Carbon Dioxide Uptake Rates Research Articles

Functional Data Analysis for Predicting Pediatric Failure to Complete Ten Brief Exercise Bouts.

Physiological response to physical exercise through analysis of cardiopulmonary measurements has been shown to be predictive of a variety of diseases. Nonetheless, the clinical use of exercise testing remains limited because interpretation of test results requires experience and specialized training. Additionally, until this work no methods have identified which dynamic gas exchange or heart rate responses influence an individual's decision to start or stop physical activity. This research examines the use of advanced machine learning methods to predict completion of a test consisting of multiple exercise bouts by a group of healthy children and adolescents. All participants could complete the ten bouts at low or moderate-intensity work rates, however, when the bout work rates were high-intensity, 50% refused to begin the subsequent exercise bout before all ten bouts had been completed (task failure). We explored machine learning strategies to model the relationship between the physiological time series, the participant's anthropometric variables, and the binary outcome variable indicating whether the participant completed the test. The best performing model, a generalized spectral additive model with functional and scalar covariates, achieved 93.6% classification accuracy and an F1 score of 93.5%. Additionally, functional analysis of variance testing showed that participants in the 'failed' and 'success' groups have significantly different functional means in three signals: heart rate, oxygen uptake rate, and carbon dioxide uptake rate. Overall, these results show the capability of functional data analysis with generalized spectral additive models to identify key differences in the exercise-induced responses of participants in multiple bout exercise testing.

Estimation of Growth and Photosynthetic Performance of Two C4 Species (Pennisetum spicatum (L.) K鰎n. and Zea mays L.) under a Low Temperature Treatment

Pearl millet (Pennisetum spicatum (L.) Körn.) and maize (Zea mays L.) are C4 grass species grown for feeding humans and animals in Almadinah Almunawwarah, which is in the western part of Saudi Arabia. During the winter, the mean temperature, which drops to 14°C, represents a major problem for the growth of these species in this region. Therefore, the objectives of this research were to investigate the growth response and the photosynthetic performance of P. spicatum and Z. mays under a low temperature stress. The treatments involved daytime and nighttime temperatures of 14/12°C (low temperature) and 24/22°C (optimum temperature). The results indicated that low temperature significantly reduced all growth and physiological parameters, including seed germination, leaf expansion, leaf area, shoot length and root length of the two species compared to those of the control. Additionally, the low temperature significantly decreased the light-saturated assimilation rate (Asat), quantum yield (ϕ), saturated rate of carbon dioxide uptake (Amax) and efficiency of carboxylation on both species compared to those of the control. Moreover, the values of Fv/Fm and the chlorophyll contents of both species were significantly reduced by low temperature compared to those of the control. It can be concluded that both species had little tolerance to low temperatures.

Salt marsh climate change adaptation: Using runnels to adapt to accelerating sea level rise within a drowning New England salt marsh

Sea level rise within New England is accelerating at a rate faster than the global average, leaving salt marshes particularly susceptible to degradation. Hydrological alteration is a type of climate change adaptation technique that can be used to combat the effects of sea level rise within salt marshes. Runnels (shallow channels) are a type of climate adaptation strategy used to enhance drainage in drowning marshes. In this study, we investigated the impacts of runnel installations, 3–5 years post‐implementation, on soil properties, vegetation composition, and greenhouse gas fluxes. We studied two runnel treatments (Low Elevation Runnel and High Elevation Runnel) and found that in the Low Elevation Runnel areasSpartina alterniflorastem density significantly increased in the three growing seasons after runnels were installed, and the high marsh plant,Spartina patens, persisted in the High Elevation Runnel areas. There was a significant difference in carbon dioxide uptake rates among treatments, with the unmanipulated (Reference) areas having the highest uptake rates and an increase in CO2uptake over time seen in the Low Elevation Runnel treatment. These findings highlight the potential use of a climate change adaptation strategy to combat sea level rise impacts and provide insights for future adaptation efforts.

Improving Jatropha curcas L. photosynthesis-related parameters using poultry litter and its biochar

Poultry litter and biochar contribute to improved plant growth due to their high nutrient content. However, to the best of our knowledge, how incorporating poultry litter and its biochar in soil affects photosynthesis-related parameters of Jatropha curcas L. has not been reported. Therefore, a greenhouse pot experiment was conducted using a complete randomised design with three replicates per treatment to determine the effects of poultry litter, biochar pyrolysed at 350 °C and 750 °C at different application rates (0, 0.5, 1, 2, 3 gkg-1) on Jatropha curcas L. photosynthesis parameters. The control plants recorded the lowest values of photosynthesis-related parameters compared to the treated plants except for water use efficiency. The study observed a significant (P < 0.05) increase in leaf surface area (1807 m2, PL), dark-adapted Fv/Fm ratio, carbon dioxide uptake, and transpiration rate for PL and BC350 with increased application rates, compared to BC750 treatments. BC350 treated plants exhibited higher values (0.79) of Light-adapted Fv’/Fm’. The quantum yield of PSII electron transport displayed an increase with an application rate of 3 gkg-1in PL (0.75) treated soils. Comparing organic amendments used, BC350 exhibited a significantly higher value of carbon dioxide uptake rate (2.67 μmol m-2 s-1) and transpiration rate (2.20 mmol m-2 s-1); however, WUE increased at an application rate of 3 gkg-1 in BC750 (3.8 µmol (CO2) mol-1(H2O)) treated plants. The study results indicate that poultry litter and biochar produced at a lower temperature significantly improved photosynthesis parameters than biochar produced at a higher temperature.

Alkalinization Scenarios in the Mediterranean Sea for Efficient Removal of Atmospheric CO2 and the Mitigation of Ocean Acidification

It is now widely recognized that in order to reach the target of limiting global warming to well below 2°C above pre-industrial levels (as the objective of the Paris agreement), cutting the carbon emissions even at an unprecedented pace will not be sufficient, but there is the need for development and implementation of active Carbon Dioxide Removal (CDR) strategies. Among the CDR strategies that currently exist, relatively few studies have assessed the mitigation capacity of ocean-based Negative Emission Technologies (NET) and the feasibility of their implementation on a larger scale to support efficient implementation strategies of CDR. This study investigates the case of ocean alkalinization, which has the additional potential of contrasting the ongoing acidification resulting from increased uptake of atmospheric CO2 by the seas. More specifically, we present an analysis of marine alkalinization applied to the Mediterranean Sea taking into consideration the regional characteristics of the basin. Rather than using idealized spatially homogenous scenarios of alkalinization as done in previous studies, which are practically hard to implement, we use a set of numerical simulations of alkalinization based on current shipping routes to quantitatively assess the alkalinization efficiency via a coupled physical-biogeochemical model (NEMO-BFM) for the Mediterranean Sea at 1/16° horizontal resolution (~6 km) under an RCP4.5 scenario over the next decades. Simulations suggest the potential of nearly doubling the carbon-dioxide uptake rate of the Mediterranean Sea after 30 years of alkalinization, and of neutralizing the mean surface acidification trend of the baseline scenario without alkalinization over the same time span. These levels are achieved via two different alkalinization strategies that are technically feasible using the current network of cargo and tanker ships: a first approach applying annual discharge of 200 Mt Ca(OH)2 constant over the alkalinization period and a second approach with gradually increasing discharge proportional to the surface pH trend of the baseline scenario, reaching similar amounts of annual discharge by the end of the alkalinization period. We demonstrate that the latter approach allows to stabilize the mean surface pH at present day values and substantially increase the potential to counteract acidification relative to the alkalinity added, while the carbon uptake efficiency (mole of CO2 absorbed by the ocean per mole of alkalinity added) is only marginally reduced. Nevertheless, significant local alterations of the surface pH persist, calling for an investigation of the physiological and ecological implications of the extent of these alterations to the carbonate system in the short to medium term in order to support a safe, sustainable application of this CDR implementation.

Increasing efficiency of carbon dioxide sequestration through high temperature carbonation of cement-based materials

Abstract The increasing global carbon dioxide concentration has received significant public attention in recent decades, with various sequestration methods being proposed by researchers all around the world. The enormous amount of generated cement-based building materials, such as waste concrete aggregate, particularly exhibits a great carbon dioxide fix potential through the carbonation process. However, the slow reaction speed in a normal environment prohibits its further application. On the other hand, exhaust carbon dioxide emitted from a cement factory is usually in a broad temperature range. Therefore, high temperature carbonation is proposed to accelerate the carbon dioxide sequestration speed in this study. The carbonation performance and carbon dioxide uptake rate of cement paste blocks and powder were evaluated in a broad temperature range (20 °C–300 °C), demonstrating that carbonation speed was greatly affected by temperature, with optimum carbonation temperature appearing at around 100 °C, at which cement paste samples could achieve the fastest carbonation speed at a rate of 16.4%/h in the first 1 h compare to 2.4%/h at normal temperature. In addition, the inner liquid water content of samples was another great influential factor which is related closely to the calcium carbonate generation rate from different hydrate substances, and adding liquid water into samples at appropriate time intervals could enhance carbonation reaction effectively, with the maximum improvement of 34.1%. Consequently, it was found that an improvement could be achieved at 100 °C in comparison with normal temperature, with a further increase of 721% being attained by adding liquid water every 20 min to 2 mm thickness cement paste blocks.

Investigation into the CO2 concentrating step rates within the carbon concentrating mechanism of Synechocystis sp. PCC6803 at various pH and light intensities reveal novel mechanistic properties

Synechocystis sp. PCC6803 actively uptake inorganic carbon to elevate the carbon dioxide concentration around their Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) enzyme. The mechanism is comprised of a set of enzymatic complexes which act in an orchestrated manner to transfer extracellular dissolved inorganic carbon into the cell and concentrate it within a specialized compartment. NADPH dehydrogenase-1 is the first operational super-complex in light conditions and, therefore, allows a direct estimation of carbon flux into the cell, albeit before photosynthesis and the concentrating mechanism reach a chemical steady-state condition. We calculated the uptake rate during this period by fitting a nonlinear function to the sharp decline in the CO2 gas exchange profile recorded with membrane inlet mass spectrometry, considering the constant change in rate during this step. We estimate the carbon dioxide uptake rates and pool sizes, showing that these values correspond to previously reported steady state rates in the literature. During the investigation, we reveal new properties of the uptake mechanism. At pH lower than six, the enzymatic complex responsible for the CO2 uptake becomes inactive. Its activity recovers within minutes when attenuating the pH within the cell or elevating it back at or above pH 6.0 outside the cell, a characteristic which seems to be conserved in activity in Symbiodinium microadriaticum, Isochrysis galbana, Phaeodactylum tricornutum and Emiliania huxleyi. We, therefore, suggest that within the carbon concentrating mechanism, CO2 insertion processes can be deactivated when the organism faces pH 5.7.

Dynamic process model and economic analysis of microalgae cultivation in open raceway ponds

Microalgae are a promising biomass feedstock for production of value-added chemicals and bioproducts. A first principle based bioreaction kinetics and bioprocess model of microalgae production was developed for an outdoor open raceway pond that takes into account year-round geospatial characteristics and variability. The primary factors that affect microalgae growth, such as solar irradiance, temperature, biological growth parameters, nutrients and carbon dioxide uptake rates, were considered in the bioreactor process model. Microalgae productivity varied between 2000 and 7200tkm−2year−1 for the different geographical regions studied. For each location, techno-economic analysis was conducted to assess microalgae production price. The capital and operating expenses for microalgae cultivation systems were quantified based on the mass and energy balances of the process. Given the geospatial locations, cost of microalgae production varied between 1074$t−1 to 502$t−1. Results showed that cost of microalgae production depends heavily on the average areal productivity, price of nutrients, as well as design specifications of microalgae growth ponds.

Pressure reduction affects growth and morphology of Chlamydomonas reinhardtii.

Cellular perception of pressure is a largely unknown field in microalgae research although it should be addressed for optimization of a photobioreactor design regarding typically occurring pressure cycles. Also for the purpose of using microalgae as basic modules for material cycles in controlled ecological life support systems, the absence of pressure in outer space or the low absolute pressures on other planets is an abiotic factor that needs to be considered for design of integrated microalgae-based modules. The aim of this work is to study the effects of lowered pressure and pressure changes on photosynthesis as well as morphology. Two Chlamydomonas reinhardtii wild-type strains were exposed to controlled pressure patterns during batch cultivations. Sudden pressure changes should test for existing threshold values for cell survival to mimic such events during space missions. Algae were grown inside a 2 L photobioreactor with an integrated vacuum pump ensuring constant pressures down to 700 mbar. Cultivation samples were analyzed for OD750, cell dry weight, and morphology via light microscope. Chlamydomonas reinhardtii CC-1690 cells showed decreased growth rates, higher carbon dioxide uptake rates, and unchanged oxygen production rates at lower pressures. For sudden pressures changes in the range of 300 mbar no fatal threshold was determined. This study shows that pressure reduction affects growth, gas exchange rates, and morphology. Within the tested pressure range no fatal threshold value was reached.

LED spectrum optimisation using steady-state fluorescence gains

The use of light emitting diodes (LEDs) in greenhouses entails the possibility to control the light in a better way, since both spectrum and light intensity can be adjusted. We aim at developing a method to automatically find the optimal spectrum in terms of energy consumption and plant growth. Previous work shows that chlorophyll fluorescence (ChlF) at 740 nm strongly correlates with the photosynthetic rate (carbon dioxide uptake rate) and that the net efficiency of a LED group therefore is coupled to the fluorescence gain w.r.t. energy consumption, i.e., the slope of a curve depicting steady-state ChlF versus applied power to the LED group. In the present work we compare the fluorescence gains for six different LED types (wavelength peaks from 400 to 660 nm) and six different species: tomato, cucumber, basil, lettuce (two species) and dill. We also compare two different kinds of experiments: steadystate experiments, waiting for the fluorescence to reach a steady state at a few incident light intensities, and ramp experiments, where the light intensity is increased slowly. The ramp experiment gives essentially the same information as the steady-state experiment, but was found to slightly overestimate the gains of the blue LEDs. Being aware of this, it should be possible to initially use the faster (ramp) method in order to find the right light composition, possibly using steady-state experiments for a few LED colours to fine tune the lamp. The relative order of the fluorescence gains among the tested LED groups is similar, but not identical, for all species tested. LED660 has the highest fluorescence gain w.r.t. incident photon flux density, and LED400 and/or LED530 have the lowest. However, the important quantity is in fact the fluorescence gain w.r.t. applied electrical power. If the individual electrical efficiencies of the LEDs change the most efficient power split on the different LEDs might change.

Determination of Growth Rate and Yield of Nitrifying Bacteria by Measuring Carbon Dioxide Uptake Rate

Nitrifier growth parameters--the maximum growth rate (microAmax) and yield (YA)--were estimated by measuring the rate of carbon dioxide uptake and additional rates of oxygen uptake and ammonia (or nitrite) use. Batch tests in a combined titrimetric and offgas analyzer with enriched Nitrobacter and Nitrosomonas cultures and an activated sludge sample were performed. The measured microAmax values for the Nitrobacter and Nitrosomonas cultures were 0.67 +/- 0.03 day(-1) and 0.54 +/- 0.09 day(-1), while the YA values were 0.072 +/- 0.01 g volatile suspended solids (VSS) x g nitrogen (N)(-1) and 0.14 +/- 0.02 gVSS x gN(-1), respectively. For the activated sludge sample, microAmax was observed to increase with pH (microAmax = 0.72 x 3.3(pH-7.1)) over the range 6.8 to 7.1. All microAmax and YA values determined by this method were similar to those previously reported. Compared with other microAmax and YA estimation methods, this method allows for unique microAmax and YA estimations for given conditions from a single experiment.

Relations between carbon dioxide fluxes and environmental factors of Kobresia humilis meadows and Potentilla fruticosa meadows

Carbon dioxide fluxes of Kobresia humilis and Potentilla fruticosa shrub meadows, two typical ecosystems in the Qinghai-Tibet Plateau, were measured by eddy covariance technology and the data collected in August 2003 were employed to analyze the relations between carbon dioxide fluxes and environmental factors of the ecosystems. August is the time when the two ecosystems reach their peak leaf area indexes and stay stable, and also the period when the net carbon absorptions of Kobresia humilis and Potentilla fruticosa shrub meadows reach 56.2 g C·m−2 and 32.6 g C·m−2, with their highest daily carbon dioxide absorptions standing at 12.7 μmol·m−2·s−1 and 9.3 μmol·m−2·s−1, and their highest carbon discharges at 5.1 μmol·m−2·s−1 and 5.7 μmol·m−2·s−1, respectively. At the same photosynthetic photo flux densities (PPFD), the carbon dioxide-uptake rate of the Kobresia humilis meadow is higher than that of the Potentilla fruticosa shrub meadow; where the PPFD are higher than 1,200 μmol·m−2·s−1. The carbon dioxide uptake rates of the two ecosystems declined as air temperature increased, but the carbon dioxide uptake rate of the Kobresia humilis meadow decreased more quickly (−0.086) than that of the Potentilla fruticosa shrub meadow (−0.016). Soil moistures exert influence on the soil respirations and this varies with the vegetation type. The daily carbon dioxide absorptions of the ecosystems increase with increased diurnal temperature differences and higher diurnal temperature differences result in higher carbon dioxide exchanges. There exists a negative correlation between the vegetation albedos and the carbon dioxide fluxes.

Sudden increase in atmospheric CO2 concentration reveals strong coupling between shoot carbon uptake and root nutrient uptake in young walnut trees

We studied the short-term (i.e., a few days) effect of a sudden increase in CO2 uptake by shoots on nutrient (NO3-, P ion, K+, Ca2+ and Mg2+) uptake by roots during vegetative growth of young walnut (Juglans nigra x J. major L.) trees. The increase in CO2 uptake was induced by a sudden increase in atmospheric CO2 concentration ([CO2]). Twelve 2-year-old trees were transplanted and grown in perlite-filled pots in a greenhouse. Rates of CO2 uptake and water loss by individual trees were determined by a branch bag method from 3 days before until 6 days after [CO2] was increased. Nutrient uptake rates were measured concurrently by a hydroponic recirculating nutrient solution system that provided non-limiting supplies of water and nutrients. Six control trees were kept in ambient [CO2] (360 ppm), and [CO2] was increased to 550 ppm for one set of three trees and to 800 ppm for another set of three trees. Before imposing the elevated [CO2] treatments, all trees exhibited similar daily water loss, CO2 uptake and nutrient uptake rates when expressed per unit leaf area to account for the tree size effect. Daily water loss rates were only slightly affected by elevated [CO2]. Carbon dioxide uptake rates greatly increased with increasing atmospheric [CO2], and nutrient uptake rates were proportional to CO2 uptake rates during the study period, except for P ion. Our results show that, despite the important carbon and nitrogen storage capacities previously observed in young walnut trees, nutrient uptake by roots is strongly coupled to carbon uptake by shoots over periods of a few days.

Sensory transduction of blue light in guard cells

Guard cells regulate the dimension of stoma-tal pores in the leaf epidermis, and pore size regulates gas exchange between leaves and the atmosphere. Stomatal function couples the rate of water loss by transpiration with the rate of carbon dioxide uptake by photosynthesis. Guard cells sense multiple environmental and hormonal signals and transduce those signals to modulate the pore apertures to conserve water and ensure a supply of CO2 sufficient for photosynthesis.

Effect of propane on the kinetics of carbon dioxide adsorption in NaA zeolites

Results obtained in a batch adsorption system with constant volume showed that preadsorption of small amounts of propane on NaA zeolite can significantly slow down the rate of carbon dioxide uptake. The observed decrease in the carbon dioxide uptake rate was related to a decrease in accessibility of the external surface of the microcrystallites due to the effect of an external film resistance created by the preadsorbed propane molecules. Small amounts of preadsorbed propane localized inside the zeolite microcrystallites were found to have no affect on the CO2 adsorption rate. Experiments carried out using the frequency response technique confirmed the significant influence of the external film resistance on the kinetics of adsorption of carbon dioxide/propane mixtures on 4A zeolite even at low propane concentrations (2–3%).

Bio-oxidation of a refractory gold bearing high arsenic sulphide concentrate: A pilot study

A modular mobile bioleach pilot plant has been designed and constructed by Coastech Research Inc. The pilot plant was designed to be transported on two flatbed trucks to any location in North America for on-site piloting. The first bioleach demonstration pilot study was completed on a refractory gold bearing high arsenic (7.5% As) sulphide (28.9% S) flotation concentrate. A production capacity of up to 562 kg per day at a feed solids concentration of approximately 14% (w/w) solids and overall retention time of 115 h was achieved under steady operating conditions. Sulphide oxidation exceeded 75% while almost complete arsenic solubilization was observed for the final bioleached product. Oxygen and carbon dioxide uptake rates were obtained from off gas analyses. Oxygen uptake rates as high as 0.99 kg h−1 m−3 were observed. A critical dissolved oxygen concentration, to maintain chemical oxidation, within the range of 0.7–1.1 ppm was observed. Carbon dioxide uptake ratcs were decreased at dissolved oxygen concentrations below approximately 0.7–0.5 ppm. Gold extraction was enhanced from approximately 5% for the pretreated flotation concentrate, to in excess of 90% for the final bioleached product. A worker environment monitoring program was undertaken during the pilot plant operation. All samples taken for air-borne and urinary monitoring were below their respective time weighted average exposure limits.

Bio-oxidation of a refractory gold bearing high arsenic sulphide concentrate: A pilot study

A modular mobile bioleach pilot plant has been designed and constructed by Coastech Research Inc. The pilot plant was designed to be transported on two flatbed trucks to any location in North America for on-site piloting. The first bioleach demonstration pilot study was completed on a refractory gold bearing high arsenic (7.5% As) sulphide (28.9% S) flotation concentrate. A production capacity of up to 562 kg per day at a feed solids concentration of approximately 14% (w/w) solids and overall retention time of 115 h was achieved under steady operating conditions. Sulphide oxidation exceeded 75% while almost complete arsenic solubilization was observed for the final bioleached product. Oxygen and carbon dioxide uptake rates were obtained from off gas analyses. Oxygen uptake rates as high as 0.99 kg h−1 m−3 were observed. A critical dissolved oxygen concentration, to maintain chemical oxidation, within the range of 0.7–1.1 ppm was observed. Carbon dioxide uptake ratcs were decreased at dissolved oxygen concentrations below approximately 0.7–0.5 ppm. Gold extraction was enhanced from approximately 5% for the pretreated flotation concentrate, to in excess of 90% for the final bioleached product. A worker environment monitoring program was undertaken during the pilot plant operation. All samples taken for air-borne and urinary monitoring were below their respective time weighted average exposure limits.

Ex vitro photosynthetic activity in plantlets of tissue-cultured Douglas-fir.

Plantlets of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) had lower rates of net CO(2) uptake and higher rates of root respiration than greenhouse-grown seedlings. They also had a lower photosynthetic efficiency at high photosynthetic photon flux density, and exhibited significant depression of CO(2) uptake under water stress. The ability of plantlets to utilize high irradiance and to photosynthesize under conditions of water stress was enhanced by the presence of many roots. Carbon dioxide uptake rate increased with increasing root number, and decreased as the needle surface area/root surface area ratio decreased. High root number, low needle surface area/root surface area ratio, and elongated roots were associated with low apparent rates of shoot and root respiration.

Contrasting effects of sulfide and thiosulfate on symbiotic CO2-assimilation of Phallodrilus leukodermatus (Annelida)

Experiments were conducted in order to specify the reductants responsible for the carbon dioxide fixation of the symbiotic sulfur bacteria in the gutless marine oligochaete Phallodrilus leukodermatus (Annelida) from shallow calcareous sediments in Bermuda. Carbon dioxide-uptake rates were suppressed by S= and stimulated by S2O3 =. Individuals which hosted bacteria containing reserve energy substances maintained a high short-term CO2-uptake activity, while bacteria in worm homogenates and in worms treated with an antibiotic (Baypen) did not show any significant metabolic activity. Absolute uptake rates in P. leukodermatus were usually considerably higher than those reported for other animals harbouring prokaryotic sulfuroxidizing symbionts. Utilization of thiosulfate rather than sulfide is compatible with the preferred occurrence of the worms around the redox discontinuity layer and has been confirmed in other “thiobiotic” animals. Sulfur stored in the symbiotic bacteria appears to be oxidized to sulfate and be excreted when the worms are held under energy-limited conditions. The data emphasize the complexity of the possible metabolic pathways involved in the oxidation of reduced-sulfur compounds by bacterial symbionts in marine invertebrates.

Phase Shift in the Potassium Uptake Rhythm of the Duckweed Lemna gibba G3 Caused by an Azide Pulse

A 6-hour application (6-hour pulse) of 1 millimolar azide significantly changed the phase of the potassium uptake rhythm of Lemna gibba G3. The phase response curve obtained was type 0 and very similar to that caused by a 6-hour pulse of low temperature (5 degrees C) or darkness. The magnitude of the phase shift and the type of the phase response curve depended on the concentration of azide. However, 6-hour pulses of 3 millimolar cyanide or 10 micromolar (3-(3,4-dichlorophenyl)-1,1-dimethylurea) failed to shift the phase of the rhythm, while these pulses lowered the rate of carbon dioxide uptake or release. Azide, even at 3 micromolar, selectively reduced the amplitude of the rhythm without inhibiting the mean level of potassium uptake.