Distribution Of 14C Research Articles

Biosynthetic production of 13C and 14C erythronolide labeled erythromycin A

(1,3,5,7,9,11,13-13C)-Erythromycin A and [1,3,5,7,9,11,13-14C]-erythromycin A were produced in liquid fermentation broths of Saccharopolyspora erythraea CA340 after the administration of (1-13C)-sodium propionate and [1-14C]-sodium propionate, respectively. Fermentations were carried out in shake flasks. Labeled erythromycin A products were isolated by ethyl acetate extraction of the fermentation broth, followed by chromatographic purification on silica gel and Sephadex LH-20. 13C NMR verified (13C) incorporation at the seven possible positions on the erythronolide nucleus. Mass spectral analysis of (13C) labeled erythromycin A revealed a distribution of (13C) label which could be rationalized based on the known biosynthetic pathway for production of the erythronolide nucleus. © 1997 John Wiley & Sons, Ltd.

Characteristics of photosynthetic carbon metabolism of spikelets in rice

In lemmas and paleae of rice, the amount of pyruvate, Pi dikinase (PPDK) protein increased dramatically 6 d after anthesis and this change was consistent with that in the activity of PPDK. Since lemmas and paleae at this stage also showed high activities of the other marker enzymes of C4 pathway including phosphot enolpyruvate carboxylase (Imaizumi et al. (1990) Plant Cell Physiol 31: 835–843), photosynthetic carbon metabolism with lemmas at this stage were characterized. In a 14C pulse-12C chase study by photosynthetic CO2 fixation, about 35% and 25% of 14C fixed in lemmas were incorporated initially into 3-phosphoglycerate (3-PGA) and C4 acids, respectively. This suggests that lemmas participate mainly in C3-type photosynthetic metabolism, but that lemmas may also participate in the metabolism of C4 acids to some extent. To clarify this possibility, large amounts of 14C-labeled C4 acids were synthesized in vivo by a light-enhanced dark CO2 fixation (LED) method and the fate of 14C in C4 acids in the light was investigated. The percentage distribution of 14C in C-4 position of malate was about 90% and 83% after 10 s of photosynthetic 14CO2 fixation and 110 s of LED, respectively. Some of the 14C incorporated into C4 acids was transferred into 3-PGA and sugar phosphates. The possibility of direct fixation of CO2 by phosphot enolpyruvate carboxylase and metabolic pathway of CO2 released by decarboxylation of malate produced were discussed.

14C Dating of Modern Marine and Estuarine Shellfish

We measured the 14C content of 36 living marine molluscs from Tairua Harbour and the rocky coast on the Coromandel Peninsula of New Zealand. We identified species suitable for radiocarbon dating and show that the open marine intertidal zone is enriched in 14C compared to the open marine subtidal zone or estuary. We also found a uniform 14C distribution in the Tairua Harbour, by analyzing samples of the estuarine bivalve Austrovenus stutchbwyi collected up to 5 km from the harbor entrance.

Ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxylase activities of photoautotrophic callus of Platycerium coronarium (Koenig ex O.F. Muell.) Desv. under CO2 enrichment

The in vitro activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) were measured in cell-free extracts of Platycerium coronarium callus cultured for up to 42 days under photoautotrophic conditions with CO2 enrichment. With an increase in CO2 in the culture environment to 10% (v/v) at low light, the apparent photoautotrophic fixation of CO2 by Rubisco declined, whereas the non-photoautotrophic CO2 fixation by PEPC activity was enhanced. Hence, photosynthesis appears to play a lesser role in providing carbon skeletons and energy with prolonged culture in a CO2-enriched environment. Instead, the anaplerotic supply of C-skeletons by PEPC may be important under such a situation. Short-term H14CO3-fixation experiments indicated that photoautotrophic callus cultured for 3 weeks with 10% CO2 enrichment assimilated less 14CO2 than the control (0.03% CO2). Analyses of 14C-metabolites indicated that about 50% of the total soluble 14CO2 fixed was in the organic acid fraction and 35% in the amino acid fraction. Despite the changes in the in vitro Rubisco/PEPC activity-ratio, no significant change in the 14C distribution pattern was apparent in response to increasing sucrose or CO2 concentrations. The suppression of Rubisco activity and total chlorophyll content in high sucrose or elevated CO2 concentrations suggests an inhibition of the capacity for photoautotrophic callus growth under these conditions.

An in vivo13C NMR study on the effect of dietary sugar on pyruvate cycling during glucogenesis from (2- 13C)pyruvate in Manduca sexta L.

Pyruvate cycling from (2- 13C)pyruvate was detected in vivo in intact 5th instar Manduca sexta larvae by application of NMR spectroscopy. Cycling was evident from the enrichment of C3 in alanine following transamination of recycled pyruvate in larvae maintained on casein-based diets with or without sucrose. This metabolism is assumed to principally occur in the fat body. Analysis of 13C enriched metabolites released into the hemolymph indicated that isotopic dilution of recycled pyruvate was sufficiently great that further metabolism of the recycled metabolite did not occur to any significant extent under these dietary conditions. The C3/C2 13C-enrichment ratio of alanine, therefore, accurately reflected the relative degree of pyruvate cycling and indicated that the rate of cycling was approximately three-fold lower in larvae maintained on diets lacking sucrose. Moreover, based on the distribution of 13C in trehalose, these larvae displayed significantly greater rates of gluconeogenesis. Enrichment of C1, C2, C5 and C6 were principally due to carboxylation of the isotopically substituted substrate catalyzed by pyruvate carboxylase and, therefore, reflected net carbohydrate synthesis. Trehalose C3 and C4 enrichments were principally due to pyruvate dehydrogenase-catalyzed decarboxylation and reflected incorporation of label following metabolism through the TCA cycle. Pentose cycling following glucogenesis significantly affected the 13C distribution in trehalose in insects on both diets, and the relative intensity of trehalose C6 was, therefore, used for comparing the rates of gluconeogenesis and pyruvate cycling. Based on the 13C enrichment of trehalose C6 relative to C3 of alanine the mean rate of pyruvate cycling relative to the rate of gluconeogenesis was approximately 60% in larvae on the diet lacking sucrose, while the rate of pyruvate cycling in larvae maintained on the diet supplemented with sucrose was greater than the gluconeogenic flux. The results were consistent with the conclusion that pyruvate kinase likely plays an important role in regulating gluconeogenesis in M. sexta larvae.

Problems in the methods of estimation of growth and maintenance respiration

Abstract According to Thornley, J.H.M. (Nature, 227, 304-305, 1970) and McCree, K.J. (Crop Sci., 14, 509-514, 1974), respiratory substances are used only for maintenance respiration when plants are exposed to the dark conditions for a long period of time (more than 2 d). The maintenance respiration is also affected by the nitrogen status in plant, because protein turnover is one of the major energy consumption sources under maintenance process. Therefore, to determine whether respiratory substances are used only for maintenance, 14C- [U] -sucrose or a mixture of 14C- [U] -amino acids was introduced to rice and soybean plants from the tip of leaf. Plants were grown under natural light conditions and under dark conditions for 4 d with 2 nitrogen levels (0.2 and 0.02 g N L-1 soil). After the introduction of the 14C-compounds, the 14CO2 respiratory rate was monitored during 24 h, then the 14C distribution to organic acids, free amino acids, proteins, sugars, and polysaccharides was analyzed. Following results...

Distribution of 14C from ingested, radiolabelled dichlorostearic, stearic and oleic acids in body and in lipids of perch, Perca fluviatilis

The body distribution and incorporation into complex lipids of 14C from orally administered, radiolabelled dichlorostearic, stearic and oleic acid were studied in perch, Perca fluviatilis. Uptake and incorporation of 14C from radiolabelled dichlorostearic acids did not differ significantly from that of unchlorinated fatty acids, implying that dichlorostearic acid most probably was not recognised as ‘abnormal’. This may distinguish chlorinated fatty acids from other known organic pollutants. High concentrations of chlorinated fatty acids in fish have been correlated with adverse biological effects and may cause disturbances in biological membranes, particularly if they become incorporated into phospholipids. Lack of recognition, and thereby elimination, of the chlorinated fatty acids by the fish may impose an increased risk for toxic effects.

Exercise and exhaustion effects on glycogen synthesis pathways.

Female Sprague-Dawley rats were infused with [1-13C]glucose to measure the effect of endurance training and the effect of various metabolic conditions on pathways of hepatic glycogen synthesis. Four metabolic states [sedentary (S), trained (T), sedentary exhausted (SE), and trained exhausted (TE)] were studied. T and TE rats were trained on a motor-driven treadmill (30 m/min, 15% grade, 1.0 h/day, 5 days/wk) for 8-10 wk. After a 24-h fast, SE and TE rats were run to exhaustion (sedentary average = 78 min, trained average = 155 min) at a training pace and immediately infused with labeled glucose for 2 h. S and T rats were infused after a 24-h fast. After infusion, tissues were removed and glycogen was isolated and hydrolyzed to glucose. The glucose was measured for distribution of 13C by using nuclear magnetic resonance. Glycogen was synthesized predominantly by the indirect pathway for all metabolic states, indicating that infused glucose was first metabolized primarily in the peripheral tissue. The direct-pathway utilization was greater in rested S than in rested T animals (30 vs. 14%); however, for exhausted animals, the trained use of the direct pathway was greater (22 vs. 9%). Both TE and rested T animals utilize the indirect pathway a comparable amount. Sedentary animals, on the other hand, dramatically decreased utilization of the direct pathway, with exhaustive exercise changing from 30 to 9%. The results indicate that endurance training modifies glucose utilization during glycogen synthesis after fasting and exhaustive exercise.

Biological factors controlling the carbon isotope record in fish otoliths: Principles and evidence

Otoliths are calcium carbonate structures in the endolymph of the inner ear of fish used to estimate age of individual fish based on the annual and the daily periodicities of the internal banding structure of the otolith. The otolith is about 10% by weight carbon and changes in carbon isotope disequilibrium in the endolymph over the life of the fish are recorded in the pattern of 13C deposition in the otolith. The record of carbon isotope disequilibrium in the otolith can provide useful information about two aspects of the biology of the fish: position in the food chain, and the changes in metabolic rate over the life of the fish. Measurements of σ 13C distribution in otoliths show an increasing 13C enrichment over the part of the life of the fish when respiration rate can be expected to increase, and a decline in 13C when respiration rate can be expected to be decreasing or stationary. Basic models of 13C fractionation in biological systems predict that 13C enrichment of metabolic CO 2 should occur at higher respiration rates. The similarity between observed and expected 13C distributions in otoliths suggests that the enrichment pattern of 13C in otoliths is an analog of the metabolic rate of fishes, which provides a useful life history record.

Fructose 2,6-bisphosphate-modulated photosynthesis in sorghum leaves grown under low water regimes

The influence of low water potentials on photosynthetic sucrose formation, fructose 2,6-bisphosphate (F2,6BP) content, partitioning of photosynthate and related enzymes was investigated in control and water-stressed leaves of Sorghum bicolor cv. CSH 5. The accumulation of F2,6BP in water-stressed plants reduced sucrose formation in the leaves. The activities of fructose 1,6-bisphosphatase and sucrose phosphate synthase were reduced in water-stressed plants. The distribution of 14C into starch increased with increasing stress, while the incorporation into sucrose was drastically reduced in the stressed plants. The activity of fructose 6-phosphate 2-kinase was very high in stressed plants, while the levels of fructose 2,6-bisphosphatase were low. The results suggest that low water potentials result in F2,6BP accumulation, which modulates sucrose synthesis and carbon-partitioning in sorghum leaves.

Distribution of organic 14C and 15N in particle-size fractions of soils incubated with 14C, 15N-labelled glucose/NH 4, and legume and wheat straw residues

Comparisons have been made of the recoveries and distributions of total and isotope-labelled microbial biomass C and organic C and N in particle-size fractions of a Vertisol and an Alfisol decomposing 14C, 15N-labelled glucose/NH 4; and of the Vertisol decomposing labelled legume and wheat residues. Recoveries after fractionation of soil weight, and of labelled organic C and N and biomass C, were high (> 96%) in most treatments. In both the strongly-aggregated Vertisol and the weakly-aggregated Alfisol, each amended with 14C-glucose/ 15NNH 4 and incubated for 3 days, the bimodal patterns of distribution of residual organic 14C and of biomass 14C were very similar to each other and to that of native biomass C. Major proportions of residual organic 14C and biomass 14C were located with the fine silt-size (20-2 μm) fraction, which, in the Vertisol under the conditions of soil dispersion, was dominated by clay microaggregates. A minor peak of residual organic 14C and biomass 14C was located in the fine sand-size (250-50 μm) fraction from both soils. Residual organic 15N was similarly bimodally-distributed. Nevertheless, the clay-size (2-0.1 μm) fraction of both soils accounted for higher proportions of organic 15N than of organic 14C, and in the case of the Alfisol, higher proportions of organic 14C than of biomass 14C. The relatively large differences in the cation exchange capacities of the clay- and fine silt-size components of the almost fully-dispersed Alfisol, in contrast to the similar cation exchange capacities of these size fractions in the partially-dispersed Vertisol, are considered to be contributory factors to the differences between the soils in residual 14C and 15N distributions. The more evident partial separation of biomass 14C, organic 14C and organic 15N in the fine particle size fractions of the Alfisol than in those of the Vertisol is thought to have arisen from the greater opportunities in the former soil for metabolic turnover and preferential adsorption of exocellular nitrogenous organics to its clay-size than its silt-size constituents. However, the cation exchange capacity of fractions was not the sole factor determining residue distribution throughout the total size-range of the fractions. Expressed as concentrations on a fraction CEC basis, organic 14C, biomass 14C and organic 15N were most concentrated in the coarse sand-size (> 250 μm) fraction of both soils (probably from the association of the labelled microbial residues with partly degraded plant fragments). In the cases of the Vertisol amended with 14C, 15N-labelled legume or wheat materials and incubated for 49 days, the distribution patterns of labelled organic residues were very similar to those respectively from the glucose/NH 4-amended Vertisol. The main exception was that the sand-size fractions from the plant residue-amended Vertisols were relatively enriched with total organic 14C, due very probably to the persistence of some undecomposed plant constituents. Both the similarities and differences in the distributions of labelled organic 14C and 15N and biomass 14C in soil particle-size fractions have been related to the influence of substrate and soil properties and of incubation time on the extent of decomposition of primary substrates and of C and N turnover. Broadly, the results suggest that there is a widespread and somewhat uniform early response of the soil microflora to a soluble substrate such as glucose and NH 4, i.e. the distribution of the new growth (and of immobilized 14C and 15N) within the soil matrix was strongly determined by the location of the existing microflora. For plant residue substrates (after an incubation period sufficient to allow turnover of the relatively unprotected components of biomass 14C), the distributions within particle-size fractions of labelled biomass and immobilized 15N were also broadly similar to that of native biomass C. In such circumstances, the distributions of biomass C and 14C in the fractions were probably determined by factors of substrate supply and protection from predation within the unfractionated soil matrix; factors which appeared to be unrelated to fraction CEC.

Observed and Simulated Solute Transport Under Varying Water Regimes: II. 2,6‐Difluorobenzoic Acid and Dicamba

AbstractSignificant interest in the fate of agrichemicals in soils has prompted the development of several transport simulation models. Our primary objective was to evaluate the simulation model LEACHM for predicting the fate of dicamba (3,6‐dichloro‐2‐methoxybenzoic acid) and a nonreactive tracer, 2,6‐difluorobenzoic acid (2,6‐DFBA) in fallow and cropped (barley, Hordeum vulgare L.) systems under different water application levels. A field study (1992) was conducted using in situ soil columns on a Borollic Calciorthid (Brocko silt loam, Gallatin Co., MT). Dicamba (14C‐labeled) and 2,6‐DFBA were surface applied at rates of 0.26 and 112 kg ha−1, respectively. Solute concentrations were measured at three depths (0.36, 0.66, and 0.96 m) using porous cup lysimeters for 70 d following chemical application. Time‐moment analysis of observed solute breakthrough curves (BTCs) generally showed increasing travel times with increasing soil depth and decreasing water application. Dicamba transport was similar to 2,6‐DFBA, with the exception that 40 to 60% of applied dicamba was degraded during transport. The distribution of 14C remaining in the soil columns showed that the primary degradate of dicamba, 3,6‐dichlorosalicylic acid (DCSA), was confined primarily to surface samples (0–0.2 while dicamba was found only at lower depths. This is consistent with a much higher sorption coefficient (Koc) determined for DCSA relative to dicamba. Comparison of observed and predicted (LEACHM) solute BTCs suggested that preferential solute transport occurred especially under high and medium water regimes. Finally, data from three field seasons at the same site suggest that the timing of chemical application relative to initial soil water content and plant stage, the presence of root channels, and temporal changes in soil hydraulic properties in the absence of tillage may significantly affect the degree of preferential flow and subsequent agreement between predicted and observed BTCs.

ESPAS — An advanced phytotron for measuring carbon dynamics in a whole plant-soil system

The use of carbon isotopes as tracers is essential for measuring carbon flows in an intact whole plant-soil system. Here, we describe and Experimental Soil Plant Atmosphere System (ESPAS) to perform pulse-labelling and steady-state labelling experiments with 13CO2 and 14CO2. The ESPAS facility is an environmental research tool that is used to measure the carbon fluxes from the atmosphere to the roots and into the soil and the microbial biomass and to study decomposition of plant residues and soil organic matter. The influence of environmental conditions in the atmosphere or in soil on the carbon allocation and turnover in the plant-soil ecosystem can be quantified. The design and the technical description of the phytotrons is presented and evidence is provided that the phytotrons are equivalent. For this purpose, Triticum aestivum plants were cultivated in the phytotrons for 39 days and shoot growth, root growth and water use were compared. No significant differences were observed for plant growth and water use. As an example of the practical application of the equipment, an experiment with elevated atmospheric CO2 is presented. Data are given on the uptake of 14C under ambient (350 μL L−1) and elevated (700 μL L−1CO2) in Lolium perenne and Festuca arundinacea and the distribution of 14C among different plant-soil compartments i.e. shoot, root, root-soil respiration, and soil. We conclude that these phytotrons yield detailed information on gross carbon flows in a whole plant-soil system that can not be obtained without sensitive carbon tracers. Such data are important for proper calibration of simulation models on soil organic matter.

Carbon allocation patterns in fungi in the presence of chitin in the external medium

A number of fungi were grown on defined media containing uniformly labelled [14C]glucose with and without additional chitin, to examine how the presence of chitin affected growth and the distribution of 14C between cytoplasm and cell wall. Chitin significantly enhanced the growth of Heterobasidion annosum Boletinus cavipes and Paxillus inoolutus. These fungi, with the exception of H. annosum, also incorporated a lower proportion of assimilated 14C into their cell walls. The specific activity of 14C in the fungi screened was generally reduced in the presence of chitin indicating incorporation of C from chitin in fungal tissue. In the case of Phytophthora cinnamomi there was a significant increase in specific activity. These data indicate that patterns of carbon allocation in both chitinous and non-chitinous fungi change in the presence of chitin in the external medium.

WOCE AMS Radiocarbon I: Pacific Ocean Results (P6, P16 and P17)

AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14C since the time of the GEOSECS survey (8/73–6/74). Near-surface Δ14C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25–50‰, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25‰. Changes in the 250–750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75‰ since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50‰.

Transect Along 24°n Latitude of 14C in Dissolved Inorganic Carbon in the Subtropical North Atlantic Ocean

The distribution of bomb-produced 14C in the ocean provides a powerful constraint for circulation models of upper ocean mixing. We report 14C measurements from an east-west section of the main thermocline at 24°N latitude in the subtropical North Atlantic Ocean in summer 1992, and one profile from the Gulf of Mexico in 1993. Observed gradients reflect the transient invasion of bomb 14C into the thermocline via mixing along isopycnals from the poleward outcrop, with progressively more sluggish mixing at greater depths. A slight deepening of the profile is observed over the 20-yr period since the GEOSECS survey at one location where the comparison is possible.

Biosynthesis of ML-236C and the hypocholesterolemic agents compactin by Penicillium aurantiogriseum and lovastatin by Aspergillus terreus: determination of the origin of carbon, hydrogen and oxygen atoms by 13C NMR spectrometry and observation of unusual labelling of acetate-derived oxygens by 18O2

Sodium [1-13C, 2-2H3]-, [2-13C, 2-2H3]- and [1-13C, 18O2]-acetate are incorporated in separate experiments into ML-236C 2 and the hypocholesterolemic agent compactin 3 by cultures of Penicillium aurantiogriseum, and the regiochemical distribution of 2H, 13C and 18O is determined by 13C NMR spectrometry. In addition, sodium [1-13C, 18O2]acetate and 18O2 are incorporated into lovastatin (mevinolin) 4 by cultures of Aspergillus terreus to re-examine the origin of oxygen atoms. The results show that the main-chain oxygen atoms of 2–4 originate from acetate, and the C-8 oxygen atom of 3 and 4 is derived from molecular oxygen. However, detailed mass spectral analysis shows that significant amounts of aerobic oxygen can be incorporated at sites normally labelled by acetate oxygen, presumably through generation of [18O]acetate by ω-oxidation of fats. On the basis of labelling results, a mechanism is proposed to account for the formation of the bicyclic ring system in these compounds.

Photosynthesis and export during steady-state photosynthesis in bean leaves infected with the bacterium Xanthomonas campestris pv. phaseoli

Export and distribution of 14C assimilates during a period of steady-state photosynthesis were studied in attached leaflets of the second trifoliate of Phaseolus vulgaris for 2 weeks following inoculation with Xanthomonas campestris pv. phaseoli. The control was the leaflet opposite the inoculated leaflet at the same node. On either a chlorophyll or leaf area basis, photosynthesis and export declined over time after inoculation compared with control tissue. By 14 days after inoculation, photosynthesis and export were 25 and 50% lower, respectively, in the infected leaflets, even though only 7–10% of the total area measured was necrotic. The reduced synthesis of sucrose was sufficient to account for the reduced export from the infected leaflets. Starch synthesis was reduced to a greater extent in infected leaflets than was sucrose synthesis. In leaf disks infiltrated with mannose or xanthan, an extracelluar bacterial polysaccharide containing mannose, photosynthesis was reduced by 20% and the partitioning of 14C photoassimilates into sucrose was reduced but was slightly increased into starch. This is consistent with an effect of xanthan at least in vitro on the triose phosphate/phosphate exchange at the chloroplast envelope. Keywords: photosynthesis, export, xanthan, Phaseolus vulgaris, Xanthomonas campestris pv. phaseoli.

Forest- and pasture-derived carbon contributions to carbon stocks and microbial respiration of tropical pasture soils.

The clearing of tropical forest for pasture leads to important changes in soil organic carbon (C) stocks and cycling patterns. We used the naturally occurring distribution of13C in soil organic matter (SOM) to examine the roles of forest- and pasture-derived organic matter in the carbon balance in the soils of 3- to 81-year-old pastures created following deforestation in the western Brazilian Amazon Basin state of Rondônia. Different δ13C values of C3 forest-derived C (-28‰) and C4 pasture-derived C (-13‰) allowed determination of the origin of total soil C and soil respiration. The δ13C of total soil increased steadily across ecosystems from -27.8‰ in the forest to -15.8‰ in the 81-year-old pasture and indicated a replacement of forest-derived C with pasture-derived C. The δ13C of respired CO2 increased more rapidly from -26.5‰ in the forest to -17‰ in the 3- to 13-year-old pastures and indicated a faster shift in the origin of more labile SOM. In 3-year-old pasture, soil C derived from pasture grasses made up 69% of respired C but only 17% of total soil C in the top 10 cm. Soils of pastures 5 years old and older had higher total C stocks to 30 cm than the original forest. This occurred because pasture-derived C in soil organic matter increased more rapidly than forest-derived C was lost. The increase of pasture-derived C in soils of young pastures suggests that C inputs derived from pasture grasses play a critical role in development of soil C stocks in addition to fueling microbial respiration. Management practices that promote high grass production will likely result in greater inputs of grass-derived C to pasture soils and will be important for maintaining tropical pasture soil C stocks.

Simulation of bomb radiocarbon in two global ocean general circulation models

We perform simulations using global ocean general circulation models of the uptake by the ocean and the transport within the ocean of 14C from atmospheric nuclear bomb tests. The models used are (1) a modified version of the Geophysical Fluid Dynamics Laboratory modular ocean model (MOM) configured with simple horizontal subgrid‐scale mixing of tracers; (2) the same model using the Gent and McWilliams (1990) subgrid‐scale tracer transport parameterization, which includes isopycnal mixing; and (3) the ocean isopycnal (OPYC) model of J. M. Oberhuber, which uses isopycnal subgrid‐scale mixing of tracers within an isopycnal coordinate system. In addition, we perform another MOM simulation which illustrates the sensitivity of the results to the thickness of the surface mixed layer. To minimize differences in the results due to differences in forcings, all our simulations use the same formulations for surface fluxes of heat and fresh water and the same sea ice model. In addition, the numerical grids are all nearly the same. Comparison of our model results to revised estimates of oceanic bomb 14C from the Geochemical Ocean Sections Study observations and to annual mean temperature observations shows that our results are sensitive to the models' treatments of subgrid‐scale mixing of tracers and to the thickness of the surface mixed layer. Since the simulated vertical profiles of bomb 14C or temperature alone can be brought close to the observed profile by adjusting model parameters, we judge the realism of modeled vertical transport by how well vertical profiles of both bomb 14C and temperature can be simulated with one set of parameter values. By this standard, OPYC does better than our baseline configuration of MOM, which we attribute to the unrealistic treatment of subgrid‐scale mixing of tracers and of the surface mixed layer in this simulation. We illustrate this by showing that the use of the Gent and McWilliams tracer transport parameterization results in a markedly improved vertical temperature profile and almost no change in the bomb 14C distribution, compared with our baseline MOM simulation. Finally, we show that increasing the effective mixed layer thickness in MOM from 30 to 76 m results in a slightly more realistic vertical profile of bomb 14C but a slightly less realistic temperature profile.