Study of metal- and metalloid-bearing nanoparticles in shallow hydrothermal systems

Abstract

Nanomaterials are structures with at least one of their dimensions in the nanometer scale, conventionally between 1 and 100 nm. The size confinement effect of materials can generate unique physical properties i.e. higher solubility or acute antimicrobial effects. Nanomaterials can naturally occur in the environment, for instance, hydrothermal systems have been described as a source of amorphous metal (Fe, Cu, Zn) bearing nanoparticles to the ocean. Elements with detrimental effects, like arsenic (As), are also enriched in hydrothermal fluids, yet, As bearing nanoparticles have not been considered. The aim of this doctoral dissertation is to identify and characterize As based nanomaterials present in a hydrothermal system (Chapter 3), evaluate environmental parameters controlling the formation and stability of the colloids (Chapter 4) and the possible toxic implications of the material to the marine microbial communities (Chapter 5).

The study site corresponds to the island of Milos (Greece), where the shallow hydrothermal systems with hot fluids rich in As represent an ideal environment to understand the formation, stability and toxicity of As based colloidal material under a gradient of parameters (temperature, pH or organic matter: OM). Hydrothermal fluids, porewater and seawater collected (Paleochori and Spathi bay, Milos) showed a significant percentage of As in the size fraction between 200 and 20 nm, suggesting the presence of As bearing nanoparticles. Further characterizations, revealed the presence of nanospheres with a size diameter distribution between 250 and 50 nm, an elemental composition rich in As, S and O; and the absence of a crystal phase, which classifies it as an amorphous material. The particles where observed in samples with the highest As content, elevated temperatures and the lowest pH values, suggesting a critical role of environmental parameters in the formation and stability of the colloids. Therefore, synthesis of As colloidal particles was evaluated under environmental hydrothermal conditions. The results indicated precipitation of amorphous As and S rich nanospheres as a natural phenomenon occurring in marine environments at low pH. During As and S precipitation, temperature and OM (thiol rich additives; cysteine and glutathione) can dictate the morphology, size and As content of the particles. At high temperatures and in the absence of additives, the morphology, size and As content of the material highly resemble the particles described in the hydrothermal system off Milos. The results indicate the formation of colloids being favored within the first 24 hours of incubation at high temperatures and with a distinctive S and As ratio, confirming hydrothermal systems as a source of As bearing nanoparticles to the ocean. However, stability of the material was further evaluated, establishing a total dissolution of the particles at high temperature after 72 hours. No dissolution was observed when temperatures of incubation where below 75°C, suggesting stabilization of the material once mixed with seawater.

The toxicity of synthetic As bearing nanoparticles was evaluated in marine bacterial cultures (Shewanella oneidensis MR1). A decrease in cell density and growth rate after cultivation with the colloids was observed. Furthermore, the bacteriostatic effect, was characterized by variations of intact polar lipids, specifically, changes in abundance of aminolipids and aminophospholipids. When compared to soluble forms of As, the bacteriostatic and the lipid response found within treatments of dissolved As(III) species was very similar to those observed with the nanoparticles. The outcomes suggest the release of dissolved As(III) species from the colloidal structure as a mechanism of toxicity.

In conclusion, this thesis reveals the existence of amorphous arsenic bearing nanoparticles in a hydrothermal system. The formation and stability of the colloids strictly depends on environmental parameters like pH, temperature and organic matter (additives rich in -SH groups), and finally, this study suggests a bacteriostatic effect of the nanoparticles, characterized by changes in aminolipids and aminophosholipid abundance. The results and discussions highlight the importance of a deeper understanding in colloidal chemistry of harmful elements, like As, present in hydrothermal fluids (e.g., Hg, Sb). A greater comprehension of nanomaterial behavior in the ocean will further contribute in the conservation of the marine ecosystems.