New research published in the Proceedings of the National Academy of Sciences reveals how some of the ocean’s tiniest organisms, such as phytoplankton and bacteria, are swept into underwater currents that shuttle them from the surface to deeper depths. This phenomenon plays a crucial role in affecting the ocean’s chemistry and ecosystem. The study, conducted in subtropical regions of the Mediterranean Sea from 2017 to 2019, challenges traditional understanding of how carbon is transported to depth in the ocean.
The currents responsible for this transport are called intrusions and are found to carry microscopic single-celled organisms beyond their natural habitat, where there is not enough sunlight for photosynthesis. Lead researcher Mara Freilich, a professor at Brown University, worked closely with Dr. Amala Mahadevan from Woods Hole Oceanographic Institution and Dr. Alexandra Z. Worden from the Marine Biological Laboratory to conduct the study. The intrusions not only change the types of food available in the deeper layers of the ocean but also transport a significant amount of carbon from the surface, helping feed other organisms in the marine food chain and increasing ecosystem complexity at deeper depths.
The study found that intrusions occur year-round and are widespread in the world’s subtropical oceans, acting as continuous conduits for the transportation of carbon and oxygen from the sunlit ocean to depth. These intrusions form a mechanism that transports small, single-celled organisms, including bacteria that feed on carbon fixed by photosynthesizing cells, to depths where they are not typically found. The researchers used specialized tools to measure properties like water temperature, salinity, and organism abundance at various depths, along with computer models to simulate ocean currents and understand how tiny plant and bacteria communities move in the water.
The ecological importance of intrusions in shaping oceanic biodiversity is highlighted in the study, alongside considerations of how climate change may impact these mechanisms. As Earth’s oceans warm, the proportion of carbon in tiny cells is expected to increase, potentially affecting their transport in intrusions. Understanding how intrusions regulate carbon storage and microbial dynamics in the deep ocean is crucial for predicting how changes in the microbial community composition could impact the global carbon cycle in a changing climate.
The researchers discovered that the majority of microbes in the intrusions were bacteria feeding on carbon fixed by photosynthesizing cells, showing a significant amount of biomass being transported to depth. This finding challenges previous estimates of carbon transport and highlights the role of these three-dimensional conduits in bringing surface microbes to the dark ocean in warm waters. By establishing this process in the Mediterranean, the researchers have been able to identify similar export mechanisms in major open ocean regions, suggesting a global significance of intrusions in marine ecosystems.
The study underscores the importance of further exploration to understand how changes in microbial community composition may influence carbon transport and the global carbon cycle. By uncovering the role of intrusions in shaping oceanic biodiversity and regulating carbon storage beneath the surface, researchers hope to advance our understanding of how the marine ecosystem functions and adapts to environmental changes. Ultimately, these findings could provide valuable insights into managing and protecting the delicate balance of the oceans in a rapidly changing world.
