Carbon dioxide fixation in the dark by photosynthetic bacteria in sulfide-rich stratified lakes with oxic-anoxic interfaces

Carbon fixation was analyzed in a series of stratified lakes with oxygen-sulfide interfaces following C-14-bicarbonate incorporation into oxygenic phototrophic, anoxygenic phototrophic (photosynthetic sulfur bacteria; PSB), and chemolithoautotrophic guilds (dark fixation processes), respectively. One of the lakes (Lake Ciso, Banyoles) was studied in more detail using microautoradiography of C-14-labeled cells and microscopic observations during day and night. Dark carbon fixation was high for many of the depths sampled in all the lakes studied and there was a correspondence between carbon (C) fixation in the dark and abundance of photosynthetic purple sulfur bacteria (PSB). After in-situ dark incubations, microautoradiographies revealed labeled cells actively incorporating inorganic carbon belonging to both unidentified rod bacteria but also to PSB (Amoebobacter sp. and Thiocystis sp.). The concentration of labeled rod bacteria was very similar in light and dark incubations, as well as in day and night samplings, and had a tendency to increase in the anoxic dark hipolimnion. Surprisingly, PSB consistently incorporated inorganic carbon in dark incubations at high rates during the day but much less at night. This suggests that photosynthetic organisms are capable of substantial dark C fixation after being subjected to light, and that "light dependent'' dark carbon-fixation activity is an issue that has to be carefully addressed in future experiments.