Hydrogenosomes of Metopus contortus physiologically resemble mitochondria

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Abstract
The anaerobic free-living ciliated protozoonMetopus contortusis a grazer in anoxic marine sediments. It does not possess mitochondria, but it does have specialized organelles termed hydrogenosomes which release hydrogen gas. The cationic lipophilic cyanine dye DiOC7(3) is an indicator of transmembrane electrochemical potential. With the aid of confocal laser scanning microscopy (CLSM), the association of this dye with hydrogenosomesin situwas followed. Flow cytometric measurements showed that fluorescence of the membrane potential dye decreased in response to an elevatedpH2in the cell. CLSM also revealed localization of fluorescence of the calcium probe Fluo 3-AM, and of the transmembrane pH gradient probe BCECF-AM, within the lumen of the hydrogenosomes. In addition, hydrogenosomal inclusions were detected. X-ray microanalysis of these electron-dense granules revealed high levels of calcium, phosphate and magnesium. It is concluded thatM. contortushydrogenosomes are calcium stores, have a membrane potential, and an alkaline lumen. These physiological features resemble those of mitochondria in aerobic protozoa.