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.
Article co-author Benjamin Van Mooy, a senior scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution (WHOI) said: “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”.
The biological carbon pump is a process where biomass sinks from the ocean surface to the deep ocean
Image credit: Helen Fredricks, ©Woods Hole Oceanographic Institution
About 5 to 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 for improving 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.
Distinct dietary preferences for deep sea bacteria
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 200-1000 meters below the ocean surface.
“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.”
Communities of bacteria exhibit different degradation rates
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.”
Lars Behrendt et al. ,Microbial dietary preference and interactions affect the export of lipids to the deep ocean.Science385,eaab2661(2024).DOI:10.1126/science.aab2661