Fungi in deep subsurface bedrock - an overlooked
geobiological agent?
Ivarsson, M. , Drake, H. , Bengtson, S. , Belivanova, V. , Heim, C.
1
2
1
1
3
1
Swedish Museum of Natural History, Department of Natural History and Nordic Center for Earth Evolution (NordCEE), Stockholm, Sweden. email:
[email protected]
2
Department of Biology and Environmental Sciences, Linnaeus University, Kalmar.
3
Department of Geobiology, Geoscience Centre, University of Göttingen, Germany.
Introduction
Introduction
The deep biosphere of the subsurface crust is usually considered to be dominated by
Prokaryotes (Bacteria and Archaea) but recently fungi have been recognized as an
important component as well. Fossilized fungi have been reported from several drill
cores from the ocean igneous crust and a few live strains have been isolated from
both marine and continental crust. Fungi from deep igneous crust have been shown
to be involved in mineral weathering of carbonates and zeolites, mineral precipitation, mobilization of elements like C, P, Fe and Mn, as well as engaging in symbiotic
relationships with prokaryotes, most likely Archaea. Thus, fungi in the deep subsurface play a significant ecological role that is far from understood yet.
A
B
E
D
F
C
G
H
Anaerobic fungi
ESEM images of A) Fossilized fungal mycelium.
B) One mineralized hyphae next to a carbonaceous hyphae. C) Transition zone where a hyphae
is mineralized. D) Tomographic reconstruction
of a mycelium showing
branching and anastomosis. E) Tomographic
reconstruction of a mycelium showing anastomosis between branches. F)
ESEM image of a zeolite
weathered by the presence
of hyphae. G) ESEM image of a hyphae leaving a
weathered negative elongated pit-structure in a
zeolite surface. H) A zeolite surface weathered by a
fungal biofilm.
Anoxic conditions prevail at depths greater than 20 m and thus, the
fungal remains most certainly represent anaerobic fungi. Anaerobic fungi, in contrast to the more common aerobic fungi, have hydrogenosomes instead of mitochondria and produce H2 during their
respiration. In rumen of herbivores (the only environment in which
anaerobic fungi has been well studied) anaerobic fungi exist in vital
symbiosis with H2-dependent Archaea like acetogens and methanogens. A similar symbiotic relationship would be possible in other
anoxic environments such as the deep subsurface.
Conclusions
Results
Recently, fossilized fungal communities were discovered in drilled cores from Laxemar, Sweden,
representing a depth of 740 meters. These fungi
occur in granite fracture voids and have been fossilized to partly fossilized by clays and Fe-oxides.
Microanalytical data (SIMS) of co-genetic carbonate (δ13C) and pyrite (δ34S) reveal that degradation
of organic matter by microorganisms, bacterial
sulfate reduction (BSR) and anaerobic oxidation of
methane (AOM) has occurred in the local fracture
system. Pyrite with BSR signatures occur frequently on or closely associated with the fungal hyphae
and indicate co-existence of the fungi and sulfate
reducing bacteria in the system.
A
B
A) ESEM image of a hyphae with
pyrites with BSR signatures. B)
Tomographic reconstruction of a
mycelium and the distribution of
pyrites (bright spots) along the hyphae.
We propose that the Laxemar communities consist of the remains of anaerobic fungi and H2-dependent sulfate reducing bacteria, represented by pyrites with BSR signatures. The growth of the
sulfate reducing bacteria has been supported by the fungal H2 production, while the fungi have been
using the prokaryotic biomass for their metabolism. In similarity with deep basalt habitats the hyphae readily dissolve and penetrate secondary zeolites and calcite in the voids. This has implications
for the long term stability of nuclear waste repository barriers, such as buffers and natural retention
materials, but also other waste repositories (CO2 sequestration) and underground constructions
and mines.