Blue algae live without light

Researchers find first living cyanobacteria in 613 meters of deep rock

The purple and red spots indicate active cyanobacteria in the rock drill sample © PNAS
Read out

Survival despite darkness: Actually, cyanobacteria need sunlight to survive - like other plants too. But now researchers have discovered in a deep well in Spain viable blue-green algae even at a good 600 meters depth. The cyanobacteria found themselves in tiny pores and cracks of the rock and were there active despite permanent dark. The oldest plant organisms on earth are surprisingly adaptable.

The "cellar" of the earth is anything but empty and dead: Although deep in the earth's crust permanent dark, tight, high pressure and heat prevail, it is teeming there even before life. Even 2, 500 meters below the seabed, researchers have detected living bacteria and other microbes. And in the "basement" of our continents there could even be as many as quadrillions of living cells.

However, all previously known inhabitants of this deep biosphere had one thing in common: they belonged to the bacteria or archaea and were not dependent on sunlight and photosynthesis for their energy and nutrients. In the persistent darkness of the earth's crust, these microbes instead gain their energy through chemical reactions with hydrogen and other substances present in the rock.

Blue-green RNA at a depth of 600 meters

But now researchers around Fernando Puente-Sánchez of the Spanish National Institute of Technology and Aerospace in Madrid have discovered a dweller of depth who is unlikely to exist there: cyanobacteria. These protozoan protozoans are considered the first organisms to use photosynthesis and to supply oxygen to the earth's atmosphere.

Proportions of cyanobacteria and other mokrobes in the rock. © Puente-Sanchez et al. / PNAS

"Cyanobacteria have long been known as ecologically highly adaptable organisms, " explain the scientists. "But until now, their ecological range has been limited to environments where there are at least some sunlight." The researchers were all the more surprised when they examined the rocks from a hole in the southwest of Spain from 420 and 607 meters depth on traces of life. display

Analysis of the samples for microbial RNA revealed: "The most common organisms in both samples were cyanobacteria, " report Puente-S nchez and his colleagues. Whole clumps of several blue-green algae grew on the mineral surfaces of tiny pores and cracks in the rock, as revealed by further investigation.

Hydrogen instead of photosynthesis

But how do these cyanobacteria survive there? At these depths and in the middle of the rock there is neither sunlight nor any other source of light that could use the blue-green algae for photosynthesis. Indeed, fluorescence analysis revealed that these subterranean cyanobacteria appear to be out of active photosynthesis: their photosynthetic pigments are reduced, the researchers report.

Massive Rock: The cyanobacteria live in tiny pores and crannies of the crustal rock. PNAS

Instead, the cyanobacteria have switched their energy supply to purely chemical processes. They were helped by an already existing emergency mechanism, as the scientists found out: if the blue-green algae receive too much light, this triggers a process in which electrons are released from the cell and redirected to external electron acceptors Similar to known by electrogenic bacteria.

"This protective mechanism could allow blue-green algae to anaerobically oxidize hydrogen and other compounds by donating electrons to acceptors such as iron, manganese oxides, or phenolic compounds, " the researchers say. Through this reaction, the cyanobacteria then gain the energy they need to build up high-energy cell constituents and their metabolic processes.

Blue algae also in the Mars underground?

But this means that cyanobacteria could play a much more important role in the deep biosphere than previously thought, as Puente-S nchez and his colleagues emphasize. By being able to colonize even dry, low-nutrient and dark environments and using hydrogen as their main source of energy, blue-green algae are likely to be an important primary producer for the deep lifecycle.

This is also exciting for astrobiology: "The discovery of this hitherto unknown ecological niche for cyanobacteria opens up new perspectives on its potential presence in the biosphere of other planets and species Moons, "say the researchers. In theory, blue-green algae could exist in the background of other celestial bodies such as Mars. (Proceedings of the National Academy of Sciences, 2018; doi: 10.1073 / pnas.1808176115)

(PNAS, 02.10.2018 - NPO)