Bystrow’s Paradox – gills,fossils, and the fish‐to‐tetrapod transition

Schoch, R.R. and Witzmann, F. 2011. Bystrow’s Paradox – gills, fossils, and the fish‐to‐tetrapod transition. —Acta Zoologica (Stockholm) 92 : 251–265. The issue of which breathing mechanism was used by the earliest tetrapods is still unsolved. Recent discoveries of stem tetrapods suggest the presence of internal gills and fish‐like underwater breathing. The same osteological features were used by Bystrow to infer a salamander‐like breathing through external gills in temnospondyl amphibians. This apparent contradiction – here called Bystrow’s Paradox – is resolved by reviewing the primary fossil evidence and the anatomy of the two gill types in extant taxa. Rather unexpectedly, we find that internal gills were present in a range of early crown tetrapods (temnospondyls), based on the anatomy of gill lamellae and location of branchial arteries on the ventral side of gill arch elements (ceratobranchials). Although it remains to be clarified which components are homologous in external and internal gills, both gill types are likely to have been present in Palaeozoic tetrapods – internal gills in aquatic adults of some taxa, and external gills in the larvae of these taxa and in larvae of numerous forms with terrestrial adults, which resorbed the external gills after the larval phase. Future developmental studies will hopefully clarify which mechanistic pathways are involved in gill formation and how these might have evolved.     

Metabolism,gas exchange,and acid‐base balance of giant salamanders

The giant salamanders are aquatic and paedomorphic urodeles including the genera Andrias and Cryptobranchus (Cryptobranchidae), Amphiuma (Amphiumidae), Siren (Sirenidae), and Necturus (Proteidae, of which only N. maculosus is considered ‘a giant'). Species in the genera Cryptobranchus and Necturus are considered aquatic salamanders well adapted for breathing water, poorly adapted for breathing air, and with limited abilities to compensate acid‐base disturbances. As such, they are water‐breathing animals with a somewhat fish‐like respiratory and acid‐base physiology, whose habitat selection is limited to waters that do not typically become hypoxic or hypercarbic (although this assertion has been questioned for N. maculosus). Siren and Amphiuma species, by contrast, are dependent upon air‐breathing, have excellent lungs, inefficient (Siren) or no (Amphiuma) gills, and are obligate air‐breathers with an acid‐base status more similar to that of terrestrial tetrapods. As such, they can be considered to be air‐breathing animals that live in water. Their response to the aquatic hypercarbia that they often encounter is to maintain intracellular pH (pH_i) and abandon extracellular pH regulation, a process that has been referred to as preferential pH_i regulation. The acid‐base status of some present‐day tropical air‐breathing fishes, and of Siren and Amphiuma, suggests that the acid‐base transition from a low PCO₂‐low [] system typical of water‐breathing fishes to the high PCO₂‐high [] systems of terrestrial tetrapods may have been completed before emergence onto land, and likely occurred in habitats that were typically both hypoxic and hypercarbic.     

Functional genomics and sexual differentiation in amphibians

To succeed on land rather than in water, crabs require a suite of physiological and morphological changes, and ultimately the ability to reproduce without access open water. Some species have modified gills to assist in gas exchange but accessory gas exchange organs, usually lungs, occur in many species. In accomplished air-breathers the lung becomes larger and more vascularised with pulmonary vessels directing oxygenated haemolymph to the heart. The relative abundance of O₂ in air promotes relative hypoventilation and thus an internal hypercapnia to drive CO₂ excretion. Land crabs have a dual circulation via either lungs or gills and shunting between the two may depend on respiratory media or exercise state. During their breeding migration on Christmas Island Gecarcoidea natalis maintained arterial Po₂ by branchial O₂ uptake, while pulmonary O₂ pressure was reduced; partly because exercise doubled relative haemolymph flow through the gills. Related species rely on elevated haemocyanin concentration and affinity for O₂ to assist uptake but this compromises unloading at the tissues and thus the aerobic scope of tissues. Aquatic crabs exchange salt and ammonia with water via the gills but in land crabs this is not possible. Birgus latro has adopted uricotelism but other species excrete ammonia in either the urine or as gas. Land crabs minimise urinary salt loss using a filtration-reabsorption system analogous to the kidney. Urine is redirected across the gills where salt reabsorption occurs in systems under hormonal control, although in G. natalis this is stimulatory and in B. latro inhibitory. While crabs occupy a range of habitats from aquatic to terrestrial, these species do not comprise a physiological continuum but across the crab taxa individual species possess appropriate and specific physiological features to survive in their individual habitat.     

Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates

Regardless of the mode of reproduction, three things must occur at birth or hatching in amniote vertebrates: initiation of breathing, pulmonary fluid elimination and reabsorption, and adequate perfusion of pulmonary circulation. Although data on these events are few, there appears to be no fundamental difference in them that can be associated with the oviparity to viviparity transition. There are, however, differences in the timing of these events in oviparous and viviparous amniotes. The transition to neonatal respiration tends to be very quick in viviparous species because the vascular support for oxygen uptake provided by the mother is rapidly disassociated from the mechanism for uptake by the embryo. By contrast, hatching often is a slow process, taking 24 h or more in some species, as chorioallantoic blood flow slowly gives way to clearing of the lungs and pulmonary gas exchange. Little is known of the mechanisms of pulmonary fluid elimination and reabsorption or lung inflation in reptiles, but the cellular structures and surfactant systems are similar in all amniote vertebrates. Nevertheless, there are differences, particularly of timing and maturation of various systems, but there has been no exploration of the functional (or phylogenetic) bases of these differences. Perfusion of the neonatal pulmonary system to support respiration in reptiles remains to be investigated. In mammals and birds, closure of the ductus arteriosus is important, but the role played by the ductus arterioisus in reptilian hatching or birth is not known.     

Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high-resolution x-ray microcomputed tomography,and the origin of the amniote crown group

The origin of amniotes was a key event in vertebrate evolution, enabling tetrapods to break their ties with water and invade terrestrial environments. Two pivotal clades of early tetrapods, the diadectomorphs and the seymouriamorphs, have played an unsurpassed role in debates about the ancestry of amniotes for over a century, but their skeletal morphology has provided conflicting evidence for their affinities. Using high-resolution X-ray microcomputed tomography, we reveal the three-dimensional architecture of the well preserved endosseous labyrinth of the inner ear in representative species belonging to both groups. Data from the inner ear are coded in a new cladistic matrix of stem and primitive crown amniotes. Both maximum parsimony and Bayesian inference analyses retrieve seymouriamorphs as derived non-crown amniotes and diadectomorphs as sister group to synapsids. If confirmed, this sister group relationship invites re-examination of character polarity near the roots of the crown amniote radiation. Major changes in the endosseous labyrinth and adjacent braincase regions are mapped across the transition from non-amniote to amniote tetrapods and include: a ventral shift of the cochlear recess relative to the vestibule and the semicircular canals; cochlear recess (primitively housed exclusively within the opisthotic) accommodated within both the prootic and the opisthotic; development of a distinct fossa subarcuata. The inner ear of seymouriamorphs foreshadows conditions of more derived groups, whereas that of diadectomorphs shows a mosaic of plesiomorphic and apomorphic traits, some of which are unambiguously amniote-like, including a distinct and pyramid-like cochlear recess.     

Early tetrapod relationships revisited

In an attempt to investigate differences between the most widely discussed hypotheses of early tetrapod relationships, we assembled a new data matrix including 90 taxa coded for 319 cranial and postcranial characters. We have incorporated, where possible, original observations of numerous taxa spread throughout the major tetrapod clades. A stem‐based (total‐group) definition of Tetrapoda is preferred over apomorphy‐ and node‐based (crown‐group) definitions. This definition is operational, since it is based on a formal character analysis. A PAUP* search using a recently implemented version of the parsimony ratchet method yields 64 shortest trees. Differences between these trees concern: (1) the internal relationships of aistopods, the three selected species of which form a trichotomy; (2) the internal relationships of embolomeres, with Archeria crassidisca and Pholiderpeton scutigerum collapsed in a trichotomy with a clade formed by Anthracosaurus russelli and Pholiderpeton attheyi; (3) the internal relationships of derived dissorophoids, with four amphibamid species forming an unresolved node with a clade consisting of micromelerpetontids and branchiosaurids and a clade consisting of albanerpetontids plus basal crown‐group lissamphibians; (4) the position of albenerpetontids and Eocaecilia micropoda, which form an unresolved node with a trichotomy subtending Karaurus sharovi, Valdotriton gracilis and Triadobatrachus massinoti;(5) the branching pattern of derived diplocaulid nectrideans, with Batrachiderpeton reticulatum and Diceratosaurus brevirostris collapsed in a trichotomy with a clade formed by Diplocaulus magnicornis and Diploceraspis burkei. The results of the original parsimony run ‐ as well as those retrieved from several other treatments of the data set (e.g. exclusion of postcranial and lower jaw data;character reweighting; reverse weighting) ‐ indicate a deep split of early tetrapods between lissamphibian‐ and amniote‐related taxa. Colosteids, Crassigyrinus, Whatcheeria and baphetids are progressively more crownward stemtetrapods. Caerorhachis, embolomeres, gephyrostegids, Solenodonsaurus and seymouriamorphs are progressively more crownward stem‐amniotes. Eucritta is basal to temnospondyls, with crown‐lissamphibians nested within dissorophoids. Westlothiana is basal to Lepospondyli, but evidence for the monophyletic status of the latter is weak. Westlothiana and Lepospondyli form the sister group to diadectomorphs and crown‐group amniotes. Tuditanomorph and microbrachomorph microsaurs are successively more closely related to a clade including proximodistally: (1) lysorophids; (2) Acherontiscus as sister taxon to adelospondyls; (3) scincosaurids plus diplocaulids; (4) urocordylids plus aïstopods. A data set employing cranial characters only places microsaurs on the amniote stem, but forces remaining lepospondyls to appear as sister group to colosteids on the tetrapod stem in several trees. This arrangement is not significantly worse than the tree topology obtained from the analysis of the complete data set. The pattern of sister group relationships in the crownward part of the temnospondyl‐lissamphibian tree re‐emphasizes the important role of dissorophoids in the lissamphibian origin debate. However, no specific dissorophoid can be identiffed as the immediate sister taxon to crown‐group lissamphibians. The branching sequence of various stem‐group amniotes reveals a coherent set of internested character‐state changes related to the acquisition of progressively more terrestrial habits in several Permo‐Carboniferous forms.     

Cyclic model of respiration applied to asymmetrical ventilation and periodic breathing

A model taking into account the cyclic character of respiration in humans is developed using two classical simplifications: CO₂ is the only respiratory gas involved; and respiration is regulated only by a CO₂ linear controller. The model is used to investigate two important clinical aspects of respiratory disease: asymmetrical ventilation and periodic breathing. We show that asymmetry in ventilation significantly influences the time course of the CO₂ partial pressure in the expired alveolar air at the mouth and the elimination of CO₂ through the lungs. Furthermore, the CO₂ controller delay plays a major role in periodic breathing.     

Measurement of the lung and skin excretion of CO<Subscript>2</Subscript> during anesthesia

A mass spectrometer with capillary and membrane interfaces was used during anesthesia to deliver the gas mixture from the breathing circuit of the inhalation anesthesia machine (IAM) into the ionic source of an analyzer and to measure the concentration of CO₂ released from the skin in real time. The extent of the stress response during surgery correlated with the time course of changes in the concentrations of CO₂ released from the lungs and skin. The CO₂ concentration and the BIS index of changes in EEG frequency spectrum were measured simultaneously during intravenous total propofol–fentanyl anesthesia. The BIS index and the concentrations of CO₂ released from the lungs and skin were found to correlate with the most traumatic steps of the surgical procedure.     

Regulation of ventilation in the caiman (Caiman latirostris): effects of inspired CO2 on pulmonary and upper airway chemoreceptors

In order to study the relative roles of receptors in the upper airways, lungs and systemic circulation in modulating the ventilatory response of caiman (Caiman latirostris) to inhaled CO₂, gas mixtures of varying concentrations of CO₂ were administered to animals breathing through an intact respiratory system, via a tracheal cannula by-passing the upper airways (before and after vagotomy), or via a cannula delivering gas to the upper airways alone. While increasing levels of hypercarbia led to a progressive increase in tidal volume in animals with intact respiratory systems (Series I), breathing frequency did not change until the CO₂ level reached 7%, at which time it decreased. Despite this, at the higher levels of hypercarbia, the net effect was a large and progressive increase in total ventilation. There were no associated changes in heart rate or arterial blood pressure. On return to air, there was an immediate change in breathing pattern; breathing frequency increased above air-breathing values, roughly to the same maximum level regardless of the level of CO₂ the animal had been previously breathing, and tidal volume returned rapidly toward resting (baseline) values. Total ventilation slowly returned to air breathing values. Administration of CO₂ via different routes indicated that inhaled CO₂ acted at both upper airway and pulmonary CO₂-sensitive receptors to modify breathing pattern without inhibiting breathing overall. Our data suggest that in caiman, high levels of inspired CO₂ promote slow, deep breathing. This will decrease dead-space ventilation and may reduce stratification in the saccular portions of the lung.     

Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates.

Regardless of the mode of reproduction, three things must occur at birth or hatching in amniote vertebrates: initiation of breathing, pulmonary fluid elimination and reabsorption, and adequate perfusion of pulmonary circulation. Although data on these events are few, there appears to be no fundamental difference in them that can be associated with the oviparity to viviparity transition. There are, however, differences in the timing of these events in oviparous and viviparous amniotes. The transition to neonatal respiration tends to be very quick in viviparous species because the vascular support for oxygen uptake provided by the mother is rapidly disassociated from the mechanism for uptake by the embryo. By contrast, hatching often is a slow process, taking 24 h or more in some species, as chorioallantoic blood flow slowly gives way to clearing of the lungs and pulmonary gas exchange. Little is known of the mechanisms of pulmonary fluid elimination and reabsorption or lung inflation in reptiles, but the cellular structures and surfactant systems are similar in all amniote vertebrates. Nevertheless, there are differences, particularly of timing and maturation of various systems, but there has been no exploration of the functional (or phylogenetic) bases of these differences. Perfusion of the neonatal pulmonary system to support respiration in reptiles remains to be investigated. In mammals and birds, closure of the ductus arteriosus is important, but the role played by the ductus arterioisus in reptilian hatching or birth is not known.     

The ventilatory response to environmental hypercarbia in the South American rattlesnake,<Emphasis Type="Italic"> Crotalus durissus</Emphasis>

To study the effects of environmental hypercarbia on ventilation in snakes, particularly the anomalous hyperpnea that is seen when CO₂ is removed from inspired gas mixtures (post-hypercapnic hyperpnea), gas mixtures of varying concentrations of CO₂ were administered to South American rattlesnakes, Crotalus durissus, breathing through an intact respiratory system or via a tracheal cannula by-passing the upper airways. Exposure to environmental hypercarbia at increasing levels, up to 7% CO₂, produced a progressive decrease in breathing frequency and increase in tidal volume. The net result was that total ventilation increased modestly, up to 5% CO₂ and then declined slightly on 7% CO₂. On return to breathing air there was an immediate but transient increase in breathing frequency and a further increase in tidal volume that produced a marked overshoot in ventilation. The magnitude of this post-hypercapnic hyperpnea was proportional to the level of previously inspired CO₂. Administration of CO₂ to the lungs alone produced effects that were identical to administration to both lungs and upper airways and this effect was removed by vagotomy. Administration of CO₂ to the upper airways alone was without effect. Systemic injection of boluses of CO₂-rich blood produced an immediate increase in both breathing frequency and tidal volume. These data indicate that the post-hypercapnic hyperpnea resulted from the removal of inhibitory inputs from pulmonary receptors and suggest that while the ventilatory response to environmental hypercarbia in this species is a result of conflicting inputs from different receptor groups, this does not include input from upper airway receptors.Communicated by G. Heldmaier     

PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID‐TOLERANT,FREE‐LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)1

The processes of CO₂ acquisition were characterized for the acid‐tolerant, free‐living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO₃^? uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO₂ formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO₂ uptake and O₂ evolution by membrane‐inlet mass spectrometry demonstrated that air‐grown cells reduced the CO₂ concentration in the medium to an equilibrium concentration of 15 μM, but AZA‐treated cells caused a drop in extracellular CO₂ concentration to a compensation concentration of 27 μM at pH 8.0. CO₂‐pulsing experiments with cells in the light indicated that the cells do not actively take up CO₂. An internal pool of unfixed inorganic carbon was not detected at the CO₂ compensation concentration, probably because of the lack of active CO₂ uptake, but was detectable at times before compensation point was reached. These results indicate that this free‐living Coccomyxa possesses a CO₂‐concentrating mechanism (CCM) due to an active bicarbonate‐uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.     

Gross and fine anatomy of the respiratory vasculature of the mudskipper,Periophthalmodon schlosseri (Gobiidae: Oxudercinae)

To illustrate vascular modification accompanying transition from aquatic to amphibious life in gobies, we investigated the respiratory vasculatures of the gills and the bucco‐opercular cavities in one of the most terrestrially‐adapted mudskippers, Periophthalmodon schlosseri, using the corrosion casting technique. The vascular system of Pn. schlosseri retains the typical fish configuration with a serial connection of the gills and the systemic circuits, suggesting a lack of separation of O₂‐poor systemic venous blood and O₂‐rich effluent blood from the air‐breathing surfaces. The gills appear to play a limited role in gas exchange, as evidenced from the sparsely‐spaced short filaments and the modification of secondary lamellar vasculature into five to eight parallel channels that are larger than red blood cell size, unlike the extensive sinusoidal system seen in purely water‐breathing fishes. In contrast, the epithelia of the bucco‐opercular chamber, branchial arches, and leading edge of the filaments are extensively laden with capillaries having a short (<10 μm) diffusion distance, which strongly demonstrate the principal respiratory function of these surfaces. These capillaries form spiral coils of three to five turns as they approach the epithelial surface. The respiratory capillaries of the bucco‐opercular chamber are supplied by efferent blood from the gills and drained by the systemic venous pathway. We also compared the degree of capillarization in the bucco‐opercular epithelia of Pn. schlosseri with that of the three related intertidal‐burrowing gobies (aquatic, non‐air‐breathing Acanthogobius hasta; aquatic, facultative air‐breathing Odontamblyopus lacepedii; amphibious air‐breathing Periophthalmus modestus) through histological analysis. The comparison revealed a clear trend of wider distribution of denser capillary networks in these epithelia with increasing reliance on air breathing, consistent with the highest aerial respiratory capacity of Pn. schlosseri among the four species. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Inference of the Protokaryotypes of Amniotes and Tetrapods and the Evolutionary Processes of Microchromosomes from Comparative Gene Mapping

Comparative genome analysis of non-avian reptiles and amphibians provides important clues about the process of genome evolution in tetrapods. However, there is still only limited information available on the genome structures of these organisms. Consequently, the protokaryotypes of amniotes and tetrapods and the evolutionary processes of microchromosomes in tetrapods remain poorly understood. We constructed chromosome maps of functional genes for the Chinese soft-shelled turtle (Pelodiscus sinensis), the Siamese crocodile (Crocodylus siamensis), and the Western clawed frog (Xenopus tropicalis) and compared them with genome and/or chromosome maps of other tetrapod species (salamander, lizard, snake, chicken, and human). This is the first report on the protokaryotypes of amniotes and tetrapods and the evolutionary processes of microchromosomes inferred from comparative genomic analysis of vertebrates, which cover all major non-avian reptilian taxa (Squamata, Crocodilia, Testudines). The eight largest macrochromosomes of the turtle and chicken were equivalent, and 11 linkage groups had also remained intact in the crocodile. Linkage groups of the chicken macrochromosomes were also highly conserved in X. tropicalis, two squamates, and the salamander, but not in human. Chicken microchromosomal linkages were conserved in the squamates, which have fewer microchromosomes than chicken, and also in Xenopus and the salamander, which both lack microchromosomes; in the latter, the chicken microchromosomal segments have been integrated into macrochromosomes. Our present findings open up the possibility that the ancestral amniotes and tetrapods had at least 10 large genetic linkage groups and many microchromosomes, which corresponded to the chicken macro- and microchromosomes, respectively. The turtle and chicken might retain the microchromosomes of the amniote protokaryotype almost intact. The decrease in number and/or disappearance of microchromosomes by repeated chromosomal fusions probably occurred independently in the amphibian, squamate, crocodilian, and mammalian lineages.     

Development and evolution of the tetrapod skull–neck boundary

The origin and evolution of the vertebrate skull have been topics of intense study for more than two centuries. Whereas early theories of skull origin, such as the influential vertebral theory, have been largely refuted with respect to the anterior (pre‐otic) region of the skull, the posterior (post‐otic) region is known to be derived from the anteriormost paraxial segments, i.e. the somites. Here we review the morphology and development of the occiput in both living and extinct tetrapods, taking into account revised knowledge of skull development by augmenting historical accounts with recent data. When occipital composition is evaluated relative to its position along the neural axis, and specifically to the hypoglossal nerve complex, much of the apparent interspecific variation in the location of the skull–neck boundary stabilizes in a phylogenetically informative way. Based on this criterion, three distinct conditions are identified in (i) frogs, (ii) salamanders and caecilians, and (iii) amniotes. The position of the posteriormost occipital segment relative to the hypoglossal nerve is key to understanding the evolution of the posterior limit of the skull. By using cranial foramina as osteological proxies of the hypoglossal nerve, a survey of fossil taxa reveals the amniote condition to be present at the base of Tetrapoda. This result challenges traditional theories of cranial evolution, which posit translocation of the occiput to a more posterior location in amniotes relative to lissamphibians (frogs, salamanders, caecilians), and instead supports the largely overlooked hypothesis that the reduced occiput in lissamphibians is secondarily derived. Recent advances in our understanding of the genetic basis of axial patterning and its regulation in amniotes support the hypothesis that the lissamphibian occipital form may have arisen as the product of a homeotic shift in segment fate from an amniote‐like condition.     

Proposed habitats of early tetrapods: gills, kidneys, and the water-land transition

Recent finds of early tetrapods have established that the most primitive form, Acanthostega, retained internal gills and other fish-like features; this has led to the conclusion that it was a primarily aquatic animal. Other Late Devonian tetrapods, such as lchthyostega and Tulerpeton, provide no evidence of internal gills, but have also been interpreted as inhabiting an aquatic environment. The probable aquatic habits of a diversity of Devonian tetrapods has led to the suggestion that the entire early tetrapod radiation may have been an aquatic one, with terrestriality having evolved in later forms. However, consideration of the physiology of living amphibious vertebrates suggests that this scenario is unlikely. The use of the gills for the excretion of carbon dioxide and ammonia appears to be a fundamental feature of all primarily aquatic vertebrates. No living fish loses its internal gills, even if it excretes a significant portion of its nitrogenous waste as urea via the kidney in the water. Gills are simply too valuable to be lost by an aquatic animal, even in those air-breathing fishes that no longer use the gills for oxygen uptake. We suggest that the apparent loss of the gills in tetrapods more derived than Acanthostega signals their descent from a more terrestrial phase in tetrapod evolution, following the primary assumption by the kidney of the excretion of nitrogenous wastes. Without this new role of the kidney, loss of the gills would have been impossible. With this new kidney role, loss of the gills may have been advantageous in reducing desiccation on land.     

The concentration and efflux of tree stem CO2 and the role of xylem sap flow

The accurate assessment of actual tree stem respiration and its relation with temperature plays a considerable role in investigating the forest carbon cycle. An increasing number of research reports have indicated that tree stem respiration determined with the commonlyapplied chamber gas exchange measuring system does not follow expectations regarding temperature relationships. This method is based on the nowadays widely-accepted theory that the respired CO₂ in a tree stem would all diffuse outward into the atmosphere. However, it neglects partial CO₂ that is dissolved in the xylem sap and is carried away by the transpirational stream. Scientists have started to realize that the respired CO₂ measured with the chamber gas exchange method is only a portion of the total stem respiration (CO₂ efflux), while the other portion, which is sometimes very substantial in quantity (thought to occupy maybe 15%–75% of the total stem respiration), is transported to the upper part of the stem and to the canopy by sap flow. This suggests that the CO₂ produced by respiration is re-allocated within the stem. Accordingly, the change in CO₂ efflux could be reflected in the rates of sap flow in addition to its dependence on temperature. Proper methods and instruments are required to quantify the internal and external CO₂ fluxes in the trunk and their interaction with related environmental factors.     

SOURCE OF INORGANIC CARBON FOR PHOTOSYNTHESIS IN TWO MARINE DINOFLAGELLATES1

Inorganic carbon uptake was investigated in two marine dinoflagellates, Amphidinium carterae Hulburt and Heterocapsa oceanica Stein. Mass spectrometric and potentiometric assays indicated that both species lacked external carbonic anhydrase (CA). The presence of internal CA was demonstrated by potentiometric assay and by the inhibition of photosynthesis upon the addition of 500 μM ethoxyzolamide a membrane‐permeable inhibitor of CA. The capacity for bicarbonate transport was investigated by comparing the calculated rate of spontaneous CO₂ formation at pH 8.2 and 25°C with the rate of photosynthesis after the addition of 100 μM NaHCO₃. Both species appeared to have a very limited capacity for direct bicarbonate uptake. Monitoring of CO₂ and O₂ fluxes in both species by mass spectrometry demonstrated a rapid uptake of CO₂ on illumination, to concentrations below the CO₂ equilibrium concentration, indicating an effective selective uptake of CO₂. This dependence of photosynthesis on free CO₂ alone suggests that these species are CO₂ limited in their natural environment because the CO₂ concentration of seawater is very low.     

Respiratory control in bullfrogs: Cutaneous versus pulmonary response to selective CO2 exposure

Summary Bullfrogs,Rana catesbeiana, were exposed to high ambientP _Co2 (15–22 Torr) at 20°C through either their skin or their lungs. The objective was to evaluate the effectiveness of the gas exchange surface not exposed to high CO₂ to excrete the excess CO₂ load. Frogs exposed to high CO₂ through their skin increased pulmonary ventilation and controlled arterialP _CO2, close to the normal value. In contrast, frogs that breathed CO₂ did not increase their skin CO₂ conductance, and arterialP _CO2, increased significantly. These results support the concept that the lungs are the primary effector mechanism for respiratory control in the bullforg while the skin is a passive, poorly controlled avenue for CO₂ loss. They also reveal that the lungs of this anuran are affective eliminators of CO₂.     

树干皮层光合作用--生理生态功能和测定方法

大部分植物的树干(枝条)等部位含有能进行光合作用的绿色组织,树皮叶绿素含量最高可达750 mg/m2。这些绿色组织能够再固定树干内部的CO2(来源于自身组织呼吸或者木质部液流运输),使树干向大气排放的CO2量减少60%—90%皮层光合作用是树干生理活动的重要组成部分,其与树干呼吸和液流速率之间均有密切关系,对植物的碳平衡有重要作用。概述了皮层光合作用的生理生态功能;介绍了皮层光合作用测定和计算方法;讨论了皮层光合作用研究存在的问题;通过加入皮层光合作用的测量修正质量平衡法,以减少树干呼吸测定的不确定性。建议综合运用稳定碳同位素示踪、CO2和O2微传感器、树干液流技术等,准确地区分树干内部CO2的来源及比例,分析各个组分与影响因素的关系。同时,在微观上揭示皮层光合作用的基因组调控功能,在宏观上探讨尺度扩展、模型模拟,并与涡度协方差技术和遥感技术相融合以提高区域尺度估算的精度。