Can marine phytoplankton evolve to cope with ocean acidification?

One of the most pressing questions in evolutionary biology is whether or not species will be able to evolve in response to global climate change and its associated effects such as ocean acidification. A paper published earlier this year in Nature Geoscience provides experimental evidence that one of the ocean’s most important species, Emiliania huxleyi, is capable of quickly adapting to increased levels of acidification through natural selection.

Tiny organism, big impact

The fate of certain organisms will have an outsized impact on the world’s ecosystems, and E. huxleyi, a species of phytoplankton, is one of them. It’s hard to overstate the importance of phytoplankton; these tiny photosynthesizing drifters form the base of nearly all the ocean’s food webs, and are responsible for half of all photosynthetic activity on the planet. Specifically, E. huxleyi is a coccolithophore, a type of single-celled algae that makes up an important part of the ocean’s phytoplankton. The “coccoliths” that cover the cells are plates made of calcium carbonate, the same substance stony coral reefs are made of. Just as ocean acidification threatens the ability of corals to produce their reef-building calcium carbonate structures, it threatens to do the same for coccolithophores.

The coccoliths themselves have their own ecological significance. When these calcium carbonate plates sink toward the ocean floor, they can push organic particles down with them, impacting the amount of organic matter that reaches the deep sea. Though individuals are only visible with a microscope, when conditions are right algal blooms can occur that can be seen from space because the coccoliths reflect light and change the seas to a beautiful milky blue.

E.huxlyei under SEM microscope vs. bloom in Barents Sea from space

E. huxlyei is both tiny and gigantic. Left: one individual under an SEM microscope; photo by Alison R. Taylor. Right: a bloom in the Barents Sea from space; photo by NASA.

Previous short-term studies have shown slower growth rates and lower levels of calcium carbonate production for E. huxleyi exposed to acidic (high carbon dioxide) conditions. Lead author Kai Lohbeck and his colleagues wanted to see if natural selection under acidic conditions could result in adaptations that would partially or fully restore growth and a calcification levels to normal. Adapting to increasing ocean acidification is a race against time, but fortunately phytoplankton have very short generation times.

The researchers took advantage of this and ran a year-long experiment that allowed for about 500 generations of asexual reproduction. They exposed populations of E. huxleyi to three different “selection conditions” — the conditions under which each population would be allowed to grow and reproduce, and to which they would have the chance to evolve adaptations.

The selection conditions were: ambient (the control, which is our current level of carbon dioxide), medium (projected carbon dioxide levels for early next century), and high (higher than any projected carbon dioxide levels). At the end of the year-long experiment, the researchers tested all three “evolved” populations in their selection condition and in the control condition. Algae grown in the control condition were tested in all three conditions.

Adaptations for acidification

Not surprisingly, increased carbon dioxide conditions led to slower growth rates and calcium carbonate production than ambient conditions for all populations, even algae from the medium and high selection conditions. However, when tested in the acidic conditions, algae from the higher acidity selection conditions had significantly higher growth rates than those from ambient selection conditions, showing that natural selection had led to adaptations that partially restored growth rate.

Similarly, in when tested in acidic conditions, algae that evolved in acidic conditions produced significantly higher amounts of calcium carbonate compared to controls grown under ambient conditions. This is particularly interesting because a common prediction was that during adaptation to acidification, coccolith formation would become too “costly” to maintain and would become reduced, through either genetic drift or direct selection against coccolith formation. This outcome would have major effects on ocean nutrient cycling, due to the role that coccoliths play in transporting organic particles to the ocean bottom, so it is encouraging to see indications against this possibility.

The results of this study are especially exciting as they represent the first evidence for the potential for evolutionary adaptation to ocean acidification by a key phytoplankton species. Of course, the lab is not the field, and questions remain about how adaptation of E. huxleyi would play out in the ocean’s complex ecosystems.

It’s worth noting that in some respects this laboratory experiment is probably a conservative estimate for the adaptive capacity of E. huxleyi. Given a period longer than one year, populations grown in acidic conditions could possibly accrue more beneficial adaptations and fully recover the growth rates and calcium carbonate production levels seen under ambient conditions. The authors also note that genetic diversity of natural populations is far greater than the levels that were present in the experimental populations. E. huxleyi is also known to sexually reproduce, which would further increase genetic variation. That means that the E. huxleyi in the ocean right now have even better chances for adaptive evolution than what was demonstrated in the experiment.

Climate change and ocean acidification present many frightening unknowns for the future of ecology and evolution. Having experimental evidence that one key species has the capacity to quickly evolve adaptations to an acidic ocean is perhaps small comfort, but it’s something.

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