Acidic oceans prompt evolution
It's no secret that greenhouse gases warm the planet and that this has dire consequences for the environment whole islands swallowed up by rising seas, animal and plant species stressed by higher temperatures, and upsets in ecological interactions as populations move to cooler areas. However, carbon dioxide has another, less familiar environmental repercussion: making the Earth's oceans more acidic. Higher levels of carbon dioxide in the atmosphere mean that more carbon dioxide dissolves in the ocean. This dissolved carbon dioxide forms carbonic acid the same substance that helps give carbonated beverages their acidic kick. While this process isn't going to make the ocean fizzy anytime soon, it is introducing its own set of challenges for marine organisms like plankton and coral. Recent research suggests that evolution may have the power to temper some of this shake-up ...
Where's the evolution?
This past summer, a group of researchers from Germany reported on a study that aimed to determine how one particularly important algae species might respond as the oceans continue to acidify. Coccolithophores are single-celled organisms with a calcium carbonate skeleton that live near the surface of the ocean, soaking up sunlight and carbon dioxide. They are so abundant that their blooms can be seen from space, and they seem to play a similarly over-sized role in biologic and Earth systems processes. As photosynthesizers, coccolithophores provide food for many other species and thus, indirectly for the predators that feed on those species. Furthermore, when coccolithophores die, their skeletons settle to the bottom of the ocean, moving carbon from the surface of the ocean to the sediment. Unfortunately, these roles may be compromised by ocean acidification. Experiments have shown that under increasingly acidic conditions, coccolithophores grow more slowly and build flimsier, malformed skeletons.
Researchers wondered if these negative effects of acidification on coccolithophores would continue for the long term. After all, species are not fixed entities, but evolving populations that can interact with their environments and change through time. Perhaps, over many generations of natural selection for higher acid tolerance, coccolithophore species would adapt in ways that dampen or cancel out the worst effects of ocean acidification.
Evolution via natural selection requires four ingredients: variation, inheritance, selection, and time. It seemed likely that acid tolerance varies among coccolithophore individuals and is heritable. The selection component would be provided by increasing ocean acidification as humans continue to overproduce greenhouse gases. And since some important coccolithophore species have a generation time of less than a day, adaptation could occur on relatively short time scales. All in all, the idea that coccolithophores species could evolutionarily adapt as oceans acidify seemed plausible.
To test this idea, the researchers set up an experiment. They started 15 separate populations of the coccolithophore Emiliania huxleyi, which can reproduce asexually. To provide some initial genetic variation (as would be present in the wild), each population was begun from six different genotypes. These populations experienced conditions mimicking different degrees of ocean acidification. Some were kept in conditions with the current atmospheric level of carbon dioxide, some were kept in conditions with more than twice that level of carbon dioxide (the projected level in the year 2100), and some were kept in conditions with even higher levels. The populations were then allowed to reproduce asexually for one year about 500 generations.
Of course, the populations evolved over the course of the experiment genetic drift and mutation act on any reproducing population, even those not undergoing natural selection but did they actually adapt to the different carbon dioxide levels? To test for adaptation, the researchers examined how the different populations performed when exposed to the same carbon dioxide levels at the end of the experiment. The graph below summarizes some of their results.
First off, compare results from the three different test conditions. You can see that all the populations (even those that experienced high levels of carbon dioxide over the course of the year) grow more quickly under normal levels of carbon dioxide than they do under higher levels. This confirms what was already known: a more acidic ocean means slower growing coccolithophores. Now, focus on the two test conditions with higher carbon dioxide levels. In both cases, the population that experienced selection for survival in a more acidic environment grows faster than the population that experienced a normal level of acidity. This indicates that adaptation did occur. A year's worth of selection for acid tolerance leads to populations that grow better in more acidic conditions.
What does all this mean for the future of our oceans? While it is good news that at least some species may evolve as oceans acidify, there is still much uncertainty. How will the complex environment of the real ocean with its multitude of interacting species affect this process? How many species will experience adaptive evolution? Will the adaptations that come about be sufficient to maintain key ecosystem processes? Will slower-reproducing species experience any adaptation at all? Unfortunately the answers to these questions are not nearly as clear cut as the changes that are taking place in ocean chemistry. Since before the industrial revolution the ocean's pH has dropped from 8.21 to 8.10, with an even larger drop expected over the next hundred years. If our carbon dioxide production continues without change, we have only seen the tip of the proverbial iceberg.
Discussion and extension questions
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Emiliana huxleyi cell photo by Alison R. Taylor (University of North Carolina Wilmington Microscopy Facility); satellite photo from NASA/SeaWiFS project
Understanding Evolution © 2020 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California