Distributions and activities of chemolithoautotrophic bacteria in aphotic waters of the North Pacific Subtropical Gyre

Abstract

The dark, interior waters of the world's oceans form some of the largest habitable space on Earth, yet knowledge of deep-sea biology and biogeochemistry remains rudimentary. In this thesis, I quantified and sequenced form II cbbM ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) genes in seawater samples from the surface through the bathypelagic (0-4000 m) of the North Pacific Subtropical Gyre (NPSG) to determine the distributions of chemoautotrophic bacteria in the ocean. In addition, I conducted two substrate addition experiments to determine whether an increase in a reduced sulfur substrate (thiosulfate) would change rates of carbon fixation (based on assimilation of 14C-bicarbonate), bacterial production (based on incorporation of 3H-leucine), or the abundance of cbbM genes. Polymerase chain reaction (PCR) amplification and sequencing of cbbM genes revealed a diverse assemblage of bacteria, including members of γ-and δ-proteobacteria, in the mesopelagic waters of the NPSG. Quantitative PCR (qPCR) analyses of cbbM genes revealed low abundances in the epipelagic transitioning to high abundances throughout the meso-and bathypelagic. Results from the thiosulfate addition experiments indicated that in one of the two experiments, elevated concentrations of thiosulfate stimulated rates of both 14C-bicarbonate assimilation and 3H-leucine incorporation, but resulted in no significant changes in cbbM abundances. Such results hint that mixotrophy may be a common strategy among chemoautotrophic bacteria dwelling in the energy-limited waters of the deep sea. Moreover, my results indicate that largely unexplored chemoautotrophic microbial metabolisms may play an important role in biogeochemical transformations of the dark ocean.