PHOTOLITHOTROPHY, PHOTOHETEROTROPHY AND CHEMOHETEROTROPHY DURING SPRING PHYTOPLANKTON DEVELOPMENT (LAKE PAVIN)

In order to determine the relative importance of autotrophic and heterotrophic activities in both bacterial and phytoplanktonic communities in an oligomesotrophic lake, the size fractionation by differential filtration and the use of a bacterial inhibitor (gentamycin) were combined. The study was carried out at Lake Pavin during the spring planktonic bloom. Photosynthetic and photo- and chemohcterotrophic activities were measured from the assimilation of (NaHCO3)-C-14 and glucose-H-3, using a double labeling technique. The bacterial community was at low cell concentration (0.6 to 7 X 10(5) cells ml-1) and represented very low biomass values (0.9 to 11.5 mugC liter-1). The abundance of the phytoplankton varied between 0.5 and 1.8 X 10(6) cells liter-1, and biomass values ranged between 19 and 118 mugC liter-1. The diatom Melosira italica sp. subarctica (O. Mueller) was the largely dominant species in the meta- and hypolimnion. Inorganic fixation by photolithotrophy (mean value: 1.66 mg C m-3 hour-1) largely predominates over assimilation by photoheterotrophy (mean value: 0.93 mug C m-3 hour-1) or chemoheterotrophy (mean value: 2.42 mug C m-3 hour-1). However, because of the considerable underestimation of heterotrophic assimilation due to the experimental methods used, and because of the spatial and temporal separation of photolithotrophic and photo- and chemoheterotrophic activities, it is likely that the fixation of organic carbon by microalgae plays an important role in the survival of species and/or in competitive interactions, as the results with Monoraphidium contortum (Pasch. et Korschik.), the prevailing species in the epilimnion, would suggest.