Description:

Few investigations of extreme environments target eukaryotes because biologists generally consider them to be less diverse and incapable of growing in hostile conditions. Eukaryotes growing in the iron-rich Rio Tinto may provide clues about novel physiological adaptations necessary for survival in extreme conditions, such as acid mine drainages or in extraterrestrial ecosystems that may exist on the planet Mars. Using a combination of molecular approaches with the functional assays available through the BRC we have begun to unravel the adaptive strategies that allow organisms to live in these extreme environments.

Perhaps the most interesting questions include: How do eukaryotes survive such a strong H+ ion concentration gradient and how did they adapt to these conditions over relatively short periods of evolutionary time? Unless the internal pH of protists living in acid environments is lower than 6.5, they must have evolved mechanisms to prevent protons from freely diffusing into the cell. We are trying to identify the kinds of ion regulatory machinery including ion pumps and porters required for survival of eukaryotes at low pH, and to explore their diversity and evolutionary history in protists isolated from the Rio Tinto. We hypothesize that a diversity of ATPases in a variety of acidophilic protists have allowed these organisms to adapt to extreme environments.

NCRR BioCurrents Research Center resource used: Physiological experiments use advanced imaging techniques to assess and confirm intracellular pH levels for the Rio Tinto isolates, as well as for their closest relatives from standard culture collections. BRC culture facilities are used to maintain the organisms.

A comprehensive study of the photosynthetic green alga Chlamydomonas sp. (Rio Tinto Isolate RT1no2) indicates a relatively neutral cytosolic pH. The phagotrophic Vannella sp. (Rio Tinto Isolate Vn4) maintains an alkaline cytosolic pH in extracellular pH 2. The cytosolic pH range is 7.2-7.8. These gradients raise questions about how such differentials are maintained across the membrane and body wall.

Current progress:

Studying Chlamydomonas for oxygen consumption we recorded a small increase in aerobic metabolism (7%) when specimens were transferred from a neutral pH to the acidic environment. We have also been able to show a difference in membrane permeability between acid loving and normophilic protists. Also, and critically, we have confirmed that the resting potential of Chlamydomonas is actually positive. Thus by a combination of electrochemical and permeability solutions the metabolic cost associate with any proton leak has been kept to a minimum.

This work was published in the current year and featured in "Inside the JEB" in July 2005.