FOSSIL PRESERVATION OF DEEP-BIOSPHERE FUNGI

The presence of fungi in the deep biosphere, hundreds of meters below the ocean floor, has only recently been established. Their ecological role is still unknown, owing to the difficulties of obtaining, observing and culturing organisms from sub-seafloor settings. Natural fixation through fossilization provides means of observing morphological characteristics and deducing life conditions of these elusive organisms. Synchrotron-based X-ray tomographic microscopy (SRXTM) of exquisitely preserved fungi from Eocene basaltic rocks in the Emperor Seamounts chain (northern Pacific Ocean) reveal the three-dimensional organization of the fungi and their spatial relationship to the surrounding basaltic rock. The fossils occur in veins and cracks in the basalt, and are preserved both through embedding in hydrothermally precipitated minerals and as montmorillonite replacements within unfilled voids. Complex mycelial networks with anastomosing septate hyphae occur with clusters of cell-like objects, suggesting that both hyphal and yeast-like organisation is represented. The organisms were able to bore into mineral substrates (calcite and zeolites), forming characteristic galleries. Morphological diversity suggests symbiotic relationships between organisms having close and specific spatial relationships. The fossilization environments represented by the Emperor Seamounts present a means to study the evolution of the deep biosphere in deep time, in particular the remarkable participation of eukaryotes in what was long considered a prokaryotic recluse.