Viable cyanobacteria in the deep underground of Rio Tinto

  • A study led by the Centro de Astrobiología (CAB, CSIC-INTA) has detected for the first time the presence of viable cyanobacteria in the deep subsurface (below 600 meters of depth)
  • The finding took place at the rocky massif sulphide ore of the Iberian Pyrite Belt (IPB; at Río Tinto, Huelva), an area in the South of Spain considered in astrobiology as a terrestrial analogue of early and wet Mars.
  • The found cyanobacteria apparently use as an energy source a mechanism that at the Earth surface protects them from the light.

Cyanobacteria are very versatile photosynthetic organisms that live in the majority of ecosystems, from sea systems to extremely arid deserts. Until now, their ecological range seemed to be restricted to environments with at least occasional occurrence of sunlight. A study led by researchers of the Centro de Astrobiología (CAB) and published by the journal Proceedings of the National Academy of Sciences (PNAS), has detected the presence of viable cyanobacteria in samples of deep rocky cores of the Iberian Pyrite Belt (area in which Rio Tinto originates, in the Huelva province of Spain).

In the study, done in the framework of the projects Advanced Grant of the European Research Council (ERC) and the Spanish RETOS-MINECO, molecular, microscopic, and metagenomic evidence is presented of the preponderance of cyanobacteria in some niches of the deep underground of IPB. The analysed core samples were obtained from two purposed designed deep drillings , within the ERC’s IPBSL project (Iberian Pyrite Belt Subsurface Life). This project, undertaken by CAB between the years 2010 and 2015, had as the main objective to characterize the geomicrobiology and operating metabolisms in the deep ecosystems of the IPB.


During the drillings, the core samples extracted were analysed with the SOLID-LDchip system, a biochip used to detect signs of life that is under development at CAB for planetary exploration. Researchers detected from the first moment and on site some immunologic clues of the presence of cyanobacteria. The initial result of the analysis confirmed later by means of other techniques, such as the sequencing of the ribosomal 16S gene extracted from the analysed rocks, or the visualization of cyanobacteria with microscopy and the use of specific fluorescent probes capable of binding to them. Later on, the sequence of two metagenomes (collective pool of genes from an environmental sample) at different depths, 420 and 607 m, respectively, allowed researchers to fully confirm the presence of cyanobacteria.

As energy source, the found cyanobacteria seem to use a natural “security valve” that at the Earth surface protects them from the light. The system redirects excess energy towards the outside of the cell, transferring electrons towards substances such as oxidised metals or organic matter. Paradoxically, the same system would become activated in conditions of darkness and anoxia present in the deep underground, allowing them to obtain energy despite the absence of light.

Rock sample cross-section (left), LDChip immunoassay results (right) and fluorescence plot of quantified cyanobacteria (below)
Rock sample cross-section (left), LDChip immunoassay results (right) and fluorescence plot of quantified cyanobacteria (below)

The results of the study suggest that cyanobacteria can play a very relevant role as primary producers in the deep biosphere of the Earth. Also, this ecological niche, until recently unknown, highlights the versatility of cyanobacteria, one of the most ancient groups of microorganisms on Earth. It as well allows to propose new models on their origin and evolution, and to suggest the possible presence of similar organisms in current or primitive biospheres in other planets and their moons.

Image credits:

All images were kindly provided by the Centro de Astrobiología.