Phosphorous and Nitrogen Science communication

2018.10.07

Digging in the mud

I currently spend most of my time in the office, working with large datasets and computer models. So, I jumped at the opportunity to join a research cruise. It was a great to be out of the office and learn first-hand about interesting research being conducted in the Baltic Sea.

For me, the best part of this research cruise was collecting sediment (aka mud)! Most people probably find sediments to be the least interesting aspect of any lake or coastal area. But to me, they are fascinating. 

Sediments in deep water don’t receive enough light to support plants, but they are full of all sorts of organisms, like worms, clams and different microorganisms (such as bacteria and archaea, which are tiny single-celled organisms similar to bacteria). Microbes play an important role in decomposing organic matter (carbon-rich material formed by the decay of living things) and freeing up nitrogen and phosphorus (nutrients) that can be used by algae and other microbes. 

Microbes also that play an important role in the production and consumption of greenhouse gases, like nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4). Personally, I am most interested in nitrogen-related processes (because it is my favorite element), but this research cruise focused on carbon.

Oxygen from the water column diffuses into the top layers of sediment. Organic matter settles to the sea floor and accumulates as sediment. Together, oxygen and organic matter can fuel the production of carbon dioxide by microbes as they break down the organic matter. Oxygen that is consumed by microbes in deeper sediments is too deep to be replaced by diffusion of oxygen from the overlying water.

sediment coreThis device is dropped to the sea floor using a winch to collect two 1 m sediment cores

The story doesn’t end here, however, because microbes can deal with oxygen-free environments in deep sediments. For example, in the absence of oxygen, certain archaea can convert carbon compounds into methane. They do this to get energy, just was we take in oxygen and food and convert it to carbon dioxide. Methane is about 30 times more potent than carbon dioxide as a greenhouse gas, so it is important to understand where methane production is occurring and the factors that influence it.

sediment coreThe long core (basically a long metal pipe) is dropped to the sea floor using a heavy-duty winch. It brought to the side of the ship where it can be cut into smaller sections for easier handling. The white tube, a core sleeve, is used to line the corer and will hold the sediments.

During the cruise, we collected sediments using long (6 m) corer and short (1 m) corers (see above pictures). The sediment cores were collected at locations that have been studied by researchers at Tvärminne Zoological Station (University of Helsinki). These prior results will be combined with findings from this cruise to get a better understanding of ecological processes in the sea.

The seawater that is trapped in the sediment is called pore water. On this cruise, pore water was collected using syringes (see picture below) and will later be analyzed in the laboratory for dissolved methane gas. Methanogenesis (the production of methane) is interesting to study because it interacts with the iron, phosphorus, and sulfur cycles in sediments.   

sediment coreThe long core was cut into smaller sections and pore water was extracted using specialized syringes. The pore water will later be analyzed for concentrations of methane.

The top centimeters of the short cores (1 m) were removed with a core slicer (see picture below). In the laboratory on the SU campus, DNA will be extracted from the sediments. Abord the ship, the samples were stored in liquid nitrogen (about -195 C) to prevent degradation. The extracted DNA will later be genetically sequenced at another laboratory in Stockholm. My colleague who is doing this work hopes to learn which microorganisms are in the sediments and what they are doing. Also, because we collected sediments at a number of points, from close to shore to close to open waters, he will examine how the composition of microorganisms varies across this gradient. 

sediment coreThe core slicer was used to remove sediments for further analysis in the laboratory

Another colleague will measure the stable isotopes of nitrogen and carbon in the surface sediments. Isotopes are atoms of the same element that have different weights because they contain different numbers of sub-atomic particles called neutrons. For example, most carbon atoms, for example, have 6 protons and 6 neutrons (called 12C); about 1% of all carbon atoms have 7 neutrons (called 13C). The ratios between 13C and 12C in organic matter from land (e.g. soils, leaves) are different from the ratios of organic matter produced in the sea (e.g. algae) and can provide a clue about the sources of the carbon. Nitrogen isotopes can tell us something about how quickly nitrogen is transformed by microbial processes in the sediments. 

Michelle McCrackin

Limnologist
michelle.mccrackin@su.se