Energy transfer from antenna pigments to the reaction center is common in chlorophyll-based photosynthesis but never been observed in retinal-based ion pumps and photoreceptors. Here we describe xanthorhodopsin, a retinal protein/carotenoid complex in the eubacterium Salinibacter ruber, a proton pump. Difference absorption spectra measured under a variety of conditions and action spectra for pumping indicate that this protein contains two chromophores: retinal and the carotenoid, salinixanthin, in a molar ratio of about 1:1. The two chromophores strongly interact, and light energy absorbed by the carotenoid is efficiently transferred to the retinal and used for transmembrane proton transport.The extreme halophile Salinibacter ruber isolated from salt crystallizer ponds (1,2) can be grown in aerobic heterotrophic culture in 4 M NaCl. This eubacterium accumulates high concentrations of KCl to adapt to the high ionic strength (3), as the haloarchaea in the same environment. After several days of growth, Salinibacter ruber acquires a deep red color, from salinixanthin which constitutes nearly 100% of its carotenoid content and whose chemical structure was recently established (4). It was proposed to provide protection from photodamage and to stabilize the cell membrane because both the polyene and the fatty acid part of this carotenoid acyl glycoside will be immersed in the lipid bilayer (4). We report here that salinixanthin is not the only pigment in the Salinibacter ruber cell membrane, and heterotrophy is not the only source of energy for this organism. These cells contain an unusual retinal protein, which uses salinixanthin to assist harvesting light energy in a wider spectral range and utilizes it for transmembrane proton transport. Thus, it is a light-driven proton pump similar to bacteriorhodopsin (5) and the archaerhodopsins (6) of the archaea, the proteorhodopsins of planktobacteria (7), and leptosphaeria rhodopsin of an eukaryote (8), but with two chromophores. We term it here xanthorhodopsin. Its novel carotenoid antenna is a feature shared with chlorophyll-based light-harvesting complexes and reaction centers (9,10).Illumination of cell membrane vesicles prepared from Salinibacter produces acidification of the medium, which is abolished by the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) (Fig. 1A). When assayed in 1 M Na 2 SO 4 , these light-dependent pH changes are unaffected by the presence of chloride ions (not shown). Thus, the vesicles contain an outward-directed light-driven proton pump like bacteriorhodopsin, and lack a

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This paper surveys results of the authors and others concerning estimates for the Hausdorff dimension of strange attractors, particularly in the case of (generalized) Lorenz systems and ROssler systems. A key idea is the interpretation of Hausdorff measure as an analogue of a Lyapunov function. (1991). 58F13 Mathematics Subject and Classification