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

The effects of light spectral distribution on the composition of phototroph ic microbial communities were analyzed in three metalimnetic levels (relati ve depth positions) of 41 lakes. Principal Component Analysis was used to c ompare light quality conditions reaching the populations of phototrophic mi cro-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 a daptations were defined, according to the wavebands that could be related t o the selection of these microbial groups. The micro-organisms adapted to u se red and near-infrared light - eukaryotic phytoplankton, Chloronema spp. and green-coloured Chlorobiaceae - predominated at shallow depths (speciall y in waters containing high gilvin contents) using their respective Q(y) ab sorption bands. The micro-organisms adapted to green-yellow light - phycoer ythrin-containing cyanobacteria, Chromatiaceae and brown-coloured Chlorobia ceae - were dominant in deep metalimnetic communities. Laboratory experimen ts with cultures of Chlorobium limicola and C. phaeobacteroides growing und er different light quality conditions showed that the green-coloured specie s had higher photosynthetic activity under red light, while the brown-colou red species was more active under green light. These results demonstrated t hat physiological differences between micro-organisms with different light- harvesting adaptations are responsible of their selection under different l ight quality conditions. This selection is experimented by Chlorobiaceae (a s it was previously indicated by other investigators) at the deepest positi ons of the metalimnetic communities (level 3), but also by Chromatiaceae an d Chloronema spp. at level 2 and by the eukaryotic phytoplankton and cyanob acteria at level 1.