Ecophysiology of microbial communities associated with marine sponges Ircinia strobilina and Mycale laxissima

Abstract

Marine sponges are hosts to many microorganisms that can constitute up to 60% of the sponge biomass. Complex symbiotic interactions may exist between sponges and associated microorganisms. My primary goal was to develop a model sponge system for laboratory studies of complex symbioses between bacteria and marine invertebrates. Two sets of aquaculture systems were designed in order to optimize the conditions for culturing marine sponges Mycale laxissima and Ircinia strobilina. Bacterial communities associated with the sponges were characterized using culture-based and molecular techniques. There was a substantial change in the diversity and composition of bacterial communities upon transfer into aquaculture. This work shows a potential for maintaining healthy marine sponges in closed aquaculture systems, a necessary foundation for using sponges in aquaculture as a laboratory model. There is a limited knowledge of the contributions of bacteria to sponges, and thereby indirectly to the coral reef ecosystem. I examined the role of sponge-associated bacteria in nitrogen fixation. Nitrogen fixation was demonstrated in sponges using nitrogen isotopic composition. The potential for nitrogen fixation by symbionts was assessed by amplification of nifH gene fragments from total DNA and RNA extracted from sponges. Diverse nifH genes were detected, and gene expression studies proved that nifH genes were expressed in sponge-associated bacteria. These nifH transcripts were closely related to cyanobacterial nifH genes. This is the first demonstration of the expression of bacterial genes in sponge symbionts. Cyanobacterial symbionts are likely fixing nitrogen and provide fixed nitrogen to their hosts. Quorum sensing in sponges is hypothesized to play a role in colonization by symbionts and the regulation of symbiosis between sponges and associated bacteria. Alpha- and gamma-proteobacterial isolates were tested for the production of acyl homoserine lactones (AHLs) using diffusion bioassays coupled with thin-layer chromatography fractionation. Isolates affiliated with the Silicibacter-Ruegeria subgroup of the Roseobacteria clade were the main producers of AHLs. These findings demonstrate that sponge associated bacteria show a high incidence of production of complex suites of AHLs involved in cell signaling. This research contributes to the field of sponge microbiology by advancing the understanding of the roles of symbionts and interactions with their hosts.