Award: OCE-1536927

Dissolved organic matter (DOM) plays an essential role in a variety of biogeochemical processes within the oceans. An important component of the DOM, known as colored or chromophoric dissolved organic matter (CDOM), absorbs light across the ultraviolet and visible wavelengths and is a critical determinant of the underwater light field and photochemical reactions occurring within the upper water column of the oceans. To date, however, scientists know very little about the structure and diversity of the compounds within the DOM and CDOM. This study examined the role of Sargassum seaweed as a source of DOM and CDOM in the open ocean. The team selected Sargassum natans and Saragssam fluitans as study organisms as these are macro brown algal species known to be truly holopelagic, that is, it floats throughout its lifespan and can, therefore, be found in the surface waters of the open ocean. These species are found in the Gulf of Mexico, the Western Atlantic and the Sargasso Sea where they form extensive mats sufficiently large to be observed by satellites and contribute a significant quantity of biomass to the regional ecosystem. We found that Sargassum sp release large quantities of dissolved organic carbon (DOC) and CDOM to the ocean and hence may play significant role in shaping the regional ecosystem. We calculated the amount of released carbon and were also able to quantify a specific pool of aromatic and deeply colored organic carbon contributing to the total CDOM pool. This class of materials, the so-called polyphenols, was previously believed to largely originate from land-based woody plants. This class of compounds can strongly absorb light and undergo photochemical reactions in sunlight. The optical and photochemical properties of this material was characterized and was found to be similar but not identical to the terrestrially-sourced materials. Determining the extent of Sargassum-derived dissolved organic carbon release, its chemical composition and changes in composition over time are particularly important because these results can lead to a better understanding of distribution and impact that these compounds may have on the local to regional scales. The major finding of this study was that Sargassum sp. acts as a large regional source of marine dissolved organic carbon and that specific polyphenols called phlorotannins, may contribute significantly to the regional CDOM pool, although they were found to degrade quickly under sunlight. These phlorotannins may 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 where the oxygen is very quickly depleted in the presence of Sargassum, leading 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 the lagoons. The results acquired from this study may provide information allowing faster remediation/clean up during future inundation events. Last Modified: 11/14/2019 Submitted by: Neil V Blough