12 páginas, 3 tablas, 4 figuras ; Partitioning of CO2 incorporation into oxygenic phototrophic, anoxygenic phototrophic, and chemolithoautotrophic guilds was determined in a freshwater lake (Lake Ciso´ , Banyoles, Spain). CO2 incorporation into the different types of microorganisms was studied at different depths, during diel cycles, and throughout the year. During winter holomixis, the whole lake became anoxic and both the anoxygenic and chemolithoautotrophic guilds were more active at the surface of the lake, whereas the activity of the oxygenic guild was negligible. During stratification, the latter guild was more active in the upper metalimnion, whereas the anoxygenic guild was more active in the lower metalimnion. Specific growth rates and doubling times were estimated for the most conspicuous phototrophic microorganisms. Doubling times for Cryptomonas phaseolus ranged between 0.5 and 192 days, whereas purple sulfur bacteria (Chromatiaceae-like) ranged between 1.5 and 238 days. These growth rates were similar to those calculated with a different approach in previous papers and indicate slow-growing populations with very large biomass. Overall, the annual total CO2 incorporation in Lake Ciso´ was 220 g C m _2. Most of the CO2 incorporation, however, was due to the chemolithoautotrophic guild (61% during holomixis and 56% during stratification), followed by the anoxygenic phototrophic guild (35 and 19%, respectively) and the oxygenic phototrophs (4 and 25%, respectively), making dark carbon fixation the key process in the autotrophic metabolism of the lake. ; This work was supported by both CAICYT grant 646/84 to RG and CICYT grant PB87-0183 to CPA, from the Spanish Ministry of Education and Science. During the final writing of this work, EOC was supported by the Programa Ramo´n y Cajal and VIARC project (REN2003-08333-C02-02/GLO) from the Spanish Ministerio de Ciencia y Tecnologı´a (MCYT) and JMG by MCYT project MICRODIFF REN2001-2120/MAR. ; Peer reviewed