Inhibition of photosynthesis and bleaching of zooxanthellae by the coral pathogen Vibrio shiloi

Vibrio shiloi is the causative agent of bleaching (loss of endosymbiotic zo oxanthellae) of the coral Oculina patagonica in the Mediterranean Sea. To o btain information on the mechanism of bleaching, we examined the effect of secreted material (AK1-S) produced by V. shiloi on zooxanthellae isolated f rom corals. AK1-S caused a rapid inhibition of photosynthesis of the algae, as measured with a Mini-PAM fluorometer, The inhibition of photosynthesis was caused by (i) ammonia produced during the growth of V. shiloi on protei n-containing media and (ii) a non-dialysable heat-resistant factor. This la tter material did not inhibit photosynthesis of the algae by itself but, wh en added to different concentrations of NH4Cl, enhanced the inhibition appr oximately two- to threefold. Ammonia and the enhancer were effective to dif ferent degrees on zooxanthellae isolated from four species of coral examine d. In addition to the rapid inhibition of photosynthesis, AK1-S caused blea ching (loss of pigmentation) and lysis of zooxanthellae. Bleaching was more rapid than lysis, reaching a peak (25% bleached algae) after 6 h, The fact ors in AK1-S responsible for bleaching and lysis were different from those responsible for the inhibition of photosynthesis, because they were heat se nsitive, non-dialysable and active in the dark. Thus, the coral pathogen V. shiloi produces an array of extracellular materials that can inhibit photo synthesis, bleach and lyse zooxanthellae.