Comparing bacterial communities in two closely related sponge species.

A review of the paper : Sponge associated bacteria are strictly maintained in two closely related but geographically distant sponge hosts, Montalvo, N and Hill, T (2011) Applies and environmental microbiology, 77, p7207-7216.

Marine sponges are invertebrates that feed by actively pumping water through pores to filter out the microbes from the seawater, but have no true organs or gut. Previous studies have shown that there may be symbiotic bacteria that play a role in digestion containing chemicals with bioactive properties. Several of these compounds have shown promise for pharmaceutical leads, which is one reason why the study of sponge-related symbioses is important. They also make good models for studying ancient and complex symbioses.

This paper is a study of two closely related species of giant barrel sponges associated with coral reefs: Xestospongia testudinaria, which can be found in the indo-pacific, and Xestospongia muta, which can be found in the Caribbean. These two species have been geographically separated for over 3 million years but are so closely related, they could potentially be thought of as the same species if not for the location. This means that they would once have had a common ancestor. The aim of this paper is to compare the bacterial communities between these two species and gain an insight into the specificity of the relationship between sponges and their associated bacterial communities. The method was to take full length bacterial 16S rRNA gene sequences of 3 individuals of each species and classify the bacteria found. The bacteria retained by both species are likely to be symbionts rather than part of the microbial film.

The study found that the bacteria Chloroflexi dominated both species whereas some other bacteria were abundant in one species but rare in the other and vice versa. When looking at all 6 individuals, there was no significant difference in the bacterial communities. However when the individuals were pooled into species, there was a significant difference (P<0.01), which showed that there was a specific difference between species.

These results show that although the bacterial communities of the two species are very similar, they are not identical, so must have specific communities. It was also shown that the bacterial communities of the two species were more similar to each other than to the surrounding seawater and to other more distantly related sponges. The authors conclude that this supports the hypothesis that the bacteria are likely to also have a common ancestor and the difference between the two species may have arisen due to speciation in the last 3 million years because of the geographical separation.

This paper is successful in concluding that the bacterial communities of sponges are species specific so therefore must have specific requirements even though the environments that the sponges provide are very similar. However, I think that although the data from this paper may support evolutional theories, this conclusion is made to strongly for the evidence at hand and further study would have to be done to be able to make this conclusion more confidently.