CHEMICAL CONTROL OF BACTERIAL EPIBIOSIS ON ASCIDIANS

Two co-occurring ascidians of the Pacific subtidal, Polyclinum planum and Cystodytes lobatus, were found to exhibit remarkably different num bers of surface bacteria. On average, epibacterial densities on P. pla num were 140 times greater than those on C lobatus as measured by plat e-counting methods. Organic extracts of both ascidian species were tes ted for antimicrobial activities and effects on bacterial settlement. Bacterial settlement was measured using a new bioassay described in th is paper. The results of the settlement bioassay clearly demonstrate t hat extracts of the little-colonized C. lobatus deter bacterial settle ment while extracts of the heavily colonized P. planum induce settleme nt relative to the control. In addition to reducing bacterial settleme nt, extracts of C. lobatus colonies exhibited varying, but generally l ow, 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-deter ring activities were correlated with epibacterial abundances while ant imicrobial activity was not. It is concluded that the chemical control of bacterial settlement, possibly complemented by antimicrobial toxic ity, provides an accurate model to explain the dramatically different bacterial abundance on the surfaces of the ascidian species studied. T his investigation presents evidence that non-toxic metabolites influen ce bacterial settlement and, in this way, may function to regulate bac terial epibiosis on the surfaces of some marine invertebrates.