Filtering activity on a pure culture of Vibrio alginolyticus by the solitary ascidian Styela plicata and the colonial ascidian Polyandrocarpa zorritensis: a potential service to improve microbiological seawater quality economically

We investigated and compared, by laboratory experiments, the filter-feeding activity on bacteria by the solitary ascidian Styela plicata and the colonial ascidian Polyandrocarpa zorritensis. Clearance rates and retention efficiencies were estimated by using, as only food source, the bacterial species Vibrio alginolyticus selected on account of its importance in aquaculture pathogenicity. The Cmax was 1.4±0.17Lh(-1)g(-1) DW for S. plicata and 1.745Lh(-1)g(-1) DW for P. zorritensis. The highest retention efficiency was 41% corresponding to a removed bacterial biomass of 16.34+1.71 μgCL(-1)g(-1) DW for P. zorritensis and 81% corresponding to a bacterial biomass of 32.28+2.15 μgCL(-1)g(-1) DW for S. plicata. Styela plicata resulted higher efficient than P. zorritensis in removing V. alginolyticus from seawater in experimental tanks, thus representing a more suitable biofilter to restore the quality of microbiologically contaminated waters including those where aquaculture is practiced. Present laboratory experiments represent the first contribution to the comparison of the filtration activity of the two ascidians, as well as to characterize the filtration process on bacterioplankton and pone the basis for future field works aimed to restore bacteriological polluted seawater.