Copper is an essential element for primary productivity in the ocean. However, at concentrations observed surface seawater, copper likely would be toxic to most microbiota if not buffered by a pool of mostly unidentified ligands. These compounds of presumed biological origin can reduce toxicity either by being directly exuded to chelate extracellular bioavailable copper, or by mitigating the toxic effect of copper inside the cell via complexation and subsequent export. This dissertation examines the role of thiols, a presumed component of this pool of ligands, in maintaining copper homeostasis in marine algae. Stability constants of Cu(I) complexes with cysteine, glutathione, Arg-Cys, and Gln-Cys were measured using a new analytical method that employs fluorescent ion indicators and minimized oxidation of Cu(I). Computational methods were used to support reported constants. Speciation models predict that while thiolate ligands can significantly buffer intracellular copper, at concentrations typically observed in surface seawater, their effect on lowering copper is negligible. Uptake experiments in the presence of thiols confirmed speciation predictions. Thiols were only partially effective in reducing Cu(I) uptake, while the Cu(II) ligands EDTA and GSSG were able to effectively block uptake. However, cysteine enhances copper uptake in copper-limited Emiliania huxleyi cells and can increase the bioavailability of copper bound to EDTA. The use of stable isotopes to measure direct uptake of copper in the absence of a ligand, revealed a constitutive copper efflux mechanism in the coccolithophore E. huxleyi. This mechanism allows E. huxleyi to maintain relatively low cellular levels under exposure to a wide range of high copper concentrations, while the diatom Thalassiosira pseudonana only appears to employ export at higher concentrations. Cu-limitation minimizes efflux. These results suggest that E. huxleyi maintains low levels of cellular copper through an intracellular buffering and efflux mechanism. Such a mechanism would give E. huxleyi an ecological advantage in water prone to high copper, such as coastal and those experiencing an upwelling event. Results from a survey of marine algae and estuarine field measurements also demonstrate that the thiols Arg-Cys and Gln-Cys may be biologically and geographically widespread.