Who’s a sponge(r)?
Microbiomes are the communities of microorganisms that live in, or on, host plants or animals. They are a subject quite literally close to our hearts (and every other major organ).
Over the past four years, the Human Microbiome Project has spent billions trying to define the species of bacteria that are associated with different parts of our bodies, known as the human reference microbiome. The researchers were looking for species that are consistent between humans, but what they actually discovered is that there is no such thing as a typical human microbiome. The bacteria living in your mouth could be very different to those living in my mouth, but they both do the same job. How could this be?
A recent study of marine sponges has begun to address this question. Sponges are probably best known for the fact that you can separate them into single cells (in a food blender) and they will re-aggregate to form a whole sponge, but it turns out they are also pretty useful models for studying microbiomes.
Six very different marine sponges, each with their own individual microbial communities, were collected in Sydney and on the Great Barrier Reef. The microbes associated with the sponges were very different to those found in the surrounding seawater, suggesting they have formed stable interactions (symbioses) with their hosts.
Remarkably, the authors reported that each community had conserved core functions. For example, they could all break down creatinine, which is produced by sponges and provides an important carbon and nitrogen source for the microbial symbionts, but they all used different biochemical pathways to do it. In addition, all of the communities were capable of anaerobic respiration – either through denitrification or through ammonium oxidation – but each used different enzymes to achieve the same result.
Finally, the authors revealed that the microbes in each sponge had a large number of systems for sharing DNA, which were mostly absent from microbes in the surrounding seawater. This led them to conclude that sharing genes, and gene functions, allows the microbes to evolve so they can better serve their hosts (known as convergent evolution).
This study supports the hypothesis that it’s the collection of genes (and gene functions) that are important to a host and it doesn’t really matter which microbes they are in, so long as they perform the correct jobs. This is a fascinating insight into the functions of microbiomes, communities that can literally mean the difference between life and death for their hosts – including you and me.
Fan L, Reynolds D, Liu M, Stark M, Kjelleberg S, Webster NS, & Thomas T (2012). Functional equivalence and evolutionary convergence in complex communities of microbial sponge symbionts. Proceedings of the National Academy of Sciences of the United States of America, 109 (27) PMID: 22699508