DISPERSAL OF DEEP BIOSPHERE THERMOPHILES INTO COLD MARINE SEDIMENTS

Subsurface habitats are leaky due to geological features such as hydrocarbon seeps, hydrothermal vents, discharging seamounts, and mud volcanoes. Geofluids may passively transport deep subsurface dwelling microorganisms through and/or out of the subsurface. Consistent with this is the discovery of thermophilic Firmicutes present as dormant endospores in surface marine sediments where in situ conditions are too cold to support their germination and growth. These “thermospores” are phylogenetically and physiologically diverse and many are closely related to Clostridia commonly found in deep anoxic habitats such as petroleum reservoirs or mid ocean ridges. The distribution of thermospores in marine sediments is spatially variable, both globally and regionally. In Arctic sediments near Svalbard, certain thermospore phylo- and phenotypes appear to be unique to certain fjords, while others are more widespread. This sequence and trait based variability has also been observed in sediments along the Tyne estuary (northeast England). In addition to surviving the cold temperatures in surface sediments, certain thermospores display remarkable heat resistance and remain viable after serial autoclaving and/or long-term exposure to temperatures above their growth limit. Several different thermospore taxa are typically detected in a single surface sediment location, suggesting multiple sources and dispersal vectors may contribute to this microbial diversity. Thermospores provide a tractable model for quantitative studies of microbial dispersal with the potential to highlight previously unconsidered dispersal mechanisms for deep biosphere organisms in particular, and for marine microorganisms in general.