The movement of carbon dioxide (CO₂) from the surface of the ocean, where it is in active contact with the atmosphere, to the deep ocean, where it can be sequestered away for decades, centuries, or longer, depends on a number of seemingly small processes.

One of these key microscale processes is the dietary preferences of bacteria that feed on organic molecules called lipids, according to a journal article, “Microbial dietary preference and interactions affect the export of lipids to the deep ocean,” published in Science.

“In our study, we found incredible variation in what the different microbes preferred to digest. Bacteria seem to have very distinct diet preferences for different lipid molecules. This has real implications for understanding carbon sequestration and the biological carbon pump,” said journal article co-author Benjamin Van Mooy, a senior scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution (WHOI).

“This study used state-of-the-art methods to link the molecular composition of the sinking biomass with its rates of degradation, which we were able to link to the dietary preferences of bacteria,” continued Van Mooy. The biological carbon pump is a process where biomass sinks from the ocean surface to the deep ocean.

About 5–30% of surface ocean particulate organic matter is composed of lipids, which are carbon-rich fatty acid biomolecules that microbes use for energy storage and cellular functions. As the organic matter sinks to the deep sea, diverse communities of resident microbes degrade and make use of the lipids, exerting an important control on global CO₂ concentrations.

Understanding this process is vital to improve our ability to forecast global carbon fluxes in changing ocean regimes. Geographic areas where more lipids reach the deep ocean undegraded could be hotspots for natural carbon sequestration.

“Bacteria isolated from marine particles exhibited distinct dietary preferences, ranging from selective to promiscuous degraders,” the article states. “Using synthetic communities composed of isolates with distinct dietary preferences, we showed that lipid degradation is modulated by microbial interactions. A particle export model incorporating these dynamics indicates that metabolic specialization and community dynamics may influence lipid transport efficiency in the ocean’s mesopelagic zone.” The mesopelagic zone extends about 200-1,000 meters below the ocean surface.

“I was thrilled to see how much there is to learn about the functioning of the ocean by combining two technologies—high-end chemical analysis and microscale imaging–that have historically never been used together,” said co-author Roman Stocker, professor at the Institute of Environment Engineering, Department of Civil, Environmental and Geomatic Engineering, ETH Zurich, Switzerland.

“I believe that work at the interface between the exciting technologies we now have available in microbial oceanography will continue to yield important insights into how microbes shape our oceans, now and into the future.”

“Scientists are starting to understand that lipids in the ocean can vary significantly depending on different environments, such as the coast versus the open ocean, and the season,” said Van Mooy. “With this information, researchers can start to consider whether there are places in the ocean where lipids sink and are sequestered very efficiently, while there may be other locations where lipids are barely sequestered at all or are very inefficiently sequestered.”

“What excites me about this paper is that it shows bacteria are not just eating any type of lipid, but are very specialized, and like us, have specific food preferences,” said article co-author Lars Behrendt, associate professor and SciLifeLab fellow at the Science for Life Laboratory, Department of Organismal Biology, Uppsala University, Sweden.

“This changes how we think about how microorganisms consume food in their natural environment and how they might help each other or compete for the same resource. It also supports the idea that combinations of bacteria better break down specific compounds, including lipids, or to achieve other desired functions.”

In addition to studying specific bacteria species in isolation, the researchers also looked at how dietary preference affects degradation rates by multispecies communities of bacteria, which they stated is ecologically more relevant than species in isolation. The researchers found that simple synthetic co-cultures exhibited different degradation rates and delay times when compared to monocultures. The researchers also noted that the degradation of particulate organic matter in the natural environment is even more complex than what is described in the study.

“Phytoplankton are the main reason the ocean is one of the biggest carbon sinks. These microscopic organisms play a huge role in the world’s carbon cycle—absorbing about as much carbon as all the plants on land combined,” said co-author Uria Alcolombri, senior lecturer, Alexander Silberman Institute of Life Sciences, Department of Plant and Environmental Sciences, The Hebrew University of Jerusalem, Israel. “It’s fascinating that we can study tiny microbial processes under the microscope while uncovering the biological factors that regulate this massive ‘digestive system’ of the ocean.”

The common practice of building dams to prevent flooding can actually contribute to more intense coastal flood events, according to a new study.

The study, published in the Journal of Geophysical Research: Oceans, studied the effects of dams built in coastal estuaries, where rivers and ocean tides interact. Those massive infrastructure projects are surging in popularity globally, in part to help offset intensifying storms, salt intrusion and sea-level rise fueled by climate change.

By analyzing data and measurements from Charleston Harbor, South Carolina, dating back more than a century, researchers determined that coastal dams don’t necessarily mitigate flooding. Dams can either increase or decrease flood risks, depending on the duration of a surge event and friction from the flow of water.

“We usually think about storm surges becoming smaller as you go inland, but the shape of the basin can actually cause it to become larger,” said lead author Steven Dykstra, an assistant professor at the University of Alaska Fairbanks College of Fisheries and Ocean Sciences.

Estuaries are typically shaped like a funnel, narrowing as they go inland. Introducing a dam shortens the estuary with an artificial wall that reflects storm surge waves moving inland. The narrowing channel shape also makes small reflections that change with the surge duration. Dykstra compared those storm-fueled waves to splashes in a bathtub, with certain wave frequencies causing water to slosh over the sides.

After using Charleston Harbor as a case study, researchers used computer modeling to gauge the flood response at 23 other estuaries in diverse geographic areas. Those encompassed both dammed and naturally occurring estuary systems, including Cook Inlet in Alaska.

The models confirmed that the basin shape and alterations that shorten it with a dam are the key component in determining how storm surges and tides move inland. At the right amplitude and duration, waves in dammed environments grow instead of diminishing.

The study also determined that areas far from coastal dams could still be directly influenced by human-created infrastructure. In the Charleston area, the highest storm surges routinely occurred more than 50 miles inland.

“One of the scary things with this is that sometimes people don’t realize they are in a coastal-influenced zone,” Dykstra said. “Sea-level rise is making people far inland aware that they’re not free from coastal effects—and it usually happens with a massive flood.”

Other contributors to the study included Enrica Viparelli, Alexander Yankovsky and Raymond Torres from the University of South Carolina, and Stefan Talke from California Polytechnic State University, San Luis Obispo.