Microbe-Mineral Interactions: Mercury Homogenization, Bacterial Colloid Surface Acidity, and Protocol Transfeminism.

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Microorganisms, humans, and minerals are in constant interaction. Whether within human biology or in the extra-human extracellular environment. Environmental engineers are interested historically in minerals that have particular consequence for environmental health. This work seeks to explore an interstitialality between environmental engineering and our subject matter as well as the people who perform that work. This interstitialality is represented in a variety of ways in this research and will largely take place in three case studies that are defined by the follow questions: (1) What are the relative rates of homogenization of environmentally relevant mercury (Hg) in a freshwater wetland? (2) What do bacterial nanoparticles contribute to the pH buffering of the extracellular environment as compared to their parent cells? (3) Can we incorporate lessons from queer and feminist science studies into environmental engineering and what does a transfeminist protocol look like in environmental engineering? The first being uh an application of a range of Hg species into wetland simulated mesocosms freshwater, here Hg is transformed and homogenized in the interstitial spaces between cells, the mineral portions of soil, the water column and finally other forms of biota. Much of these transformations are mediated bacterially, specifically the transformation of inorganic species Hg into monomethylmercury (MeHg). These wetland mesocosms were dosed with isotopically labelled Hg, here forward referred to as endmembers. We performed experiments in simulated freshwater wetland mesocosms that were dosed with four isotopically labeled mercury forms: two dissolved (Hg2+ and Hg-humic acid) and two particulate (nano-HgS and Hg adsorbed to FeS). Over the course of one year, we monitored the four Hg isotope endmembers for their relative distribution between surface water, sediment, and fish in the wetland mesocosms, partitioning between soluble and particulate forms, reactivity by sequential extractions, and uptake flux in diffusive gradient in thin-film (DGT) passive samplers. We observed that the four isotope spikes were relatively similar in concentration (ca. 3000 ng/L) immediately after spike addition. At 1 to 3 months after dosing, Hg concentrations were 1 to 50 ng/L and were greater for the initially dissolved isotope endmembers than the initially particulate endmembers. In contrast, the Hg isotope endmembers in surface sediments were similar in concentrations for all time periods after spike addition. However, the uptake fluxes of the Hg in DGT samplers, which is a measure of the reactive fraction of souble Hg, were generally greater for initially dissolved Hg endmembers and lower for the initially particulate endmembers. At one year post-dosing, the DGT-uptake fluxes were converging toward similar values between the Hg isotope endmembers. However, the relative distribution of isotope endmembers were still significantly different in both the water column and sediment (p<0.01 according to one-way ANOVA analysis). For MeHg concentrations in surface sediment and fish, the relative contributions by each endmember were significantly different at all sampling time points, including the final one year time point. Altogether, these results provide insights to the timescales for distribution for different Hg species that enter a wetland ecosystem. While these inputs attain homogeneity in concentration in the primary ecosystem storage compartment (i.e., sediments) within weeks after addition, these input pools remain differentiated for more than one year in terms of reactivity for passive samplers, methylmercury concentration and bioaccumulation. The second case study examines another interstitial space focused on the cell surface and extracellular environment with respect to proton buffering. The acid-base characteristics of bacterial surfaces are relevant for key processes such as cation exchange and pH buffering at the cell envelope. Microorganisms also produce nanoparticulate soft material such as extracellular vesicles (EVs) and strands of flagellin. Given their high surface area to volume ratio, these biogenic particles may contribute additional surfaces for proton buffering and cation exchange for the external cellular environment. In this work, biocolloid suspensions were isolated from two ubiquitous environmental bacteria, the gram-negative Pseudomonas fluorescens and the gram-positive Bacillus subtilis. These resulting colloidal suspensions were evaluated via alkalimetric titrations and surface proton modeling of the data to derive surface acidity constants and site concentrations. Biocolloids isolated from P. fluorescens cultures contributed 2.18 (+/- 0.65) µmole/g cell surface acidity. This proton exchange density corresponded to 1 surface acid site for every 113 surface sites at the cell envelope of the whole cell. In contrast, biocolloids isolated from B. subtilis contributed surface acidity of 3.89 (+/- 0.82) µmole/g cell, which corresponded to a ratio of 1 site to every 64 cell surface sites. While lower than two orders of magnitude in total surface acidity, we note that these particles have measurable contributions to the extracellular environmental proton buffering and cation exchange. Additionally, even between these two species, surface active biocolloid production can vary by a factor of 2. Surface acidity and cation exchange have profound impacts on bacterial and human health, sequestering toxic metals as one example. Given these results, further study into the general and more specific surface functionality is needed to constrain the possible functions these nanoparticles could perform in pristine and engineered environments. The final exploration in this chapter lends itself to a more theoretical lens, while still residing in a highly interstitial space. The technical study of extracellular vesicles from bacteria prompted further questions about how scholars relate to their organisms. This prompted a discursive intervention in environmental engineering and science studies. I propose that EVs can be a productive object for theoretical exploration using frameworks critical to feminist science studies and queer of color critique. I sought to disrupt masculinist interactions with extracellular vesicles in biotechnological context and to prompt the science studies community to explore the theory making capacity of microbial (largely bacterial and fungal) extracellular vesicles. Feminist and anticolonial studies have shown that protocol can be a productive starting point for materially enacting theoretical goals around ethics. I propose a transfeminist protocol for the isolation of biocolloids from Pseudomonas fluorescens and Bacillus subtilis. Reflecting my own lived reality while culturing these bacteria, this protocol assisted me in knowing my organisms and their capacity for surface acidity as well as knowing myself and the role of an environmental engineer. I hope for this work to inspire deeper interactions and questioning. These three case studies show how microbe-mineral-human interactions profoundly shape material and theoretical conditions. In tandem, the research here shows several paths for these interactions. One shows how different species of Hg can be introduced into freshwater wetland environments through anthropogenic activities. Those environments then themselves react and transform that Hg and resulting partitioning and fate of that Hg has long lasting impacts. The second shows the contribution of biogenic soft nanoparticles like EVs and strands of flagellin to the extracellular pH buffering capacity. This capacity is influenced by the surfaces of the bacteria that produce them, as well as mineral surfaces and dissolved molecules. These materials have capacity for buffering protons and sorbing metals. Finally, I offer a perspective to pull environmental engineers closer to the organisms that we study. The materiality of the organisms that I studied show me that we are already very intimately connected. If we acknowledge this connection, point to it, and take agency while allowing our organisms to express their own—we have much to gain.





Wadle, Austin Jacob (2023). Microbe-Mineral Interactions: Mercury Homogenization, Bacterial Colloid Surface Acidity, and Protocol Transfeminism. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/29134.


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