MAJOR AND COMPARABLE ROLES FOR FREE-LIVING AND ATTACHED BACTERIA IN THE DEGRADATION OF PHAEOCYSTIS-DERIVED ORGANIC-MATTER IN BELGIAN COASTAL WATERS OF THE NORTH-SEA

Microbial degradation of Phaeocystis globosa colonies and their derive d organic matter by free-living and attached bacteria was investigated in Belgian coastal waters during the spring development of diatom-Pha eocystis colonies in 1994. Results obtained show concomitant evolution of hydrolytic ectoprotease and beta-ectoglucosidase ectoenzymatic act ivities with respect to the phytoplankton bloom, suggesting that the l ow biodegradability of Phaeocystis colonies leading to transient accum ulations of Phaeocystis-derived material in the coastal North Sea was not due to a lag phase required for the induction of beta-ectoglucosid ase. Up to 66 % of total bacterial biomass was found attached to parti cles larger than 10 mu m. While occurring always in low abundance comp ared to free-living bacteria, both the average specific biomass and gr owth rate of particle-attached bacteria were very high, i.e. 60 fg C c ell(-1) and 0.28 h(-1), respectively. Similarly, specific ectoenzymati c activities of particle-attached bacteria were on average about 5 tim es higher than those characterising free-living bacteria. Budget calcu lations show a 53 % contribution of Phaeocystis-attached bacteria to t he mineralisation of Phaeocystis-associated production, i.e. a 53:47 % role for attached and free-living bacteria, respectively.