The many sources and fates of dissolved organic carbon in the ocean interest scientists because of the numerous important roles this material plays in marine ecosystems and in Earth’s carbon cycle. In new research, Powers et al. suggest that brown algae, such as kelp and other types of seaweed, might be a key source of dissolved organic compounds called polyphenols.
Previous research has shown that macroscopic brown algae produce a class of polyphenols known as phlorotannins—an important part of algal cell walls—during their normal biological activities. However, the relative importance of brown algae as a source of marine polyphenols and other forms of dissolved organic carbon has been unclear.
Researchers studied a brown alga species known as Sargassum natans, which is commonly found in the Gulf of Mexico, the western Atlantic Ocean, and the Sargasso Sea. They conducted experiments with the alga under natural and artificial sunlight, monitoring how much and which types of dissolved organic carbon it released.
The scientists found that S. natans releases large amounts of dissolved organic carbon in response to natural sunlight, which might help protect it against damage from UV irradiation. Accounting for the estimated biomass of S. natans in nature, the findings suggest that the species is a major contributor of dissolved organic carbon in the Gulf of Mexico and the western North Atlantic.
In the experiments, polyphenols made up about 5%–18% of the compounds released by the algae, making S. natans the biggest biological source of open-ocean polyphenols currently on record. Further research is needed to clarify what happens to these polyphenols after they are released, but the researchers noted that the findings challenge earlier conclusions that all polyphenols found in the ocean originate from land-based life. (Global Biogeochemical Cycles, https://doi.org/10.1029/2019GB006225, 2019)
—Sarah Stanley, Freelance Writer