Phylogenetic and physiological diversity of subseafloor microbial communities at deep-sea seamounts.

摘要

Recent studies strongly suggest the existence of a subseafloor biosphere in oceanic crust, yet little is known about the overall phylogenetic diversity of microbial communities in the subseafloor and the physiological diversity of microorganisms specifically adapted to this biotope. This thesis proposes that the physiological and phylogenetic diversity of microbes in low temperature hydrothermal diffuse fluids can be used to describe and understand the subseafloor community and habitat. Two contrasting subseafloor environments are examined, Axial Seamount and Baby Bare Seamount, both located in the northeast Pacific Ocean. The former is a young active mid-ocean ridge volcano with magma-driven fluid circulation, while the latter is a sedimented outcrop located on 3.5 Ma oceanic crust. The thesis is divided into four main topics focused on the subseafloor. First, an examination of the temporal changes in bacterial diversity and geochemistry in the subseafloor at Axial in response to an eruptive event found an increase over time both in the overall bacterial diversity in vent fluids and in microbes that appear to be indigenous to the vent environment. Second, genetic signatures and enrichment cultures of microorganisms from Baby Bare crustal fluids demonstrated that ridge flanks host a microbial community composed of species indigenous to the subseafloor and from other deep-sea habitats. Third, physiological characteristics of a novel cultured microorganism from diffuse fluids were identified, such as exopolysaccharide production and autotrophy, and indicate adaptation to the subseafloor habitat. Finally, a group of subseafloor indicator organisms belonging to the Thermococcales were studied from multiple vent sites using culturing and molecular methods and their in-situ diversity and distribution determined. These studies combine to support the existence of a warm anaerobic subseafloor biosphere in both mid-ocean ridge and ridge flank crust. Key characteristics of both subseafloor microbial communities and their habitat were identified, including high phylogenetic and physiological diversity as an adaptation to wide fluctuations and gradients in temperature and chemistry, the ability to form biofilms and attach to surfaces to compensate for both high fluid flow rates and low levels of nutrients, and the importance of autotrophy in the low organic carbon, volatile-rich subseafloor environment.