Award: OCE-1459406

Award Title: Collaborative Research: Planktonic Sources of Chromophoric Dissolved Organic Matter in Seawater
Funding Source: NSF Division of Ocean Sciences (NSF OCE)
Program Manager: Henrietta N. Edmonds

Outcomes Report

Chromophoric dissolved organic matter (CDOM) is the light absorbing component of natural waters. CDOM's distribution and dynamics can influence CO2 fluxes between the surface ocean and the atmosphere. However, the origins of CDOM found in the ocean are a matter of debate. CDOM in the surface ocean is generated through phytoplankton production and heterotrophic grazing, but it also flows into the ocean via rivers and estuaries. The manner in which it absorbs and fluoresces light are two key optical properties used to understand CDOM?s origins. These optical properties have been attributed to humic substances – which are formed in soils and sediments on land – and explained by a model of intramolecular charge transfer. Because no prior work has investigated the photophysical or photochemical properties of the CDOM produced by plankton or microbes, we proposed to test the charge transfer model on phytoplankton origins of CDOM. In this project, we hypothesized that the organic matter produced by ocean phytoplankton can be transformed into CDOM via degradation by bacteria that will exhibit absorbance and fluorescence properties consistent with the charge transfer model. We conducted experiments on organic matter produced from cultures of individual phytoplankton taxa and from cultures of mixed assemblages of phytoplankton. We tracked the changes in absorbance and fluorescence properties of the CDOM generated as phytoplankton grew and were then degraded by bacteria. We then treated the planktonic CDOM with sodium borohydride and evaluated the changes induced to absorbance and fluorescence. Further, we studied how bacteria growth and enzyme activity changed contemporaneously as CDOM formed. Finally, we examined the relationship between amino acids and planktonic CDOM formation. Intellectual merits of our project contributed to a deeper understanding in the role of CDOM formation in the oceans C and N cycles. We discovered that when planktonic CDOM is formed, bacteria are key mediators in this process. In cultures of individual phytoplankton taxa, bacterial abundances increased as CDOM was formed from planktonic particulate organic matter (POM). Strong correlations between carbohydrate enzyme activities and fluorescence indicated that either carbohydrate hydrolysis products were a major source of CDOM or that bacterial formation of CDOM is an energetically demanding process. Further, high rates of aminopeptidase activity were surprising and indicated rapid turnover of nitrogen compounds and recycling of nutrients throughout the phytoplankton growth experiments. We then discovered that formation of planktonic CDOM in a mixed assemblage of phytoplankton from the Atlantic Ocean produced the light absorbance and fluorescence patterns which fit the charge transfer model. The spectral properties of the CDOM and their response to reaction with borohydride mimicked results found for terrestrial humic substances. Results thus confirmed our hypothesis that phytoplankton produce organic substances which bacteria may then transform into products that absorb light and fluoresce consistent with a charge transfer model. We estimated that up to 30% of the CDOM in the mesopelagic ocean is attributable to phytoplankton. Finally, we discovered clear connection between CDOM formation and Earth?s N cycle through transformation of particulate and dissolved amino acids, the building blocks of proteins. We found strong correlations between fluorescence signals in planktonic CDOM and the aromatic amino-acids tryptophan and tyrosine. Amino acid ratios changed consistently with CDOM formation in a manner suggesting their degradation. Aminopeptidase enzyme results indicated that degradation of POM and formation of planktonic CDOM is strongly linked to peptide/protein formation. The broader impacts of this project included application to management and monitoring of aquatic ecosystems, mentoring of postdocs, and training of graduate and undergraduate students, in addition to inclusion of project results in courses taught by the PIs. Phytoplankton are key organisms in virtually all aquatic ecosystems. Understanding better their role in the cycling of important elements such as C and N can lead to new insight into freshwater ecosystem dynamics (lakes, rivers) – perhaps extending to management of freshwater resources. The optical signals we associated with processing of planktonic organic matter are based on techniques used to develop and calibrate in situ sensors that are used in monitoring and management of aquatic ecosystem resources. Insight gained from our study benefits society by providing a deeper understanding of how Earth?s C and N cycles operate and how they can respond to environmental and global change. Four PIs from the project used results from this project in teaching and outreach at host institutions. Several undergraduates were supported by this project and assisted with experiments and measurements. One undergraduate student used part of this project for her honor?s thesis. A female postdoc and a female graduate student each were supported by this project. The postdoc served as liaison between the PIs and coordinated field sampling and experiments. She trained several graduate and undergraduate students are part of her experience during the project. Graduate students contributed to a public outreach and teaching module entitled "Light and Life in the Ocean." Last Modified: 06/24/2019 Submitted by: Christopher Osburn

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Principal Investigator: Christopher Osburn (North Carolina State University)

Co-Principal Investigator: Astrid Schnetzer