Photochemistry of marine and fresh waters: A role for copper–dissolved organic matter ligands

Abstract

The photodegradation of marine dissolved organic matter (DOM) plays a critical role in the global carbon cycle. The photodegradation of both DOM and particulate organic matter (POM) leads to the formation of carbon monoxide and/or the production of labile organic matter that can be rapidly biodegraded. DOM also regulates metal speciation, particularly important for the bioavailability of nutrient metals required by phytoplankton. Copper is one of these essential nutrients but is also toxic in elevated concentrations. Its speciation is regulated by DOM photodegradation however, DOM–copper ligands are not well characterized because of their low concentrations in natural waters. Copper immobilized metal affinity chromatography (IMAC) was used to isolate high and low affinity DOM ligands from both Pacific near-shore seawater and freshwater from the Black River (NC). Laser flash photolysis (LFP) was used to characterize excited state species from these fractions and to identify excited state species in the open ocean through a depth profile from 5 to 4532m (North Atlantic, Bermuda Atlantic Time Series Station (BATS)). LFP generated transients characteristic of the solvated electron (τ=3.5±0.1μs), a microbial-sourced triplet excited state (τ=1.1μs±0.1μs), and a long-lived transient, DOM*, with a millisecond lifetime. Unlike the seawater triplet excited state, a copper ligand from Black River water generated a triplet excited state with a lifetime of 3.4±0.1μs. Fractions were also characterized by excitation–emission matrix fluorescence spectroscopy (EEMs), UV–visible, and 1H NMR spectroscopy. EEMs spectra correlated the marine excited state species with the protein-like, microbial fraction while the freshwater transients were associated with chromophoric DOM (CDOM). 1H NMR with spectral database matching identified ninety-seven compounds as potential sources of these excited state species in the Pacific seawater IMAC fractions. Aqueous extracts of marine macro-algae (Sargassum natans (Atlantic Ocean), Macrocystis pyrifera), and surf grass (Phyllospadix torreyi) were analyzed to investigate potential additional sources of these excited state transients. To our knowledge, this is the first study to characterize DOM–copper ligands in seawater using a combination of EEMs, NMR and laser flash photolysis.