Terraforming—the act of transforming an inhospitable world into one that we can live on—has been a staple of science-fiction for decades. But some living things have been carrying out their own version of the practice for far longer.
The bacterium Vibrio fischeri is a squid terraformer. Although it can live independently in seawater, it also colonises the body of the adorable Hawaiian bobtail squid. The squid nourishes the bacteria with nutrients and the bacteria, in turn, act as an invisibility cloak. They produce a dim light that matches the moonlight shining down from above, masking the squid’s silhouette from predators watching from below. With its light-emitting microbes, the squid becomes less visible.
Margaret McFall-Ngai from the University of Wisconsin has been studying this partnership for almost 25 years and her team, led by postdoc Natacha Kremer, have now uncovered its very first moments. They’ve shown how the incoming bacteria activate the squid’s genes to create a world that’s more suitable for their kind. And remarkably, it takes just five of these microbial pioneers to start the terraforming (teuthoforming?) process.
Inside its egg, an embryonic squid is sheltered in a sterile bubble. When it hatches, it starts pumping seawater into its body, bringing a flood of bacteria into contact with a special light organ. Among these teeming hordes is V.fischeri. It gathers in the layer of mucus on the light organ’s surface, squeezes through six tiny pores, and multiplies to fill the various nooks and crannies within.
To understand the first moments of this partnership, Kremer raised baby squid in either normal Hawaiian seawater, or water that contained all the usual bacteria except for V.fischeri. By comparing the two groups, she could “eavesdrop into the very first conversations of an animal host with its coevolved partner”.
She found that the light organs can sense the presence of just five V.fischeri cells among millions of other bacteria. These microbes touch just two or three of the squid’s own cells at most, but that’s enough to change the activity of 84 genes across the whole light organ.
Several of these genes are involved in the squid’s immune system. The team suspects that they may activate antimicrobial chemicals in the mucus, to create an environment that’s inhospitable for other bacteria besides V.fischeri. This might explain why the light organ is exposed to hundreds of bacterial species, but only V.fischeri can colonise it.
Kremer also showed that V.fischeri switches on a squid gene that breaks down chitin, a large molecule found in the mucus around the light organ. The chitin is converted into a smaller molecule called chitobiose, which the bacteria can sense. And once they detect chitobiose, they become attracted to it.
So, when V.fischeri reaches the light organ, it starts destroying chitin and making chitobiose. Chitin is especially abundant near the pores and internal ducts, so these areas eventually teem with chitobiose. And that produces an alluring signal that draws other V.fischeri towards the pores and into the light organ itself. The first few bacteria that go down this route lay down a chemical trail that many more follow.
All of this happens in the hours after hatching. By tweaking the genome of their hosts, a few bacteria can make the squid more attractive to their peers and less conducive to their competitors.
Studies like this aren’t just relevant to squid. We are also colonised by trillions of bacteria in our first moments of life. The squid gets its bacteria from the surrounding water, and we get ours from our mothers—from her vagina if we’re born naturally, or from her skin if we’re born through C-section. Our microbes might not glow or hide us like the squid’s partners, but they do change the properties of our guts, help to control our immune system, and might even shape our behaviour as we grow up. Perhaps by studying the squid, we’ll learn more about how our own terraformers shape our bodies to their needs.
Reference: Kremer, Philipp, Carpentier, Brennan, Kraemer, Altura, Augustin, Hasler, Heath-Heckman, Peyer, Schwartzman, Rader, Ruby, Rosenstiel & McFall-Ngai. 2013. Initial Symbiont Contact Orchestrates Host-Organ-wide Transcriptional Changes that Prime Tissue Colonization. Cell Host and Microbe http://dx.doi.org/10.1016/j.chom.2013.07.006
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