Microbial Community Structure in Hydrothermal Sediments: The Guaymas Basin Field Site

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading, hydrothermal activity, and organic matter accumulation on the seafloor due to high sedimentation rates. In the hydrothermal sediments of Guaymas Basin, microbial community compositions and coexistence patterns change across steep gradients of temperature, potential carbon sources, and electron acceptors. After introducing the geological and geochemical context, this chapter discusses the phylogenetic diversity, temperature range, and occurrence patterns of characteristic bacteria and archaea in the hydrothermal sediments of Guaymas Basin. Here, ample carbon substrates, nutrients, and energy-rich redox couples coexist within dynamic thermal gradients and sustain uncommonly diverse microbial communities. The availability of sulfate, introduced via hydrothermal circulation, favors in particular sulfate-reducing bacteria and archaea, and sulfate-reducing, methane- and alkane-oxidizing microbial consortia; perhaps no other site has yielded such a wide range of thermophilic, hydrocarbon-degrading specialists. Guaymas Basin continues to yield microbiological discoveries, for example novel types of Asgardarchaeota. This archaeal lineage gave rise to the early ancestors of eukaryotic cells and retains genomic and biochemical similarities to modern eukaryotes. In analogy to Jurassic Park, Guaymas Basin serves as an Archaeal Park where microbiologists may search for evolutionary "missing links" that thrive in this primordial habitat.