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First published online June 16, 2005
Journal of Experimental Biology 208, 2569-2579 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01660

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Life at acidic pH imposes an increased energetic cost for a eukaryotic acidophile

Mark A. Messerli^1,2,*, Linda A. Amaral-Zettler¹, Erik Zettler^3,4, Sung-Kwon Jung², Peter J. S. Smith² and Mitchell L. Sogin¹

¹ The Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA 02543, USA
² BioCurrents Research Center, Program in Molecular Physiology, Marine Biological Laboratory, Woods Hole, MA 02543, USA
³ Sea Education Association, PO Box 6, Woods Hole, MA 02543, USA
⁴ Centro de Biología Molecular, Universidad Autónoma de Madrid, Cantoblanco, Madrid 28049, Spain

^* Author for correspondence (e-mail: mmesserli{at}mbl.edu)

Accepted 25 April 2005

Organisms growing in acidic environments, pH <3, would be expected to possess fundamentally different molecular structures and physiological controls in comparison with similar species restricted to neutral pH. We begin to investigate this premise by determining the magnitude of the transmembrane electrochemical H⁺ gradient in an acidophilic Chlamydomonas sp. (ATCC® PRA-125) isolated from the Rio Tinto, a heavy metal laden, acidic river (pH 1.7-2.5). This acidophile grows most rapidly at pH 2 but is capable of growth over a wide pH range (1.5-7.0), while Chlamydomonas reinhardtii is restricted to growth at pH 3 with optimal growth between pH 5.5 and 8.5. With the fluorescent H⁺ indicator, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), we show that the acidophilic Chlamydomonas maintains an average cytosolic pH of 6.6 in culture medium at both pH 2 and pH 7 while Chlamydomonas reinhardtii maintains an average cytosolic pH of 7.1 in pH 7 culture medium. The transmembrane electric potential difference of Chlamydomonas sp., measured using intracellular electrodes at both pH 2 and 7, is close to 0 mV, a rare value for plants, animals and protists. The 40 000-fold difference in [H⁺] could be the result of either active or passive mechanisms. Evidence for active maintenance was detected by monitoring the rate of ATP consumption. At the peak, cells consume about 7% more ATP per second in medium at pH 2 than at pH 7. This increased rate of consumption is sufficient to account for removal of H⁺ entering the cytosol across a membrane with relatively high permeability to H⁺ (7x10^-8 cm s^-1). Our results indicate that the small increase in the rate of ATP consumption can account for maintenance of the transmembrane H⁺ gradient without the imposition of cell surface H⁺ barriers.

Key words: acidophile, cytosolic pH, membrane potential, energetic cost, Chlamydomonas sp

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