Depth-dependent photodegradation of marine dissolved organic matter

Abstract

Marine dissolved organic matter (DOM) in surface and deep waters of the eastern Atlantic Ocean and Sargasso Sea was analyzed by excitation emission matrix (EEM) fluorescence spectroscopy and parallel factor analysis (PARAFAC). Photo-degradation with semi-continuous monitoring of EEMs and absorbance spectra was used to measure the photo-degradation kinetics and changes of the PARAFAC components in a depth profile of DOM at the Bermuda Atlantic Time Series (BATS) station in the Sargasso Sea. A five component model was fit to the EEMs, which included traditional terrestrial-like, marine-like, and protein-like components. Terrestrial-like components showed the expected high photo-reactivity, but surprisingly, the traditional marine-like peak showed slight photo-production in surface waters, which may account for its prevalence in marine systems. Surface waters were depleted in photo-labile components while protein-like fluorescent components were enriched, consistent with previous studies. Ultra-high resolution mass spectrometry detected unique aliphatic compounds in the surface waters at the BATS site, which may be photo-produced or photo-stable. Principle component and canonical analysis showed strong correlations between relative contributions of unsaturated/aromatic molecular formulas and depth, with aliphatic compounds more prevalent in surface waters and aromatic compounds in deep waters. Strong correlations were seen between these aromatic compounds and humic-like fluorescent components. The rapid photo-degradation of the deep-sea fluorescent DOM in addition to the surface water relative depletion of aromatic compounds suggests that deep-sea fluorescent DOM may be too photochemically labile to survive during overturning circulation.

Drivers of fluorescent dissolved organic matter in the global epipelagic ocean

“…c,d). In fact, there appears to be consistent dominance of amino acid‐like fluorescence intensity dominance in the surface waters of the Atlantic Meridional Transect (Kowalczuk et al ), the eastern Atlantic Ocean and in the northern Sargasso Sea (Timko et al ) or in the southern Canada Basin and in the East Siberia Sea (Guéguen et al ). Here, we find larger values of the tyrosine‐like C4 at the surface than in the dark ocean, with half of the data exceeding 10.2 × 10 −3 RU, whereas for the dark ocean half of the data exceeded 4.9 × 10 −3 RU.…”

“…Consequently, the photochemical degradation will affect equally in all biogeographic provinces studied regardless of the solar radiation received. In fact, previous irradiation experiments with natural waters showed the highest humic‐like substances depletion rates during the first hours (Del Vecchio and Blough ; Nieto‐Cid et al 2006; Timko et al ), then a slowing down after 20 d (Helms et al ). Likewise, the humic‐like peak A/C experienced the lowest losses after 24‐h irradiation in the surface samples compared with greater depths (Timko et al ), which is likely a result of the highly photodegraded material that was already present.…”

“…In our circumnavigation, the minimum values of C1 and C2 at both surface and deep ocean were found in the South Indian Ocean. Regarding the depletion of the humic‐like components in surface waters and the increase with depth, Timko et al () found that in the eastern North Atlantic Ocean and the northern Sargasso Sea, the humic‐like C1 and C2 were on average 5.9 and 3.4 times higher below 1000 m than at the surface.…”

Picocyanobacteria and deep-ocean fluorescent dissolved organic matter share similar optical properties

“…Reproducible photodegradation experiments1041 undertaken on the deep-sea SPE-DOM (Sargasso Sea 4,530 m), SRNOM and three different strains of Synechococcus SPE-DOM (Supplementary Fig. 8) showed that the photodegradation of the Synechococcus SPE-DOM samples behaved in a similar manner to the deep-sea SPE-DOM, whereas the maximum fluorescence loss of SRNOM occurred at longer emission wavelengths.…”

From Fresh to Marine Waters: Characterization and Fate of Dissolved Organic Matter in the Lena River Delta Region, Siberia

“…As a result, the humic-like-dominated, highly photoreactive DOM (Helms et al, 2014;Timko et al, 2015) is exposed longer to photochemical degradation (Fichot and Miller, 2010). This process is evidenced by the relationship between S Ratio and salinity ( Figure 5E), with high S Ratio values (>0.95) observed in intermediate salinity (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20).…”