Phytoplanktonic excretion and bacterial reassimilation in an oligomesotrophic lake: Molecular weight fractionation

Abstract

The release of photosynthetically produced compounds by phytoplankton and their reassimilation by bacteria were studied in the oligomesotrophic Lake Pavin (Massif Central of France). Samples were collected monthly from April to November 1993 at the central station of the lake at depths of 1, 10 and 15 m. Release of dissolved organic products and bacterial reassimilation were assayed by the 14C method which permits the quantitative and qualitative study of phytoplankton exudates. Gel-permeation chromatography was used to separate phytoplankton exudates into different molecular weight products. Size fractionation was used to measure 14C fixation in three fractions: 160-1 μm (mainly phytoplankton), 1-0.2 μm (mainly bacteria) and <0.2 μm. The average per cent release of photosynthetically produced dissolved organic compounds by chlorophyllian protists is 10 % of algal primary production (range 2-45 %). Gel-permeation chromatography showed that low-molecular-weight products (<700 Da) dominated this excreted organic matter pool and were an important source of carbon for heterotrophic microorganisms, allowing the establishment of a microbial loop, owing to the heterotrophic bacterial reassimilation of exudates constituting up to 90% (mean 42 ± 22%) of excreted products.