Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr Norman PA Huner

Abstract

Photoautotrophs are predisposed to maintain a balance between light energy absorption with the capacity to consume this energy through metabolism. An imbalance in energy flow may be a consequence of increased light intensity and is sensed as modulation of excitation pressure (EP). Chlorella vulgaris acclimated to continuous high EP exhibits a yellow-green phenotype characterized by reduced chlorophyll content and high chlorophyll a/b ratio with reduced light-harvesting complex abundance relative to the dark green phenotype of low EP-acclimated cultures. Previous studies on acclimation to EP in green algae have been conducted under constant growth light. To determine the role of EP in the regulation of photoacclimation in response to photoperiod cells were exposed to sudden and sustained changes in the light environment. Using variable light exposure, it becomes evident that EP is not the sole regulator of photoacclimation. Rather photoacclimation is dependent on direct light availability at the level of chlorophyll biosynthesis and is photoperiod-dependent. While photoperiod influences the cell cycle, C. vulgaris exhibits minimal plasticity in metabolic sinks capacity. I propose that perception of light in response to growth under a variable photoperiod may be involved in limiting the extent of the acclimation response. Phenotypic plasticity and photoacclimation appears to be dependent on a network of intracellular sensors and signal transduction pathways integrating direct perception of light and perception of light as an energy source through modulation of the redox state of the photosynthetic apparatus balanced against the capacity to consume the products of photosynthesis through metabolism and growth.

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