UC Santa Cruz

Large brown algae in the class Phaeophyceae (Heterokontophyta) form the structural and energetic foundation of temperate and subtropical nearshore marine forests of high productivity and ecological diversity. This dissertation examines the carbon uptake and transport physiology of large brown algae with a particular focus on the plastic or adaptive responses of these physiological traits to their abiotic environment. Chapter 1 takes an anatomical and modeling approach to investigate the structure and function of photosynthate transport networks (analogous to phloem) in diverse members of the Laminariales. To evaluate the existence of scaling and optimization of the kelp vascular system, a model of optimized transport anatomy was developed and tested with a diverse suite of kelp species in the Laminariales. Results revealed a surprising lack of universal scaling in the kelps and the presence of optimized transport anatomy in the giant kelp (Macrocystis pyrifera) only. Chapter 2 focuses on the dynamics of carbon uptake in M. pyrifera, which can acquire both carbon dioxide and bicarbonate as carbon substrates for photosynthesis. To evaluate whether the proportion of carbon dioxide and bicarbonate utilized by M. pyrifera is constant or a variable function of their fluctuating environment, oxygen evolution experiments were carried out on entire blades from several targeted populations in the Monterey Bay. Results indicated that M. pyrifera possesses a plastic carbon uptake physiology in which proportionally more bicarbonate is used in high irradiance and high flow conditions, but that local populations have not yet developed fixed genetic differences. Chapter 3 investigates the mechanism and patterns of carbon stable isotope discrimination in M. pyrifera. Results of a dual field and laboratory incubation approach indicate that 13C discrimination patterns are determined by a complex interaction of light intensity, dissolved inorganic carbon limitation, and fractionation occurring during transport of polysaccharides. Overall, this dissertation informs patterns and mechanisms of carbon uptake and transport in kelps, and highlights the many ways in which kelps may impact and structure their ecosystems.