16S rRNA phylogeny of sponge-associated cyanobacteria

Phylogenetic analyses of 16S rRNA sequences of sponge-associated cyanobacteria showed them to be polyphyletic, implying that they derived from multiple independent symbiotic events. Most of the symbiont sequences were affiliated to a group of Synechococcus and Prochlorococcus species. However, other symbionts were related to different groups, such as the Oscillatoriales. Although both cyanobacteria and sponges have a very long evolutionary history (1, 3), little is known about the identity and phylogeny of cyanobacterial sponge symbionts. For marine sponges, symbiosis with cyanobacteria can be obligate or non-obligate according to the sponge species (2). Thus far, all attempts at culturing sponge-associated cyanobacteria have failed (13; unpublished data). It is thus not known whether sponge-associated cyanobacteria are able to survive outside their host. The most common sponge-associated cyanobacte-rium was not found in water samples (13, 15). In Chondrilla australiensis, this cyanobacterium is transmitted verticall