Methane is a hot topic for a reason: its atmospheric concentration has more than doubled since the beginning of the industrialization. This increase in methane is a key contributor to climate change. Coastal environments are an important source of methane. Professor at Radboud University and expedition leader on board of Pelagia Caroline Slomp explains: ‘We know that microbes in the seafloor are responsible for both the production and the removal of methane. While rates of methane production in coastal areas are quite high, the removal of methane in the seafloor is often quite efficient. This means that only a small fraction of the total methane that is produced in the sediment is thought to escape to the atmosphere.’
Increased production
That’s why it is important to analyse how this process in the seafloor works exactly. Especially because global warming is dramatically changing coastal systems. Slomp: ‘The higher temperatures and increased input of organic matter are leading to increased methane production in coastal sediments. We do not know if the microbes that currently remove the methane will be able to keep up with the increasing methane production.’ The goal of the expedition to the Bothnian Sea is to characterize the various types of microbes in the seafloor that consume methane and understand what controls their growth and their efficiency to remove methane.
Methane samples
On Pelagia, the team collected samples from both the water column and the seafloor at locations throughout the Bothnian Sea. For the water sampling, they used a “CTD-Rosette” system. ‘This is a device that, among other parameters, measures the temperature, salinity and oxygen concentration of the seawater with water depth,’ explains researcher from Radboud University Niels van Helmond. The system contains 24 sample bottles – so-called Niskin bottles - that can be closed remotely. Van Helmond: ‘This allows us to bring up water from different water depths. On deck and in the laboratory containers the water from the Niskin bottles is transferred into different smaller bottles and vials to determine the chemical composition of the water, including methane.’ The methane samples will be analyzed in the laboratories at Radboud University when the team is back. Slomp: ‘The question we want to answer is: will there be methane leaking from the sediments into the water column? That would say something about the methane eating microbes.’
Seafloor
To collect surface sediments, a special “multicoring” device is used that allows vertical sections (up to ~60 cm) of undisturbed sediment to be collected, says Van Helmond. ‘Besides the multicorer the team also uses a gravity coring device which captures up to ~10 meters of sediment. All kinds of samples are taken from these sediment cores, ranging from porewaters (the water in the sediment), to samples for DNA analysis.’ When the team is back on shore, the microbiology and chemistry of the collected samples will be studied in great detail. This will take months (and sometimes years) to complete.
Lab containers
Most samples that are taken on board are stored for later chemical and microbiological analysis. But some chemical analyses are done directly on board too. Slomp: ‘Microsensors are used to measure oxygen concentrations in the surface sediment, for example. Furthermore we are making use of the great support facilities of Royal NIOZ: this means that important nutrients, such as ammonium and phosphate and other solutes in the seawater are being measured directly in special lab containers on board.’ The first exciting results are expected soon.