Biochemical and spectroscopic characterization of photocomplexes from the extremely halophilic purple sulfur bacterium, Halorhodospira halophila strain SL1T.

The purple sulfur phototrophic bacterium Halorhodospira (Hlr.) halophila type strain SL1T (= DSM244T) is among the most halophilic bacteria known and has been used as a model for exploring the osmoadaptive and photosynthetic strategies that enable phototrophs to thrive in hypersaline environments. Here we report the isolation and characterization of four photocomplexes from this organism: a conjoined core and peripheral light-harvesting complex (LH1-LH2), a light-harvesting-reaction center complex (LH1-RC), and two individual light-harvesting complexes, LH1-only (LH1-B880) and LH2-only. Although the LH1-LH2 and LH1-RC complexes displayed similar spectral features to those of their corresponding complexes from Hlr. halophila strain BN9622, the LH2 B850 absorption maxima in both LH1-LH2 and LH2-only complexes of strain SL1T differed from those of strain BN9622. This difference could be linked to the substitution of a key amino acid residue in the strain SL1T LH2 α-polypeptides. The LH1-B880 complex from strain SL1T was unstable in the absence of salts and converted to a unique intermediate species with an absorption maximum at 850 nm. These findings are compared with those from strain BN9622 and discussed with respect to the electrostatic charge interactions that maintain thermal and spectral stabilities of these photocomplexes and underlie the adaptation of halophilic phototrophs to their hypersaline habitats.