In 2005, Science Magazine published a special issue exploring 125 big unanswered questions in science in celebration of their 125th anniversary. One of the questions that I keep thinking about (as many biologists do, I’m sure), is this question of, “what determines species diversity?“.
I know: This is an overwhelmingly big and important question in biology. So I decided to focus on just one hypothesis addressing this question. That is trade-offs across heterogeneous environment promoting species diversity. For example, imagine there are two species: species A grows slow but has a thick shell to protect itself against predators. Species B grows fast but has a very thin shell and therefore is susceptible for predation. In this scenario, there is a trade-off between growth and predation resistance. When there are many predators, Species A will survive and reproduce more but when there are no predators, species B will perform better. So when predation is variable (spatially and/or temporally), two species can coexist. Similarly, when there are many species, trade-offs across heterogeneous environments can promote diversity. This pattern, in fact, has been demonstrated in some animal and plant species.
But how about microbes? Considering their fast generation times, large population size and high gene flow, it’s not clear whether they face similar trade-offs as macro-organisms. In fact, in lab conditions, trade-offs in microbial systems are undetectable. Unfortunately, we know very little about how microbes respond to heterogeneous environments in field conditions, mainly because it’s very difficult to test it. But recently, Porter and Rice took up this challenge.
In this study, Porter and Rice (Evolution 2013) examined a trade-off between tolerance to heavy-metal nickel and growth rate (in absence of nickel stress) among rhizobium strains collected from two distinct soil types: serpentine and non-serpentine. Serpentine soil typically has a high nickel level as well as an extremely low calcium:magnesium ratio. Rhizobium strains were isolated from the roots of leguminous plants (Acmispon wrangelianus or Medicato polymorpha L.) growing in either serpentine or non-serpentine soil (N = 450 isolates!). Using these isolates, they conducted an experimental growth assay to calculate nickel tolerance by growing them in presence and absence of nickel. They predicted that rhizobia are adaptively differentiated between soil types and that the trade-off between nickel tolerance and growth maintains phenotypic diversity of rhizobium strains in heterogeneous environments.
What they found:
They detected a trade-off between nickel tolerance and growth for all the isolates collected from A. wrangelianus (but not for the invasive M. polymorpha. Check out the paper for more details). They also found that rhizobia from serpentine soils had higher fitness in the presence of nickel and rhizobia from nonserpentine soils had higher fitness in the absence of nickel, evidence for adaptive differentiation.
Why it’s cool:
This study shows that trade-offs are important for microbes too. Trade-offs may be one of the drivers of promoting microbial diversity in nature. With recent progress in molecular biology, I’m sure we’ll learn more cool things about microbial communities in nature. However, this study is elegantly designed to test the hypothesis without the help of fancy molecular techniques (well, they still sequenced 16S to identify rhizobium genus).
Stephanie S. Porter and Kevin J Rice 2013. Trade-offs, spatial heterogeneity, and the maintenance of microbial diversity. Evolution. doi:10.1111/j.1558-5646.2012.01788.x