Microbial and photochemical reactivity of fluorescent dissolved organic matter in a coastal upwelling system

We observed significant changes in the dissolved oxygen content and the fluorescence of humic substances and dissolved aromatic amino acids after 24 h light and dark incubations in the coastal upwelling system of the Rı́a de Vigo under a wide variety of meteorologic and oceanographic conditions. Respiration rates were inversely correlated with the net production of humic fluorescence in the dark at a net rate of 20.027 6 0.003 mg equivalents of quinine sulphate per mmol of O2, suggesting that marine humics are a by-product of the bacterial respiration of dissolved organic matter (DOM). On the contrary, humic fluorescence consumption in the light minus dark incubations was positively correlated with the net production in the dark, indicating a rapid photodegradation of recently produced marine humic substances. Parallel incubation experiments demonstrated that daily photodegradation rates and residual humic fluorescence levels followed a seasonal pattern characterized by a marked autumn maximum. Finally, a significant linear correlation between the gross primary production (Pg) and the net production of aromatic amino acids fluorescence in the light pointed to the rapid consumption of dissolved protein-like materials at a net average rate of 21.4 6 0.2 ppb equivalents of tryptophan per day, which accumulates in the water column only when Pg exceeds 80 6 20 mmol kg21 d21. Marine dissolved organic matter (DOM) constitutes the main substrate for bacterioplankton growth and respiration (Azam and Cho 1987). DOM sources in estuarine and coastal waters include phytoplankton exudation, cell autolysis, and grazing pressure (Nagata 2000), as well as allochthonous organic matter of terrestrial and oceanic origin (Wollast 1993). The diversity of sources produces a myriad of different compounds, with a microbial reactivity ranging from hours for the dissolved free amino acids (DFAA), monosaccharides, and other labile molecules (Fuhrman 1987) to thousands of years for the most refractory humic compounds upwelled from the deep sea (Williams and Druffel 1987). The resistance of humic substances to microbial degradation contrasts with their susceptibility to photochemical decomposition (Benner and Biddanda 1998). Fluorescence is a useful, simple, and quick technique to characterize and quantify two different classes of DOM: the labile DFAA (Yamashita and Tanoue 2003) and the recalcitrant humic substances (Coble et al. 1990). These two classes of compounds can be used to trace diverse biogeochemical processes such as labile organic matter production (Nieto-Cid et al. 2005), respiration (Chen and Bada 1992; Nieto-Cid et al. 2005), and photobleaching (Skoog et al. 1996; Moran et al. 2000). All these processes play a key role in the accumulation, recycling, and export of DOM in marine ecosystems (Carlson 2002). This work proposes using fluorescence to deal with the following three partially unresolved questions about the dynamics of marine DOM: 1. The origin of dissolved labile organic matter can be autotrophic, via extracellular release (Myklestad 1995) and cell lysis (Kirchman et al. 1993), or heterotrophic, via grazing losses (Storm et al. 1997). Rapid turnover maintains these compounds at nanomolar concentrations in the open ocean, but they support a large portion of the heterotrophic bacterial growth and respiration (Skoog et al. 1999). We propose using the fluorescence of dissolved aromatic amino acids (FDOMT) to discriminate whether the accumulation of labile DOM in the marine environment is preferentially due to anabolic (related to primary production) or catabolic (related to respiration) processes. 2. The origin of marine humic substances is not completely resolved. An alternative to the classical polyphenol and melanoidin condensation models (Hedges 1978) and the photo-oxidation of lipids (Kieber et al. 1997) is the formation of humic substances as a byproduct of microbial respiration (Brophy and Carlson 1989; Kramer and Herndl 2004). A fraction of the respired organic carbon is transformed into biologically refractory organic matter instead of CO2 (Chen and Bada 1992; Hayase and Shinozuka 1995). We propose using the fluorescence of humic compounds (FDOMM) to trace the daily production of humic substances during microbial respiration processes in the dark. 1 Corresponding author ([email protected]). Acknowledgments We thank the captain, crew, and technician of RV Mytilus and the members of the Department of Oceanography (Instituto de Investigacións Mariñas) and the Group of Physical Oceanography (Universidade de Vigo) for their help. Special thanks to S. Brea, J. Gago, and T. Rellán. Comments by two anonymous reviewers helped improve the manuscript. Financial support came from the Spanish Ministerio de Ciencia y Tecnologı́a (MCyT) grant REN2000-0880-C02-01 and Xunta de Galicia grant PGIDT01MAR40201PN. A fellowship from the MCyT and the I3P-CSIC Program funded M.N.-C. Limnol. Oceanogr., 51(3), 2006, 1391–1400 E 2006, by the American Society of Limnology and Oceanography, Inc.