Actual and potential dark respiration rates and different electron transport pathways in freshwater aquatic plants

The rates of respiratory O₂ uptake have been studied in leaves, stems and whole shoots of several freshwater plants: 6 angiosperms, 2 bryophytes and one alga. For angiosperm leaves, rates varied widely with species (30–142 μmol O₂ (gDW)⁻¹ h⁻¹), were correlated with chlorophyll content and were higher than those of the stems (13–71 μmol O₂ (gDQ)⁻¹ h⁻¹). The rates for the shoots of bryophytes (53–66 μmol O₂ (gDW)⁻¹ h⁻¹) and for the alga Cladophora glomerata (L.) Kütz. (96 μmol O₂ (gDW)⁻¹ h⁻¹) were slightly higher than those of most angiosperm stems, but lower than those for most leaves.

These plants had a significant cyanide-resistant respiration, suggesting the existence of an alternative pathway to the “classic” cytochrome system. This pathway was found to be active in all the species studied, as judged by responses to a specific inhibitor, SHAM (salicylhydroxamic acid). Measurement of electron-transport system (ETS) activity showed that there is a large electron-transport capacity which is not normally used by respiration in vivo.     

Influence of phosphorus concentration on the growth kinetics and stoichiometry of the microalga Scenedesmus obliquus

The growth of the freshwater microalga Scenedesmus obliquus was studied at 30°C in a mineral culture medium with phosphorus concentrations of between 0 and 372 μ . The values for the specific growth rates, between and , fitted a semistructured substrate-limitation model with μ_m1 = 0·0466 h⁻¹, μ_m2 = 0·0256 h⁻¹ and . The specific uptake rate of phosphorus reached a maximum value of q_Sm1 = 658·01 × 10⁻⁴ μmol P mg⁻¹ biomass h⁻¹.     

Photosynthetic response of Myriophyllum salsugineum A.E. orchard to photon irradiance, temperature and external free CO2

The photosynthetic capacity of Myriophyllum salsugineum A.E. Orchard was measured, using plants collected from Lake Wendouree, Ballarat, Victoria and grown subsequently in a glasshouse pond at Griffith, New South Wales. At pH 7.00, under conditions of constant total alkalinity of 1.0 meq dm⁻³ and saturating photon irradiance, the temperature optimum was found to be 30–35°C with rates of 140 μmol mg⁻¹ chlorophyll a h⁻¹ for oxygen production and 149 μmol mg⁻¹ chlorophyll a h⁻¹ for consumption of CO₂. These rates are generally higher than those measured by other workers for the noxious Eurasian water milfoil, Myriophyllum spicatum L., of which Myriophyllum salsugineum is a close relative. The light-compensation point and the photon irradiance required to saturate photosynthetic oxygen production were exponentially dependent on water temperature. Over the temperature range 15–35°C the light-compensation point increased from 2.4 to 16.9 μmol (PAR) m⁻² s⁻¹ for oxygen production while saturation photon irradiance increased from 41.5 to 138 μmol (PAR) m⁻² s⁻¹ for oxygen production and from 42.0 to 174 μmol (PAR) m⁻² s⁻¹ for CO₂ consumption. Respiration rates increased from 27.1 to 112.3 μmol (oxygen consumed) g⁻¹ dry weight h⁻¹ as temperature was increased from 15 to 35°C. The optimum temperature for productivity is 30°C.     

On the trophic fate of Phaeocystis pouchetii (Hariot). II. Grazing rates of Calanus hyperboreus (Krøyer) on diatoms and different size categories of Phaeocystis pouchetii

Experiments were conducted with CIV and C V copepodites of Calanus hyperboreus (Krøyer) to determine if they would feed on the prymnesiophyte Phaeocystis pouchetii (Hariot). We used analysis of gut pigment to estimate ingestion and clearance rates. In applying this methodology we have demonstrated that pigments can be completely extracted from whole animals within 90 min, and that laborious procedures of tissue homogenization and centrifugation are not required. We conducted two experiments. In the first experiment Stage IV copepodites were exposed to ≈1 mg C·1⁻¹ of either P. pouchetii flagellates, small colonies (25–200 μm), large colonies (> 200 μm) or mixed diatoms > 25 μm (primarily Chaetoceros socialis Lauder and Nitzschia grunowii Hasle). Ingestion rates and daily rations were almost four times greater on both sizes of colonies than on either Phaeocystis pouchetii flagellates or mixed diatoms. Daily rations of copepodites feeding on colonies ranged from 8.1 to 12.4% · day⁻¹, well within the range previously reported for Calanus hyperboreus or sympatric copepods of similar size. From the second experiment we determined that Stage V copepodites obtained a daily ration of 6.2 to 10.8% · day⁻¹ when feeding on small colonies of Phaeocystis pouchetii. We conclude that a diet of P. pouchetii colonies should sustain the metabolic and growth requirements of Calanus hyperboreus copepodites.     

Activity of oxyntomodulin on gastric acid secretion induced by histamine or a meal in the rat

Conscious rats with chronic gastric fistula were trained for drinking a 14-ml milk meal. The activity of an intestinal hormone, oxyntomodulin (OXM), was studied in this model and compared to that observed when histamine was the stimulus. Under histamine (0.25 mg·kg⁻¹·h⁻¹) stimulation, OXM at doses (60–120 pmol·kg⁻¹·h⁻¹) that induced physiological circulating levels inhibited gastric acid secretion up to 50%. Under meal stimulation, OXM reduced up to 29% acid secretion at doses (1–1.5 nmol·kg⁻¹·h⁻¹) inducing supraphysiological levels. We conclude that at physiological concentrations OXM cannot counteract the complex processes triggered by a meal. OXM would be a component of enterogastrone, a combination of several intestinal hormones acting in synergy. The OXM action is related to pathways recognizing the C-terminal 19–37 moiety of the molecule.     

Bipyridylium quaternary salts and related compounds. V. Pulse radiolysis studies of the reaction of paraquat radical with oxygen. Implications for the mode of action of bipyridyl herbicides

1. Rate constants for reduction of paraquat ion (1,1′-dimethyl-4,4′-bipyridy-lium, PQ²⁺) to paraquat radical (PQ⁺·) by e⁻_aq and CO₂⁻· have been measured by pulse radiolysis. Reduction by e⁻_aq is diffusion controlled (k = 8.4·10¹⁰ M⁻¹·s⁻¹) and reduction by CO₂⁻· is also very fast k = 1.5·10¹⁰ M⁻¹·s⁻¹).

2. The reaction of paraquat radical with oxygen has been analysed to give rate constants of 7.7·10⁸ M⁻¹·s⁻¹ and 6.5·10⁸ M⁻¹·s⁻¹ for the reactions of paraquat radical with O₂ and O₂⁻·, respectively. The similarity in these rate constants is in marked contrast to the difference in redox potentials of O₂ and O₂⁻· (− 0.59 V and + 1.12 V, respectively).

3. These rate constants, together with that for the self-reaction of O₂⁻·, have been used to calculate the steady-state concentration of O₂⁻· under conditions thought to apply at the site of reduction of paraquat in the plant cell. On the basis of these calculations the decay of O₂⁻· appears to be governed almost entirely by its self-reaction, and the concentration 5 μm away from the thylakoid is still 90% of that at the thylakoid itself. Thus, O₂⁻· persists long enough to diffuse as far as the chloroplast envelope and tonoplast, which are the first structures to be damaged by paraquat treatment. O₂⁻· is therefore sufficiently long-lived to be a candidate for the phytotoxic product formed by paraquat in plants.     

Effect of growth hormone and γ-aminobutyric acid on Brachionus plicatilis (Rotifera) reproduction at low food or high ammonia levels

Growth hormone (GH, 0.0025 and 0.025 I.U. ml⁻¹) and γ-aminobutyric acid (GABA, 50 μg ml⁻¹) enhance rotifer population growth in batch cultures. In order to further understand the mechanism of their actions, we conducted experiments culturing isolated females at low food and high free ammonia levels. At an optimum food level of 7×10⁶ Nannochloropsis oculata cells ml⁻¹ or at low free ammonia level of 2.4 μg ml⁻¹, the F₁ offspring of rotifers treated with GH at 0.0025 I.U. ml⁻¹ had significantly higher population growth rate (r) and net reproduction rate (Ro), and shorter generation time than untreated rotifers. At a lower food level of 7×10⁵ cells ml⁻¹ or at high free ammonia level of 3.1 μg ml⁻¹, rotifers treated with GABA at 50 μg ml⁻¹ had significantly higher r and Ro, and shorter generation time. These results indicate that GABA is effective in enhancing rotifer reproduction when rotifers are cultured under stress whereas GH enhances rotifer reproduction when culture conditions are optimal. Significant effects were also observed in F¹ and F² generations which were not treated with hormones. These data may be useful for treating rotifer mass cultures to mitigate the effects of stress caused by high population densities.     

Production of the antitumoral retamycin during continuous fermentations of Streptomyces olindensis

Continuous fermentations were performed in order to correlate the production of retamycin, an anthracycline antibiotic produced by Streptomyces olindensis in submerged cultures, with the dilution rate. Maximum retamycin production was achieved at a dilution rate of 0.05 h⁻¹ (D=μ_x=0.05 h⁻¹), while higher dilution rates caused a decrease in antibiotic production, which ceased completely at a dilution rate of 0.30 h⁻¹. Otherwise, biomass productivity was favoured by high dilution rates, achieving a maximum at D=0.25 h⁻¹, whereas retamycin productivity reached a maximum at D=0.05 h⁻¹. Dilution rate influenced morphology, which was assessed by image analysis. The percentage of clumps decreased with an increase in dilution rate, with a correspondent increase in pellet percentage.     

Antimicrobial activity of steady-state cultures of Bacillus sp. CCMI 1051 against wood contaminant fungi

The effect of changing dilution rate (D) on Bacillus sp. CCMI 1051 at dilution rates between 0.1 and 0.55 h⁻¹ in a glucose-limited medium was studied. Biomass values varied between 0.88 and 1.1 g L⁻¹ at D values of 0.15–0.35 h⁻¹. Maximal biomass productivity was found to be 0.39 g L⁻¹ h⁻¹, obtained at D = 0.35 h⁻¹ and corresponding to a 54.4% conversion of the carbon into cell mass. The highest rate of glucose consumption was 4.45 mmol g⁻¹ h⁻¹ occurring at D = 0.4 h⁻¹. The glucose concentration inside the chemostat was below the detection level starting to accumulate around 0.4 h⁻¹. Growth inhibition of fifteen strains of fungi by the broth of the steady-state cell-free supernatants was assessed. Results showed that the relative inhibition differ among the target species but was not influenced by the dilution rate changing.     

Halophila ovalis (R. Br.) Hook. f. from a submarine hot spring in southern Japan

Colonies of the seagrass Halophila ovalis are found growing adjacent to coral Acropora sp. and Seriatopora hystrix in a submarine hot spring (at 15.7 m depth, 28.6°C) at the north coast of Taketomi Island, near the southern tip of Japan. Halophila plants grow in sea water containing sulphide 930 μg S ml⁻¹ and on the substratum with fine precipitates of the submarine hot spring which have sulphide content up to 5400 μg S g⁻¹ DW. The accumulated sulphide concentration reaches as high as 8400 μg S g⁻¹ DW in under ground tissues and 5700 μg S g⁻¹ DW in above-ground tissues, respectively. It is suggested that, not the sulphide concentration but light and possibly water temperature are the limiting factors for the Halophila colonization in the submarine hot spring.     

Cultivation and preservation of vinegar bacteria

Ten strains of acetic acid bacteria were investigated for their characteristics of growth and metabolism. The strains were identified as those presently in use for industrial vinegar production in southern Germany. At the time of isolation from industrial acetators the total concentrations, i.e. acetic acid (w/v) plus ethanol (v/v), of the fermenting vinegars were 6.1–14.9%. The applicability of a previously described method for starter preparation was examined for the various isolates as well as for the type strains of species of the genera Gluconobacter and Acetobacter. Isolates from cider or wine vinegar fermentations grew readily in RAE-medium to total counts of >1×10⁹ cells ml⁻¹. For the cultivation of strains isolated from spirit vinegar fermentations AE-medium proved most suitable. Cultures of these strains exhibited lag phases of 2–5 days and grew up to total counts of <1×10⁹ cells ml⁻¹. All type strains could be grown on RAE-agar. The use of 20% malt extract as cryo-protectant was effective for the preservation of all strains. Upon revitalization the cultures were suitable as inoculum for starting fermentations in pilot acetators. 16S rRNA-targeted oligonucleotide probes were constructed which were species specific for Gluconobacter oxydans or Acetobacter aceti or group specific for Acetobacter europaeus/Acetobacter xylinum. The probes hybridized with the DNA of the respective type strains. Four isolates were allotted to A. europaeus/A. xylinum applying the group specific probe. The DNA of six of the Acetobacter sp. hybridized with none of the probes.     

Oestradiol and the preovulatory surges of luteinising hormone and follicle stimulating hormone in ewes during the breeding season and transition into anoestrus

This study was designed to see if giving exogenous oestradiol, during the follicular phase of the oestrous cycle of intact ewes, during the breeding season or transition into anoestrus, would alter the occurrence, timing or magnitude of the preovulatory surge of secretion of luteinising hormone (LH) or follicle stimulating hormone (FSH). During the breeding season and the time of transition, separate groups of ewes were infused (intravenously) with either saline (30 ml h⁻¹; n = 6) or oestradiol in saline (n = 6) for 30 h. Infusion started 12 h after removal of progestin-containing intravaginal sponges that had been in place for 12 days. The initial dose of oestradiol was 0.02 μg h⁻¹; this was doubled every 4 h for 20 h, followed by every 5 h up to 30 h, to reach a maximum of 1.5 μg h⁻¹. Following progestin removal during the breeding season, peak serum concentrations of oestradiol in control ewes were 10.31 ± 1.04 pg ml⁻¹, at 49.60 ± 3.40 h after progestin removal. There was no obvious peak during transition, but at a time after progestin removal equivalent to the time of the oestradiol peak in ewes at mid breeding season, oestradiol concentrations were 6.70 ± 1.14 pg ml⁻¹ in ewes in transition (P < 0.05). In oestradiol treated ewes, peak serum oestradiol concentrations (24.8 ± 2.1 pg ml⁻¹) and time to peak (41.00 ± 0.05 h) did not differ between seasons (P > 0.05). During the breeding season, all six control ewes and four of six ewes given oestradiol showed oestrus with LH and FSH surges. The two ewes not showing oestrus did not respond to oestrus synchronisation and had persistently high serum concentrations of progesterone. During transition, three of six control ewes showed oestrus but only two had LH and FSH surges; all oestradiol treated ewes showed oestrus and gonadotrophin surges (P < 0.05). The timing and magnitude of LH and FSH surges did not vary with treatment or season. In blood samples collected every 12 min for 6 h, from 12 h after the start of oestradiol infusion, mean serum concentrations of LH and LH pulse frequency were lower in control ewes during transition than during mid breeding season (P < 0.05). Oestradiol treatment resulted in lower mean serum concentrations of LH in season and lower LH pulse frequency in transition (P < 0.05). We concluded that enhancing the height of the preovulatory peak in serum concentrations of oestradiol during the breeding season did not alter the timing or the magnitude of the preovulatory surge of LH and FSH secretion and that at transition into anoestrus, oestradiol can induce oestrus and the surge release of LH and FSH as effectively as during the breeding season.     

Continuous production of lactic acid from whey permeate by Lactobacillus helveticus in two chemostats in series

The effect of dilution rate on the production of lactic acid from whey permeate by Lactobacillus helveticus has been investigated. In the first chemostat of a two-stage system, total conversion (98.1%) and maximum lactic acid concentration (43.7 g l⁻¹) were obtained at a dilution rate (D_Itot) of 0.06 h⁻¹. Maximum volumetric productivities of lactic acid (8.27 g l⁻¹ h⁻¹) and biomass (1.90 g l⁻¹ h⁻¹) occurred at D_Itot of 0.40 h⁻¹. The fraction of -lactate in the product was found to increase with dilution rate and reached a maximum of 66% at the same dilution rate. The maximum specific growth rate (μ_max) on this medium was 0.7 h⁻¹. A Y_ATP (max) value of 22.4 g dry weight (mol ATP)⁻¹ and a maintenance coefficient of 8.0 mmol ATP (g dry weight h)⁻¹ were determined. The second stage, in series with the first, confirmed these results and further showed that the total residence time could be reduced by 50%, compared with a single chemostat for the same nearly complete level of substrate conversion.     

Elemental composition and food intake of Phialidium hydromedusae in the laboratory

Elemental composition and feeding rate of hydromedusae Phialidium sp. on copepods were studied in the laboratory. Regression equations for both mature and immature medusae allowed the estimation of their dry weight (DW), total C and N content as a function of their diameter. The mean C content as percentage of the DW varied from 13.13% ( ) for the immature to 19.38% (5.68) for the mature individuals. The mean N content is 4.03% (2.49) of DW of immatures and 5.85% (2.70) of the matures. Ingestion rate of Phialidium sp. fed on copepods (200–500 μm) increased with prey density but reached a maximum at high prey concentrations. A maximum ingestion rate of 8.55 (1.6) copepods · medusa ⁻¹ · h⁻¹ was reached for prey concentrations of > 140 copepods · 1 ⁻¹ for both immature and mature medusae. Maximum daily consumption of prey weight varied from 1.41 to 978% C body weight for mature medusae and from 2.90 to 975% for the immature individuals.     

Nuclear maturation of ovine oocytes in cultured preantral follicles

Small (150–250 μm in diameter) and large (251–400 μm in diameter) preantral follicles (PFs) in sheep were cultured for 6 days in four different concentrations of transforming growth factor-alpha (TGF-), epidermal growth factor (EGF), FSH and LH. Proportions of follicles exhibiting growth, antrum formation and increase in follicular and oocyte diameter were the initial indicators of development. The ability of the oocytes isolated from these cultured follicles to mature to metaphase II (MII), after 24 h culture in a known in vitro maturation medium was the final criterion of success. TGF- 2.5 ng ml⁻¹, EGF 50 ng ml⁻¹ and FSH 1 and 2 μg ml⁻¹ supported good initial growth of the PFs. Thirty and seventeen percent of the oocytes from the large PFs cultured in TGF- 2.5 ng ml⁻¹ and FSH 2 μg ml⁻¹ respectively, matured to the MII stage. These proportions for oocytes from small PFs were 11 and 6%, respectively. Oocytes from follicles cultured in EGF did not mature to the MII stage. LH at all concentrations tested and TGF-, EGF and FSH above 5, 50 ng ml⁻¹ and 2 μg ml⁻¹, respectively, induced degeneration of the PFs. It was concluded that (i) TGF- 2.5 ng ml⁻¹ supports development of large PFs in sheep to obtain meiotically competent oocytes, (ii) PFs > 250 μm in initial diameter develop better in vitro, and (iii) in vitro development of sheep PFs could be obtained independent of gonadotropin stimulation.     

Halothiobacillus neapolitanus strain OSWA isolated from "The Old Sulphur Well" at Harrogate (Yorkshire, England)

The “Old Sulphur Well” has a subterranean input of water containing 5.5 mM total sulfide, which would be inhibitory to the growth of most bacteria. The obligately chemolithoautotrophic Halothiobacillus neapolitanus is a sulfur bacterium known to tolerate and metabolize high sulfide concentrations, and we report the isolation of H. neapolitanus strain OSWA from this source. Strain OSWA grows well on thiosulfate and tetrathionate as energy sources, and tolerates at least 5 mM sulfide. Its specific growth rates and yields in batch culture were 0.22 h⁻¹ and 5.3 g mol⁻¹ (thiosulfate), and 0.23 h⁻¹ and 9.5 g mol⁻¹ (tetrathionate). Its 16S rRNA gene sequence shows >99% identity to reference sequences of H. neapolitanus, and it shares morphological and physiological characteristics typical of the species. It is one of a very small number of strains of H. neapolitanus described to date, and the first to be isolated from an ancient sulfide-rich natural spa.     

Catalysis of plastocyanin electron self-exchange by redox-inert multivalent cations

Electron self-exchange in solutions of the ‘blue’ copper protein plastocyanin is catalysed by the redox-inert multivalent cations Mg²⁺ or Co(NH₃)³⁺₆. Measurements of specific ¹H-NMR line broadening with 50% reduced solutions in the presence of these cations show that electron exchange proceeds through encounters of cation-protein complexes which dissociate at high ionic strength. In the presence of 8mM (5 equivalents/total protein) Co(NH₃)³⁺₆, with 10 mM cacodylate (pH^*6.0) as background electrolyte, the bimolecular rate constant at 25°C is 7 × 10⁴ M⁻¹·s⁻¹. For comparison, the ‘electrostatically screened’ rate constant measured in 0.1 M KCl in the absence of added multivalent cations is ˜ 4 × 10³ M¹·s⁻¹.

Plastocyanin Electron self-exchange NMR Protein-protein interaction Multivalent cation Blue copper protein     

Effect of leaf age on CAM activity in Littorella uniflora

In this study the effect of ontogenetic drift on crassulacean acid metabolism (CAM) was investigated in the aquatic CAM-isoetid Littorella uniflora. The results of this study strengthen the general hypothesis of CAM being a carbon-conserving mechanism in aquatic plants, because high-CAM capacity (45–183 μequiv. g⁻¹ FW) was present in all leaves of L. uniflora irrespective of age. Since possession of CAM in aquatic plants allows CO₂ uptake throughout the light/dark cycle, presence of CAM in all leaves influences the carbon balance of L. uniflora positively. On average for all lakes, different leaf classes accounted for 11–36% of the total dark CO₂ uptake by the individual plant.

The capacity for both CAM and photosynthesis declined with increasing leaf age, and was in the oldest leaves only 25–53% of the capacity in the youngest. The photosynthetic capacity was estimated to be sufficiently high to ensure refixation of the CO₂ released from malate during decarboxylation in the daytime. In line with this, a linear coupling between CAM capacity and photosynthetic capacity was found. Parallel to the change in photosynthetic capacity, an age-related change in total ribulose-bisphosphate carboxylase/oxygenase (rubisco) activity from 732 μmol C g⁻¹ DW h⁻¹ in the youngest leaves to 346 μmol C g⁻¹ DW h⁻¹ in the oldest was observed. In contrast, no significant change in phosphoenolpyruvate carboxylase (PEPcase) activity with leaf age was observed (means ranged between 46 and 156 μmol C g⁻¹ DW h⁻¹).     

Arachidonic acid production by Mortierella alpina with growth-coupled lipid synthesis

The fungus Mortierella alpina LPM 301, a producer of arachidonic acid (ARA), was found to possess a unique property of a growth-coupled lipid synthesis. An increase in specific growth rate (μ) from 0.03 to 0.05 h⁻¹ resulted in a two-fold increase in the specific rate of lipid synthesis (milligram lipid (gram per lipid-free biomass) per hour). Under batch cultivation in glucose-containing media with urea or potassium nitrate as nitrogen sources, the ARA content was 46.0 and 60.4% of lipid; 16.4 and 18.8% of dry biomass; and 4.2 and 4.5 g l⁻¹, respectively. Under continuous cultivation of the strain, the productivity of ARA synthesis was 16.2 and 19.2 mg l⁻¹ h⁻¹ at μ=0.05 and 0.03 h⁻¹, respectively.     

A nitrite reductase with cytochrome oxidase activity from Micrococcus denitrificans

The formation of nitrite reductase and cytochrome c in Micrococcus denitrificans was repressed by O₂. The purified nitrite reductase utilized reduced forms of cytochrome c, phenazine methosulphate, benzyl viologen and methyl viologen, respectively, as electron donors. The enzyme was inhibited by KCN, NaN₃ and NH₂OH each at 1 mM, whereas CO and bathocuproin, diethyl dithiocarbamate, o-phenanthroline and ,'-dipyridyl at 1 mM concentrations were relatively ineffective. The purified enzyme contains cytochromes, probably of the c and a₂ types, in one complex. A K_m of 46 μM for NO₂⁻ and a pH optimum of 6.7 were recorded for the enzyme. The molecular weight of the enzyme was estimated to be around 130000, and its anodic mobility was 6.8·10⁻⁶ cm²·sec⁻¹·V⁻¹ at pH 4.55.

The most highly purified nitrite reductase still exhibited cytochrome c oxidase activity with a K_m of 27 μM for O₂. This activity was also inhibited by KCN, NaN₃ and NH₂OH and by NO₂⁻.

A constitutive cytochrome oxidase associated with membranes was also isolated from cells grown anaerobically with NO₂⁻. It was inhibited by smaller amounts of KCN, NaN₃ and NH₂OH than the cytochrome oxidase activity of the nitrite reductase enzyme and also differed in having a pH optimum of about 8 and a K_m for O₂ of less than 0.1 μM. Spectroscopically, cytochromes b and c were found to be associated with the constitutive oxidase in the particulate preparation. Its activity was also inhibited by NO₂⁻.

The physiological role of the cytochrome oxidase activity associated with the purified nitrite reductase is likely to be of secondary importance for the following reasons: (a) it accounts for less than 10% of total cytochrome c oxidase activity of cell extracts; (b) the constitutive cytochrome c oxidase has a smaller K_m for O₂ and would therefore be expected to function more efficiently especially at low concentrations of O₂.