Little is known about which microorganisms have the most impact on biogeochemical cycles in the subsurface, and practically nothing is known about microbial eukaryotes (mEuks) in these ecosystems. During the first year of the C-DEBI funded postdoctoral fellowship project “World-wide Exploration of Microbial Eukaryote Diversity and Activity in the Marine Subsurface” we produced the first survey of active subsurface mEuks across a globally distributed sample collection of sediments from up to 48 meters below seafloor (mbsf). The data reveal a dramatic increase in fungal diversity with increasing sediment depth. Unique communities of fungi inhabit different locations and are selected for as a result of geographic isolation and differential responses to in situ geochemical conditions. These findings support the hypotheses that the diversity of subsurface fungi increases substantially with sediment depth, and that fungi may play an important role in large scale elemental cycling and organic substrate turnover in the marine subsurface. We propose to test these hypotheses by 1) surveying fungal diversity across five depths from Iberian Margin sediments spanning 10-120 mbsf, and 2) sequencing metatranscriptomes for analysis of fungal derived message RNA coding for functional proteins in Peru Margin (IODP site 1229) sediments from 5 and 50-mbsf.