Observation of Microstructure and Microbial Cells in Marine Sediments

Abstract

Marine subsurface sediments, which cover more than two thirds of the Earth’s surface, harbor a remarkable number of microbial cells that comprise over 10²⁹ cells on our planet. As little is known about deep subseafloor life, sedimentological and mineralogical characteristics of their geological habitats remain poorly documented despite of its vast significance in life and earth sciences. This was mainly because of the limitation of available methods and technologies on observing both fine sedimentary structures and small microorganisms. To address this issue, we developed a resin-embedding sample preparation method for fixing fine structure of soft sediments, and successfully visualized well preserved sedimentary microstructures with electron microscopy and microfocus X-ray computed tomography. We also developed a new quantitative method for microbial cells in marine sediment samples. Using a flow cytometry, our new protocol enable us to perform high-throughput and precise enumeration of microbial cells in deep and low-biomass sediment samples. These technological progresses have significantly expanded our analytical capabilities to understand microbial communities and their localizations in the deep subseafloor biosphere.