In the words of Carl Sagan, “The universe is a pretty big place. If it's just us, it seems like an awful waste of space.” That’s just it, it’s not ‘just us’ and we’re not alone. By this, I mean that each of us and all other animals and plants inhabiting our pale blue dot in the vast cosmic soup of this Universe is inhabited by upwards of trillions of microbes. In fact, some say that we’re just “animals in a bacterial world” (McFall-Ngai et al. 2013). Alas, don’t fear these microbes – that we have categorized into the bacteria, viruses, Archaea, and fungi, and form communities called a ‘microbiome’ – because many are beneficial!
The functional diversity of this community is astounding and spans from nutrient acquisition and metabolism to serving as a protective barrier from pathogens and buffer against environmental variation (McFall-Ngai et al. 2013). As a marine ecologist, it’s the role of environmental variation that has particularly intrigued me – or ‘caught my eye' – because the ocean is unbelievably dynamic and ever changing. Not all plant and animal life stages are equally susceptible or resistant to variation in the vagaries of the ocean such as temperature, salinity, and food availability.
Fig. 1: Left: Sputnik, the first artificial Earth satellite launched by the Soviet Union in 1957; Right: a sea urchin larva.
Young animal larvae are commonly considered the most vulnerable life stage and often enter the water column as eggs, where they are responsible for finding phytoplankton (food) and ‘real estate’ on the seafloor to begin adult life. One challenge is that the type and quantity of foods they prefer are difficult to come by. Sputnik-shaped sea urchin larvae (Figure 1) have a solution for this: when food availability is low, they increase the overall size of their feeding apparatus in order to filter a larger volume of seawater (Miner, 2005). This plasticity in the larval phenotype is an animal-centric way of acclimating to variation in the feeding environment.
A microbiome, on the other hand, can help ‘cope’ with this same type of environmental variation by changing which bacterial taxa are present (community membership) and how many of each taxon are part of the community (relative proportion). In the early stages of my PhD dissertation I quickly recognized that there are host- and microbiome-centric means to acclimate to the environment, but what was unknown was whether these mechanisms were linked, or synergistic. Specifically, do larval sea urchins associate with a phenotype-specific microbial community?
Fig. 2: Left: Purple sea urchin, Strongylocentrotus purpuratus; Center: Red sea urchin, Mesocentrotus franciscanus; Right: Green sea urchin, S. droebachiensis. Larval forms for each are below.
In a paper recently published in Nature Communications (Carrier and Reitzel, 2018), I present data from the larvae of three species of sea urchins found in the Salish Sea: Strongylocentrotus purpuratus, Mesocentrotus franciscanus, and S. droebachiensis (Figure 2). What these experiments show is that, when each species of urchin larvae expresses plasticity in the larval phenotype, the microbial communities they associate with are distinct from larvae that have yet to express plasticity. This response is directly correlated to the magnitude at which phenotypic plasticity is expressed. Furthermore, the microbiome of larval sea urchins is specific to diet concentration (number of phytoplankton cells in the water) and each developmental stage. This phenotype-specific pattern can also be detected independently of the other factors (Figure 3), and is bi-directional.
Fig. 3: Decoupling phenotype-specific microbial communities from diet, development, and time for two species of echinoid larvae. Community similarity of the associated microbiota for Strongylocentrotus purpuratus (a) and Mesocentrotus franciscanus (b) larvae at the 8- (maroon), 6- (orange), and 4-arm (yellow) stage having been fed 10,000; 1000; and 100 cells mL−1, respectively, in comparison with larvae pre-expression (blue) and post-expression (purple and yellow) of phenotypic plasticity. This figure and the figure caption were taken figured from Carrier and Reitzel (2018).
Host- and microbiome-centric means of acclimating to environmental variation appear to be synergistic, and sea urchin larvae likely form specific associations/symbioses with bacteria to increase their odds of survival in the water column. If and how these phenotype-specific microbial communities benefit the Sputnik-shaped urchin larvae is unknown. But after all, “Somewhere, something incredible is waiting to be known.”
Tyler Carrier is a PhD candidate in Dr. Adam Reitzel’s laboratory at the University of North Carolina at Charlotte. His first work at Friday Harbor Labs in the summer of 2016 was thanks to support from the Charles Lambert Memorial Endowment.
Carrier T.J. and A.M. Reitzel. 2018. Convergent shifts in host-associated microbial communities across environmentally elicited phenotypes. Nature Communications: 9: 952.
McFall-Ngai M., Hadfield M.G., Bosch T.C.G, and 23 others. 2013. Animals in a microbial world, a new imperative for the life sciences. Proceeding of the National Academy of Science: 110, 3229-3236.
Miner B.G. 2005. Evolution of feeding structure plasticity in marine invertebrate larvae: a possible trade-off between arm length and stomach size. Journal of Experimental Marine Biology and Ecology: 315, 117-125.
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