ALLOCHTHONOUS DISSOLVED ORGANIC-MATTER AS AN ENERGY-SOURCE FOR PELAGIC BACTERIA AND THE CONCEPT OF THE MICROBIAL LOOP

Substantial evidence exists that allochthonous dissolved organic matte r (DOM) can provide an important carbon source for pelagic bacteria. O n the other hand, it is implicit in the concept of the 'microbial loop ' that the degradation of recalcitrant, allochthonous DOM should be re tarded in the pelagic environment, as bacteria able to utilize recalci trant DOM compounds for slow growth would be out-competed by faster-gr owing bacteria utilizing more labile DOM compounds. Several possible s olutions of this apparent paradox are suggested in this paper, includi ng formation of labile DOM from recalcitrant DOM by e.g. photochemical reactions, and mechanisms enabling the maintenance of a metabolically diverse bacterioplankton. These mechanisms include an explanation ana logous to Hutchinson's classical solution to the 'paradox of plankton' , and differential mortality of different populations within the bacte rioplankton enabled by selective grazing, infections by bacteriophages and predatory bacteria, and spatial micropatchiness.