CHEMICAL MEDIATION OF BACTERIAL SURFACE COLONIZATION BY SECONDARY METABOLITES FROM THE RED ALGA DELISEA-PULCHRA

We investigated the effects of halogenated furanones from the red alga Delisea pulchra on colonisation of surfaces by marine bacteria. Bacte rial abundance on the surface of D. pulchra, assessed using scanning e lectron microscopy (SEM), was significantly lower than on the surfaces of 3 co-occurring algal species, all of which lack furanones. There w as also a strong inverse correlation between bacterial abundance and f uranone content (previously determined) for different sections of the thallus of D. pulchra, consistent with inhibition of bacteria by furan ones. Based on these observations we experimentally investigated inhib ition of marine bacteria by furanones, initially testing the effects o f crude extract of D. pulchra (about 50 % of which is furanones) on th e growth of 144 strains of bacteria isolated from the surfaces of D. p ulchra, nearby rocks, or a co-occurring alga (Sasgassum vestitum). Thi s crude extract did not strongly inhibit growth of these bacteria; 79% of the strains grew at 50 pg ml(-1) of crude extract, and 63 % grew a t 500 mu g ml(-1). Inhibition of growth that did occur was strongly so urce dependent, with bacteria isolated from rocks the least affected, and strains from D. pulchra the most. As inhibition of growth did not provide an adequate explanation for the inverse relationship between l evels of furanones and bacteria abundance on D. pulchra, we proceeded to investigate the effects of these metabolites on other bacterial cha racteristics relevant to colonisation - attachment, swarming, and swim ming. Individual furanones or crude extract at natural concentrations strongly inhibited bacterial attachment in the laboratory and in the f ield. In laboratory assays, attachment of 3 strains isolated from rock s was much more strongly affected than that of 3 isolates from D. pulc hra, in contrast to the pattern for growth inhibition. We also tested individual furanones against swimming and swarming of the same 6 bacte rial isolates (3 from rocks, 3 from D. pulchra) used in the attachment assays. At least some furanones inhibited swarming or swimming at non -growth-inhibitory concentrations for all isolates, again indicating s pecific effects against bacterial characteristics. As for attachment, there were significant differences in the responses of different isola tes to furanones. We also found that the ability to swarm was widespre ad among these surface associated marine bacteria, suggesting that swa rming may be ecologically important in these systems. Overall, we foun d that the effects of furanones on bacteria varied among (1) furanones , (2) bacterial phenotypes, (3) different isolates and (4) different s ources of isolation (e.g. rocks or algae). This differential inhibitio n of different bacterial isolates or phenotypes by furanones, as well as affecting overall bacterial abundance on the alga, should have stro ng effects on the species composition of the bacterial community on th e alga's surface. The effects of furanones on specific bacterial colon isation traits are discussed in the light of recent evidence demonstra ting that furanones interfere with bacterial acylated homoserine lacto ne regulatory systems.