Fish naturally experience fluctuating levels of O2 and CO2 in their environment. To cope with the deleterious effects of lowered O2 (hypoxia) or elevated CO2 (hypercapnia), fish exhibit an array of cardiorespiratory adjustments aimed at preserving homeostasis. One of the most significant of these responses is reflex hyperventilation. In zebrafish (Danio rerio), hyperventilation during hypoxia or hypercapnia is thought to be initiated by the activation of chemoreceptor cells, termed neuroepithelial cells (NECs) which detect changes in ambient levels of O2 or CO2. The NECs of larval zebrafish are found throughout the integument and recent studies have shown that these NECs likely mediate the ventilatory responses to hypoxia and the cardiac responses to hypercapnia. However, no study has yet examined the ventilatory response of larval zebrafish to hypercapnia and regardless of developmental stage, the signalling pathways involved in CO2 sensing remain unclear. In the mouse, a background potassium channel (TASK-2) was shown to contribute to the sensitivity of chemoreceptor cells to CO2. Zebrafish have two specific TASK-2 channel paralogs encoded by kcnk5a and kcnk5b. The purpose of this thesis was to determine whether TASK-2 channels are expressed in NECs of larval zebrafish and whether they are involved in CO2 sensing. Immunohistochemical approaches were used to visualize TASK-2 protein (encoded by kcnk5a) within NECs of larvae and adult gill. TASK-2 protein was observed on NECs in both larvae and adult gill. Exposure of larvae to hypercapnia caused an increase in cardiac and breathing frequencies; these responses were blunted in fish experiencing either TASK-2 and/or TASK-2b knockdown. The results of these experiments suggest that TASK-2 has a role in activating NECs thus eliciting cardiorespiratory responses, when larvae are exposed to hypercapnia.

Carbon dioxide sequestration is one of many mitigation tools available to help reduce carbon dioxide emissions while other disposal/repurposing methods are being investigated. Geologic sequestration is the most stable option for long-term storage of carbon dioxide (CO2), with significant CO2 trapping occurring through mineralization within the first 20-50 years. A fiber optic based monitoring system has been proposed to provide real time concentrations of CO2 at various points throughout the geologic formation. The proposed sensor is sensitive to the refractive index (RI) of substances in direct contact with the sensing component. As RI is a measurement of light propagating through a bulk medium relative to light propagating through a vacuum, the extraction of the effects of any specific component of that medium to the RI remains very difficult. Therefore, a requirement for a selective barrier to be able to prevent confounding substances from being in contact with the sensor and specifically isolate CO2 is necessary. As such a method to evaluate the performance of the selective element of the sensor was investigated. Polybenzimidazole (PBI) and VTEC polyimide (PI) 1388 are high performance polymers with good selectivity for CO2 used in high temperature gas separations. These polymers were spin coated onto a glass substrate and cured to form ultra-thin (>10 μm) membranes for gas separation. At a range of pressures (0.14 –0.41 MPa) and a set temperature of 24.2±0.8 °C, intrinsic permeabilities to CO2 and nitrogen (N2) were investigated as they are the gases of highest prevalence in underground aquifers. Preliminary RI testing for proof of concept has yielded promising results when the sensor is exposed exclusively to CO2 or N2. However, the use of both PBI and VTEC PI in these trials resulted in CO2 selectivities of 0.72 to 0.87 and 0.33 to 0.63 respectively, for corresponding feed pressures of 0.14 to 0.41 MPa. This indicates that both of the polymers are more selective for N2 and should not be used in CO2 sensing applications as confounding gas permeants, specifically N2, will interfere with the sensing element. / Graduate / 0428 / 0495 / 0542 / ben.t.davies@gmail.com