Ultrastructure of the Acidophilic Aerobic Photosynthetic Bacterium Acidiphilium rubrum Yukiko Matsuzawa, 1 Toshio Kanbe, 2 Jiro Suzuki, 1 Akira Hiraishi 1 1 Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan 2 Laboratory of Medical Mycology, Research Institute for Disease Mechanism and Control, Nagoya University School of Medicine, Nagoya 466-8550, Japan Received: 24 November 1999 / Accepted: 5 January 2000 Abstract. The ultrastructure of cells of Acidiphilium rubrum, which is an acidophilic aerobic photosyn- thetic bacterium containing zinc-complexed bacteriochlorophyll a, was studied by electron microscopy with the rapid substitution technique. Thin-section electron microscopy indicated that any type of internal photosynthetic membranes was not present in this organism despite a relatively high content of the photopigment. The majority of cells had poly-b-hydroxybutyrate granules and electron-dense spherical bodies identified as being polyphosphate granules. When the organism was grown chemotrophically with 0.1% FeSO 4 , it produced another group of electron-dense granules that were associated with the inner part of the cytoplasmic membrane. An energy-dispersive X-ray analysis showed that these membrane-bound, electron-dense granules contained iron. Acidiphilium rubrum is an acidophilic aerobic photosyn- thetic bacterium inhabiting strongly acidic environments such as mine drainage water [8, 17]. This organism is unique in containing zinc-complexed bacteriochlorophyll a (Zn-BChl a; for abbreviations of bacteriochlorophylls, see [15]) as the major photopigment associated with a fully active photosynthetic reaction center and a core light-harvesting complex [16]. It has recently been shown that Zn-BChl a is present in all established species of the genus Acidiphilium, among which A. rubrum produces the highest amount of the photopigment [4]. Although aerobic photosynthetic bacteria produce much lower amounts of BChls than anaerobic phototrophs in general [for reviews, 14, 18], some species of the former organ- isms form chromatophore-like photosynthetic membrane systems [2, 3, 13, 19]. In this connection, it is of great interest to note whether or not A. rubrum has any type of internal photosynthetic membranes. Wichlacz et al. [17] showed an electron micrograph of ultrathin sections of A. angustum cells in their report of the species description, but did not have any comments on internal membrane systems of this organism. Another subject of interest in relation to the cytologi- cal properties of A. rubrum and allied acidophiles is their ability to accumulate and deposit some metals within the cells. Acidiphilium species have been reported to have high metal tolerance for Cd 21 , Zn 21 , Cu 21 , and Ni 21 [11]. Recently, Ito et al. [5] reported that A. rubrum deposited Fe, Cr, and Ni as high electron-dense globules in the cytoplasm. Microbial formation of intracellular polyphos- phate (polyP)-metal complex bodies has been suggested as a metal accumulating system to remove toxic effects of excessive metals [6, 12]. More recently, A. cryptum JF-5 was reported to have blebs on the cell surface when grown anaerobically with Fe(III) hydroxides [9]. From the viewpoints noted above, we studied the ultrastructure of A. rubrum cells by electron microscopy with the rapid freeze-substitution method. The elements produced in the organism were detected by an energy- dispersive X-ray (EDX) analysis. Materials and Methods Bacterial strain and cultivation. The type strain of Acidiphilium rubrum (ATCC 35905) was used throughout this study. GYS medium [4], which contained 0.3% glucose and 0.1% (NH 4 ) 2 SO 4 as the sole carbon and nitrogen sources, respectively, and 0.03% yeast extract as the growth factor, was used for cultivation. The medium was supple- Correspondence to: A. Hiraishi CURRENT MICROBIOLOGY Vol. 40 (2000), pp. 398–401 DOI: 10.1007/s002840010077 An International Journal R Springer-Verlag New York Inc. 2000