Award: OCE-1536888

Dissolved organic matter plays an essential role in influencing underwater light fields and facilitating biogeochemical reactions in the ocean but to date, scientists know very little about the structure and diversity of these compounds. We have been working on a National Science Foundation-funded study researching the role of Sargassum seaweed as a source of dissolved organic matter in the open ocean, but also during inundation events in the Caribbean. The team selected Sargassum natans and Sargassum fluitans as study organisms as these are macro brown algal species known to be truly holopelagic, that it, it floats throughout its lifespan and can, therefore, be found in the open ocean. These species live in the surface waters of the Gulf of Mexico, the Western Atlantic and the Sargasso Sea where they form extensive mats large enough to be observed by satellites and contribute a significant biomass to the regional ecosystem. We found that Sargassum sp. release large quantities of dissolved organic carbon to the ocean and hence play a substantial role in shaping the ecosystem. We calculated the amount of released carbon and we also were able to quantify a specific pool of aromatic and deeply colored organic carbon which was previously believed to largely originate from land-based woody plants. These so-called polyphenols strongly absorb light and undergo photochemical degradation in sunlight. The distribution of Sargassum-derived dissolved organic carbon and its changes in composition within extensive mats are particularly interesting because results are aiding a better understanding of the life-span or reactivity of these chemical compounds. The major finding of this study was that Sargassum sp. act as a large regional source of marine dissolved organic carbon and polyphenols and that specific polyphenols, called phlorotannins, do degrade quickly under sunlight. These phlorotannins likely act as a sunscreen to the Sargassum, because release rates increased under prolonged sunlight exposure. We also looked at the local impact of Sargassum during an inundation event in Puerto Rico and we were able to show that the extremely high release of dissolved organic carbon has a profound effect on the coastal ecosystem and in particular on semi-enclosed lagoons were the oxygen is very quickly depleted in the presence of Sargassum, which leads to episodic anoxia. The disappearance of bioluminescent algae has been observed in some lagoons previously, which is detrimental to these economically valuable tourist destinations in Puerto Rico. Large quantities of Sargassum may partly explain these "black out" events in bio bays, and will hopefully lead to faster remediation/clean up during future inundation events. Last Modified: 09/30/2019 Submitted by: Michael Gonsior