Invulnerability Cloak

Grand Prismatic Hot Spring in Yellowstone National Park

Grant Ordelheide

The acidic hot springs of Yellowstone National Park are deadly to most living organisms, yet single-celled microbes of the domain Archaea flourish in them. Archaea are superficially similar to, but biochemically distinct from, bacteria, and their unique cellular membranes differ from those of bacteria, plants, and animals. New work has shed light on the unique structures in archaea membranes that allow them to survive in extreme environments.

Paula Welander, an assistant professor of Earth System Science at Stanford University, and colleagues studied Sulfolobus acidocaldarius, a species of archaea that lives in such foreboding places as volcanic craters, deep-sea hydrothermal vents, and hot springs.

The team focused on a structure of S. acidocaldarius’s membrane, composed in part by a ring-shaped molecule named calditol. To gauge calditol’s importance, the researchers combed through the genome of S. acidocaldarius and singled out three proteins apparently related to the construction of calditol. By mutating the genes behind the proteins, the researchers created S. acidocaldarius strains whose membranes lacked calditol.

When exposed to the typical acidities in their native environments, the calditol-deficient S. acidocaldarius fared just fine. But when these mutants were exposed to extremely low pH acidities occasionally endured in their habitats, their growth rate plummeted, whereas the normal S. acidocaldarius strain was relatively unscathed. Restoring calditol in the mutant strain also restored growth at the extreme acidities, indicating calditol’s key protective role.

The experiment was not designed to reveal exactly how calditol protects archaea in low-pH environments. Based on previous computer simulation studies, however, Welander and colleagues suspect that calditol is a critical ingredient for building more stable and tightly packed membranes. Such membranes would keep the outside world at bay, safely ensconcing the cell’s delicate internal components.

Fleshing out the biochemical characteristcs of archaea will help resolve the debate about where they fit in with other forms of life and how they carve out their own ecological niches. “Archaea are widespread in nature and severely understudied,” said Welander. “We’ve really become fascinated by the use of radical chemistry by archaea to modify their membranes as a protective mechanism from environmental stress.” (Proceedings of the National Academy of Sciences)

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