Microbial Transformations of Sulfur: Environmental and (Paleo) Ecological Implications

Citation

Osorio Rodríguez, Daniela (2023) Microbial Transformations of Sulfur: Environmental and (Paleo) Ecological Implications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9bn5-z794. https://resolver.caltech.edu/CaltechTHESIS:06092023-201353115

Abstract

This thesis is centered around the role that sulfur plays in the cycling of carbon and in microbial energetics. In the oceans, sulfate is the most important electron acceptor for the remineralization of organic matter after oxygen has been depleted, and sulfate reduction is particularly relevant in coastal environments and in marine and freshwater sediments. The opposite process, reduced sulfur oxidation, allows autotrophic microorganisms to fix carbon in environments where oxygen is scarce. Organic sulfur is also a relevant component of the sulfur cycle, since sulfur is the sixth most abundant element in biomass, it can protect organic matter from degradation, and it is composed of hundreds of molecules that are produced mainly by microorganisms, with potentially relevant ecological roles.

This work has been divided into two parts. In the first one, we attempt to expand our understanding on different aspects of the sulfur cycle. In Chapter 2, published in Limnology and Oceanography, we focus on dimethylsulfoniopropionate (DMSP), the most abundant organic sulfur compound in the oceans with roles of UV, cryo, and osmoprotection, and involved in the formation of sulfate aerosols. We propose a framework to differentiate between the microbial degradation pathways of DMSP based on the sulfur isotope fractionations imprinted by each one of them. In Chapter 3, we perform a survey of sulfate, sulfide, and reduced sulfur intermediates, as well as redox-sensitive elements, in porewaters of a ~40 cm core from the San Clemente Basin (California) and three 1.2-2 m cores near Cocos Ridge (Costa Rica). We correlate these concentrations with the sediment microbial community composition to unveil the specifics of organic matter and sulfur cycling at these localities. In Chapter 4, we explore the utility of sulfur isotope fractionations to characterize different pathways involved in microbial sulfur oxidation (MSO), and examine the role of nutrient limitation and growth rates on the magnitude of the fractionation.

In the second part of this thesis, we aim at understanding biomineralization by consortia between anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB), which comprise more than 90% of the microbial biomass in deep sea sediments around hydrocarbon seeps. In Chapter 5 (in review at Proceedings of the National Academy of Sciences) we establish that modern ANME-SRB aggregates precipitate amorphous silica in undersaturated solutions in sediments and carbonates, often in the form of rims, which pinpoints to a potentially new microbial biomineralization mechanism. In Chapter 6, we posit the use of this proxy, together with distinctive spectral and isotopic signals, to find potential microfossils of ANME-SRB aggregates in the rock record of the Earth and other planetary bodies where methane seepage has occurred throughout geologic time. This suite of tools is used in conjunction to identify ANME-SRB aggregates in the Tepee Buttes (Colorado, 75 Mya) seep carbonates.

Thesis Files