in both species. The whole blood HCO₃⁻ dehydration rate was depressed significantly following rapid oxygenation in the presence of acetazolamide indicating that the pathway of CO₂ excretion included activation of intracellular carbonic anhydrase and an adrenergic receptor.     

Anaerobic ammonia oxidation with nitrogen dioxide by Nitrosomonas eutropha

Nitrosomonas eutropha, an obligately lithoautotrophic bacterium, was able to nitrify and denitrify simultaneously under anoxic conditions when gaseous nitrogen dioxide (NO₂) was supplemented to the atmosphere. In the presence of gaseous NO₂, ammonia was oxidized, nitrite and nitric oxide (NO) were formed, and hydroxylamine occurred as an intermediate. Between 40 and 60% of the produced nitrite was denitrified to dinitrogen (N₂). Nitrous oxide (N₂O) was shown to be an intermediate of denitrification. Under an N₂ atmosphere supplemented with 25 ppm NO₂ and 300 ppm CO₂, the amount of cell protein increased by 0.87 mg protein per mmol ammonia oxidized, and the cell number of N. eutropha increased by 5.8 × 10⁹ cells per mmol ammonia oxidized. In addition, the ATP and NADH content increased by 4.3 μmol ATP (g protein)^–1 and 6.3 μmol NADH (g protein)^–1 and was about the same in both anaerobically and aerobically grown cells. Without NO₂, the ATP content decreased by 0.7 μmol (g protein)^–1, and the NADH content decreased by 1.2 μmol (g protein)^–1. NO was shown to inhibit anaerobic ammonia oxidation. Received: 9 October 1996 / Accepted: 5 December 1996