Award: OCE-1547976

The dissolved organic matter (DOM) pool in the ocean is one of largest carbon reservoirs on earth. Understanding its fate in the marine environment helps in predicting global carbon fluxes under current and future climate scenarios. Marine microbes that obtain their growth energy from organic compounds are responsible for processing most of the DOM in the ocean. Microbial transformation of organic matter can change geochemical properties of some compounds making them susceptible to interactions with solar radiation. This fraction is a small but important fraction of the oceanic DOM pool, and it is known as chromophoric DOM or CDOM. Other sources include terrestrial runoff, as land-derived compounds, such as lignin, obtain geochemical properties that also result in the formation of CDOM. One of the reasons why CDOM is important with respect to the oceans carbon cycle stems from the fact that it is ?visible? from space by means of satellite remote sensing. Thus, CDOM can be monitored over large spatial scales in the ocean. Measuring CDOM absorbance and fluorescence in discrete water samples is relatively fast and comparatively inexpensive to other chemical techniques; however, their interpretation in terms of specific chemical functionality related to the structure and source of CDOM is rather complicated and has resulted in conflicting points of view with respect to the source of open ocean CDOM (either terrestrial sources or the result of microbial transformation of DOM). This project focused on the role of bacterial transformation of DOM as a source for open ocean CDOM. We focused on the organic matter pool produced by marine phytoplankton as one of the major sources of DOM in the ocean. Bacterial transformation of organic compounds was measured as activities of hydrolytic enzymes which are the major tools for marine bacteria to access and degrade the complex compounds and molecules within the DOM pool. To systematically decipher microbial and geochemical interactions of phytoplankton organic matter that lead to CDOM production, the research team (4 PIs and their group members from 3 different institutions) focused on laboratory experiments under controlled incubation conditions. The experiments contained phytoplankton (monocultures or natural assemblages) and their associated bacterial communities and were conducted in incubation vessels over time courses of several weeks to mimic phytoplankton growth and degradation. One of the major results with respect to bacterial activities was that enzymatic processing of phytoplankton organic matter, particularly the nitrogen-rich fraction, correlated well with the loss of fluorescence in organic microparticles > 0.7 um in diameter (particulate organic matter - POM) and the development of fluorescence in DOM. A variety of nitrogen-containing molecules produced by phytoplankton could serve as electron-rich donors (e.g. tryptophan) that could stimulate intramolecular electronic interactions of charge transfer with electron-poor acceptors. Such geochemical interactions would result in a CDOM pool that has optical properties like those from terrestrial sources. Our work provides important experimental evidence for the biological rather than terrestrial source of open ocean CDOM, underlining the importance of phytoplankton-bacterial interactions in the marine carbon cycle. This project provided research opportunities for undergraduate students involved in the processing of the experimental samples. Moreover, the results of the project will be part of a marine microbial ecology class that is currently under development by the PI. Last Modified: 07/19/2018 Submitted by: Kai Ziervogel