Culturable epibacteria of the marine sponge Ircinia fusca: temporal variations and their possible role in the epibacterial defense of the host

Abstract

The influence of environmental changes on the epibacterial population of the marine sponge Ircinia fusca (Carter) was evaluated by using a bacterial culture-based approach. Sponge surface-associated bacteria were isolated, enumerated and cultured during 5 different collection periods. These bacteria were assessed for antibacterial activity against fouling bacteria and for auto-inhibition with their extracts, using the ecologically relevant agar plug method. Morphological studies of all culturable sponge-associated bacteria were carried out and the isolates with antibacterial potential were further identified using 16S rRNA gene sequences. It was found that the epibacterial population of the sponge is influenced by temporal changes in the environment. However, the bacterial strain belonging to the genus Bacillus, which displayed antibacterial activity against fouling bacteria, was found to be associated with the sponge in every collection period, indicating close association of this strain with the sponge I. fusca. The sponge seems to have selective control of the bacterial epibiosis on its surface. The antibacterial assay of sponge extract against its associated bacteria showed that the sponge promotes the growth of certain antibiotic producing strains of the genus Bacillus, which may indirectly help the sponge in governing epibacterial populations on its surface by deterring settlement of other microbes from the vicinity. In summary, this investigation explores the possible role of sponge surface-associated bacteria in the epibacterial defense of the host, by considering the temporal variations in the habitat.