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As Earth's largest carbon reservoir, the ocean locks carbon away from the atmosphere. However, scientists still struggle to measure and monitor exactly how much carbon is stored in the ocean, hindering efforts to model and respond to our changing climate.

MBARI researchers and collaborators from the University of Rhode Island, the University of Maine, and the University of California, Santa Barbara have conducted a comprehensive analysis of the ecological mechanisms that drive the transport of carbon into the deep sea, marking a major advance in efforts to monitor the ocean-climate connection. The team shared their findings in a new research publication in .

Supported by NASA's interdisciplinary EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign, researchers sequenced DNA from 800 individual particles of marine snow and identified two groups of plankton that can be used to predict the magnitude of carbon transport from the atmosphere to the deep sea. This groundbreaking work sets the stage for improving satellite-based models for ocean carbon export.

"This research represents a major advance in quantifying the ocean carbon cycle. We've developed a predictive model for ocean carbon export that links surface ocean phytoplankton communities with the ecological mechanisms that occur in the deep ocean. This model will improve efforts to link satellite observations of the surface ocean to the deep sea, better predict the impacts of climate change, and evaluate the effectiveness of future ocean-based climate interventions," said Sasha Kramer, a postdoctoral fellow at MBARI and lead author of the study.

The ocean acts like a carbon superhighway—it soaks up carbon dioxide at the surface, then marine life transforms and transports it down to deeper waters in the form of sinking organic material. Marine snow—a mixture of dead plankton, waste, mucus, and other organic material slowly sinking from the ocean's surface—is an important part of the ocean carbon cycle. However, sinking particles of organic material are difficult to observe, and flurries of marine snow are challenging to predict.

The sequestration of carbon into the ocean's depths remains one of the most uncertain parts of the global carbon cycle. Because we do not fully understand the export of carbon from the surface through the midwater, we cannot accurately quantify the ocean's carbon cycle, how it is changing, and the impacts on ocean ecosystems.

MBARI's Carbon Flux Ecology Team, led by Scientist Colleen Durkin, studies the microscopic biological interactions that control carbon export in the ocean. The team's research focuses on the intricate sinking particles found throughout the ocean depths and the diverse communities of organisms that produce, transform, and feed on these particles as they descend from the surface to the deep ocean. This work is helping to advance global-scale observations and models of Earth's climate and ocean ecosystems.

"The ocean and its inhabitants export carbon on a massive scale, all driven by tiny phytoplankton. Microscopic processes translate to global impact," explained Durkin. "By shifting our focus to individual particles, probing the biology and ecology of individual phytoplankton groups, we found the surprising connections that can transform efforts to measure and monitor ocean carbon export."

The EXPORTS program is a large-scale multi-institutional effort led by NASA that brings together a variety of disciplines and perspectives—including ocean optics, remote sensing, and molecular biology—to deploy diverse state-of-the-art technologies to understand the transport of carbon from the surface ocean to the deep sea.

Kramer and Durkin joined EXPORTS field campaigns in the North Atlantic and North Pacific. With a team of researchers from MBARI, the University of Rhode Island, the University of Maine, and the University of California, Santa Barbara, they meticulously sorted the individual sinking particles sampled by sediment traps deployed at five depths between 100 to 500 meters (330 to 1,640 feet). Previous studies have typically examined bulk-filtered biomass, not individual particles.

The team's novel approach allowed them to compare finer details of how carbon was packaged and transported to the information contained in bulk particles. They then examined 18S rRNA gene sequences from phytoplankton sampled in surface seawater, bulk-filtered particles, and 800 individual particles of marine snow. These genetic tags allowed researchers to detect specific phytoplankton groups in the surface ocean and follow their transport into deeper waters where their carbon is sequestered.

Pairing the relative abundance of genetic sequences with chemical measurements of sinking carbon allowed the team to identify predictive relationships between the export of specific phytoplankton taxa and the magnitude of carbon flux with depth. They found that two key groups of phytoplankton—diatoms and photosynthetic Hacrobia—can be used to predict the magnitude of carbon export into the deep ocean.

The relationships found in this study expand the utility of satellite observations of the ocean. Satellites are the most powerful tool for visualizing marine processes on a global scale. New ocean color satellites, like NASA's PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission, are providing a fresh perspective on the ocean from space. PACE is equipped with a hyperspectral Ocean Color Instrument—a radiometer that allows scientists to quantify the pigments in different taxonomic groups of phytoplankton in the surface ocean.

Scientists can now look for blooms of diatoms and Hacrobia, specifically, to develop better models that estimate carbon export to the ocean's depths at a global scale.

Since both of the key phytoplankton groups identified here can be reliably detected with DNA sequencing and satellite observations, the relationship with carbon export can be tested in other regions and ecosystems.

"Responding to the climate crisis will require major leaps forward in our ability to monitor the ocean ecosystem. We must find new ways to observe the processes occurring on the microscopic scale and integrate that perspective with the climate drivers occurring on the global scale," said Durkin.

"This work demonstrates the value of translating across scientific disciplines and physical scales. By identifying the tiny microbes inside marine snow particles, we can propose strategic use of our global observing technologies that may improve monitoring of the ocean carbon cycle."