Award: OCE-2046799

While bacteria and archaea are known to mediate biogeochemical cycles in the deep marine subsurface, they share this habitat with eukaryotes, in particular microbial fungi, which are decomposers of organic matter, and also function in symbiosis with chemoautotrophic prokaryotes and in mineral weathering. This project completed the first assessment of fungal activities in the organic carbon- and hydrocarbon-rich deep hydrothermal subsurface of Guaymas Basin, Gulf of CA, Mexico, along with a direct assessment of fungal cell abundance and of the active fraction of the whole microbial community at different depths and at sites with different sediment characteristics. The study leveraged samples collected during International Ocean Discovery Program (IODP) Expedition 385. It also established a collection of 253 fungal isolates, primarily affiliated to Ascomycota and Basidiomycota (higher fungal lineages), and obtained an inventory of ~200 saturated hydrocarbons, polyaromatic hydrocarbons (PAHs) and alkylated PAHs in the sediments. Based on observed rapid downcore decrease in concentration of recovered DNA and on decreasing cell counts of both prokaryotes and fungi,  temperature is the most likely limiting factor controlling both fungal and prokaryotic cell numbers. Hydrocarbon concentrations  were quite similar (100-200 mg/kg) over wide depth ranges, increasing ( to > 1000 mg C9-C44/kg sediment sample) at depths of deep buried magmatic sill intrusions. There are many unclassified fungal taxa in the sediments, suggesting a high degree of novelty. Chytrids were relatively abundant among fungi in shallower sediments, and microscopy shows that they are alive.  Bioorthogonal non-canonical amino acid tagging (BONCAT) provided supporting evidence for metabolically active fungi. Counts of fungi were up to 10E3/gm sediment, and became too low for detection below ~90 meters below seafloor.  Filamentous fungi were the most difficult to count due to difficulties associated with separating intact filaments from sediment samples, but they were successfully imaged within undisturbed Guaymas sediment core material using a novel protocol involving fluorescently-labeled antibodies to chitin and micro-CT scanning to confirm the presence of intact filaments. A higher genetic potential for hydrocarbon degradation was detected for fungal representatives within the genera Penicillium and Aspergillus, two genera known for their ability to degrade hydrocarbons. These showed the highest ability to utilize different PAHs (chrysene, naphthalene, phenanthrene, anthracene, pyrene, fluoranthene) as sole carbon sources and to produce biosurfactants that are likely important for this degradation. Analyses of expressed genes for the whole microbial community revealed insights into their physiological adaptations to hydrothermal subsurface conditions. In particular, it appears that tRNA, mRNA, and rRNA modifications are important for survival of bacteria and archaea in these hydrothermal sediments because they permit modulation of cell activity under elevated temperatures and along steep and sometimes fluctuating geochemical gradients. In sum, microbial Fungi are present and active within the near surface (within min. 20 meters below seafloor) of Guaymas Basin and must be factored into our understanding of active carbon cycling below the seafloor. 

Our access to students was restricted due to COVID-19, however one undergraduate was mentored in the PI’s laboratory and has gone on to pursue a PhD in microbiology. An early career scientist received mentoring at Woods Hole Oceanographic on project management and molecular approaches for RNA work and hydrocarbon analyses, and a Research Associate learned new DNA isolation skills. A graduate student participated in BONCAT work at Montana State University. Numerous undergraduates at University of North Carolina Chapel Hill were exposed to objectives and data from this project, and a postdoc participated in the project. High school students were mentored in the PI’s laboratory on hydrothermal vent microbiology and marine science.

 

 

Last Modified: 10/07/2025
Modified by: Virginia P Edgcomb