A New Approach to Quantifying the Metabolic Rates of Coral

BIOS assistant scientist Yvonne Sawall and research specialist Tim Noyes were chosen in early 2020 from among five applicants to receive funding from the Cawthorn Innovation Award for a project designed to advance understanding of the metabolic rates of corals and other reef organisms. The award was established in 2016 by trustee emeritus Rob Cawthorn to support innovative and challenging research ideas among scientists at BIOS.

By studying these organisms in their natural environment with new instruments and measurement techniques, they plan to address emerging research questions about how reef organisms function and contribute to ecosystem processes.

Sawall, who was promoted to assistant scientist in March, joined the Institute in 2016 as a postgraduate researcher. She is a marine benthic ecologist whose research focuses on key metabolic processes in coral reefs and seagrass meadows. Noyes, a research specialist, has worked at BIOS since 2008 on projects related to the physical, chemical, and biological assessments of the marine environment.

All living organisms, from animals and plants to microscopic organisms, exist in a perpetual dance with their environment, exchanging oxygen, carbon dioxide (CO2), inorganic nutrients, and carbon compounds. The two fundamental metabolic processes involved in these exchanges are photosynthesis (the uptake of CO2  and the release of oxygen, or O2) and respiration (the uptake of O2 and the release of CO2).

For corals and other organisms with calcium carbonate shells, such as mollusks and crustaceans, there is yet another important process: calcification. Calcifying organisms absorb dissolved inorganic carbon and calcium from the seawater and use them to build calcium carbonate skeletons.

Numerous laboratory-based studies have shown that a variety of environmental factors influence the metabolic processes of reef organisms, including corals, algae, seagrass, and minute plants and animals living in the sediment. But scientists have a limited understanding of how these findings translate to the real world.

“In a controlled setting, it’s easy to manipulate a few variables at a time, such as water chemistry and temperature,” Sawall said. “But in the ocean, there are numerous environmental and biological factors acting at the same time on a variety of spatial and time scales.”

To address these shortcomings, Sawall and Noyes proposed to test a new instrument designed to collect continuous measurements of metabolic rates in the field, for up to a week at a time. The setup, called a BIO-RESORT, was developed in a collaborative effort between BIOS and GEOMAR Helmholtz Center for Ocean Research, Kiel, Germany, and is in its final stages of construction.

The BIO-RESORT comprises a series of six incubation chambers that can hold individual reef organisms. Each chamber is connected to an oxygen sensor and an automated water sampler, which allow researchers to measure photosynthesis and respiration, and determine rates of nutrient uptake and release, calcification, and feeding.

The team is planning to deploy the BIO-RESORT at six locations across the Bermuda reef platform to gain insight into the variability of metabolic processes among marine species and their locations. The results are expected to significantly improve our understanding about the flow of energy, carbon, and nutrients through organisms in their natural habitat, as well as the relative contribution of different organisms to ecosystem processes.

The second phase of the project is designated to improving measurements of the total amount of energy acquired by an organism from sunlight, known as gross photosynthesis (GP) rates. Current measurement techniques are based on changes in oxygen concentrations in experimental setups and tend to underestimate GP rates.

“We plan to investigate the potential utility of the oxygen isotope tracing technique, first in the laboratory and later in the field using the BIO-RESORT,” Sawall said. “This technique has been successfully used for other organisms under laboratory conditions and has the potential to significantly improve calculations of organism energy budgets.”

Isotopes are versions of an element that differ in the number of neutrons they contain. More than 99.9 percent of the oxygen on earth (called 16-O) has eight protons and eight neutrons; however, trace amounts of oxygen contain one extra neutron (17-O) or two extra neutrons (18-O). These are referred to as “rare stable isotopes.” In isotope tracer experiments, rare isotopes are added to a study system, such as an incubation chamber, where the study organisms utilize them in biological processes. After a period of time, the concentration of the isotopes can be measured in the water or the organisms, and the loss or addition of rare isotopes allows researchers to calculate metabolic rates. 

Sawall and Noyes plan to use water (H2O) that is labeled with 18-O (H218-O) and add it to incubation chambers containing live corals. Here, the labeled water is split into hydrogen and oxygen during photosynthesis by the symbiotic algae living in the coral, called zooxanthellae. 18-O then accumulates in the incubation chamber, allowing it to be measured and used to calculate GP rates. Collaborator Michael Bender, a professor emeritus of Princeton University and oxygen isotope geochemist, will advise and provide the instrumentation necessary for isotope measurements.

Work and travel restrictions related to the COVID-19 pandemic have delayed the project, which is now expected to start in July. At that point, Sawall and Noyes will begin testing the BIO-RESORT and conducting the first phase of the project. Work on the isotope tracing technique will likely start in early 2021.