On Thursday 26 November, Xing (Jason) Ji will defend his PhD thesis
Phytoplankton convert CO2 and sunlight into biomass, and form the base of freshwater and marine food webs. Cyanobacteria evolved three billions years ago, and belong to the most widespread phytoplankton groups. However, some cyanobacteria are toxic and their dense blooms are often deteriorating water quality in many lakes and reservoirs across the globe. Cyanobacterial blooms are predicted to increase with global warming, but how they are affected by rising atmospheric CO2 levels has not yet been resolved.
Climate change scenarios predict that the atmospheric CO2 concentration will have doubled by the end of this century. We know that cyanobacteria and other phytoplankton groups have evolved sophisticated CO2-concentrating mechanisms (CCMs) that enable them to transport and utilize both CO2 and bicarbonate. Intriguingly, cyanobacteria use several CO2 and bicarbonate transporters with different kinetic properties and can combine these transporters in different ways. This variation may lead to adaptive changes in their carbon uptake kinetics in response to elevated CO2, may affect their competitive interactions with other species, and may ultimately alter phytoplankton community composition. In this thesis, I use a combination of laboratory experiments, mathematical models, and field data to obtain more insight into the impacts of rising CO2 concentrations on cyanobacterial growth and phytoplankton community composition.
First, phenotypic plasticity of the carbon uptake kinetics was investigated for the common bloom-forming cyanobacterium Microcystis (Chapter 2). Our results revealed a strong increase of the maximum CO2 uptake rate of Microcystis at elevated pCO2. Furthermore, a new mathematical model incorporating this phenotypic plasticity enabled accurate prediction of the population dynamics observed in our chemostat experiments. We up-scaled our model from laboratory chemostats to lakes. The model predicts that rising pCO2 will intensify Microcystis blooms across a wide range of eutrophic lakes.
Subsequently, the extent to which rising CO2 concentrations may induce micro-evolutionary changes in cyanobacteria was investigated (Chapter 3). We detected, both in the laboratory and in a Dutch lake, a striking change in the genotype composition of Microcystis in response to rising CO2, with an increase of genotypes with high-flux carbon uptake systems. These results show that elevated CO2 levels will lead to a genetic reshuffle of Microcystis strains, such that future blooms will most likely have a genetic composition that differs from current blooms.
Thereafter, competition between a cyanobacterium and three species of green algae was explored at low and high pCO2 (Chapter 4). Our model predictions and experiments showed that two of the three green algae were superior competitors at low pCO2, whereas a high-flux specialist Microcystis strain became a strong competitor at high pCO2. These results contradict one of the classic paradigms in aquatic ecology, i.e., that cyanobacteria would be superior competitors at low pCO2 whereas eukaryotic phytoplankton like green algae would be stronger competitors at high pCO2. Instead, our results demonstrate that bloom-forming cyanobacteria with high-flux carbon uptake systems will benefit from elevated CO2. We therefore call for a more comprehensive understanding of the diversity of CCMs, if we are to predict how the species composition of natural phytoplankton communities will respond to rising pCO2.
As a next step, I zoomed out from cyanobacteria to the phytoplankton community level (Chapter 5). Data from ~1,000 lakes across the United States were analyzed to investigate how natural variation in dissolved CO2 and lake temperature may affect the relative abundances of the main phytoplankton groups. Our results suggest that increasing dissolved CO2 concentrations will most likely benefit species that lack CCMs or have a very ineffective CCM such as chrysophytes and euglenophytes, whereas higher temperatures are likely to favor dinoflagellates and euglenophytes at the expense of chrysophytes and diatoms.
In conclusion, this thesis shows that future phytoplankton blooms are likely to differ significantly from present-day blooms, not only in terms of their intensity and frequency but also in terms of their physiological traits, genetic composition, and species composition (Chapter 6). For this reason, investigating and incorporating phenotypic and genetic variation into existing theoretical frameworks is highly recommended to improve predictions of how plankton communities will respond to environmental change. I hope this thesis will not only provide solid insights into the effects of rising CO2 concentrations on the phytoplankton community, but will also inspire ecologists, microbiologists, evolutionary biologists, and water quality managers to integrate this knowledge in their future studies and decision-maki