This week’s BEACON Researchers at Work post is by MSU graduate student Mridul Thomas.
Diatoms, one of the most important & prettiest groups of phytoplankton
Every day, a staggering quantity of carbon is drawn out of the atmosphere into the oceans as a result of the silent actions of massive numbers of photosynthetic microbes. These microbes, called phytoplankton, incorporate around one hundred million tonnes of carbon per day into their cells, almost as much as all land plants put together. This community plays such a large role in the global cycle of carbon that fertilizing the oceans to boost their numbers is increasingly discussed as a potential solution to climate change (there’s serious problems with that idea, though). The flipside of this vision is that changes in ocean conditions that lower phytoplankton growth may lead to faster carbon accumulation in the atmosphere and more rapid climate change. And we are driving changes in the oceans, with measurable changes in acidity and temperature having taken place during our lifetimes.
As a student of oceanography, I’m ultimately interested in understanding how the biological, physical and chemical pieces of this story will unfold. It’s a fascinatingly complex system of interconnected parts, with human economic policy playing a major role. My own research concerns a modest chunk of this: I’m trying to understand the effects of changing temperatures on the ecology & evolution of phytoplankton species.
Work that I am currently doing with other members of the Klausmeier-Litchman lab at MSU’s Kellogg Biological Station shows that phytoplankton taken from around the world grow fastest near the average temperature that they experience in their part of the oceans. This is a sign that they’ve adapted to their environmental conditions in the past. As temperatures rise, they will no longer be well-adapted to local conditions, prompting both ecological and evolutionary changes. These evolutionary changes are the focus of a BEACON-funded project I am working on.
Adapting to higher temperatures is a little different from adapting to other environmental stresses, due to some interesting peculiarities of thermal physiology. These flow from the effects of temperature on biochemistry. In organisms, reactions are controlled by the action of enzymes, which change shape as part of the process. Higher temperatures increase their flexibility, allowing metabolic reactions to speed up. Beyond a certain temperature, though, the increased flexibility causes the enzymes to malfunction and degrade. One this starts to occur, fitness decreases very rapidly till the organism can no longer grow. As the example below illustrates, the fitnesses of all ectothermic species increase slowly with increasing temperatures till a peak (the optimum temperature), after which they drop off rapidly. This means that changes in temperature above the optimum of a species can be much more harmful than those below.
A number of important experiments examining this topic were done in Rich Lenski’s lab here at MSU using E. coli, but there are plenty of interesting questions remaining to be answered. Some are specific to the phytoplankton we’re interested in but nonetheless important when trying to understand these communities, such as, ‘How fast can these species adapt to changes in temperature?’ As is common, the more interesting questions transcend the specific taxon we’re working on. These include 1) how much of the genome undergoes changes as part of this adaptation?, 2) are there multiple genetic ways to adapt to high temperatures?, and 3) is it more ‘costly’ in terms of physiology to adapt to a fluctuating environment than a constant one?
So, in collaboration with the lab of Virginia Armbrust at University of Washington, we are performing an experiment on a marine diatom called Thalassiosira pseudonana to examine adaptation to four different temperature conditions (high and low temperatures, and within each of those, constant and fluctuating environments). Thalassiosira is a large (well, for a microbe) eukaryote that doesn’t growth quite as fast as bacteria like E. coli does, but it nonetheless divides more than once per day, which should allow us to see evolution take place relatively quickly. The experiment is fairly simple. We’ll have a number of test tubes filled with identical populations of T. pseudonana suspended in water baths maintaining the temperature. Every two days, we’ll supply the populations with fresh nutrients to grow on. Mutations and new genetic combinations from sexual reproduction will produce cells that differ in physiology, some of which will tend to tolerate higher temperatures better. These will grow fastest will slowly take over the test tubes kept at high temperatures, which we will track by measuring how temperature affects their fitness over time. Tracking changes in the other groups of tubes as well and subsequent genetic analysis will help us answer the questions I posed.
As I mentioned earlier, learning about the costs of adaptation through lab experiments should help us predict how phytoplankton communities will be affected by climate change. But one of the nice things about addressing questions such as these is that the answers can sometimes have broader implications. For example, our attempts to understand adaptation to environmental temperatures in marine phytoplankton showed patterns that were strikingly similar to those seen in terrestrial insects. This probably reflects similar underlying processes and physiology. Similarly, we expect that answers we get will not just be specific to phytoplankton, but will reflect patterns that can help us understand temperature adaptation in larger ectotherms as well, both aquatic and terrestrial. That’s a large part of the pleasure of this work: the experiments are small-scale, but the processes they help illuminate are global.
For more information about Mridul’s work, you can contact him at thomasmr at msu dot edu.
