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The influence of microorganisms on the atmosphere and tools for the study of transacylase domains


Specific biochemical influences on sea spray aerosol (SSA) are not well characterized and are vital to understanding the complex and variable chemical composition and atmospheric properties of SSA. Several studies reported in this thesis explore how microorganisms affect SSA both by examining their metabolic products in seawater and how they transfer from the seawater to aerosol, as well as the impacts of airborne microbes on SSA properties.

To investigate the influence of microbial metabolism on aerosol composition, direct measurements of aerosols revealed a correlation between marine bacteria in seawater and the production of gas-phase even-chain (C:2, C:4) alkyl nitrates (RONO2). Alkyl nitrates are a large contributor to the production of tropospheric ozone and prior to these studies generally thought to originate from anthropogenic sources and produced through uv-dependent mechanisms. The results show that bacterial metabolism can contribute to the production of climate warming ozone and demonstrate an important biochemical influence on atmospheric processes. In separate studies, we explored the role of enzymatic processing of lipid metabolites by lipase in seawater and the resulting changes to aerosol morphology and composition. We observed that lipase driven digestion of lipids in seawater vastly changes the composition of SSA in simple triacylglycerol and diatom lysate systems, identifying a possible mechanism for organismal control of aerosolization.

The structural organization within SSA, taxon-specificity of air-sea transfer, and ice nucleation (IN) characteristics of whole bacteria, viruses, and vesicles was explored in several studies. One study elucidated the detailed composition of seawater (bulk), SSML, and SSA using cryogenic transmission electron microscopy. The unique technique allowed the preservation and detailed examination of soft biological particles, facilitating observations of complex structures of diatoms, bacteria, viruses, and vesicles, their hydration state, and their orientation within particles. SSML derived vesicles had a more multi-lamellar structure in contrast to bulk and aerosol samples. This was the first demonstration of vesicles within SSA, indicating a possible role of these structures in climate processes. Another study examined bacterial and viral transfer across the air-sea interface using metagenomics, revealing marine bacteria aerosolize more efficiently than viruses and taxon-specific properties governing selective transfer of viruses and bacteria delineated by class and order. Several species of bacteria and viruses were consistently enriched in aerosols, and hydrophobic cell- and virus- surfaces positively influenced aerosolization. The results provide a genomic framework to elucidate aerosolization mechanisms and like the lipase studies, identify additional microbial controls of SSA composition and properties. Lastly, we examined the IN properties of halotolerant bacteria and fungi isolated from rain and SSA samples collected from Scripps Pier in La Jolla, CA. Most rain isolates possess moderate IN activity, and reveal that a possible unidentified source of marine IN activity contributing to precipitation. The studies together have yielded important information of how microbial biochemistry and aerosolization influences SSA composition and properties. This work will serve as a foundation to examine the biochemical mechanisms of SSA composition, SSA properties, climate relevant biomarkers, and aerosolization mechanisms that will inform climate models, civil engineering, and public health.

Acyltransferase (AT) domains are serine hydrolases and vital enzymes in the biosynthesis of fatty acids, polyketides and hybrid polyketide nonribosomal peptides. Experiments utilizing broad activity-based serine hydrolase probes to label cells surface proteases and lipases of marine microbial membranes, provoked development of specific probes for the study of AT domains. To date, inhibitors and probes specific to AT domains have not been reported in literature. AT specific probes have proven challenging to develop due to their characteristically small and restrictive active sites. This dissertation details the design, synthesis, and testing of five activity-based AT probes. Comparisons of these five probes by binding efficacy, specificity, optimization, and competition in FAS and PKS systems provides a basis for future development of crosslinking probes to examine carrier protein-AT interactions and AT inhibitors. These small molecules will permit the study of specific AT-carrier protein interactions in a variety of systems and will provide possible novel therapeutics against drug-resistant bacteria.

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