What will you do at Askö?

Following up on a recent field experiment on microenvironments of Trichodesmium sp. colonies (Stn ALOHA, North Pacific Oce an), which showed pronounced O2 and pH gradients, were now intend to perform similar measurements on Nodularia aggregates in the Baltic Sea, to account for the effects of different carbonate chemistry in the respective habitats. Specifically, we are going to characterize chemical parameters in the microenvironment of these aggregates as well as rates of photosynthesis and respiration under different pCO2 levels using microsensors. To this end, gradients of pH, O2 and redox potential will be measured in Nodularia aggregates collected from station B1. Microsensor measurements will be performed under different instantaneous pCO2 levels as well as after several days' acclimation to a future pCO2 scenario in roller tanks, which is achieved by pre-bubbling the seawater with gas of different pCO2 levels. Samples for DIC, pH and TA from the bulk seawater will be taken for determination of carbonate chemistry at the sampling station as well as in the different CO2 treatments.

Project background

N2-fixing cyanobacteria play an important role for marine primary productivity and therewith for the oceans capability of taking up anthropogenically produced CO2. Previous laboratory studies showed an exceptional sensitivity of N2-fixing cyanobacteria towards CO2 concentrations predicted for the end of this century. However, these studies did not account for the fact that many N2 fixers in their natural environment form aggregates, producing a microenvironment differing strongly in the chemical conditions compared to the surrounding seawater. In view of the possible modulation of CO2 responses by these microenvironment conditions, it is crucial to complement previous studies of ocean acidification effects on single filaments by measurements under the actual microenvironment conditions in aggregates.

In the current project, we study CO2 responses of different N2-fixing, aggregate-forming cyanobacteria, characterizing their microenvironments by use of microsensors. Important chemical parameters in the microenvironment as well as key physiological processes are characterized in aggregates acclimated to current and future CO2 scenarios. Experiments are conducted with two species (Nodularia sp. and Trichodesmium sp.) from habitats with different carbonate chemistry, and therewith different capacity for bufferi! ng changes due to the uptake of anthropogenic CO2 as well as cellular activity. These experiments will give insight into the mechanisms that determine chemical conditions in aggregates under varying conditions. Trichodesmium, which is the most important N2 fixer in nutrient depleted open ocean regions, as well as the toxic Baltic Sea species Nodularia, play a key role in their respective ecosystems by making nitrogen available for a range of associated organisms. To account for the complex interactions with these associated organisms in aggregates, our laboratory experiments are supplemented with field studies. The results will contribute to more realistic predictions of CO2 effects on N2 fixers.