Characterization of phytoplankton extracellular products (PDOC) and their subsequent uptake by heterotrophic organisms in a mesotrophic forest lake

The magnitude of phytoplankton release of photosyntheticallyproduced dissolved organic carbon (PDOC), the size distributionand chemical nature of the released substances, and the quantityand composition of PDOC that were immediately assimilated byheterotrophic bacteria were investigated in a meso- to oligotrophic,mesohumic forest lake by means of the ¹⁴CO₂ technique. The percentageextracellular release (PER) was comparatively high (up to {smalltilde}40%). The bulk of the released substances was of highmol. wt (>5000 dalton), with a minor contribution from lowmol. wt (<1000 dalton) compounds. Heterotrophic bacteriapreferentially assimilated the high molecular weight componentsof PDOC, and a polysaccharide of {small tilde}6000 dalton witha very short turnover time was a key substance in the carbonflow from autotrophic phytoplankton to heterotrophic bacteria.The shortcomings of the ¹⁴CO₂ method to assess phytoplanktonrelease of PDOC is critically evaluated and discussed.