Although TEM analyses revealed that in some cases, the mineralized reticulated filaments contain bacterial cells, extensive culturing and molecular biology analyses were elusive to identification. We focused on enigmatic reticulated filaments found in Ardales cave (Spain), a granite spring water tunnel in Porto (Portugal) and four lava tubes from Canary Islands (Spain), Easter Island (Chile) and Galápagos Islands (Ecuador).
In this study, microscopy techniques, such as FESEM-EDS, TEM-EDS, CLSM, and ancillary chemical and mineralogical techniques were conducted to recognize biominerals, microbial fabrics or fossil bacteria as possible biosignatures valuable for astrobiology. They were characterized as mineralized hollow filaments with hexagonal and diamond-shaped chambers resembling honeycombed structures. Intriguing filamentous structures consisting of long reticulated filaments rich in C, Ca, Mn, Fe or Si have been found concealed within speleothems in limestone caves and lava tubes all over the world. Subsurface filamentous fabrics, fossil bacteria preserved in minerals, mineralized nano-sized structures and biologically generated textures have been some of the proposed models for extraterrestrial biosignatures. Caves on Earth host an especially interesting variety of mineral-utilizing microorganisms and biological processes, which may be recognized as biosignatures. Recently reported signs of liquid water on early Mars and volcanic activity suggest that Mars’s subsurface could harbour traces of microbial life, making the tracing of life on Earth’s subsurface even more compelling.
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