DOM degradation by light and microbes along the Yukon River-coastal ocean continuum

Brice K Grunert,Peter Hernes,Maria Tzortziou,Patrick Neale,Alana Menendez

doi:10.1038/s41598-021-89327-9

Abstract

The Arctic is experiencing rapid warming, resulting in fundamental shifts in hydrologic connectivity and carbon cycling. Dissolved organic matter (DOM) is a significant component of the Arctic and global carbon cycle, and significant perturbations to DOM cycling are expected with Arctic warming. The impact of photochemical and microbial degradation, and their interactive effects, on DOM composition and remineralization have been documented in Arctic soils and rivers. However, the role of microbes, sunlight and their interactions on Arctic DOM alteration and remineralization in the coastal ocean has not been considered, particularly during the spring freshet when DOM loads are high, photoexposure can be quite limited and residence time within river networks is low. Here, we collected DOM samples along a salinity gradient in the Yukon River delta, plume and coastal ocean during peak river discharge immediately after spring freshet and explored the role of UV exposure, microbial transformations and interactive effects on DOM quantity and composition. Our results show: (1) photochemical alteration of DOM significantly shifts processing pathways of terrestrial DOM, including increasing relative humification of DOM by microbes by > 10%; (2) microbes produce humic-like material that is not optically distinguishable from terrestrial humics; and (3) size-fractionation of the microbial community indicates a size-dependent role for DOM remineralization and humification of DOM observed through modeled PARAFAC components of fluorescent DOM, either through direct or community effects. Field observations indicate apparent conservative mixing along the salinity gradient; however, changing photochemical and microbial alteration of DOM with increasing salinity indicate changing DOM composition likely due to microbial activity. Finally, our findings show potential for rapid transformation of DOM in the coastal ocean from photochemical and microbial alteration, with microbes responsible for the majority of dissolved organic matter remineralization.

Highlights

The major Arctic Rivers—Ob’, Yenisey, Lena, Kolyma, Yukon and Mackenzie—deliver over 18 Tg of terrestriallyderived dissolved organic carbon (DOC) per year to the coastal ocean, with nearly half of this material delivered during the spring f reshet[1]
The size-fractionation of microbes has been shown to impact Dissolved organic matter (DOM) composition, with a larger size fraction encompassing the entire microbial community in mesocosm experiments resulting in DOM composition more representative of that observed in the e nvironment[27]
We manipulated DOM using three experiments explicitly characterizing photochemical, microbial, and microbial after photochemical transformation of DOM, with DOM compositional shifts assessed via changes in DOM optical properties (CDOM and fluorescent DOM) and DOM concentration assessed through changes in DOC

Summary

Introduction

The major Arctic Rivers—Ob’, Yenisey, Lena, Kolyma, Yukon and Mackenzie—deliver over 18 Tg of terrestriallyderived dissolved organic carbon (DOC) per year to the coastal ocean, with nearly half of this material delivered during the spring f reshet[1]. Our fluorescence results further supported our hypothesis that photochemical alteration increases microbial transformation pathways of DOM, primarily through photodegradation of humic DOM and formation of terrestrial derivatives (e.g., lignin depolymerization products).

Results

Conclusion