For microscopic organisms, ocean currents act as 'expressway' to deeper depths, study finds

PROVIDENCE, R.I. [Brown University] - Some of the ocean's tiniest organisms get swept into underwater currents that act as a conduit that shuttles them from the sunny surface to deeper, darker depths where they play a huge role in affecting the ocean's chemistry and ecosystem, according to new research.

Published in the Proceedings of the National Academy of Sciences and based on fieldwork during three research cruises spanning 2017 to 2019, the study focuses on subtropical regions in the Mediterranean Sea. It uncovered how some microscopic single-celled organisms that are too light to sink beyond 100 meters or so - like phytoplankton and bacteria - end up going deeper into the ocean where there's not enough sunlight for these photosynthetic organisms to grow, live and eat.

"We found that because these organisms are so small, they can be swept up by ocean currents that then bring them deeper than where they grow," said Mara Freilich, an assistant professor in Brown University's Division of Applied Mathematics and Department of Earth, Environmental and Planetary Sciences who launched the work as a Ph.D. student a joint program at MIT and the Woods Hole Oceanographic Institution. "It's often a one-way trip for these organisms, but by taking this trip, they play a critical role in connecting different parts of the ocean."

Freilich conducted the research during her Ph.D. with Amala Mahadevan, senior scientist at Woods Hole, in a close collaboration with senior scientist at the Marine Biological Laboratory Alexandra Z. Worden and her team.

The currents the team found are called intrusions and by sweeping up the tiny organisms, they help change the types of food available in the deeper layers of the ocean while also transporting a significant amount of carbon from the water surface. This helps feed other organisms in the ocean's food chain and increases the complexity of the ecosystem at deeper depths, influencing how life and chemistry work underwater.

Altogether, the study challenges conventional understanding of how carbon, which is turned into organic matter by photosynthesis in the sunlit layer of the ocean, gets transported to depth.

"The majority of photosynthesis - by which light is converted into organic carbon, a food source for living organisms - happens in the upper 50 meters of the ocean, so the question has always been: How does the carbon that gets fixed through photosynthesis get into the deep ocean?" Freilich said. "The sinking of carbon-rich particles has always been thought to be the only answer to this question. But what we found is that tiny, single-celled organisms get caught in the oceanic flow to form intrusions… Such intrusions are significant features of the subtropical ocean - while they extend tens of kilometers laterally, they also descend hundreds of meters in the vertical, bringing cells and carbon with them. This mechanism has been unaccounted in previous estimates of carbon transport."