Earth microbe grows in a test that mimics Enceladus’s ocean

A microbe from Earth grew in a laboratory mixture designed to reproduce the chemistry of the ocean on Enceladus, Saturn’s icy moon. The organism had failed to grow in a laboratory medium at similarly high alkalinity. The contrast, reported on September 25 in Science Advances, shows that this combination of ingredients can support a known metabolism under conditions previously considered too extreme for the organism.

Beneath Enceladus’s icy crust lies a salty ocean in contact with a rocky core. Water-rock reactions may release hydrogen, a potential energy source for microorganisms that produce methane. Because Cassini found salts, organic compounds and evidence of hydrothermal activity in material ejected from the moon, researchers can use those clues to build laboratory simulations of the ocean’s chemistry.

In the experiment, conducted by researchers at Ludwig Maximilian University of Munich, with Vanessa Helmbrecht among the authors, water, salts, carbonates, and powdered rock were combined to reproduce alkaline conditions and hydrogen-producing reactions between water and rock. The researchers introduced Methanothermococcus okinawensis, an archaeon: a microorganism without a cell nucleus, originally isolated near hydrothermal vents in the Okinawa Trough, between Japan and Taiwan. Its metabolism uses hydrogen and carbon dioxide, produces methane, and obtains energy from that process without relying on light.

The small amount of available carbon dioxide posed an obstacle because the organism needs the compound both for its energy metabolism and as a source of carbon to build cellular material. According to the experimental account released by Freie Universität Berlin, it did not grow in the laboratory medium with a high pH and scarce dissolved carbon dioxide. In the mixture mimicking Enceladus, however, it grew at pH values as high as 11—a strongly alkaline condition—and produced methane using hydrogen supplied by water-rock reactions. Growth occurred over experiments lasting days; the result was more than the mere survival of cells.

Analysis of gene expression revealed a response to the scarcity of dissolved carbon dioxide. Although the mixture contained a high concentration of dissolved inorganic carbon, only a small amount was available in the form of carbon dioxide. In growing cells, expression increased along a carbon-fixation pathway: a sequence of reactions that incorporates carbon from that compound into organic matter. This response enabled the cells to use carbon dioxide even at extremely low concentrations. The study thus links the observed growth to efficient use of that compound and to energy supplied by reactions between minerals and water. The demonstration is limited to the conditions reproduced and the days covered by the experiments; sustained growth or metabolism over geological timescales was not tested.