New Frontiers: The Evaluation of Eukaryotic Populations in the Deep Marine Subsurface: Scientific Goals and Approaches for Future Ocean Drilling Projects Edgcomb,

The composition and activities of microbial communities from diverse and often ‘extreme’ habitats have been the focus of intense research during the past decade, spurred on largely by advances in molecular biology. These studies have placed microbes (used in the largest sense to include viruses, archaea, bacteria and protists) as essential participants in virtually all biogeochemical processes on our planet. Most molecularbased research to date has focused on bacteria, archaea and viruses, while studies of the diversity and activities of protistan assemblages has lagged behind. Recently there has been a concerted effort to study life in the deepest parts of the oceans and even below the seafloor in the deep subsurface. Bacteria and archaea have been shown to comprise a major portion of Earth’s total living biomass in the deep subsurface and their activities catalyze the recycling of buried organic matter. The contribution of microbial eukaryotes (protists and fungi) in this ecosystem is unknown, as these assemblages have been studied in deep-sea sediments only to a few centimeters into the seafloor. Protists and fungi may impact carbon cycling in the marine subsurface through consumption of dissolved organic matter (fungi) and through bacterial/archaeal grazing. Protists may control bacterial and archaeal abundances and community composition, and thereby impact microbial production and nutrient cycling. Despite the pivotal roles played by these organisms, little is known regarding the presence, abundance, diversity and activities of these species in deep biosphere environments. This white paper proposes goals and approaches for initiating research on the deep biosphere with a focus on eukaryotic microbes to complement existing microbial community research in this ecosystem. Research Focus Our current view of microbial diversity in the deep ocean is largely shaped by microscopy and molecular information available for bacteria and archaea. The evaluation of diversity and ecology of the single-celled eukaryotic microbes (the protists and fungi) has been much slower despite their important role in marine microbial communities and in expanding our understanding of the evolution of multicellular taxa (Caron et al. 2009a,b). Protistan lineages represent one and a half billion years of evolution and comprise the bulk of eukaryotic phylogenetic diversity and an astounding array of morphologies, physiologies and ecological activities. In the upper water column of the World’s oceans protists play pivotal roles in global food webs as primary producers and consumers. Much less is known about the presence and activities of microbial eukaryotes in the deep ocean, although recent studies have provided preliminary insights into microbial eukaryote diversity in the bathypelagic (Countway et al. 2007; Not et al. 2007) and deep-sea sediments (Arndt et al. 2003; Edgcomb et al. 2002). Bacterial and/or archaeal grazing by protozoa in subsurface horizons may significantly impact carbon cycling in the deep oceans and deep-subsurface environments by limiting bacterial and archaeal abundances, altering community composition and controlling net production and