Sandrini, G., Ji, X., Verspagen, J.M., Tann, R.P., Slot, P.C., Luimstra, V.M., Schuurmans, J.M., Matthijs, H.C. and Huisman, J., 2016. Rapid adaptation of harmful cyanobacteria to rising CO2. Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1602435113
Background: the winners and losers of climate change
Climate change will generate both winners and losers, but who will the winners be? We need to understand what organisms are benefited and harmed by climate change in order to predict how ecosystems will look in the future. Higher CO2 concentrations as a result of fossil fuel emissions is one example of how environmental conditions are changing. CO2 dissolved in surface waters is a key ingredient in photosynthesis for algae and aquatic plants, so their ability to compete for it against their neighbors determines in part whether or not they survive and proliferate.
Different organisms have different strategies of competing for CO2 and other nutrients. Some algae (often larger organisms) have a large storage capacity: they take up nutrients relatively slowly when nutrients are scarce, but are able to increase their uptake more when nutrients are more abundant. You can imagine these organisms do better when nutrients are plentiful! In contrast, other algae are good at scavenging nutrients when they are scarce, but reach their storage limit quickly. These are often smaller algae with a higher ratio of surface area to body size, giving them in an advantage in absorbing scarce nutrients. Increasing CO2 concentrations in the ocean could therefore disrupt the current competitive balance between types of algae, tipping the scales in favor or organisms adapted to higher CO2 concentrations.
Cyanobacteria are the tiniest algae in the ocean and can form toxic blooms when they outcompete other types of algae (Figure 1). Due to their small cell size, they are usually considered to be well adapted to low CO2 concentrations. They therefore might be expected to be losers in a future high-CO2 ocean. However, different types of cyanobacteria have different mechanisms for acquiring CO2, each with a different tradeoff between how quickly they take up CO2 and their storage capacity. This raises the possibility that cyanobacteria could adapt to increasing nutrient concentrations and maintain their competitive advantage in the future.
Can cyanobacteria adapt to higher CO2?
A new paper in the Proceedings of the National Academy of Science tests the potential for the cyanobacterial community to adapt in response to increasing CO2. First, the authors conducted a lab experiment with 5 different strains of the cyanobacterium microcystis, each containing a different mechanism for CO2 uptake. Those with the BicA gene specialized in rapid uptake at high concentrations, while those with SbtA could better compete for CO2 at low concentrations. They used both a high (1000 ppm) and low (100 ppm) CO2 treatment to test how increasing the concentration would affect competition among the different strains.
Overall, the cyanobacteria grew faster at higher CO2 concentrations, and the total biomass at the end of the experiment was 2.5 time higher than in the low concentration experiment. The different concentrations also changed the competition among the different strains. At low concentrations, a toxic strain with both the BicA and SbtA genes completely took over and outcompeted the other cyanobacteria. At high concentrations, the toxic strain with only BicA coexisted with a nontoxic strain with both genes (Figure 2). This showed that under higher CO2 concentrations, natural selection will favor organisms able to capitalize on the additional nutrients.
A field study was also conducted in Lake Kennemermeer in the Netherlands. This lake regularly experiences cyanobacteria blooms so severe the water is not considered safe to swim in. CO2 concentrations in the lake are very low during the summer due to high algal activity, and increase in the fall when the population is smaller. Strains with genes for both uptake mechanisms dominated in the summer when CO2 was scarce. But in the fall, as concentrations increased, they were replaced with strains with only BicA, adapted to the higher CO2 (Figure 3). The replacement of the population was very rapid, occurring within 2 months or 12-42 generations of the cyanobacteria. Both the lab and field experiments indicate that at higher concentrations of CO2, organisms able to take up CO2 more rapidly outcompete organisms adapted to lower CO2 conditions.
The study demonstrates rapid evolution of a cyanobacterial species, both in the lab and in natural conditions—not from new mutations, but by natural selection acting on existing diversity within the population. We like to think of evolution as something happening over thousands or millions of years. But for bacteria with short lifespans, it can happen in just months. This means that when thinking about the winners and losers of climate change, we can’t just think about what species will do well and which will suffer. Instead, we must consider that species themselves will adapt to new conditions as they occur. Unfortunately, toxic cyanobacteria are among the most genetically diverse and capable of rapid evolution. They will probably be fine, at the expense of more desirable algal species.