A major part of the biologic activity on Earth is hidden underneath our feet in an environment coined the deep biosphere which stretches several kilometers down into the bedrock. The knowledge about life in this vast energy-poor deep system is, however, extremely scarce, particularly for micro-eukaryotes such as fungi, as most studies have focused on prokaryotes. Recent findings suggest that anaerobic fungi indeed thrive at great depth in fractures and cavities of igneous rocks in both the oceanic and the continental crust. Here we discuss the potential importance of fungi in the deep biosphere, in particular their involvement in fundamental biogeochemical processes such as symbiotic relationships with prokaryotes that may have significant importance for the overall energy cycling within this vast subsurface realm. Due to severe oligotrophy, the prokaryotic metabolism at great depth in the crust is very slow and dominantly autotrophic and thus dependent on e.g. hydrogen gas, but the abiotic production of this gas is thought to be insufficient to fuel the deep autotrophic biosphere. Anaerobic fungi are heterotrophs that produce hydrogen gas in their metabolism and have therefore been put forward as a hypothetical provider of this substrate to the prokaryotes. Recent in situ findings of fungi and isotopic signatures within co-genetic sulfide minerals formed from bacterial sulfate reduction in the deep continental biosphere indeed seem to confirm the fungi-prokaryote hypothesis. This suggests that fungi play a fundamental biogeochemical role in the deep biosphere.

地球上大部分的生物活动都隐藏在我们深深的生物圈创造的环境中的脚下一直延伸到基岩几公里。然而，由于大多数研究都集中在原核生物上，因此在如此庞大的能量贫乏的深层系统中，关于生命的知识极为匮乏，尤其是对于真菌等微型真核生物而言。最近的发现表明，厌氧真菌确实在海洋和大陆地壳的火成岩的裂缝和空洞中生长很深。在这里，我们讨论了真菌在深层生物圈中的潜在重要性，尤其是它们参与基本生物地球化学过程（例如与原核生物的共生关系）的参与，这可能对于这个巨大的地下领域内的整体能量循环具有重要意义。由于严重的寡营养，地壳中深层的原核代谢非常缓慢，并且主要是自养的，因此依赖于例如 氢气，但据认为这种气体的非生物生产不足以为深部自养生物圈提供燃料。厌氧真菌是在其新陈代谢中产生氢气的异养菌，因此被提出作为原核生物底物的假设提供者。最近的在深部大陆生物圈中细菌硫酸盐还原形成的共生硫化物矿物中的真菌和同位素特征的原位发现确实似乎证实了真菌-原核生物的假说。这表明真菌在深层生物圈中起着基本的生物地球化学作用。