Roles of the symbiotic microbial communities associated with sponge hosts in the nitrogen and phosphorus cycles

Abstract

Marine sponges are habitat-forming organisms in coral reefs. Many sponge species host highly abundant microorganisms inside their bodies, forming symbiotic relationships. Efficient nutrient cycling between the symbiotic microbial communities and their hosts is considered to be a vital mechanism to retain limited resources inside the holobiont, providing a competitive edge in an environment where ambient nutrient availability is extremely low. In this dissertation, I describe microbially mediated nitrogen fixation, ammonia oxidation and phosphorus accumulation in keystone sponge species, combining culture dependent and independent methods to characterize these functional pathways. Firstly, I characterized the symbiotic diazotrophic communities using nitrogenase gene marker nifH and by culturing representative diazotrophs. I found that various groups of cyanobacteria and heterotrophic bacteria actively express nifH genes during the entire day-night cycle, an indication that the nitrogen fixation potential was fully exploited by different N-fixing bacterial groups associated with their hosts. Archaea associated with marine sponges can actively affect the fate of fixed nitrogen in the holobiont. In order to elucidate the relative importance of host specificity and biogeographic background in shaping the symbiotic archaeal communities, I investigated these communities in sympatric sponges from the Mediterranean and the Caribbean. Based on 16S rRNA and amoA genes, the community structure in M. laxissima differed from that in Ircinia spp., including the sympatric sponge I. strobilina, indicating that host-specific processes control the sponge-archaeal communities. Compared with the nitrogen cycle, the phosphorus cycle has been little studied in sponge microbiology. I found significant accumulation of polyphosphate (polyP) granules in three common sponge species from Caribbean coral reefs. The identity of the polyP granules was confirmed by energy-dispersive spectroscopy and by fluorescence properties of the granules. Microscopic images revealed that a large proportion of microbial cells associated with sponge hosts contained intracellular polyP granules. Based on these findings, I propose a potentially important phosphorus sequestration pathway through symbiotic microorganisms of marine sponges. Considering the widespread sponge population and abundant microbial cells associated with them, these pathways are likely to have a significant impact on the nitrogen and phosphorus cycle in benthic coral reef ecosystems.