Temporal variability of Chlorobium phaeobacteroides antenna pigments in a meromictic karstic lake

The changes in the antenna pigment composition of Chlorobium phaeobacteroides growing in 2 meromictic basins of Lake Banyoles have been studied during a seasonal growth period. Changes involved both the increase in carotenoid content and the enrichment of the antenna in highly alkylated bacteriochlorophyll e (BChl e) homologs. Interestingly, the homolog composition at the end of the growth period was similar to those measured in laboratory cultures incubated at low Light intensity, which are characterised by high amounts of BChl e homologs possessing larger alkyl substituents at the C-8 position of the macrocycle (i.e, isobutyl and neopentyl). Furthermore, differences in the vertical distribution of both pigments were also observed. Seasonal variation in the antenna pigment composition could be interpreted as a response to gradual changes in light regime during growth, mainly caused by self-shading and by the development of an overlying population of the purple sulphur bacterium Chromatium minus, which acts as a biological filter for Light. The increase in both antenna pigments is discussed on the basis of recent findings on the spectral properties of BChl e and carotenoids and the role in the light adaptation processes is accordingly interpreted.