Heat-loving sporeforming sulfate reducers in the cold

Sulfate-reducing microbes (SRM) are of global relevance for sulfur cycling and the mineralization of carbon in the upper, anoxic sediment layers of the seafloor. In addition to the common psychro- and/or mesophilic microbial SRM populations, some cold and temperate marine sediments unexpectedly host a cryptic community of presumably inactive spores from thermophilic SRM, whose source and mode of dispersal are currently unknown. One hypothesis is that these heat-loving, sporeforming SRM grow in the hot subsurface and are transported to the surface as spores via locally defined seepages, which could be areas where also deep gas and possibly oil occasionally penetrate up to the sediment surface. The apparent inability of the thermophilic SRM to grow in the cold sediments where they were discovered provides a unique possibility to exemplarily study the biogeography and dispersal of microbial cells in the oceans. This project thus aims to shed light on the origin of thermophilic SRM spores by revealing their diversity and distribution in ocean sediments and waters of various geographic regions using a combination of molecular, biogeochemical, and cultivation-based methods. Practical molecular and biogeochemical assays will be developed for the detection and quantification of thermophilic, sporeforming SRM and applied to test if these organisms are bioindicators for fluid flow from the hot subsurface and thus potentially also for deep oil/gas deposits.

Microbial biogeography is influenced by the combined effects of passive dispersal and environmental selection, but their individual impacts can be difficult to discern. Inactive spores of thermophilic microorganisms that are found at cold or temperate seafloor sites were established as indicators to selectively reveal the consequence of passive dispersal on the biogeography of marine microorganisms. High-temperature germination experiments combined with molecular and biogeochemical analysis revealed the presence of a genetically and metabolically diverse community of spores in marine sediments. Closest relatives to these bacteria come from subsurface petroleum reservoir and marine hydrothermal vent ecosystems, suggesting that seabed fluid flow from these hot spots is seeding the cold ocean with thermophiles and may broadly influence the composition of marine microbial communities. Thermophile spores are constantly supplied to the cold or temperate seafloor via oceanic currents and sedimentation. Deposited spores survive for a long-time and potentially become metabolically active in the warm, deep biosphere. A global biogeography survey of thermophile spores demonstrated that the distribution of spores in marine surface sediments is differentially influenced by the relative connectivity of regional water masses to global ocean circulation. Hydrothermal sediments in the Guaymas Basin harbored a large fraction of globally distributed spore species and were identified as typical thermophile source environments. Limitations in passive dispersal to certain ocean regions, as observed for thermophile spores, will have considerable impact on marine biogeography of vegetative cells, dependent on the population sizes and levels of resistance against environmental stress of individual marine microbial taxa.