Temperature thresholds for bacterial symbiosis with a sponge.

The impact of elevated seawater temperature on bacterial communities within the marine sponge Rhopaloeides odorabile was assessed. Sponges were exposed to temperatures ranging between 27 and 33 degrees C. No differences in bacterial community composition or sponge health were detected in treatments between 27 and 31 degrees C. In contrast, sponges exposed to 33 degrees C exhibited a complete loss of the primary cultivated symbiont within 24 h and cellular necrosis after 3 days. Furthermore, denaturing gradient gel electrophoresis (DGGE) and clone sequence analysis detected a dramatic shift in bacterial community composition between 31 and 33 degrees C. Within the first 24 h most of the DGGE bands detected in samples from 27 to 31 degrees C were absent from the 33 degrees C sponges whereas eight bands were detected exclusively in the 33 degrees C sponges. The 16S rRNA sequencing revealed that most of the microbes from sponges exposed to 27-31 degrees C had highest homology to known sponge-associated bacteria. In contrast, many of the microbes from sponges exposed to 33 degrees C were similar to sequences previously retrieved from diseased and bleached corals. The 16S rRNA clone library analysis also detected a significant shift in bacterial community structure. The 27 degrees C library was composed of Proteobacteria, Actinobacteria, Nitrospira, Acidobacteria and Chloroflexi whereas the 33 degrees C library contained sequences from the Proteobacteria, Bacteroidetes and Firmicutes. The clear shifts in community composition at elevated temperatures can be attributed to the loss of symbionts and to the establishment of alien microbial populations including potential pathogens. Breakdown of symbioses and stress in the sponge occurred at temperatures identical to those reported for coral bleaching, indicating that sponges may be similarly threatened by climate change.