Endospores in the deep marine biosphere: Distribution, abundance and function

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Scientific drilling has demonstrated the abundance of Earth's deep microbial biosphere, and its importance in driving carbon and nutrient cycles. A major question presented by these large and understudied communities is how they deal with progressive burial and associated energy starvation. One important piece of this puzzle may reside in the gray area between extremely slow growth, dormancy, and microbial death; bacterial endospores may play a critical role in this context. Endospores are resistant cellular entities: metabolically inactive, dormant, and structurally unique, with the ability to monitor their habitat in order to resume growth under favorable conditions. Endospores have so far been poorly accounted for in censuses of subseafloor biomass. Indeed, it remains controversial to what extent DNA stains penetrate into endospores, which therefore might either have remained invisible or been included in the pool of counted cells. An alternative chemical approach, targeting the specific biomarker dipicolinic acid (DPA), has recently produced the first data of endospore abundance in the subseafloor and suggests that the vast population of vegetative cells is accompanied by an equally large population of dormant endospores. Building on this observation we propose a DPA-based survey of endospore concentration in subseafloor sediments, using our large archive of appropriately preserved samples resulting from our group's participation in ODP and IODP expeditions (e.g., ODP Leg 201, IODP Exps. 301, 311, 315, 316, 317, 322, 329, 337), and other expeditions dedicated to subseafloor life. We aim to tackle the ecological relevance of endospores in the deep biosphere and to address two of the global challenges defined by the IODP Science Plan for 2013-2023: #5 What are the origin, composition, and global significance of subseafloor communities and #6 What are the limits of life in the subseafloor? Our research is driven by the hypothesis that endospores are major and insufficiently accounted for contributors to the deep biosphere. To test this we will a) survey DPA-based endospore abundance in a large set of deep subseafloor samples for which contextual cell concentration and geochemical data exists, and b) investigate to which extent cell counts include endospores. While the analytical method for DPA analysis is fully established in our lab, reliable enumeration of endospores in the deep biosphere requires refinement of DPA to spore conversion factors and information on the partitioning of DPA in the sediment. We also hypothesize that endospores not only survive extreme conditions during burial, but may colonize deep, activity-stimulating habitats. To test this we will a) experimentally evaluate the ratio of sedimentary endospores that remain viable over geological time scales and the conditions under which they may germinate and b) confirm the ecological relevance of endospore germination in sediment layers where microbial activity is stimulated.