A BACTERIAL FACULTATIVE PARASITE OF GRACILARIA-CONFERTA

Bacterial epiphytes of Gracilaria conferta were quantified. Saprophyti c bacteria reached 350 times and agar degraders 25000 times higher num bers g-1 algal wet wt on tissues infected with the 'white tips disease ', as compared to healthy tissues. A bacterial inducing agent of the ' white tips disease' was detected. Addition of 10(2) to 10(3) cells of this isolate ml-1 medium led to increased rates of infection. This eff ect did not occur if the isolate was autoclaved before addition. The v irulent bacteria could always be isolated from infected tissues. It fr equently, but not always, infected G. conferta and should be regarded as a facultative parasite. Several factors influenced the disease deve lopment. Temperatures above 20-degrees-C, in combination with photon f lux densities of more than 200 muE m-2 s-1, increased the rate of infe ction. Relatively low amounts (more than 25 mug ml-1) of certain organ ic nutrients (peptone and yeast extract) led to strong manifestations of the disease. Addition of agar did not cause any symptoms, while 5 m g l-1 of the antibiotic rifampicin prevented the alga from being infec ted.