Diversity in sponge holobiont functioning: the role of microbial abundance, host sponge identity, and geography

Abstract

Although marine sponges are frequently placed into functional categories based on their overall microbial abundance (High and Low Microbial Abundance [HMA and LMA]), recent research has shown that these communities and overall interactions are highly variable across individual species. To elucidate the role that overall microbial abundance, photosymbiont abundance, host sponge identity, and geography play in holobiont C and N cycling, we used a stable isotope approach in conjunction with multivariate and ellipse-based analyses. This allowed us to evaluate the core isotopic niche of a species or group and investigate factors driving the placement of individuals within isotopic space. Up to 14 sponge species were collected from three Caribbean sites (Panama, Honduras, and Belize), the Florida Keys, and a temperate reef off Georgia, USA. Some variation was attributed to geographic location, but this effect was greatly minimized when data from the temperate reef were excluded. Within individual sites, HMA and LMA groups filled unique niche space. However, host identity accounted for at least 70 percent of the variation in isotope values within a site, while photosymbiont and overall microbial abundance accounted for a much lower proportion of this variation. Together these data suggest that microbial symbionts may allow sponges to expand into novel niche space on tropical reefs, but species placement within isotopic space is relatively stable and is likely structured more by host-specific microbial community composition than by overall microbial or photosymbiont abundance.