Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy
at the
Massachusetts Institute of Technology
and the
Woods Hole Oceanographic Institution
September 2018
Description:
Life is ubiquitous in the environment and an important mediator of Earth’s carbon cycle,
but quantifying the contribution of microbial biomass and its metabolic fluxes is difficult,
especially in spatially and temporally-remote environments. Microbes leave behind an often
scarce, unidentifiable, or nonspecific record on geologic timescales. This thesis develops
and employs novel geochemical and genetic approaches to illuminate diagnostic signals
of microbial metabolisms. Field studies, laboratory cultures, and computational models
explain how methanogens produce unique nonequilibrium methane clumped isotopologue
(13CH3D ) signals that do not correspond to growth temperature. Instead, Δ13CH3D values
may be driven by enzymatic reactions common to all methanogens, the C-H bond
inherited from substrate precursors including acetate and methanol, isotope exchange, or
environmental processes such as methane oxidation. The phylogenetic relationship between
substrate-specific methyl-corrinoid proteins provides insight into the evolutionary history
of methylotrophic methanogenesis. The distribution of corrinoid proteins in methanogens
and related bacteria suggests that these substrate-specific proteins evolved via a complex
history of horizontal gene transfer (HGT), gene duplication, and loss. Furthermore, this
work identifies a previously unrecognized HGT involving chitinases (ChiC/D) distributed
between fungi and bacteria (∼650 Ma). This HGT is used to tether fossil-calibrated ages
from within fungi to bacterial lineages. Molecular clock analyses show that multiple clades
of bacteria likely acquired chitinase homologs via HGT during the late Neoproterozoic into
the early Paleozoic. These results also show that, following these HGT events, recipient terrestrial
bacterial clades diversified ∼400-500 Ma, consistent with established timescales of
arthropod and plant terrestrialization. Divergence time estimates for bacterial lineages are
broadly consistent with the dispersal of chitinase genes throughout the microbial world in
direct response to the evolution and expansion of detrital-chitin producing groups including
arthropods. These chitinases may aid in dating microbial lineages over geologic time and
provide insight into an ecological shift from marine to terrestrial systems in the Proterozoic
and Phanerozoic eons. Taken together, this thesis may be used to improve assessments of microbial
activity in remote environments, and to enhance our understanding of the evolution
of Earth’s carbon cycle.
Description:
Supported by
the National Science Foundation (NSF), the NSF Graduate Research Fellowship Program,
the MIT Energy Initiative and its partnership with Shell, the Neil and Anna Rasmussen
Foundation Fund, and the Grayce B. Kerr Fellowship. This research and its dissemination
was supported by funds from the Deep Carbon Observatory, NASA Astrobiology Institute,
WHOI Academic Programs Office, and the MIT Graduate Student Council.
Keywords:
Microorganisms
;
Microbial metabolism
;
Carbon cycle
;
Phylogeny
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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