Experimental Analysis of Diurnal Variations in Humic-Like Fluorescent Dissolved Organic Matter in Surface Seawater

Abstract

Humic-like fluorescent dissolved organic matter (FDOM) has been widely used as tracers for bio-refractory dissolved organic matter (RDOM) to understand its dynamics in the oceans. Vertical distributions of humic-like FDOM are controlled by microbial production in the ocean interiors and photobleaching in surface layers. Although humiclike FDOM is expected to be actively produced in surface layers with high bacterial activity, its production in surface seawater is not well understood. To examine the diurnal variations in humic-like FDOM due to microbial production and photobleaching in surface seawater, we conducted seven experiments from night to day using surface seawater in the subtropical Pacific and coastal regions. Parallel factor analysis (PARAFAC) determined that FDOM in the incubated seawater was composed of three components: two types of humic-like FDOM and a protein-like FDOM. The fluorescence intensity of humic-like FDOM increased to 104.0 ± 2.5% of the initial intensity during the night and decreased to 101.2 ± 2.5% under sunlight exposure during the day. Conversely, its intensity significantly increased to 114.0 ± 2.7% under dark conditions during the day. The turnover rates of humic-like FDOM by the increase and decrease in its intensity were estimated to be 0.14 and 0.11 day −1 , respectively. These comparable turnover rates indicated that the production and photobleaching of humic-like FDOM were almost in equilibrium in the surface layer, with a low level of humic-like FDOM. Linear correlations between the intensity of humic-like FDOM and concentrations of dissolved oxygen (DO) in all experiments under dark conditions indicated that humiclike FDOM were produced as the by-products of microbial respiration processes in the surface seawater. Using global bacterial respiration rates, the net production rate of humic-like FDOM in the global photic layer was estimated as 4.2-5.5 × 10 17 R.U. year −1 , contributing to 75% of its production in the entire ocean.

Dissolved Organic Matter Fluorescence as a Tracer of Upwelling and Microbial Activities in Two Cyclonic Eddies in the Eastern Tropical North Atlantic

“…In our study, C468 and C398 correlated significantly with AOU in the cruise M160 and with community respiration in both cruises. This is consistent with published studies were both components were correlated to AOU (Hayase & Shinozuka, 1995;Martínez-Pérez et al, 2019;Tanaka et al, 2014;Yamashita et al, 2007) and to bacterial respiration (Omori et al, 2020). Therefore, C468 and C398 are suitable tracers of DOM mineralization as by-product of microbial respiration.…”

CDOM spectral slope (S275–295) as tracers of water masses, CDOM heterogeneity, and Δ14C‐DOC in an oligotrophic marginal sea

“…2a). This inverse relationship could be largely ascribed to photobleaching of low-S 275-295 FDOM H in the sunlit surface ocean (Omori et al 2020;Yang et al 2020), which increases S 275-295 (Helms et al 2008;Yamashita et al 2013), and microbial production of low-S 275-295 FDOM H in the dark ocean (Yamashita and Tanoue 2008;Yang et al 2020). As C H accounts for 85% of the S 275-295 variance (Fig.…”

TreatEEM—A Software Tool for the Interpretation of Fluorescence Excitation-Emission Matrices (EEMs) of Dissolved Organic Matter in Natural Waters

“…These components appear to be microbially degradable, whereas humic-like components appear to be photodegradable but resistant to microbial degradation [5,20,21]. Humic-like FDOM has therefore been widely used as a potential tracer of refractory DOM [22], which plays an important role in carbon storage in the sea.…”