No description available.

No description available.

The lucinid bivalve Codakia orbiculata, whose gills contain sulphur-oxidizing symbiotic bacteria, occurs in high densities (500/m2) in the rhizosphere of shallow-water Thalassia testudinum sediments in Bermuda. Both sulphide and thiosulphate stimulate aerobic respiration in the isolated bacterial symbionts of C. orbiculata. Sulphide and thiosulphate stimulate anaerobic ^^COa fixation in bacteria isolated from sulphur-starved bivalves. Interstitial water sulphide concentrations in the bivalves' habitat reach 300 pM, and sulphate-reduction rates are high, but thiosulphate concentrations are low (0.66-32.27 pM) . Thiosulphate supplied to the symbionts in vivo must be produced by sulphide oxidation, possibly by the host bivalve. Isolated symbionts also respire aerobically and fix i * C02 in the absence of exogenous reduced sulphur, suggesting utilization of intracellular elemental sulphur stores. Codakia orbiculata symbiotic bacteria are able to respire nitrate. Nitrate concentrations in the interstitial water of C. orbiculata habitat can reach 36 pM. Thiosulphate stimulates nitrate respiration in the intact symbiosis, incubated in oxic and anoxic conditions, and in anoxic incubations of isolated symbionts. Intracellular elemental sulphur is also used by the •bacteria as a substrate in nitrate respiration. Nitrate respiration in the absence of exogenous nitrate suggests that the sjnnbionts may have a limited ability to store nitrate. There is no direct evidence that sulphide stimulates nitrate respiration in either the isolated symbionts or the intact symbiosis, incubated in anoxic conditions. Nitrite respiration in the symbionts is stimulated by sulphide (only), however. Because nitrate respiration was measured by nitrite accumulation, complete denitrification would explain the apparent failure of sulphide to stimulate nitrate respiration. High nitrate respiration rates in the intact symbiosis, incubated with sulphide in oxic conditions, may be in response to thiosulphate, supplied to the bacterial symbionts after host oxidation of sulphide. Nitrite respiration in the intact symbiosis, even when incubated in oxic conditions, demonstrates that the symbionts have access to some sulphide in vivo, however, and that host sulphide-oxidation may not be- 100% efficient. Nitrate and nitrite respiration in the intact symbiosis, even when incubated in oxic conditions, suggests that the bacteria may be exposed to low oxygen levels in vivo and may require the ability to utilize an alternate electron acceptor. Like some free - living bacteria , Codakia orbiculata bacterial symbionts may co-respire, or alternately respire , oxygen and nitrate . The Thalassia testudinum sediments in Bermuda may be ideal for this bacteria-bivalve symbiosis due to the availability of oxygen, nitrate and sulphide.

Typescript. / Thesis (Ph. D.)--University of Hawaii, 1966. / Bibliography: leaves [180]-199. / xxiv, 297 l illus. (part mounted), tables

Thesis (Ph. D.)--University of Cambridge, 1987. / Typescript.

A study of factors affecting gas exchange at the gills of rainbow trout during hypoxia was made. Techniques for implanting cannulae in the buccal and opercular chambers, the dorsal aorta, and the ventral aorta are described. The responses of afferent and efferent blood and water pressures, Po₂, PCo₂, pH, and the hematocrit of efferent blood were made while the unanaes thetized free swimming trout was in a sealed respirometer. The fish were allowed to consume the oxygen in the respirometer to produce conditions of hypoxia. Determination of oxygen uptake and a knowledge of the solubility of oxygen in blood and water allowed indirect estimation of ventilation volume, cardiac output and stroke volume of the heart. With this knowledge, inferences on the function of circulatory and respiratory mechanisms during hypoxia were drawn. The rainbow trout possesses a number of homeostatic mechanisms which augment oxygen uptake during hypoxia. It was found that the trout could maintain a fairly uniform oxygen uptake in environmental oxygen tensions as low as 30 to 50 mm Hg„ The major homeostatic responses to hypoxia affecting oxygen uptake were: an increase in ventilation volume, an increase in the functional capacity of the blood, and an apparent vascular shunting of blood closer to the surface of the respiratory epithelium. No significant increase in cardiac output was observed. Since there was a pronounced bradycardia with hypoxia the stroke volume was apparently increasing. The role of the bradycardia, which has usually been associated with a reduction in cardiac output, is not clear. Possible reasons and functions of the bradycardia are discussed. / Science, Faculty of / Zoology, Department of / Graduate

Unlike the respiratory organs of airbreathing vertebrates where gas exchange is perfusion limited, gas transfer across fish gills is diffusion limited. Fish can therefore enhance gas exchange by increasing the gill diffusing capacity. Previous suggestions, indicate that fish may achieve this by altering the pattern of gill blood flow to increase the area of gill perfused and to reduce the blood-water diffusion barrier. To verify these suggestions an investigation of the patterns of gill blood flow, their regulation and their significance in gas exchange in the ling cod, Ophiodori elongatus, was undertaken. The circulatory arrangement in the gill filament of the ling cod consists of an arterio-arterial respiratory network and an arterip-venous veriolymphatic system. All cardiac output passes through the respiratory exchange sites, the lamellae. Blood flow through lamellae is described by sheet blood flow equations, where flow is proportional to the vascular sheet thickness (h). The lamellar vascular sheet is very compliant .and h increases with transmural pressure (ΔP[sub lam]). It is predicted if ΔP[sub lam] and flow are raised, then intralamellar shunting of blood flow and a reduction of the blood-water barrier will result, thereby increasing the gill diffusing capacity; Not all lamellae appear to be equally perfused under certain in vivo conditions. Furthermore, if resting perfusion conditions are simulated, only 67% of the more proximal lamellae are perfused. Thus the total gill area is not utilised at rest. To account for this situation it is proposed that the critical closing pressures associated with distal lamellar units are greater than those for the proximal lamellae. The afferent arterioles were determined to be the major resistance site in the gills and they therefore control flow to lamellae. Elevations in flow and lamellar input pressure will reduce the likelihood of' critical closing and more lamellae will be perfused. Lamellar recruitment increases the gill diffusing capacity. , The demonstrated changes in flow patterns to and within lamellae are effected by elevated flow and input pressures (or ΔP[sub Lam]). Changes in cardiac performance and in the pressure profile of the gills alter flow and pressure. Cardiac performance in ling cod is influenced by intrinsic, cholinergic and adrenergic controls which alter stroke volume and heart rate. The pressure profile of the gills can be altered by cholihergically or adrenergically mediated changes in vessel dimensions. The gill outflow arteries vasoconstrict in localised regions with cholinergic .stimulation and thereby increasing gill resistance (Rg) and lamellar' input pressures. Afferent vessels apparently dilate with-g-adrenergic stimulation and thereby lower Rg. Cardiovascular changes are associated with conditions of reduced oxygen availability (hypoxia) and of increased oxygen demand (struggling) in ling cod. The cardiovascular changes are such that they alter the pattern of gill blood flow and increase the gill diffusing capacity. Increased oxygen uptake, cardiac output and gill ventilation are associated with hypoxia and struggling. The quantitative increases in cardiac output per se associated with these conditions does not fully account for the observed increase in oxygen uptake. It is concluded that change in gill diffusing capacity through alterations in gill blood flow patterns are important in enhancing oxygen uptake across the gills. / Science, Faculty of / Zoology, Department of / Unknown

published_or_final_version / Zoology / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 68-94).

Thesis (M. Phil.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 172-216).