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Physiological mechanisms facilitating morphological plasticity across hydrodynamic gradients in kelps Coleman, Liam James Marshall

Abstract

Some kelp species exhibit morphological plasticity across gradients of hydrodynamic forcing. Such kelps tend to grow narrow, flat blades in fast flow and broad, undulate blades in slow flow. This plasticity is an adaptive phenomenon that allows the kelps that show it to continuously reduce drag while enhancing productivity. While the functional consequences of this phenomenon have been relatively well studied, the developmental mechanisms that underlie it are poorly understood. The primary goal of this thesis is to improve our understanding of the developmental processes facilitating morphological plasticity across hydrodynamic gradients in kelps. I first conducted several experiments where I applied tension to blade tissue of the kelp Nereocystis luetkeana in various ways to better characterize the growth response to mechanical stimulation normally imposed by drag. The results of these experiments suggest that plasticity in kelps is probably regulated at the scale of individually stimulated cells and that changes in blade morphology are likely brought about through changes in the direction of cell growth and/or division (Ch. 2). I then examined the effects of auxin on Nereocystis blade growth and morphology and considered whether auxin could play a role in mediating kelp plasticity. I found that auxin can have morphogenic effects in Nereocystis blade tissue that are remarkably similar to the effects of tension (Ch. 3). Next, I tested whether culturing the kelp Macrocystis pyrifera in reduced concentrations of Ca²⁺ could inhibit the growth response to mechanical loading, which would suggest that Ca²⁺ signaling might play a role in regulating plasticity. No evidence arose that reducing the ambient Ca²⁺ concentration could inhibit plasticity (Ch. 4). Furthermore, all Ca²⁺ reductions greater than 50% proved lethal for kelps (Ch. 4). Finally, I investigated how prevalent phenotypic plasticity across hydrodynamic gradients is in various algal groups and considered whether mechanisms of flow perception could limit the evolution of such plasticity. I found that (1) researchers examining intraspecific variation across flow gradients in seaweeds usually have not tested for plasticity and (2) verified plasticity has been documented more frequently in brown algae with intercalary growth than it has in other macroalgae (Ch. 5).

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