Growth-rate-dependent bacteriochlorophyll c / d ratio in the antenna of Chlorobium limicola strain UdG6040

The green sulfur bacterium Chlorobium limicola UdG6040 exhibited a significant change in the spectral properties of its antenna when transferred from batch culture to a sulfide-limited chemostat. In steady-state continuous cultures, the in vivo absorption maximum of the culture changed to shorter wavelengths according to the dilution rate. The maximum difference observed was of 15 nm when cells were growing at 0.087 h–1. HPLC analyses revealed that the observed spectral change was caused by a partial enrichment of the original BChl c-containing antenna with BChl d molecules together with a change in the homolog composition of both pigments. The relative amount of BChl d reached a maximum value of 50% when cells were growing at 0.087 h–1. The content of BChl d decreased to less than the 22% when the dilution rate was diminished to 0.015 h–1. An unbalance of pigment synthesis at high dilution rates is suspected to be responsible of the changes observed in the antenna composition. Chlorosomes isolated from Chl. limicola UdG6040 growing at 0.070 h–1 contain organised pools of BChl c and BChl d in equal amounts.