Microscopical techniques reveal the in situ microbial association inside Aplysina aerophoba, Nardo 1886 (Porifera, Demospongiae, Verongida) almost exclusively consists of cyanobacteria

Sponges (Porifera) are aquatic, sessile filter feeders. As such they are permanently exposed to bacteria in the seawater. Molecular data recovered from sponges by PCR shows a high diversity in bacterial DNA. Hence, sponges are considered to live in close association with a diverse and abundant bacterial community. To recover the spatial distribution of bacteria in sponges we retrieved histological sections of Aplysina aerophoba fixed in situ. By combining signals from fluorescence in situ hybridization (FISH), light microscopy and scanning electron microscopy we revealed a detailed histological picture of the spatial organization of the sponge microbial association within the sponges. Our histological results confirm a high abundance of cyanobacteria inside A. aerophoba while other living bacteria are almost absent. This detailed insight into sponge microbiology could only be achieved by the combination of careful sample preparation and different microscopical and histological methods. It also shows the need to confirm molecular datasets in situ and with a high spatial resolution.