Productivity, metabolism and physiology of free-living chemoautotrophic Epsilonproteobacteria
Citable URI
https://hdl.handle.net/1912/8450Location
Crab Spa, East Pacific Rise9°N
DOI
10.1575/1912/8450Abstract
Chemoautotrophic ecosystems at deep-sea hydrothermal vents were discovered in 1977, but not until
1995 were free-living autotrophic Epsilonproteobacteria identified as important microbial community
members. Because the deep-sea is food-starved, the autotrophic metabolism of hydrothermal vent
Epsilonproteobacteria may be very important for deep-sea consumers. However, quantifying their
metabolic activities in situ has remained difficult, and biochemical mechanisms underlying their
autotrophic physiology are poorly described. To gain insight into environmental processes, an approach
was developed for incubations of microbes at in situ pressure and temperature (25 MPa,
24°C) with various combinations of electron donors/acceptors (H2, O2 and NO3 and 13HCO3)
as a tracer to track carbon fixation. During short (18-24 h) incubations of low-temperature vent
fluids from Crab Spa (9°N East Pacific Rise), the concentration of electron donors/acceptors and
cell numbers were monitored to quantify microbial processes. Measured rates were generally higher
than previous studies, and the stoichiometry of microbially-catalyzed redox reactions revealed new
insights into sulfur and nitrogen cycling. Single-cell, taxonomically-resolved tracer incorporation
showed Epsilonproteobacteria dominated carbon fixation, and their growth efficiency was calculated
based on electron acceptor consumption. Using these data, in situ primary productivity, microbial
standing stock, and average biomass residence time of the deep-sea vent subseafloor biosphere
were estimated. Finally, the population structures of the most abundant genera Sulfurimonas and
Thioreductor were shown to be strongly influenced by pO2 and temperature respectively, providing
a mechanism for niche differentiation in situ. To gain insights into the core biochemical reactions
underlying autotrophy in Epsilonprotebacteria, a theoretical metabolic model of Sulfurimonas denitrificans
was developed. Validated iteratively by comparing in silico yields with data from chemostat
experiments, the model generated hypotheses explaining critical, yet so far unresolved reactions supporting
chemoautotrophy in Epsilonproteobacteria. For example, it provides insight into how energy
is conserved during sulfur oxidation coupled to denitrification, how reverse electron transport produces
ferredoxin for carbon fixation, and why aerobic growth yields are only slightly higher compared
to denitrification. As a whole, this thesis provides important contributions towards understanding
core mechanisms of chemoautrophy, as well as the in situ productivity, physiology and ecology of
autotrophic Epsilonproteobacteria.
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 2016
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Suggested Citation
Thesis: McNichol, Jesse C., "Productivity, metabolism and physiology of free-living chemoautotrophic Epsilonproteobacteria", 2016-09, DOI:10.1575/1912/8450, https://hdl.handle.net/1912/8450Related items
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