The focal point of our research on board are bacteria in seawater and sediment. These bacteria are so small that they can only be seen under a microscope. But we are also interested in something that can’t even be seen with a microscope: dissolved compounds that are found in amazing variety in seawater. We want to understand where these compounds come from and how they are used by bacteria.
How do bacteria live in the ocean? They consist only of single cells, they have no teeth and no digestive tract. Everything that they take up must be in dissolved form. And the variety of dissolved compounds in the ocean is enormous! In every liter of seawater, there are many millions of different compounds.
How do we know all of this, if we can’t see these compounds? We use different chemical methods to study the DOM in the ocean. This is a difficult challenge, since these compounds are not only invisible and diverse, they are also very dilute. In the deep ocean the concentration of DOM is comparable to one cube of sugar in twenty bathtubs full of water. In even one bathtub of water, the sugar cube wouldn’t be very noticeable. Bacteria live in this very thin soup.
With modern chemical methods we can determine many things about the composition of DOM. But we don’t know much about the structure of many specific compounds. Here, we profit particularly from collaborations with microbiologists. The bacteria possess specific tools to take up and process specific compounds. When we know which tools the bacteria have and which ones they use, we can deduce which compounds they must be using, and link this knowledge to results from our chemical analyses.
The instruments that we use for chemical analyses are too large and sensitive to take aboard a research vessel. For this reason, we collect water samples aboard Sonne to transport back to Oldenburg, where we can further process and analyze them. And in concentrated form, these invisible compounds actually become visible – in the form of a golden-colored extract from seawater.
Here is a short film about sample collection from the CTD and sample preparation aboard the Sonne: