Indiana University-Purdue University Indianapolis (IUPUI) / The reaction between elemental sulfur and sulfide is a lynchpin in the biotic and abiotic cycling of sulfur. This dissertation is focused on the reactivity of elemental sulfur nanoparticles (S8weimarn, S8raffo) among other forms of elemental sulfur (S8aq, S8aq-surfactant, α-S8), and how the variation of their surface area, character and coatings reflect on the analytical, physical-chemical and geochemical processes involving sulfur cycling. A comprehensive electrochemical investigation utilizing mercury-surface electrodes showed that elemental sulfur compounds are represented by three main voltammetric signals, corresponding to potentials at -1.2V, -0.8V, and -0.6V in the absence of organics at circumneutral pH. Dissolved S8aq-surfactant signals can be found from -0.3V up to -1.0V, depending on the surfactant in the system. Variations in current response resulted from differences in electron transfer efficiency among the forms of S8, due to their molecular structural variability. Based on this observation a new reaction pathway between S8 and Hg-surface electrodes is proposed, involving an amalgam-forming intermediate step. The kinetics of the nucleophilic dissolution of S8nano by sulfide, forming polysulfides, were investigated under varying surface area, surface character and presence or absence of surfactant coatings on S8nano. Hydrophobic S8weimarn and hydrophilic S8raffo show kinetic rate laws of 𝑟𝑆8𝑤𝑒𝑖𝑚𝑎𝑟𝑛 = 10−11.33 (𝑒 −700.65 𝑅𝑇 ) (Molar(S8)/second/dm-1) and𝑟𝑆8𝑟𝑎𝑓𝑓𝑜 = 10−4.11 𝑖−0.35 (𝑒 −615.77 𝑅𝑇 ) (Molar(S8)/second), respectively. The presence of surfactant molecules can influence the reaction pathways by dissolving S8nano and releasing S8aqsurfactant, evolving the rate-limiting step as a function of the degree of the solubilization of S8nano. The reaction rate of S8biological can be compared with those of S8raffo and S8weimarn in circumneutral pH values and T=50oC, making the forms of S8nano successful abiotic analogue models of microbially produced S8biological. Field observations and geochemical kinetic modeling in the geothermal features of Yellowstone indicate that the nucleophilic dissolution reaction appears to be a key abiotic pathway for the cycling of sulfur species and the enhancement of elemental sulfur bioavailability. Furthermore, in situ and ex situ voltammetry in the same geothermal waters disclosed chaotic variability in chemical gradients of sulfide (observed over small temporal and spatial scales) which can be considered as an ecological stressor capable of influencing single cell physiology and microbial community adaptation.

Perçues jusqu’au XIXe siècle comme des masses gélatineuses aux propriétés urticantes, les méduses ont été classées dans le groupe des zoophytes, organismes intermédiaires entre les animaux et les végétaux. Les savants s’en sont alors servis pour démontrer la continuité entre les êtres vivants végétaux et animaux et elles sont devenues des arguments essentiels dans l’établissement de l’échelle des êtres. A partir du XIXe siècle, on découvre, en particulier grâce à la microscopie, la complexité de leur anatomie, ce qui permet enfin de les caractériser de manière positive. Au milieu du XIXe siècle, on met en évidence les différentes phases de leur cycle de vie et l’organisation en colonie de certaines méduses. Les méduses deviennent alors l’un des principaux centres d’intérêt des zoologistes et des embryologistes qui cherchent à trouver la signification évolutive des différentes étapes de leur développement. Le terme de méduse apparaît alors pour désigner un de leurs stades de vie. En parallèle de cette histoire conceptuelle, les différentes étapes de cette classification ont été marquées par des difficultés d’étude qui ont fait que les méduses constituaient un des rares groupes non définis par des caractères positifs à la fin du XVIIIe siècle. En effet, si les expéditions scientifiques ont permis de récupérer de nombreux échantillons, bien vite s’est posé le problème de leur conservation. Face aux difficultés de maintenir leur morphologie et leurs couleurs après leur prélèvement, les images naturalistes de méduses sont donc devenues des objets d’étude essentiels dans la connaissance et la classification de ces organismes. Ces images donnent un reflet de l’évolution conceptuelle et technique, qui a accompagnée la classification des méduses / Perceived until the 19th century as gelatinous masses with stinging properties, jellyfish were classified in the group of zoophytes, intermediate organisms between animals and plants. Scientists then used it to demonstrate continuity between plant and animal living beings and became essential arguments in establishing the scale of beings. From the nineteenth century, we discover, particularly through microscopy, the complexity of their anatomy, which finally allows to characterize them positively. In the mid-nineteenth century, we highlight the different phases of their life cycle and the colony organization of some jellyfish. Jellyfish become one of the main interests of zoologists and embryologists who seek to find the evolutionary significance of the different stages of their development. The term jellyfish then appears to designate one of their life stages. In parallel with this conceptual history, the various stages of this classification were marked by study difficulties which made jellyfish one of the few groups not defined by positive characters at the end of the eighteenth century. Indeed, if the scientific expeditions allowed to recover many samples, quickly the problem of their conservation was posed. Faced with the difficulties of maintaining their morphology and their colors after their collection, the naturalistic images of jellyfish have thus become essential objects of study in the knowledge and classification of these organisms. These images give a reflection of the conceptual and technical evolution, which accompanied the classification of jellyfish