I’m going to keep draining the pool analogy until it’s dry…
I’m going to miss the biological oceanography journal club at LDEO tomorrow in order to attend the Sequencing the Urban Genome symposium at the NY Academy of Sciences. It’s a step away from my primary field, but I’ve been to some human/built environment microbiome symposia before and found them to be fascinating. All microbial ecology gets at the same fundamental set of questions, even as the environment being studied changes dramatically, so it is often useful to see how a different group of researchers are addressing the same problems that we are. I was really looking forward to our paper discussion (to be led by Kyle Frischkorn), however, so to make up for it I decided to do a quick write up.
This week’s paper comes out of Ginger Armbrust’s lab at the University of Washington. I was fortunate to know lead author Shady Amin while he was a postdoc with Ginger, he’s now an assistant professor at NYU’s Dubai campus. The paper, Interaction and signalling between a cosmopolitan phytoplankton and associated bacteria, is another great example of what can happen when marine chemists team up with microbial geneticists, something that’s been happening a lot lately. In this case they were able to uncover a two-way communication pathway between a marine diatom and a bacterial affiliate, based on a well-studied mechanism of communication between plants and bacteria in the rhizosphere. We’ve known about plant-bacteria communication for a long time, but it was not obvious that a method of communication that works in the solid soil matrix (containing upwards of a billion bacteria per gram) would work in seawater (being liquid and containing only a hundred thousand to a million bacteria per ml). So we’re several decades behind the plant people in understanding the directed influence marine bacteria can have on phytoplankton. But moving fast.
Taken from Amin et al. 2015.
As shown in the figure above, the specific mechanism of communication reported by Amin et al. is based on the plant hormone indole-3-acetic acid (IAA). The bacterium, a Sulfitobacter, produces IAA, which kicks the diatom into overdrive, upregulating genes involved in cell division and carbon fixation. In turn the diatom releases the IAA precursor tryptophan, taurine, and DMSP, the latter two compounds serving as a carbon source for the bacterium.
What is really remarkable about this is that the relationship is highly specific. The diatom was tested with many different bacteria isolated from diatom cultures, and the bacterium was grown in the presence of many different phytoplankton isolates. Only for this specific pairing, however, did the diatom and bacteria both have a positive influence on the growth of the other.
To take the study to the next level Amin et al. measured IAA abundance in surface seawater in the North Pacific and looked for evidence of IAA biosynthesis pathways in several public metatranscriptomes. Not surprisingly they found plenty of both; measurable levels of IAA in all samples and what seems to me to be a reasonably high number of transcripts associated with IAA pathways (not sure what 107 transcripts L-1 actually means though…).
This was a really cool study, but I’m having a hard time wrapping my head around the implications. The specificity implied in the experiments contrasts sharply with the abundance of IAA and IAA biosynthesis pathways in the natural environment. And if the communication pathway provides a major benefit to the bacterium and/or the diatom, why is it rare among laboratory isolates? What is the cost to both the bacterium and diatom? For example, diatom respiration is suppressed by IAA, is maintenance deferred in favor of growth?
