Time and again, microbes have opened doors for animals, allowing them to exploit niches that would have otherwise been denied to them by their basic animal-ness. By providing nutrients that are missing from the sap of plants, bacteria have allowed bugs to subsist on a diet of nothing else, turning them into the bane of greenery and greenhouses worldwide. By breaking down the tough and typically indigestible carbohydrates in plant matter, bacteria allowed mammals to become extreme grazers and gave rise to the thundering herds of Africa’s plains. By providing a source of energy that isn’t tied to sunlight, bacteria allowed worms, clams, and hundreds of other creatures to colonise the abyssal oceans, and lose their mouths and guts in the process.
These examples inform the common view that symbioses (partnerships) between animals and microbes lead to mutual benefit and expanding opportunities. But symbiosis also comes with costs and constraints. Microbes can bar animals from valuable opportunities, restrict their options, and place burdens upon them—all without causing infections or disease.
Odrade Nougué and Thomas Lenormand from the University of Montpellier have found a great example of such constraints in an animal that will be familiar to anyone who grew up in the 70s and 80s: the sea monkey.
These little creatures are more formally known as brine shrimp, or Artemia. As their name suggests, they live in salty water, but they evolved from freshwater ancestors. They cope with salt by efficiently pumping it out of their own bloodstreams. The saltier the water, the harder they have to work and the more energy they burn.
So you’d expect that Artemia does best in mildly salty water. In fact, they can’t tolerate the stuff. At more than 40 grams of salt per litre, they’re fine. Below that threshold, they’re less likely to survive. Bizarre! Surely, it should be the other way round?
Nougué discovered that Artemia’s gut microbes are behind this weird paradox. When Nougué raised Artemia larvae in sterile cultures, so they grew up without their usual coterie of microbes, these germ-free shrimp did better in low-salt water. Likewise, when she fed them a diet of yeast instead of their usual meals of algae, they also did better with less salt. Their usual preference for high-salt water only exists when they eat algae and carry microbes. Why?
These bacteria help to break down the carbohydrates in the algae, as well as detoxifying the many poisons in those mouthfuls. Without them, the shrimp wouldn’t be able to survive on their usual meals. And here’s the rub: the bacteria like salt. They grow less well at low salinities. So Artemia, to provide these partners-in-digestion with the ideal living conditions, is forced to live in water that’s saltier than it would naturally prefer, and is effectively barred from mildly salty places.
Here, then, is a case where microbes expand an animal’s ecological opportunities (by allowing it to eat an otherwise inaccessible source of food) but also constrain it (by forcing it out of low-salt environments). They provide a valuable service, but they inadvertently issue demands in return. These aren’t mild demands, either. Salinity is the single biggest factor that defines where Artemia lives, more so than temperature or predators or parasites.
There are other examples of such constraints. Insect symbionts, of the kind that allow bugs to suck on sap, tend to be more sensitive to high temperatures than their hosts, so their numbers plummet in hot weather. (What happens to those partnerships in a warming world, you might ask?) And in some cases, hosts and microbes could become so dependent on each other that they risk both becoming extinct—consider the case of the 13-year-cicada and its ridiculously degenerate bacteria.
Reference: Nougué, Gallet, Chevin & Lenormand. 2015. Niche Limits of Symbiotic Gut Microbiota Constrain the Salinity Tolerance of Brine Shrimp. American Naturalist. http://www.jstor.org/stable/10.1086/682370