Relationship between photosystem II activity and CO2 fixation in leaves

There is now potential to estimate photosystem II (PSII) activity in vivo from chlorophyll fluorescence measurements and thus gauge PSII activity per CO₂ fixed. A measure of the quantum yield of photosystem II, Φ_II (electron/photon absorbed by PSII), can be obtained in leaves under steady-state conditions in the light using a modulated fluorescence system. The rate of electron transport from PSII equals Φ_II times incident light intensity times the fraction of incident light absorbed by PSII. In C₄ plants, there is a linear relationship between PSII activity and CO₂ fixation, since there are no other major sinks for electrons; thus measurements of quantum yield of PSII may be used to estimate rates of photosynthesis in C₄ species. In C₃ plants, both CO₂ fixation and photorespiration are major sinks for electrons from PSII (a minimum of 4 electrons are required per CO₂, or per O₂ reacting with RuBP). The rates of PSII activity associated with photosynthesis in C₃ plants, based on estimates of the rates of carboxylation (v_o) and oxygenation (v_o) at various levels of CO₂ and O₂, largely account for the PSII activity determined from fluorescence measurements. Thus, in C₃ plants, the partitioning of electron flow between photosynthesis and photorespiration can be evaluated from analysis of fluorescence and CO₂ fixation.     

A model for photosynthesis and photorespiration in C3 plants based on the biochemistry and stoichiometry of the pathways

Abstract. A model is developed for photosynthesis and photorespiration in C₃ plants, using an equation for the multisubslrate ordered reaction of ribulose 1,5-bisphosphalc carboxylase-oxygenase (Farazdaghi & Edwards, 1988). The model examines net CO₂ fixation with O₂ inhibition, and mutual inhibition when equilibrium exists between carboxylation and oxygenation (at the CO₂ compensation point). It is based on the stoichiometry of energy requirements and O₂, and CO₂ exchange in the cycles, the quantum efficiency for RuBP generation, the maximum capacity for RuBP generation, the carboxylation efficiency with respect to [CO₂], and the oxygenation efficiency with respect to [O₂]. With increasing concentrations of CO₂ above the CO₂ compensation point, decreasing quantum flux density, or decreasing O₂, simulations show that the rate of photorespiration progressively decreases. The two components of O₂ inhibition of photosynthesis change disproportionately with increasing CO₂ concentration. According to the model, the energy utilized during photosynthesis at the CO₂ compensation point is about half that under atmospheric conditions.     

Effects of rapid changes in oxygen concentration on the respiration of carrot roots

Rates of CO₂ production and O₂ consumption from aged disks of carrot ( Daucus carota L.) root tissues were measured for 4 h after they were transferred from 21% to 0, 1, 2, 4 or 8% O₂ in gas mixtures. A transient peak in the rate of CO₂ production started 5 to 7 min after transfer to 2% or lower O₂ mixtures and peaked at 50 min. After the peaks in CO₂ production from the 0, 1 and 2% O₂ treatments and after the stable production from the 4 and 8% O₂ treatments, the rate of CO₂ production from all low O₂ treatments started to decline at 50 min, reaching stable rates by 160 to 240 min. Concentrations of lactate and ethanol that were significantly higher than the 21% O₂ controls had started to accumulate in disks between 10 and 50 min after exposure to atmospheres containing 2% or less O₂. Production of CO₂ started to increase 5 to 7 min after transfer to 0, 1 and 2% O₂, while the initial decline and then rise in pH and the accumulation of ethanol did not occur until 30 min after the change in atmosphere. Ethanol accumulation paralleled the increase in pH; first at 0.4 μmol g⁻¹ h⁻¹ from 30 to 60 min as the pH shifted from 5.97 to 6.11, and then at 0.08 μmol g⁻¹ h⁻¹ from 60 to 100 min as the pH stablized around 6.12. The peak at 50 min in CO₂ production roughly coincided with the shift from the rapid to the slow change in pH and ethanol accumulation.     

Effects of elevated ozone on photosynthesis and stomatal conductance of two soybean varieties: a case study to assess impacts of one component of predicted global climate change

Global climatic change scenarios predict a significant increase in future tropospheric ozone (O₃) concentrations. The present investigation was done to assess the effects of elevated O₃ (70 and 100 ppb) on electron transport, carbon fixation, stomatal conductance and pigment concentrations in two tropical soybean ( Glycine max L.) varieties, PK 472 and Bragg. Plants were exposed to O₃ for 4 h·day⁻¹ from 10:00 to 14:00 from germination to maturity. Photosynthesis of both varieties were adversely affected, but the reduction was higher in PK 472 than Bragg. A comparison of chlorophyll a fluorescence kinetics with carbon fixation suggested greater sensitivity of dark reactions than light reactions of photosynthesis to O₃ stress. The O₃-induced uncoupling between photosynthesis and stomatal conductance in PK 472 suggests the reduction in photosynthesis may be attributed to a factor other than reduced stomatal conductance. An increase in internal CO₂ concentration in both O₃-treated soybean varieties compared suggests that the reduction in photosynthesis was due to damage to the photosynthetic apparatus, leading to accumulation of internal CO₂ and stomatal closure. The adverse impact of O₃ stress increased at higher O₃ concentrations in both soybean varieties leading to large reductions in photosynthesis. This study suggests that O₃-induced reductions in photosynthesis in tropical and temperate varieties are similar.     

Direct and indirect effects of Cd2+ on photosynthesis in sugar beet (Beta vulgaris)

Sugar-beet plants ( Beta vulgaris L. cv. Monohill) were cultivated for 4 weeks in a complete nutrient solution. Indirect effects of cadmium were studied by adding 5, 10 or 20 μ M CdCl₂ to the culture medium while direct effects were determined by adding 1, 5, 20, 50 or 2 000 μ M CdCl₂ to the assay media. The photosynthetic properties were characterized by measurement of CO₂ fixation in intact plants, fluorescence emission by intact leaves and isolated chloroplasts, photosystem (PS) I and PSII mediated electron transport of isolated chloroplasts, and CO₂-dependent O₂ evolution by protoplasts. When directly applied to isolated leaves, protoplasts and chloroplasts. Cd²⁺ impeded CO₂ fixation without affecting the rates of electron transport of PSI or PSII or the rate of dark respiration. When Cd²⁺ was applied through the culture medium the capacity for, and the maximal quantum yield of CO₂ assimilation by intact plants both decreased. This was associated with: (1) decreased total as well as effective chlorophyll content (PSII antennae size), (2) decreased coupling of electron transport in isolated chloroplasts, (3) perturbed carbon reduction cycle as indicated by fluorescence measurements. Also, protoplasts isolated from leaves of Cd²⁺-cultivated plants showed an increased rate of dark respiration.     

The influence of reduced photorespiration on long-term growth and development of wheat

The long-term role of photorespiration was investigated by comparing growth, development, gas exchange characteristics and mineral nutrition of a wheat crop ( Triticum aestivum L. cv. Courtot) cultivated in a culture chamber during a life cycle, either in 4% O₂ or in normal O₂ Low O₂ pressure reduced photorespiration, but CO₂ was controlled so that net photosynthesis remained the same as in the control crop. The growth and development of the low O₂ crop was slowed down. Ear appearance was 16 days late, but the rate of tillering was the same as in the control and was maintained longer so that the final number of tillers was doubled. Pigment, ribulose bisphosphate carboxylase (EC 4.1.1.39) and soluble sugar contents were similar. The response of photosynthesis to CO₂ and O₂ was not appreciably changed by the low O₂ treatment. There was almost no seed formation, and the senescence of the leaves was delayed. It appears that in non-stress conditions most of the photorespiration can be suppressed without damage to the photosynthetic apparatus. Retardation of development and inhibition of reproduction are likely due to other effects of O₂.     

Photosynthesis of two poplar clones under long‐term exposure to ozone

The photosynthetic response was studied in two clones ( Populus deltoides × maximowiczii Eridano and Populus × euramericana I‐214), known for their differential response to ozone (O₃) in terms of visible symptoms, when exposed to O₃ (60 nl l⁻¹ 5 h day⁻¹, 7 and 15 days). The photosynthetic ability was tested using gas exchange and chlorophyll fluorescence analysis. O₃ caused a decrease in the CO₂ assimilation rate at light saturation level in mature leaves of both clones. Alterations of Chl fluorescence parameters, in particular the F_v/F_m ratio and non‐photochemical quenching were also observed. The effects were similar for both clones and it could not be concluded that differential effects on electron transport capacity were responsible for the observed reduction in photosynthesis. The reduction of photosynthetic rate in Eridano was due mainly to a reduced mesophyll activity, as evidenced by the increase in intercellular CO₂ concentration and the minimal changes in stomatal conductance. In contrast, in I‐214, stomatal effects were primarily responsible, although effects on the mesophyll cannot be excluded. Data obtained indicate that the effects observed at the mesophyll level may be attributed to indirect effects caused by membrane disorders.     

An integrated portable apparatus for the simultaneous field measurement of photosynthetic CO2 and water vapour exchange, light absorption and chlorophyll fluorescence emission of attached leaves

Abstract. A portable apparatus has been constructed to measure simultaneously the quantum yield of CO₂ assimilation, light absorption, chlorophyll fluorescence emission and water vapour exchange of attached intact leaves in the field. The core of the instrument is a light-integrating spherical leaf chamber which includes ports for a light source, photosynthetically active radiation sensor, fluorescence probes and gas inlet and outlet manifolds. Measurement of the quantum flux inside the empty chamber and with a leaf present allows determination of leaf absorptance. An open gas exchange system is employed using an infra-red analyser to measure leaf CO₂ exchange. Using a DC white light source the quantum yield of CO₂ assimilation based on absorbed light (φ_abs) may be determined rapidly in either ambient air or artificial gas mixtures. Inclusion of capacitance humidity probes into the gas inlet and outlet ports allows simultaneous determination of water vapour exchange and subsequent estimation of stomatal conductance to CO₂ and intercellular CO₂ concentration. Measurement of fluorescence emission by the sample leaf exposed to white light is achieved by a modulated fluorescence detection system. In addition to determination of the minimal, maximal and variable fluorescence levels, a further analysis allows the photochemical and non-photochemical components of fluorescence quenching, to be estimated. The theory and design of this apparatus is described in detail. The use of the apparatus in the field is demonstrated through a study of the photosynthetic performance of a maize and bean crop during the growing season and by analysis of the photosynthetic performance of crops subjected to nitrogen-stress and a herbicide treatment.     

Oxygen-induced recovery from short-term nitrate inhibition of N2 fixation in white clover plants from spaced and dense stands

Nitrogenase (N₂ase; EC 1.18.6.1) activity (H₂ evolution) and root respiration (CO₂ evolution) were measured under either N₂:O₂ or Ar:O₂ gas mixtures in intact nodulated roots from white clover ( Trifolium repens L.) plants grown either as spaced or as dense stands. The short-term nitrate (5 m M ) inhibition of N₂-fixation was promoted by competition for light between clover shoots, which reduced CO₂ net assimilation rate. Oxygen-diffusion permeability of the nodule declined during nitrate treatment but after nitrate removal from the liquid medium its recovery parallelled that of nitrogenase activity. Rhizosphere pO₂ was increased from 20 to 80 kPa under N₂:O₂. A simple mono-exponential model, fitted to the nodule permeability response to pO₂, indicated NO⁻₃ induced changes in minimum and maximum nodule O₂-diffusion permeability. Peak H₂ production rates at 80 kPa O₂ and in Ar:O₂ were close to the pre-decline rates at 20 kPa O₂. At the end of the nitrate treatment, this O₂-induced recovery in nitrogenase activity reached 71 and 82%; for clover plants from spaced and dense stands, respectively. The respective roles of oxygen diffusion and phloem supply for the short-term inhibition of nitrogenase activity in nitrate-treated clovers are discussed.     

Influence of soil O2 and CO2 on root respiration for Agave deserti

Respiration measured as CO₂ efflux was determined at various soil O₂ and CO₂ concentrations for individual, attached roots of a succulent perennial from the Sonoran Desert, Agave deserti Engelm. The respiration rate increased with increasing O₂ concentration up to about 16% O₂ for established roots and 5% O₂ for rain roots (fine branch roots on established roots induced by wetting of the soil) and then remained fairly constant up to 21% O₂. When O₂ was decreased from 21 to 0%, the respiration rates were similar to those obtained with increasing O₂ concentration. The CO₂ concentration in the root zone, which for the shallow-rooted A. deserti in the field was about 1 000 μl l^-1, did not affect root respiration at concentrations up to 2 000 μl l^-1, but higher concentrations reduced it, respiration being abolished at 20 000 μl l^-1 (2%) CO₂ for both established and rain roots. Upon lowering CO₂ to 1 000 μl l^-1 after exposure to concentrations up to 10000 μl l^-1 CO₂, inhibition of respiration was reversible. Uptake of the vital stain neutral red by root cortical cells was reduced to zero, indicating cell death, in about 4 h at 2% CO₂, substantiating the detrimental effects of high soil CO₂ concentrations on roots of A. deserti . This CO₂ response may explain why roots of desert succulents tend to occur in porous, well-aerated soils.     

Kinetics of leaf oxygen uptake represent in planta activities of respiratory electron transport and terminal oxidases

We present, for the first time, the oxygen response kinetics of mitochondrial respiration measured in intact leaves (sunflower and aspen). Low O₂ concentrations in N₂ (9–1500 ppm) were preset in a flow-through gas exchange measurement system, and the decrease in O₂ concentration and the increase in CO₂ concentration as result of leaf respiration were measured by a zirconium cell O₂ analyser and infrared-absorption CO₂ analyser, respectively. The low O₂ concentrations little influenced the rate of CO₂ evolution during the 60-s exposure. The initial slope of the O₂ uptake curve on the dissolved O₂ concentration basis was relatively constant in leaves of a single species, 1.5 mm s⁻¹ in sunflower and 1.8 mm s⁻¹ in aspen. The apparent K _0.5(O₂) values ranged from 0.33 to 0.67 μ M in sunflower and from 0.33 to 1.1 μ M in aspen, mainly because of the variation of the maximum rate, V _max (leaf temperature 22°C). The initial slope of the O₂ response of respiration characterizes the catalytic efficiency of terminal oxidases, an important parameter of the respiratory machinery in leaves. The plateau of the response characterizes the activity of the mitochondrial electron transport chain and is subject to regulations in accordance with the necessity for ATP production. The relatively low oxygen conductivity of terminal oxidases means that in leaves, less than 10% of the photosynthetic oxygen can be reassimilated by mitochondria.     

Changes in land plant function over the Phanerozoic: reconstructions based on the fossil record

Major fluctuations in the concentrations of atmospheric CO₂ and O₂, are predicted by historical long-term carbon and oxygen cycle models of atmospheric evolution and will have impacted directly on past climates, plant function and evolutionary processes. Here, palaeobotanical evidence is presented from the stomatal density record of fossil leaves spanning the past 400 Myr supporting the predicted changes in atsmopheric CO₂. Evidence from experiments on plants exposed to long-term high CO₂, environments and the newly-assembled fossil data indicate the potential for genetic modification of stomatal characters. The influence of the changes in fossil stomatal characteristics and atmospheric composition on the rates of leaf gas exchange over the course of land plant evolution has been investigated through modelling. Three contrasting eras of plain water economics emerge in the Devonian (high), Carboniferous (low) and from the Upper Jurassic to the present-day (high but declining). These patterns of change result from structural changes of the leaves and the impact of atmospheric CO₂, and O₂, concentrations on RuBisCo function and are consistent with the fossil evidence of sequential appearances of novel plant anatomical changes. The modelling approach is tested by comparing predicted leaf stable carbon isotope ratios with those measured on fossil plant and organic material. Viewed in a geological context, current and future increases in the concentration of atmospheric CO₂, might be considered as restoring plant function to that more typically experienced by plants over the majority of their evolutionary history.     

Gas exchange studies in two Portuguese grapevine cultivars

Gas exchange characteristics of leaves of Vitis vinifera L. cvs Tinta Amarela and Periquita, two grapevine cultivars grown in distinct climatic regions of Portugal, were studied under natural and controlled conditions. Daily time courses of gas exchange were measured on both a hot, sunny day and a cooler, partly cloudy day. Responses of net photosynthesis to irradiance and internal partial pressure of CO₂, were also obtained. A strong correlation between net photosynthesis (P_N) and leaf conductance (g_s) was found during the diurnal time courses of gas exchange, as well as a relatively constant internal partial pressure of CO₂ (P_i), even under non-steady-state conditions. On the cloudless day, both P_N and g_s were lower in the afternoon than in the morning, despite similar conditions of leaf temperature, air to leaf water vapor deficit and irradiance. The response curves of net photosynthesis to internal CO₂ showed linearity up to p_i values of 50 Pa, possibly indicating a substantial excess of photosynthetic capacity. When measured at low partial pressures of O₂ (1 kPa), P_N became inhibited at high CO₂ levels. Inhibition of P_N at high CO₂ was absent under normal levels of O₂ (21 kPa). Significant differences in gas exchange characteristics were found between the two cultivars, with T. Amarela having higher rates under similar measurement conditions. In particular, the superior performance of T. Amarela at high temperatures may represent adaptation to the warmer conditions at its place of origin.     

A Chromatographic Method for Analysis of the Gaseous Phase in Shake Flask Cultures and its Application to the Study of Methane Utilization

A simple gas chromatograph with a katharometer detector is described to determine O₂, N₂, methane and CO₂ in gas samples of 0·01–2·0 ml. The apparatus is inexpensive, and can be modified to determine other gases. The sensitivity to oxygen is 3 × 10⁻⁶ g. The use of the instrument is illustrated by a study of the growth kinetics of Methylococcus capsulatus grown on methane in shake flask experiments. The ratio of O₂ uptake to methane uptake is much lower in the stationary phase than in the growth phase of the culture.     

Use of the response of photosynthesis to oxygen to estimate mesophyll conductance to carbon dioxide in water-stressed soybean leaves

Methods of estimating the mesophyll conductance (g_m) to the movement of CO₂ from the substomatal airspace to the site of fixation are expensive or rely upon numerous assumptions. It is proposed that, for C₃ species, measurement of the response of photosynthesis to [O₂] at limiting [CO₂], combined with a standard biochemical model of photosynthesis, can provide an estimate of g_m. This method was used to determine whether g_m changed with [CO₂] and with water stress in soybean leaves. The value of g_m estimated using the O₂ response method agreed with values obtained using other methods. The g_m was unchanged over the tested range of substomatal [CO₂]. Water stress, which decreased stomatal conductance (g_s) by about 80%, did not affect g_m, while the model parameter V_Cmax was reduced by about 25%. Leaves with g_s reduced by about 90% had g_m values reduced by about 50%, while V_Cmax was reduced by about 64%. It is concluded that g_m in C₃ species can be conveniently estimated using the response of photosynthesis to [O₂] at limiting [CO₂], and that g_m in soybean was much less sensitive to water stress than g_s, and was somewhat less sensitive to water stress than V_Cmax.     

BRYOZOANS INCREASE AVAILABLE CO2 FOR PHOTOSYNTHESIS IN GELIDIUM SESQUIPEDALE (RHODOPHYCEAE)

A possible benefit of the presence of the epiphytic bryozoan Electra pilosa (L.) for the red macroalga Gelidium sesquipedale (Clem.) Thuret et Bornet is described. Absorption spectra and photosynthetic parameters of O₂ evolution vs. irradiance curves were determined for both epiphytized and nonepiphytized thalli. The absorptance of G. sesquipedale thalli for PAR was not modified by the presence of the epiphyte. Gross photosynthetic rates at saturating light were approximately doubled in epiphytized thalli. Photosynthesis by G. sesquipedale was enhanced when CO₂ concentration was increased in the medium by a decrease in pH. On the other hand, an increase in pH from 8.1 to 8.7 produced a significant reduction of the O₂ evolution rates indicating that G. sesquipedale has a very low capacity to use HCO₃⁻. The decrease in photosynthesis at high pH was higher in nonepiphytized thalli than in epiphytized ones, suggesting that the amount of available CO₂ is higher in the presence of E. pilosa. This positive effect was attributed to the CO₂ released by respiration of the epiphyte.     

Gas exchange in intact isolated chloroplasts from Chlamydomonas reinhardtii during starch degradation in the dark

During starch degradation in intact isolated chloroplasts from Chlamydomonas reinhardtii gas exchange was studied with a mass spectrometer. Oxygen uptake by intact chloroplasts in the dark never exceeded 1.5% of the starch degradation rate [maximum 15 nmol O₂ (mg Chl)⁻¹ h⁻¹ consumed. 1 000 nmol glucose (mg Chl)⁻¹h⁻¹ degraded]. Evolution of CO₂ under aerobic conditions [9.8–28 nmol (mg Chl)⁻¹ h⁻¹] was stimulated by addition of 0.1–0.5 m M oxaloacetate [393–425 nmol CO₂ (mg Chl)⁻¹ h⁻¹]. Pyridoxal phosphate (5 m M ) inhibited starch degradation by more than 80%, but had no effect on O₂ uptake. Starch degradation rates and CO₂ evolution did not differ under acrobic and anaerobic conditions. Increasing P_i in the reaction medium from 0.5 m M to 5.0 m M stimulated starch degradation by 230 and 260% under aerobic and anaerobic conditions, respectively. A rapid autooxidation of reduced ferredoxin was observed in a reconstituted system consisting of purified Chlamydomonas ferredoxin, purified Chlamydomonas NADP-ferredoxin oxidoreductase (EC 1.6.7.1) and NADPH. Addition of isolated thylakoids from C. reinhardtii did not affect the rate of O₂ uptake. Our results clearly indicate the absence of any oxygen requirement during starch degradation in isolated chloroplasts.     

The impact of elevated ozone and carbon dioxide on young Acer saccharum seedlings

The effects of high O₃ (200 nl l⁻¹ during the light period) and high CO₂ (650 μl l⁻¹ CO₂, 24 h a day) alone and in combination were studied on 45-day-old sugar maple ( Acer saccharum Marsh.) seedlings for 61 days in growth chambers. After 2 months of treatment under the environmental conditions of the experiment, sugar maple seedlings did not show a marked response to the elevated CO₂ treatment: the effect of high CO₂ on biomass was only detected in the leaves which developed during the treatment, and assimilation rate was not increased. Under high O₃ at ambient CO₂, assimilation rate at days 41 and 55 and Rubisco content at day 61 decreased in the first pair of leaves; total biomass was reduced by 43%. In these seedlings large increases (more than 2-fold) in glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and in anaplerotic CO₂ fixation by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were observed, suggesting that an enhanced reducing power and carbon skeleton production was needed for detoxification and repair of oxidative damage. Under high O₃ at elevated CO₂, a stimulation of net CO₂ assimilation was observed after 41 days but was no longer observed at day 55. However, at day 61, the total biomass was only reduced by 21% and stimulation of G6PDH and PEPC was less pronounced than under high O₃ at ambient CO₂. This suggests that high CO₂ concentration protects, to some extent, against O₃ by providing additional carbon and energy through increased net assimilation.     

Carbon dioxide and ethylene interactions in tulip bulbs

The effect of CO₂ on ethylene-induced gummosis (secretion of polysaccharides), weight loss and respiration in tulip bulbs ( Tulipa gesneriana L.) was investigated. A pretreatment with 1-MCP prevented these ethylene-induced effects, indicating that ethylene action must have been directed via the ethylene receptor. Treatment with 0.3 Pa ethylene for 2 days caused gummosis on 50% of the total number of bulbs of cultivar Apeldoorn, known to be sensitive for gummosis. Addition of CO₂ (10 kPa) reduced the ethylene-induced gummosis to 18%. In a second experiment the influence of ethylene and CO₂ on respiration and FW loss of bulbs of the cultivar Leen van der Mark was studied. A range of ethylene partial pressures (0.003–0.3 Pa) was applied continuously for 29 days. Ethylene caused a transient peak in O₂ consumption rate during the first days after the start of application. The relation between O₂ consumption rate and ethylene partial pressure could be described by Michaelis-Menten kinetics. Respiratory peaks were reduced by CO₂. This inhibition by CO₂ could not totally be due to competition with ethylene at the receptor binding-site, as was indicated by the use of an O₂ consumption model. Pre-treatment of bulbs with 1-MCP and subsequent exposure to CO₂ showed that CO₂ could influence respiration irrespective of any interaction with ethylene. Ethylene and CO₂ both stimulated weight loss. The effect of combined treatments of ethylene and CO₂ on weight loss was at least as strong as the sum of the separate effects, which implies that competition between ethylene and CO₂ at the receptor binding-site was unlikely.     

Effects of graded hypoxia on Atlantic salmon infected with amoebic gill disease

Atlantic salmon Salmo salar with amoebic gill disease (AGD) were exposed to a graded hypoxia (135–40 mmHg water P O₂) and blood samples analysed for respiratory gases and pH at 119, 79·5 and 40 mmHg water P O₂. There were no differences in the rate of oxygen uptake between infected and control fish. However, arterial P O₂, and pH were significantly lower in the infected fish whereas P CO₂ was significantly higher in infected fish compared with controls prior to hypoxia and at 119 mmHg water P O₂. At 79·5 and 40 mmHg water P O₂ saturation, there were no significant differences in blood P O₂ or pH although blood P CO₂ was elevated in AGD affected fish at 50% hypoxia (79·5 mmHg water P O₂). The elevated levels of P CO₂ in fish affected by AGD resulted in a persistent respiratory acidosis even during hypoxic challenge. These data suggest that even though the fish were severely affected by AGD, the presence of AGD while impairing gas transfer under normoxic conditions, did not contribute to respiratory failure during hypoxia.