Studying Microbial Metabolisms to Better Understand the Ocean’s Carbon Cycle

Research conducted as part of the multi-year, multi-institutional BIOS-SCOPE (Bermuda Institute of Ocean Sciences – Simons Collaboration on Ocean Processes and Ecology) project is shedding light on how the end products of cellular metabolism, called metabolites, could be used to track future changes in the marine environment.

An overarching goal of BIOS-SCOPE is to study the microbial processes, structure and function in the northwest Sargasso Sea to understand how organic matter (carbon) is cycled within the marine environment. This organic carbon plays a significant role in the global carbon cycle. Dissolved organic carbon (DOC) is particularly significant; the concentration of DOC in seawater is equal to the amount of carbon dioxide in the atmosphere. It’s also important for marine ecosystems, as microorganisms use DOC to meet their metabolic requirements.

“These and other interactions between biology and chemistry play a key role in defining the factors controlling organic carbon distributions in marine systems, both now and in the future ocean,” said marine geochemist Krista Longnecker, lead author on the study and research specialist at Woods Hole Oceanographic Institution in Massachusetts (U.S.). Nine other scientists contributed to the study, including oceanographer Ruth Curry and microbial ecologist Rachel Parsons, both from BIOS.

To fully understand the role that microorganisms play in the ocean’s carbon cycle, Longnecker and her BIOS-SCOPE colleagues needed to understand more about metabolites, the set of compounds that comprise DOC and are central to cellular metabolism. Metabolites include vitamin B complexes (vitamins B2 and B5) and free amino acids (such as leucine and phenylalanine), among other organic molecules.

“We had important spatial details on the nature of complex dissolved organic carbon in marine ecosystems, but we lacked information on temporal variability, as well as the distribution, quantity and composition of these compounds,” she said.

Seasonal Patterns in the Sargasso Sea

The BIOS-SCOPE project leverages an oceanographic time-series program at BIOS, the Bermuda Atlantic Time-Series Study (BATS), which has collected monthly biogeochemical data from a fixed sampling location in the Sargasso Sea since 1988. Decades of data have revealed the Sargasso Sea experiences predictable seasonal changes. Winter storms mix the water column down to depths of 1000 feet (300 meters), followed by a spring phytoplankton bloom and a stratification of the water column that begins in April and persists throughout the summer.

This seasonal variability is reflected in physical and biogeochemical processes, nutrient cycles, oxygen levels, as well as the distribution of viruses, microorganisms, and zooplankton. There was good reason to believe that metabolites would follow a similar seasonal pattern.

The metabolites investigation analyzed four years of water samples collected from July 2016 through July 2019 at the BATS location, a site east of BATS, and at Hydrostation ‘S’, another long-term oceanographic time-series program run by scientists at BIOS. The samples were collected from the ocean’s surface down to 3000 feet (914 meters) and during all four seasons (mixed, spring, stratified, fall).

A total of 41 metabolites were present in all four years of the samples. Most of these were found in higher concentrations near the surface and decreasing concentrations deeper in the water column. When looking at other environmental variables, there were strong positive correlations between the concentrations of metabolites and temperature, bacterial production, and total organic carbon, and strong negative correlations with depth and nutrients.

Some metabolites had a repeatable set of seasonal patterns over the four years, including pantothenic acid (higher in winter, lower in summer), taurocholic acid (higher in summer, lower in winter), and tryptophan (higher in summer, lower in winter). Other metabolites, such as riboflavin, demonstrated both a daily and a seasonal cycle, with concentrations lowest in the shallowest depths during the mid-day, and also higher in the winter and lower in the summer. Overall, the compounds with higher concentrations in the winter were predominantly vitamins (riboflavin, pantothenic acid), whereas amino acids, nucleic acid precursors and other metabolites dominated in the summer.

“The dissolved metabolites measured during this multi-year study are central carbon metabolites and examining their variability in time provides insight into the processes that underlie chemical variability in a marine ecosystem,” Longnecker said. “While we cannot yet observe long-term changes in our dataset, in future work we can track metabolites over time to view how specific organic compounds change within the context of a changing climate and the subsequent impact on the marine microbial food web.”