EFFECTS OF LIGHT QUALITY ON THE PHYSIOLOGY AND THE ECOLOGY OF PLANKTONIC GREEN SULFUR BACTERIA IN LAKES

The effect of light quality on the selection of natural populations of Green Sulfur Bacteria (Chlorobiaceae) is considered to be a classic factor in the determination of their ecological niches. From the comparison among phototrophic bacterial populations of lakes, it is shown that brown and green pigmented groups of Chlorobiaceae have a differential distribution depending on depth. Statistical analyses prove that green species, which dominate at shallow oxic/anoxic boundaries, are correlated to light spectra enriched in long wavelengths, while brown ones are found when light spectra are enriched in the central region of the spectrum, as in deeper lake layers. Physiological experiments have been made with Chlorobium limicola and C. phaeobacteroides cultures placed under different light quality conditions, in order to verify these hypotheses made on a field data basis. Results show that red and white light has more positive effects on the green bacterium than on the brown. Blue and green light illuminations have opposite consequences. Therefore, the effect of shallow depths and Chromatiaceae shading - which also increases the proportion of long wavelengths in light spectra - benefits the bacteriochlorophyll-based strategies of green species. On the other hand, the carotenoid-based strategies of brown ones are favored by the light climates usually dominant at greater depths. Thus, brown species are considered to be singular adaptations of Chlorobiaceae to depth, where bacteriochlorophyll light-harvesting is strongly limited by light quality.