UC San Diego

This work is divided into two parts : (I) cell surface binding, uptake, and storage of iron in brown algae, and (II) boron uptake, localization, and speciation in brown algae. Iron is an essential element for all living organisms due to its ubiquitous role in redox and other enzymes, especially in the context of respiration and photosynthesis. Although the iron uptake and storage mechanisms of terrestrial/higher plants have been well- studied, the corresponding systems in marine algae have received far less attention. Chapter I focuses on these mechanisms in two brown seaweeds : Ectocarpus siliculosus and Macrocystis pyrifera. Through the course of experiments, it was found that a significant amount of iron was bound extracellularly. While this phenomenon is widely recognized and has prompted the development of experimental protocols to eliminate its contribution to iron uptake studies, its potential biological significance as a concentrated iron source for marine algae is only now being recognized. In this study, using an interdisciplinary array of techniques, we explore the nature of the extensive and powerful iron binding on the surface of both laboratory and environmental samples of Ectocarpus and Macrocystis. We propose that the surface binding properties of Ectocarpus and Macrocystis allow it to function as a quasi-biological metal ion "buffer" facilitating iron uptake under the widely varying external iron concentrations found in coastal marine environments. Short-term radiolabeled iron uptake studies verified that iron is taken up in a time- and concentration-dependent manner consistent with an active transport process. Using a combination of Mössbauer and XAS spectroscopies, a mineral core has been identified as the likely storage form. Genomic and microscopic analyses indicate that this iron store is more likely vacuolar-based than of protein origin. In chapter II, I describe boron uptake, speciation, localization and possible biological function in Macrocystis and Ectocarpus. In contrast to the generally boron-poor terrestrial environment, the concentration of boron in the marine environment is relatively high (0.4 mM) and while there has been extensive interest in its use as a surrogate of pH in paleoclimate studies in the context of climate change-related questions, the relatively depth independent, and the generally non- nutrient-like concentration profile of this element have led to boron being neglected as a potentially biologically relevant element in the ocean. Results herein indicate that boron is taken up by a facilitated diffusion mechanism against a considerable concentration gradient. Furthermore, in both Ectocarpus and Macrocystis some boron is most likely bound to the cell wall constituent alginate and the photoassimilate mannitol located in sieve cells. These results indicate possible biological roles for boron as an osmoprotectant and/or metabolite transporter