Mechanisms of sublethal copper toxicity damage to the photosynthetic apparatus of Rhodospirillum rubrum

Magnesium (Mg²⁺) is the ubiquitous metal ion present in chlorophyll and bacteriochlorophyll (BChl), involved in photosystems in photosynthetic organisms. In the present study we investigated targets of toxic copper binding to the photosynthetic apparatus of the anoxygenic purple bacterium Rhodospirillum rubrum. This was done by a combination of in vivo measurements of flash photolysis and fast fluorescence kinetics combined with the analysis of metal binding to pigments and pigment-protein complexes isolated from Cu-stressed cells by HPLC-ICPMS (ICP-sfMS). This work concludes that R. rubrum is highly sensitive to Cu²⁺, with a strong inhibition of the photosynthetic reaction centres (RCs) already at 2 μM Cu²⁺. The inhibition of growth and of RC activity was related to the formation of Cu-containing BChl degradation products that occurred much more in the RC than in LH1. These results suggest that the shift of metal centres in BChl from Mg²⁺ to Cu²⁺ can occur in vivo in the RCs of R. rubrum under environmentally realistic Cu²⁺ concentrations, leading to a strong inhibition of the function of these RCs.