< POS520 Cape Verde Cruise
27.02.2018 13:51 Age: 140 days

POS520: Searching For Traces in the Open Ocean


Veronique Merten in the submersible JAGO. Photo: Jürgen Schauen/JAGO-Team

Our cruise on the RV POSEIDON slowly comes to an end and today is the last day of sampling. My task during this cruise is sampling environmental DNA (eDNA). eDNA is defined as genetic material obtained directly from environmental samples, in our case water samples of different depths, without any obvious signs of biological source material. When marine organisms, let’s say a jelly, swims through the ocean, they lose cells and slime which contains small DNA fragments and this is what we are looking for. By finding these fragments in the water, I can later in the lab assign them to the original organism, that lost it in the first place, like for example a specific jelly or squid species. Doing this with all DNA fragments found is called metabarcoding and, with the right background information, gives us a pretty good overview about the biodiversity of this station. Additionally, we could search for the presence or absence of a specific organism that we are interested in. But how do we get these samples? My routine on a sampling day starts with taking water samples from the CTD rosette from different depths containing all these small DNA fragments of organisms that have been at these depths before. Following, the water samples are filtered and the DNA fragments remain in the pores of the filter. So far I have filtered 263 litre of seawater and some more will follow. It is very important to work totally clean during filtration to avoid contamination between samples and the lab environment, therefore the lab smells like a swimming pool, because I need to bleach every item that came into contact with the water after every sample with chlorine. When the filtration is done, the filters are stored in the freezer and back at GEOMAR, we are going to extract the DNA from the filters to identify all DNA fragments on it. The overall objective of all these filtrations during this cruise is to compare the eDNA approach with net catches and the towed camera system, PELAGIOS, with focus on gelatinous zooplankton and cephalopods.

My personal highlight of the cruise was definitely my JAGO dive. It was my first time in a manned submersible and we descended down to 152 m into the ocean. It was a unique and fascinating experience, seeing the sun getting weaker and weaker as we sank down in the deep blue. The water was crowded with different small organisms swimming in front of the thick plexiglas of JAGO’s front window. We saw small, super active jellies and large siphonophores with long tentacles waiting to catch food. A few fishes checked us out looking through the front window before they vanished. The time passed by too quickly and after around 2h we had to come back to the surface. Nevertheless, this was an experience I will never forget and I will definitely come back.

The cruise was a great success, I loaded many filters with DNA for later analysis, we had wonderful weather, which was a nice change after a winter in Kiel. Dolphins, one ray and pilot whales accompanied us from time to time. We had a living vampire squid and a fangtooth (deep-sea fish) in our nets and, probably the most important thing on a cruise, the team was a great pleasure to work with and so much fun!

Between 14 Feb and 1 March expedition POS520 with RV POSEIDON and the submersible JAGO takes place off the coast of the cape verde island Santo Antão.  The chief scientist, the JAGO team and participants of the cruise write here as guest writers. Today: Véronique Merten – PhD Student at the Department of Evolutionary Ecology of Fishes / Oceanic Nekton and Deep-sea Biology at GEOMAR.