Foraminifera are single-celled marine organisms with calcareous shells, playing a crucial role in sediment formation and providing insights into past marine habitats. These microscopic creatures typically thrive in tropical and subtropical coastal waters, where they attach to coral rubble, seagrass, and algae.
The Mystery of Foraminifera Migration
Recent research has uncovered a fascinating journey taken by foraminifera: they can travel long distances by hitchhiking on floating seagrass or algae—a process known as “rafting.” This phenomenon, although previously hypothesized, had rarely been documented scientifically.
Groundbreaking Discovery in the Red Sea
During an expedition in the Red Sea aboard the research vessel METEOR, an international team of scientists, including Dr. Marleen Stuhr from the Leibniz Center for Tropical Marine Research (ZMT), made a breakthrough discovery. In a deep-sea brine pool, over 600 meters beneath the surface, they found well-preserved seagrass with attached foraminifera belonging to the genera Amphistegina, Sorites, and Amphisorus. These species typically inhabit shallow waters, no deeper than 100 meters, yet they had been transported to great depths via seagrass rafting.
This finding, recently published in Scientific Reports, highlights how even the smallest marine organisms can cover vast distances through passive transportation.
Expedition Details and Key Findings
In autumn 2023, the RV METEOR embarked on a four-week research mission (M193) to explore carbonate formation and sediment deposition off the Saudi Arabian coast. Scientists from ZMT, the University of Hamburg, King Abdullah University of Science and Technology (KAUST), and other institutions participated in the study.
During the cruise, the team collected extensive sediment samples from depths ranging between 400 and 1,000 meters. To their surprise, they repeatedly discovered remains of large benthic foraminifera—organisms usually found in shallow tropical waters up to 30 meters deep.
Dr. Stuhr, the lead author of the study, noted: “We regularly found calcareous shells of foraminifera in deep-sea sediment samples located 25 to 50 kilometers away from their known shallow-water habitats.” This raised an intriguing question: how did these microscopic organisms travel such long distances?
Evidence of Rafting Confirmed
The answer emerged near the end of the expedition when the team explored a deep-sea brine pool off the coastal town of Umluj. As they retrieved sediment samples, a strong sulfurous odor hinted at the unique conditions in the brine pool—high salinity, no oxygen, and extremely slow decomposition processes.
While analyzing the samples, researchers discovered intact seagrass leaves and macroalgae fragments carrying attached foraminifera. This confirmed their hypothesis: the organisms had been transported from shallow waters by floating seagrass, eventually sinking to the deep sea.
The Impact of Macrophyte Rafting
This study demonstrates that marine species can disperse through macrophyte rafting, potentially reaching new habitats. The implications extend beyond foraminifera: invasive species such as Amphistegina lobifera and the tropical seagrass Halophila stipulacea have spread from the Red Sea to the Mediterranean, likely using similar rafting mechanisms.
According to Dr. Hildegard Westphal, co-chief scientist of the expedition: “Rafting mechanisms may contribute to the spread of invasive species, highlighting the role of drifting seagrass in marine biodiversity shifts.”
Conclusion
This groundbreaking research enhances our understanding of how marine microorganisms migrate across ocean basins. The study also underscores the ecological impact of seagrass rafting, influencing species distribution and the expansion of invasive organisms. As scientists continue to explore deep-sea ecosystems, new insights into marine connectivity and biodiversity await discovery.
More Information: Marleen Stuhr et al., Seagrass-rafted large benthic foraminifera transported into the deep Red Sea, Scientific Reports (2025). DOI: 10.1038/s41598-025-90047-7