“Bottom up” ocean acidification: A study on the effects of CO2 on the bacterial community in sediments

As atmospheric concentration of CO2 continues to increase, alternatives on how to mitigate and reduce the rate of this development has received much attention. Carbon Capture and Storage (CCS) is doing just this by storing CO2 that ordinarily would have been emitted into the atmosphere. By storing the CO2 in geological storages it is isolated for a long period of time, thousands of years. Even though this type of storage is considered safe and the risk of leakage small, one can never be absolutely sure of it holding. The risk of large leakages is considered negligible, but the risk of relative small leakages is uncertain. If such a small leakage were to occur, what are the consequences and how such a leakage could be detected? These are difficult questions to answer, but the need to be able eventually answer them is important, especially considering that international guidelines (London protocol and OSPAR) has been developed so that these questions can be answered, and they eventually need to be followed. The long term aims of this project are to developing monitoring and detection methods for small leakages and assess the environmental impacts of this type of leakage.

Biological impacts are important to study since the marine ecosystem is extremely important. Bacteria are an important part of this ecosystem, and have many important tasks and constitute the foundation of a well functioning ecosystem. This study investigates the influence of increased CO2 concentration, as a result of CO2 leakage through sediments, on bacterial community structure in sediments. By studying the bacterial community in sediments and if it responds to a CO2 leakage a more realistic assumption on how natural system might react is acquired, and if consequences are observed here, then it might indirectly or directly affect other aspects of the ecosystem.

The experimental setup is designed to be as genuinely similar to the seafloor as possible. The titanium tank functions as an aquatic mesocosm where temperature, light, pressure, continuous supply of seawater, natural sediments contributes to a realistic imitation of the natural environment to the bacterial community. Two experiments were performed by injecting CO2 into the system through the sediment. The first experiment lasted two weeks, the second a month. The effects of CO2 on the bacterial community were tested by using the method PCR-DGGE. The method gives an overview of the most dominant bacterial populations that the community is constituted of. Therefore it’s used to establish how the bacteria community in samples, in this case sediment samples, responds by detecting changes in community structure.

Only experiment 2 was a success. Results from this experiment show that the bacterial community structure in the topmost layer in sediments is resisting changes even after a month with CO2 treatment. This was not the case for the bacterial communities in deeper sediment layers, meaning layers beneath (2-9 cm) of the top sediment, which was significantly changed due to CO2 treatment.

Whether this change in community structure is a result of CO2 itself, pH or a result of CO2 changing the chemistry in sediments, especially metal mobility and solubility, is discussed. However there is no way to separate these two effects with this type of experimental setup, the observe effects on the bacterial community are a consequence of the conditions in the tank as a whole and not only CO2 or increased metal concentrations.

More research is needed before one knows what the effects of leakages are, but many improvements on how to proceed with this have been suggested. By doing these experiments some basic knowledge of how a natural bacterial community in sediments might react when faced with a CO2 leakage is obtained.

Gjøsund N. S., 2011. “Bottom up” ocean acidification: A study on the effects of CO2 on the bacterial community in sediments. Norwegian University of Science and Technology, Faculty of Natural Sciences and Technology, Department of Biology. 90 pp. Master thesis.

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