Chemical control of bacterial epibiosis on ascidians

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Abstract
Two co-occurring ascidians of the Pacific subtidal, Polyclinum planum and Cystodytes lobatus, were found to exhibit remarkably different numbers of surface bacteria. On average, epibacterial densities on P. planum were 140 times greater than those on C lobatus as measured by plate-counting methods. Organic extracts of both ascidian species were tested for antimicrobial activities and effects on bacterial settlement. Bacterial settlement was measured using a new bioassay described in this paper. The results of the settlement bioassay clearly demonstrate that extracts of the little-colonized C. lobatus deter bacterial settlement while extracts of the heavily colonized P. planum induce settlement relative to the control. In addition to reducing bacterial settlement, extracts of C. lobatus colonies exhibited varying, but generally low, levels of antiraicrobial activity against, on average, one-half of the 36 strains of marine bacteria tested. On an interspecific level, including 11 ascidian species screened in a pilot study, fouling-deterring activities were correlated with epibacterial abundances while antimicrobial activity was not. It is concluded that the chemical control of bacterial settlement, possibly complemented by antimicrobial toxicity, provides an accurate model to explain the dramatically different bacterial abundance on the surfaces of the ascidian species studied. This investigation presents evidence that non-toxic metabolites influence bacterial settlement and, in this way, may function to regulate bacterial epibiosis on the surfaces of some marine invertebrates

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