BACTERIAL INDUCTION AND INHIBITION OF A FAST NECROTIC RESPONSE IN GRACILARIA-CONFERTA (RHODOPHYTA)

Of 45 bacterial isolates from healthy tips of Gracilaria conferta (Sch ousboe ex Montagne) J. et G. Feldmann, 29% were identified as 'conditi onal inducers' of an apical necrosis. That is, the isolates induced ne crotic tips in G. conferta within 16 h after elimination of most of th e resident microflora from the alga. Several disinfectants and antibio tics were screened for their ability to induce algal susceptibility to the bacteria and to suppress uncontrolled appearance of tip necrosis. Treatment with 100 mg L-1 Cefotaxim + 100 mg L-1 Vancomycin over thre e days was the least damaging and most efficient. Tip necrosis was rel ated to isolates of the Corynebacterium-Arthrobacter-group and to the Flavobacterium-Cytophaga-group. The damaging effect occurred due to th e bacterial excretion of active agents and was not correlated with a c apability to degrade agar. The damaging influence of four Cytophaga-li ke strains was inhibited by 20 of 40 isolates. This protective effect was caused by very different organisms. In five of six cases examined further, the effect was not cellbound, but due to the excretion of age nts. These were not antimicrobially active, but inactivated necrosis-i nducing excretions. These results indicate that epiphytic bacterial de gradation or inactivation of damaging agents is a protecting factor in Gracilaria, which prevents the alga from being harmed by epiphytes.