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Phototrophic purple sulfur bacteria as heat engines in the South Andros Black Hole - page 5 / 8

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Photosynth Res

Absorbance (a.u)

400 500 600 700 800 Wavelength (nm)

900

Fig. 4 Low temperature absorption spectra of whole cells of isolates Thiocapsa BH-1 (upper plot) and Allochromatium BH-2 (lower plot)

Carotenoid

Allochromatium

Thiocapsa

BH-2 (%)

BH-1 (%)

Lycopene

3

n/d

Rhodopin

65

n/d

Anhydrorhodovibrin

7

1

Rhodovibrin

\1

n/d

Spirilloxanthin

25

91

Spirilloxanthin-2-ol

n/d

8

Carotenoid to

37

28

Table 1 Carotenoid composition and carotenoid to bacteriochloro- phyll energy-transfer (ET) efficiencies from isolates Thiocapsa BH-1 and Allochromatium BH-2

bacteriochlorophyll ET efficiency

Carotenoid characterization and quantification (% mol) and ET effi- ciencies (see also Fig. 5) were determined as previously described (Cogdell et al. 1981; Noguchi et al. 1990)

n/d, Not detected

isolated from intact cells show that both isolates have some of the lowest ET efficiencies for Car ? Bchla, yet, reported for purple bacteria: 28% for Thiocapsa BH-l and 37% for Allochromatium BH-2 (Table 1 and Fig. 5) when compared to comparable bacterial species. For example, the efficiency of Car ? Bchla ET is nearly 100% in photosynthetic membranes from wild-type Rhodobacter sphaeroides, where the major carotenoid present is sphe- roidene, (Cogdell et al. 1981; Noguchi et al. 1990; Goedheer 1959). While in Rhodopseudomans palustris, where the major carotenoid present is rhodopin (with some anhydrorhodovibrin and spirilloxanthin) (Takaichi 1999) the efficiency of Car ? Bchla ET drops to 60%

0.2

Absorbance (1-T), Fluorescence (a.u)

0.1

0.0

0.2

0.1

450

500 550 Wavelength (nm)

600

Fig. 5 A comparison of the room-temperature steady-state fractional absorption spectrum (1-T) (solid lines) and fluorescence excitation spectrum (dotted lines) in isolates Thiocapsa BH-1 (upper box) and Allochromatium BH-2 (lower box). The spectra were normalized to the Qx-transition of the bacterioclorophll a absorption band as previously described, see Cogdell et al. 1981

(Nishimura and Takamiya 1966). The efficiency of Car ? Bchla ET is further reduced in spirilloxanthin-rich species such as Rhodospirillum rubrum (nearly 100% spi- rilloxanthin) to 30–40% (Goedheer 1959). Thus the BH isolates are less efficient at transferring the energy absor- bed by the carotenoid molecules to the Bchl molecules, especially when compared to species containing the ‘‘same’’ carotenoids.

Identification of a novel carotenoid

The carotenoid content of the two isolates of purple bac- teria was analyzed by HLPC. In most cases the carotenoids could be identified with reference to known carotenoids isolated from Rhodospirillum rubrum. Both the Allochro- matium BH-2 and Thiocapsa BH-1 isolates contain carotenoids of the normal spirilloxanthin series (Table 1 and reference (Takaichi and Shimada 1992)), which are optimized for capturing the accessible wavelengths at 17.8 m depth. The identification of anhydrorhodovibrin and spirilloxanthin from BH-1 were confirmed by mass spectroscopy. Their relative molecular masses are 566 and 596, respectively.

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