Characterizing marine subsurface fungi from oligotrophic South Pacific Gyre sediments

Fungal communities from the deep marine subsurface may be important in global biogeochemical cycles through remineralization of sedimentary organic matter, but this has not yet been thoroughly observed. This study analyzes the fungal role in subsurface biogeochemical cycles and understands how these organisms have adapted to extreme environments, such as the nutrient and organic matter depleted sediments of the South Pacific Gyre. Sediment cores were collected during the Integrated Ocean Drilling Program Expedition 329 to the South Pacific Gyre on board the D/V JOIDES Resolution in the Fall of 2010. Two fungal isolates were cultured from 70 million year old sediments. Previous analysis found that the two isolates were closely related to Penicillium species. To fully characterize the isolates and test their physiological boundaries, we grew them at different temperatures, salinities and pH. Whole genomic analysis was used to understand the fungi’s physiology and metabolism on a molecular level. The fungi were found to prefer growth at mesophilic temperatures and low NaCl concentrations. Growth occurred between pH 3 and pH 8. The isolate from 12 mbsf grew optimally from pH 3 to pH 8 and the isolate from 124 mbsf grew optimally from pH 3 to pH 6. Fermentation of lactose and sucrose was confirmed, but not nitrate and sulfate reduction. The fungal isolates from the South Pacific Gyre sediment had physiological capabilities that were consistent with the in situ subsurface conditions and contained genes that were capable of utilizing the recalcitrant carbon sources found in situ. The results from this study expand on the fungal limits of life and highlight their important role global carbon cycle.