Light-harvesting adaptations of planktonic phototrophic micro-organisms to different light quality conditions

Abstract

The effects of light spectral distribution on the composition of phototrophic microbial communities were analyzed in three metalimnetic levels (relative depth positions) of 41 lakes. Principal Component Analysis was used to compare light quality conditions reaching the populations of phototrophic micro-organisms containing different photosynthetic pigments. Results allowed to identify the optimal light quality conditions for the selection of each microbial group at their respective levels. Two general light-harvesting adaptations were defined, according to the wavebands that could be related to the selection of these microbial groups. The micro-organisms adapted to use red and near-infrared light – eukaryotic phytoplankton, Chloronema spp. and green-coloured Chlorobiaceae – predominated at shallow depths (specially in waters containing high gilvin contents) using their respective Qy absorption bands. The micro-organisms adapted to green-yellow light – phycoerythrin-containing cyanobacteria, Chromatiaceae and brown-coloured Chlorobiaceae – were dominant in deep metalimnetic communities. Laboratory experiments with cultures of Chlorobium limicola and C. phaeobacteroides growing under different light quality conditions showed that the green-coloured species had higher photosynthetic activity under red light, while the brown-coloured species was more active under green light. These results demonstrated that physiological differences between micro-organisms with different light-harvesting adaptations are responsible of their selection under different light quality conditions. This selection is experimented by Chlorobiaceae (as it was previously indicated by other investigators) at the deepest positions of the metalimnetic communities (level 3), but also by Chromatiaceae and Chloronema spp. at level 2 and by the eukaryotic phytoplankton and cyanobacteria at level 1.