AEROBIC DEGRADATION OF PHYTOPLANKTON DEBRIS DOMINATED BY PHAEOCYSTIS SP IN DIFFERENT PHYSIOLOGICAL STAGES OF GROWTH

The aerobic degradation of phytoplankton debris collected in Dutch coa stal waters on 2 days in 1991 (15 April and 8 May), representing 2 phy siological stages of a phytoplankton spring bloom dominated by Phaeocy stis sp., was studied in batch culture experiments. The bacterial prod uction and the concentrations of particulate organic carbon (POC) and dissolved organic carbon (DOG) were monitored over a period of 102 d. Bacterial numbers and biomass were followed for 35 d. All experiments showed a rapid metabolic response of bacteria and a sharp decrease in the concentration of POC and DOC during the first days of the experime nts. Thereafter bacterial production rates remained constant, and POC and DOC decreased slowly. Apparently, the phytoplankton debris consist ed of a labile, rapidly degradable fraction and a refractory, slowly d egradable fraction. The labile fraction comprised approximately 50 % o f the debris, and was degraded with a bacterial carbon conversion effi ciency of between 10 and 20%. There were no indications that antibioti c compounds present in the algal debris inhibited the degradation. Acr ylate, a proposed antibiotic compound which was present in the algal d ebris, was rapidly degraded in a control experiment. The percentage of the material that had been degraded after 102 d was highest in the ex periment with material collected in May. It was concluded that during the early phase of the bloom, more refractory compounds are produced.