water temperature. A night temperature profile (Fig. 2, open circles) was made and is essentially the same as the daytime profile (Fig. 2, solid circles) except that the increased temperature in the surface water mass (0–5 m depth) disappears. This is to be expected as at night there is no solar energy input and hence no surface water heating. The temperature in the microbial layer remains the same as in the day time profile indicating there is little diffusion of heat from this water mass. The dominant phototropic bacteria present in this layer produced considerable amounts of extracellular polymeric material, which further reduces diffusion within the microbial layer. This is very evident in the dive video (see supplementary data), which shows that the body outlines of the SCUBA divers remain after they have passed through this layer. As the divers pass through the viscous layer a purple haze is observed, illu- minated by the torches. The accompanying dive video also shows that the water above and below the purple bacterial layer is clear, although the only light source below the bacterial mat is provided by the torches.
With the exception of moderately thermophilic purple sulfur bacteria, such as Chromatium tepidum the optimum growth temperature for most members of the family Chromatiaceae lie in the range 20–30C (Madigan 1986; Pfennig and Tru¨per 1989). In order to test the hypothesis that these indigenous phototropic bacteria gain an ecolog- ical advantage by increasing the temperature of their immediate environment we have determined the optimum growth temperature of both isolates (see ‘‘Methods’’). Data presented in Fig. 3 show that the optimum growth tem- perature for both isolates. For clarity, the absorbance data has been normalized to the maximum values for each profile. It is evident from these profiles that under labora- tory conditions Thiocapsa BH-1 (closed circles) and Allochromatium BH-2 (open circles) have temperature optima of about 35C, which is similar to the prevailing in situ temperature at 17.8–19 m depth (compare the position of the hatched bars in Figs. 2 and 3). Although this evidence is still preliminary it does nonetheless demon- strate that both isolates have temperature optima attuned to the prevailing thermal conditions which may give them a selective advantage and enable them to out compete other phototropic bacterial populations present at this depth horizon in the water column.
Purple sulfur bacteria belonging to the genera Allo- chromatium and Thiocapsa are able to harvest solar energy, using their bacteriochlorophyll and carotenoid chromoph- ores and transduce it into a useable form for the living cell. In vivo absorption spectra (Fig. 4) of both isolates show typical absorption maxima of Bchla, in the blue and near- IR, and carotenoids that absorb between 420 and 550 nm. The carotenoids absorb at wavelengths which are optimal for capturing the photons that penetrate to a depth of
Normalized Absorbance at 650nm
15 20 25 30 35 40 45 Temperature (°C)
Fig. 3 Growth temperature profiles of isolates Thiocapsa BH-1 (closed circles) and Allochromatium BH-2 (open circles). For clarity, the data has been normalized to the maximum absorbance value for each bacterium. The hatched vertical bar represents the in situ temperature domain where the phototropic purple sulfur bacteria are located in the natural water column, see the hatched horizontal bar in Fig. 2
17.8 m. Strong light absorption by the carotenoid-rich cells also prevents light scattering and explains why the water column of the South Andros Black Hole appears black (Fig. 1) not blue as observed in other Bahamian cave systems.
The in vivo absorption spectra of photosynthetic bac- teria show several electronic transitions in the near infra- red which correspond to their complement of light-har- vesting (LH) antenna complexes (Hawthornthwaite and Cogdell 1991; Zuber and Cogdell 1995). In BChla-con- taining photosynthetic bacteria the transitions between ca. 800 and 860 nm are attributed to peripheral light-harvest- ing (LH2) complexes. Transitions between ca. 870 and 920 nm are attributed to core light-harvesting (LH1) complexes. All photosynthetic bacteria contain LH1 com- plexes, however, not all species contain LH2 antennae. It is evident from Fig. 4 that both Black Hole isolates contain absorption peaks attributed to LH2 (Thiocapsa BH-1: 798, 821, and 867 nm; Allochromatium BH-2: 797, 809, and 874 nm). The absorption maxima for the LH1 complexes are at 914 and 912 nm for Thiocapsa BH-1 and Allo- chromatium BH-2, respectively.
The major carotenoid in BH-1 is spirilloxanthin, while in BH-2 it is rhodopin (Table 1). The Car ? Bchla effi- ciencies of ET have been determined by room-temperature steady-state, carotenoid/bacteriochlorophyll excitation- induced, bacteriochlorophyll fluorescence (Cogdell et al. 1981). Laboratory studies using membrane vesicles