1. Long-term trends
The first study was conducted in cooperation with the EMSForce project at GEOMAR and involved the supervised Master’s thesis of Sofia Modesti (University of Hamburg). We analysed deep-sea sediment samples collected in 2024 during the Mediterranean deep-sea cruise METEOR M197. Due to limited sediment availability and the extremely oligotrophic conditions of the eastern Mediterranean Sea, discoververed were mostly unicellular eurkaryotes instead of animal groups. The genetic data are currently being integrated with geological datasets within the EMSForce community to improve understanding of this vulnerable ecosystem. Initial results already indicate a patchy and highly sensitive system in the eastern Mediterranean.
In addition, sediment samples from comparable cruises conducted 25 and 15 years earlier, which had been archived at GEOMAR, were also analysed. This demonstrated that modern genetic techniques can successfully be applied to historical samples, enabling retrospective reconstructions of ecosystem developments over decades. While overall community diversity did not change drastically over time, a noticeable decline in animal abundance was observed in the most recent samples, highlighting the vulnerability of the eastern Mediterranean deep sea. Importantly, genetically identical species could be detected across time, and changes in their intraspecific genetic variation were observable. Although limited sample material prevented robust statistical conclusions, this study demonstrated the potential of using intraspecific variation as an indicator in long-term assessments.
2. Natural variability: the HAUSGARTEN observatory
The second study aimed to test whether standardised analyses of genetic population variability reflect natural variability in marine systems. In cooperation with the Alfred Wegener Institute (AWI, Bremerhaven), the Fellow participated in the HAUSGARTEN Long-Term Ecological Research cruise with RV Polarstern in the Arctic Ocean during its 25th anniversary year in summer 2024. The cruise consisted of two legs, allowing sampling during the peak of the phytoplankton bloom and several weeks later. Algae and plankton were sampled in the upper water column, as well as sediments at depths of up to 2600 m at the same stations. Genetic analyses revealed clear shifts in algal communities over time in surface waters and showed that genetically identical species could be detected from the surface down to the deep seafloor. Heavier planktonic organisms, such as Dinoflagellata and small crustaceans, were particularly prominent in deep-sea samples, reflecting the rapid export of biomass following the algal bloom.
Genetic diversity within species was highest during the bloom, lower in surface waters several weeks later, and lowest in deep-sea sediments, matching known patterns of abundance over time and depth. These results demonstrate not only the rapid transfer of biomass to the deep sea, but also that much of it is consumed before reaching the seafloor. Importantly, this study confirms that intraspecific genetic variation reflects natural ecological variability. Combining standardised genetic tools with multiple perspectives therefore provides new insights into the coupling between surface waters and deep-sea ecosystems.
3. Short-term impact study
After confirming the potential of intraspecific variation to capture long-term trends and natural variability, the final study addressed whether this approach can also detect short-term, unpredictable impacts, both natural and anthropogenic. A planned deep-sea impact experiment was conducted in February 2024 off Madeira Island aboard RV Maria S. Merian. Using the remotely operated vehicle (ROV) PHOCA (GEOMAR), a fish bait (herring) was deployed at approximately 1500 m depth to simulate a localised food input.
The bait quickly attracted scavenging fish, and water and sediment samples were collected before deployment, after 24 hours, and again after 12 days. During the second sampling, the ROV was affected by a strong current, causing substantial sediment disturbance at the experimental site. While unintended, this event effectively transformed the experiment from a positive food-input study into a disturbance impact study, highlighting the unpredictability of deep-sea research. Overall, species composition did not change dramatically following either impact. In contrast, intraspecific genetic variation responded strongly. After 24 hours, sediments showed a significant increase in genetic variability in vertebrates and tardigrades, corresponding to increased scavenger activity. Twelve days later, this variation returned to baseline levels. In contrast, sediment-dwelling organisms such as nematodes and nemerteans exhibited increased genetic variation after 12 days, although the underlying mechanisms remain unclear.
