Abstract
Sponges are one of the oldest lineages of animals on Earth and play key roles in shaping marine ecosystems. They are diverse, with more than 9600 species known to science, and come in a wide range of shapes, sizes, and colours. Sponges are, furthermore, known to host diverse communities of microbial symbionts, which play important roles in their physiology and ecology. In the present study, we sampled prokaryotic communities from 24 sponge species inhabiting coral reef flat and slope habitats off the coast of SW Celebes (Indonesia) in addition to sediment and seawater. The prokaryotic profiles of several sponge species were characterised for the first time. In line with previous studies, we revealed pronounced variation in diversity and composition among species with high microbial abundance (HMA) or low microbial abundance (LMA) status playing an important role in structuring prokaryotic communities across host sponge species. In addition to this, reef habitat (flat versus slope) also played a significant role in structuring prokaryotic communities. Most species in the reef slope habitat housed prokaryotic communities with a consistent profile of several cyanobacterial and prokaryotic OTUs, whereas these OTUs were largely absent from sponges inhabiting the reef flat habitat. Instead, they tended to house highly abundant bacterial populations related to the Synechococcus spongiarum group. We propose that specific strains of S. spongiarum may play a key role in enabling their host sponges to survive in an, otherwise, inhospitable environment (e.g., high irradiance and temperature) and, thus, help to explain differences in sponge composition between coral reef flat and slope habitats.