Responses of selected species of marine phytoplankton to increasing carbon dioxide and light

Atmospheric carbon dioxide (CO2) concentrations have been increasing since the industrial revolution and are expected to almost triple from pre-industrial values by the year 2100. CO2 enters the ocean by atmosphere-surface ocean gas exchange, decreasing carbonate ion (CO32-) concentrations and pH (ocean acidification). Additionally, the rise of CO2 concentrations and other green-house gases in the atmosphere, increase global average temperatures in the air and, consequently, in the surface ocean. This strengthens thermal stratification, decreasing mixed layer depth and changing light availability. The overarching goal of this thesis was to investigate the effects of global change, namely of increasing CO2 and light, on selected species of marine phytoplankton (cyanobacteria, coccolithophores and diatoms). At first, the focus was put on investigating the response of the evolutionarily oldest phytoplankton group (cyanobacteria) to changing CO2 concentrations. From further work with cyanobacteria during mesocosm experiments in the Baltic Sea emerged the idea of determining the time necessary for phytoplankton cells to acclimate to the changed conditions. To assess this question the best studied phytoplankton species in the context of changing carbonate chemistry, Emiliania huxleyi, was chosen. Other cellular adjustments of high interest in a future ocean are those to sudden light variation, since organisms might be exposed to an average higher light intensity and more frequent high light events. It could be possible that diatoms such as Phaeodactylum tricornutum and coccolithophores like Emiliania huxleyi differ in their ability to cope with these changes. Nitrogen fixing cyanobacteria (diazotrophs) are responsible for the input of new nitrogen into large areas of the ocean. Until the beginning of this thesis it was unknown whether and how they would respond to the expected changes in the ocean’s carbonate chemistry. The important non-heterocystous diazotroph Trichodesmium strongly responded to rising CO2 from 140 to 750 μatm, increasing cell division rate, nitrogen fixation rate per unit of phosphorus utilization and carbon fixation (publication I). In the heterocystous Nodularia spumigena (co-authored manuscript IV) and Anabaena sp. (co-authored manuscript V), however, nitrogen fixation was found to decrease with increasing CO2, potentially resulting from decreasing pH and not the CO2 concentration itself. Together these results hint to fundamental differences between heterocystous and non-heterocystous cyanobacteria. Hence, depending on their distribution, some regions could see increasing nitrogen fixation in the future while others not. This could influence regional primary productivity and possibly carbon sequestration. Globally the distribution and abundance of non-heterocystous cyanobacteria in the oceans is more significant than that of heterocystous species, so the overall feedback will be determined by the former. While the effect of increasing CO2 concentrations on cyanobacteria only started to be analyzed very recently, other phytoplankton groups, especially coccolithophores, have already been considered for a longer period of time. Often studies were performed after the cells were pre-exposed to experimental CO2 concentrations for about 9 to 12 generations. However, it is unknown how much time is actually required for cells to reach a new physiological “equilibrium” (acclimation). Hence, the frequently studied Emiliania huxleyi was exposed to abrupt variations in carbonate chemistry and its response followed over 26 hours. Cells acclimated within about 8 hours, increasing photosynthesis and decreasing calcification under elevated CO2 concentrations, similar to cells pre-exposed to those conditions (manuscript II). If such a rapid acclimation is a general phenomenon within phytoplankton species it simplifies the interpretation of short-term results for several experimental setups such as mesocosm and ship-board incubations, since in these situations it is not feasible to pre-expose the communities to the experimental conditions. As stated above, the expected rise of CO2 concentrations might indirectly change the light supply for plankton. Phytoplankton species respond differently to dramatic (abrupt and strong) changes in light intensity, influencing their competitive fitness for a certain ecological niche. Exactly how cells photoacclimate to light changes and whether there are differences between species is still not completely clear. Emiliania huxleyi and Phaeodactylum tricornutum dissipated extra energy after an abrupt rise in light intensity as heat, fluorescence and photochemistry (manuscript III). However, there were differences between the species in both magnitude and timing of their individual responses. Additionally, the coccolithophore was found to use an additional dissipation valve, calcification. Species-specific responses to dramatic increases in light intensity as those found here might be important defining competitive fitness and therefore, community composition. Results of this doctoral thesis point out the importance of the response of diazotrophs in marine feedbacks to global change. The strong increase in nitrogen fixation with rising CO2 observed for the globally important Trichodesmium might provide a negative feedback, depending on the magnitude of the effect on other changing factors, such as temperature and light. Moreover, changes in light intensity might influence community composition due to species-specific differences in response time, with potential consequences for the biological carbon pump.