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JOURNAL OF PLANKTON RESEARCH | VOLUME 19 | NUMBER 8 | PAGES 1045-1068 | 1997
© Oxford University Press
research-article |
Phytoplanktonic excretion and bacterial reassimilation in an oligomesotrophic lake: molecular weight fractionation
Laboratoire de Biologie Comparée des Protistes, UPRES A CNRS 6023, Université Blaise Pascal Clermont-Ferrand, F-63177 Aubière cedex, France
Received on November 30, 1996; accepted on April 9, 1997 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: 1601 µm (mainly phytoplankton), 10.2 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 245%). 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.
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