Turgor regulation in species of Vaucheria (Xanthophyceae, Heterokontophyta) from habitats of contrasting salinities
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Turgor regulation is the process by which walled organisms alter their internal osmotic potential to adapt to osmotic changes in the environment. Much of what we know regarding turgor regulation and osmotic adjustment in algae is limited to the green characean and chlorphytan algae. This thesis is an investigation of turgor regulation in two species of the yellow-green xanthophycean alga, Vaucheria. The first part of this study involved the collection and identification of species of Vaucheria from contrasting habitats in New Zealand. Seven species of Vaucheria were identified based on the morphology of their reproductive structures. Two were described as new species (V. aestuarii and V. edaphica) and two others were reported for the first time from New Zealand (V. erythrospora and V. litorea). The genetic variation and phylogenetic position of these species were studied using phylogenetic analyses of rbcL sequences. Two of the species from contrasting habitats were selected for a comparative study on turgor regulation. These were Vaucheria erythrospora, isolated from an estuarine habitat, and Vaucheria repens, isolated from a freshwater habitat. Using a single cell pressure probe to directly measure turgor after hyperosmotic shock, V. erythrospora was found to recover turgor after a larger shock than V. repens. Threshold shock values for this ability were > 0.5 MPa for V. erythrospora and < 0.5 MPa for V. repens. Recovery was more rapid in V. erythrospora than V. repens after comparable shocks. Growth studies showed that V. erythrospora was able to grow and maintain turgor over a wider range of NaCl concentrations. These responses are thought to underlie the ability of V. erythrospora to survive in an estuarine habitat and restrict V. repens to freshwater. xi The final part of this study investigated the mechanisms underlying turgor regulation in V. erythrospora. Different responses were observed depending on whether NaCl or sorbitol was used to elicit the shock. Membrane potential (Em) measurements showed a rapid depolarization of the plasma membrane in response to a NaCl-induced hyperosmotic shock, followed by a slower repolarization, and recovery almost back to the resting Em. MIFE recordings indicate a net K+ efflux, a response that has been reported in other systems. While recordings of Na+ fluxes were not possible due to the high external Na+, these may account for the depolarisation and recovery of turgor as turgor recovery was inhibited by the non-selective cation channels (NSCCs) inhibitor Gd3+ and was dependant on the external Na+ concentration. An equivalent sorbitol-induced hyperosmotic shock hyperpolarized the Em, followed by depolarization and recovery back to the resting Em. Net flux recordings showed that both K+ and Na+ were taken up in response to a sorbitol shock when there was a low external Na+ concentration (1mM). K+ was possibly taken up through inward rectifying K+ channels activated by membrane hyperpolarization. The ability of V. erythrospora to rapidly regulate turgor by taking up ions during hyperosmotic stress is the possible reason for its survival in an estuarine habitat.