by Johnna L. Roose
Fig. 1. A Happy Couple. Source.
Do you ever look at a couple and wonder… ‘Why are they together? What does X see in Y. I just don’t get it. Is X in it only for the money’? Who doesn’t at times ponder about such matters? There’s practically an entire economy based on it. However, you didn’t find this article while waiting to check out in the grocery line. This post isn’t about celebrities; it’s about microbes. No matter—some microbe relationships have us scratching our heads and, to the same degree, casting premature judgment.
Fig. 2. Scanning electron micrographs of “Chlorochromatium aggregatum”. (A,B) Epibionts are shown in false color green, central bacteria in false color purple. (B) The central rod is dividing, and most of the epibiont cells have dissociated from the consortium. Scale bar in (B) equals 1 μm. Source.
Two Heads Beat One
One great example of such a conundrum is represented by the phototrophic consortium Chlorochromatium aggregatum, (this invalid name derives from an ancient era, when it was thought to be a single organism). In fact this is an unusually sophisticated prokaryotic symbiosis between a central motile heterotroph (Candidatus Symbiobacter mobilis) surrounded by multiple epibiont cells of a photoautotrophic green sulfur bacterium (Chlorobium chlorochromatii) (Figure 1). At first glance, this seems like a reasonable pairing, to combine mobility with photosynthetic autotrophy for the benefit of both organisms. Taking a deeper look at this relationship raises more questions. According to Don Bryant, "The relationship of these organisms is that of a highly dysfunctional marriage between a workaholic/nitrogen fixer/primary producer that is strictly anaerobic, and an oxygen-requiring, stressed-out, addicted soccer person that provides the bus, the driver and the GPS system.” Thus, in the microbial world as elsewhere, relationships are more complicated than they seem to be.
To gain insights as to how these organisms make this tryst work, researchers have, not surprisingly, turned to genomics for answers. In Genome Biology, Liu and co-authors report the complete genome sequences of both Chl. chlorochromatii and Ca. S. mobilis (Figure 2). They uncovered plenty of interesting details on the intimate relationship between these two organisms.