Nele Martens is the first graduate of the Research Training Group 2530

5 August 2024, by RTG2530

Photo: UHH/RTG2530/Grebe

Nele Martens after defending her doctoral thesis.

‘Finally it's done‘ is written on the sash, which her colleagues ceremoniously drape around Nele Martens. Shortly before, she had successfully defended her doctoral thesis in the lecture hall of the Institute of Marine Ecosystem and Fishery Sciences.

On this summery afternoon, on the spacious balcony of the building overlooking the Elbe River, the attending staff members of the institute as well as members of the Research Training Group 2530 and the examination committee raise their glasses to toast to the newly graduated doctor. It's a great joy to see that GRK 2530’s first candidate has obtained the doctorate. ‘It was great to be part of the working group’, says Nele Martens about the time spent working together.

‘Phytoplankton in the Elbe estuary: New insights into community composition and mixotrophy, from metabarcoding, flow cytometry, and laboratory experiments’ is the title of her doctoral thesis, which she started in November 2020. During her master’s studies in Marine Ecosystem and Fisheries Sciences, she was already researching on the subject of phytoplankton. ‘I am fascinated about the fact that early forms of phytoplankton likely existed around three billion years ago and significantly contributed to make the world habitable for humans’, she says. The organisms, which are invisible to the naked eye, serve as a food source for many living beings. They absorb CO₂ from water through the process of photosynthesis, converting it into oxygen by using light energy.

For Nele Martens, it is important to understand how phytoplankton behaves, especially with regard to the global carbon cycle. Finding answers to this is a significant challenge, because, according to her research, it is still partly unknown how the phytoplankton in water bodies, especially estuaries, is composed in detail. ‘What's even there?’ was the first question she wanted to find an answer for. Traveling twice on the research vessel Ludwig Prandtl, she took water samples at six locations, from Bunthäuser Spitze in the south of Hamburg to the mouth of the river in the North Sea towards Cuxhaven. Along this route, where more and more seawater mixes with the Elbe, the environmental conditions vary for the free-floating phytoplankton.

As part of her doctoral thesis, Nele Martens analysed 17 different phytoplankton strains from the Elbe estuary. Since bodies of water contain organic carbon compounds, such as sugars and proteins, Nele Martens wanted to find out how phytoplankton reacts towards these substances and whether they are able to utilise them. ‘Normally, phytoplankton, as so-called autotrophic organisms, should use sunlight in order to produce organic compounds. As a matter of fact, absorbing organic carbon would be quite the opposite. This is what we humans do as well, we also use sugars and proteins’, explains Nele Martens.

A very special moment for Nele Martens happened when she realised during her investigations that the phytoplankton from her samples was mixotrophic, i.e. able to use both sunlight and organic carbon compounds. The observation became one of the most significant results of her doctoral thesis in a field that has been little researched and therefore harbours many unanswered questions. ‘If they absorb sugars, among other things, they no longer need as much CO₂ and they photosynthesise less. On the other hand, this also increases their ability to survive,’ says Nele Martens.

To her, the influence of phytoplankton on the CO₂ balance is a complex and important topic, but she emphasises that further research is needed to be able to make statements on this. When asked if she is now happy and relieved with her newly acquired doctoral title, she replies that she is already working on the next topic. Although the next steps have not been defined yet, Nele Martens plans to continue her research and to develop her findings further.